Inconel 718 (IN718) superalloy is widely used in aerospace engines and other high-temperature components. Improving its fatigue properties is crucial for ensuring the long-term stability of the components in service. Selective laser melting (SLM) technology is one of the best choices for the manufacturing of complex aerospace components owing to its high molding rate, high design freedom, and short production cycle time. However, the rapid melting and solidification of IN718 powder during SLM leads to the precipitation of a brittle Laves phase instead of a strengthening phase, inside the component. The characteristics of layer-by-layer scanning of SLM lead to the existence of high residual stress inside SLM-fabricated parts. Therefore, heat treatment is essential. This study investigates the influence of different heat treatment processes on the microstructure, phase distribution, and fatigue properties of SLM-fabricated IN718 alloy.

In this study, an IN718 alloy powder was prepared using a gas atomization method with particle size distributions between 15 and 55 μm. The IN718 specimens were prepared by EOSINT M280 from EOS, Germany and then cut along the plane of the substrate via wire cutting and removed. Subsequently, the IN718 specimens were subjected to three different heat treatments, as shown in Fig. 2. The three heat treatments is: 1) solution + double aging (SA); 2) homogenization + double aging (HA); 3) homogenization + solution + double aging (HSA). After heat treatment, the specimens were processed into fatigue specimens and subjected to a constant stress-controlled low-cycle fatigue test, at room temperature. Finally, after sample preparation and polishing, scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) photographic analyses were performed.

The distribution of precipitated phases differs significantly after different heat treatments (Fig. 5). After SA treatment, micron-level γ″ and γ′ strengthening phases and a small number of distributed δ phases exist in the matrix, whereas a large number of δ phases distribute at the grain boundaries. After HA treatment, nano-sized γ″ and γ′ strengthening phases exist in the matrix, and a small number of δ phases distribute at the grain boundaries. After HSA treatment, nano-sized γ″ and γ′ strengthening phases exist in the matrix, and δ phases exist at the grain boundaries. Differences in the distribution of the resolved phases leads to differences in fatigue performance (Fig. 6). Among them, the fatigue performance of the HSA treated IN718 specimen is the best, and the fatigue performance reaches 98.6% of the fatigue performance of the forged part. Subsequently, the specimens were prepared by wire cutting and the fracture morphologies were observed, and the fatigue morphologies of the specimens with different heat treatments were basically same (Fig. 7), that is, there are multiple fatigue source areas, obvious fatigue glow lines, and fatigue transient fracture areas with dimples and secondary cracks. The EBSD results (Fig. 8) show that the stress concentration is mainly in the δ-phase and grain boundary regions. Via analysis, it is found that dislocations slide freely inside the grain. When dislocations slide to the punctate γ′ phase, dislocations bypass the γ′ phase based on the Orowan strengthening mechanism and hinder the subsequent dislocation sliding. When dislocations slide to the flat elliptical γ″ phase, the γ″ phase hinders the dislocation movement and then γ″ phase will be cut with the accumulation of dislocations. Because the δ phase and matrix γ phase are non-conglomerative, dislocations accumulate around the δ phase. When dislocations slide to the grain boundary, the grain boundary can hinder the dislocation sliding and crack expansion. At the same time, the δ phase at the grain boundary can nail the grain boundary and delay the expansion of fatigue crack, thus improving the fatigue performance.

In this study, the microstructure and phase distribution of the IN718 alloy fabricated using SLM are regulated via heat treatment, to further analyze the effect of heat treatment on the low-cycle fatigue properties, at room temperature. The experimental results indicate that the alloy exhibited precipitation of the internal δ phase, as well as γ″ and γ′ strengthening phases precipitate inside the alloy following heat treatment, as opposed to the as-built conditions. The presence of these phases contributed to the alleviation of internal stresses within the alloy and led to a significant improvement in its fatigue performance. Based on the Orowan strengthening mechanism, the diffusely distributed γ″ and γ′ strengthening phases within the grain prevent the dislocations from sliding within the grain. The δ phase precipitated at the grain boundary can enhance the strength of the grain boundary, thus retarding microcrack extension in the matrix and increasing the fatigue cycles. Therefore, after HSA treatment, the IN718 specimen has optimized fatigue performance. The improvement of the microstructure and mechanical properties via heat treatment processes presented in this study provides a reference for the application of the SLM-fabricated IN718 components.

Direct laser deposition (DLD) technology is a novel technique that directly fabricates full-density near-net-shape metal components from metal powders. During the DLD process, fine metal powders are fed into a molten pool produced by a sharply focused and high-energy laser beam. Compared to conventional techniques, the DLD process has some remarkable advantages: reduction of production cycle and cost, high material utilization ratio, and excellent flexibility. It has great potential for manufacturing large complex metal components, especially for difficult-to-process materials like titanium alloys. Therefore, it is suitable for preparing high-performance and large-scale titanium alloy components.

With the continuous improvement of aircraft performance requirements and the continuous optimization of the structure, the demand for large and complex integral parts is increasing steadily, which poses a severe challenge to DLD technology. On one hand, the forming cavity size and motion mechanism capacity of DLD equipment are required to increase continuously. On the other hand, as the size of the component increases, the component is prone to deformation, cracking and even scrapping during subsequent heat treatment. Therefore, the manufacturing method of using the DLD to connect different additive manufactured parts into a whole structure can address the aforementioned problems. The technology that involves forming two or more high-performance metal components by the DLD technology is called DLD connection, and the connection area is called DLD connection area.

DLD connection technology is based on the basic principle of the DLD process; that is, through the layer-by-layer melting deposition of the connecting area, the fully dense metallurgical fusion connection of the parts is achieved. It has many unique technical advantages, the most important of which is the microstructure of the DLD connection area is finer, and the mechanical properties are also excellent. The microstructure and mechanical properties of the DLD titanium alloy have been widely reported, and the factors affecting its service performance have been fully understood. However, there are few reports on DLD connection technology, especially concerning the research and understanding of the microstructure and mechanical properties of the DLD connection area and the heat-affected zone. Therefore, this study takes TA15 titanium alloy as the research object and studies the microstructure of the DLD connection area of TA15 titanium alloy components by DLD connection technology. The mechanical properties of the DLD connection area also have been tested and analyzed.

The raw TA15 powders used in the DLD and DLD connection processes are prepared by plasma rotating electrode processing. The DLD and DLD connection processes are carried out on the DLD engineering equipment under an argon atmosphere with oxygen volume fraction of less than 80×10^-6. The processing parameters are as follows: laser beam power of 7-8 kW, scanning speed of 600-1000 mm/min, powder delivery rate of 0.8-1.2 kg/h, and overlap ratio of 30%-50%. The forming process diagram of the DLD connection TA15 sample is shown in Fig. 1. First, the body area is prepared by the DLD process. Then the connection groove is prefabricated on the TA15 body. Finally, the DLD connection TA15 titanium alloy sample is completed until the groove is filled by the DLD technology. The deposition direction of the body area is the same as that of the DLD connection area. Table 1 presents the chemical compositions of the powder and the substrate materials. The microstructures of the samples corroded by Kroll reagent after polishing are observed by optical and scanning electron microscopes. Two samples, including the T and L direction samples perpendicular to the deposition direction, as illustrated in Fig. 2, are observed. Different mechanical properties, including room temperature tensile, room temperature impact, and fracture toughness properties, are also tested. The shape and dimensions of the different specimens are illustrated in Figs. 3-5.

The microstructure of the DLD connection zone is the same as that of the additive manufactured base zone. The connection demarcation line can be observed in the low-magnification images. However, there is no obvious tissue difference between the two sides of the connection demarcation line in the high-magnification images. From the longitudinal section [Fig. 6(b)], the structure is similar to that of the base zone. On the cross-section [Fig. 6(a)], the grain morphologies of the DLD connection zone and the base zone are also equiaxed, and the grain sizes are the same. The microstructures of different regions in the DLD connection TA15 titanium alloy are shown in Fig. 7. The microstructures of these zones consist of an ultrafine α+β lamellar basketweave structure. We test the tensile, impact, and fracture toughness properties at room temperature in different directions of the bonding zone and compare them with the corresponding properties of the additive manufacturing base area. The existence of the DLD connection interface in the bonding zone exhibits no significant effect on the mechanical properties of the DLD connection TA15 titanium alloy. The tensile strength, elongation, impact toughness, and fracture toughness of the bonding zone at room temperature are 1046 MPa, 7.2%, 33.17 J/cm², and 78.2 MPa·m^1/2, respectively.

Here, the microstructure and mechanical properties of the TA15 titanium components manufactured by DLD connection are investigated. The microstructures of different regions in the DLD connection zone and the base zone are the epitaxially grown β columnar crystal and ultrafine basket α+β lamellar structure within the β grains. The properties of tensile, impact, and fracture toughness at room temperature in different directions of the bonding zone are the same as those of the base zone. The existence of the DLD connection interface in the bonding zone has no obvious effect on the mechanical properties of the DLD connection TA15 titanium alloy. The tensile strength, elongation, impact toughness, and fracture toughness of the bonding zone at room temperature are 1046 MPa, 7.2%, 33.17 J/cm², and 78.2 MPa·m^1/2,^{respectively.}

Ni-Ti shape memory alloys exhibit excellent superelasticity, shape memory properties, and biocompatibility; however, their poor processing performance, high reaction chemical activity of titanium, and high cost seriously limit their applications. It is necessary to develop low-cost shape memory alloys with sound shape memory effects to replace Ni-Ti for industrial applications. Cu-based shape memory alloys have high strength, high conductivity, excellent superelasticity, shape memory effect, a wide range of phase-transition temperatures (-180-400 °C), and low production cost (about 1/10 of Ni-Ti). Although its shape memory effect and stability are lower than those of Ni-Ti, it has apparent advantages under certain conditions (such as hot water temperature control valves, water heaters, decorations, and toys) where the requirements for its shape memory performance and stability are not too harsh. Cu-Al-Ni and Cu-Zn-Al have strong industrial applications due to their low price; however, their poor thermal stability, high-order degree of the parent phase, and high elastic anisotropy in polycrystalline alloys lead to brittleness during deformation. In recent years, Cu-Al-Mn shape-memory alloys (SMA) have attracted considerable attention because of their low price, shape-memory effect, and excellent mechanical properties. An alloy prepared by selective laser melting (SLM) has the characteristics of a fast cooling rate, small grain size, and no component segregation. It has natural advantages in the preparation of complex-shaped parts and has brought unlimited prospects for the preparation and application of Cu-Al-Mn alloys. However, shape memory alloys prepared by SLM still have problems, such as unstable mechanical properties and degradation of functional properties.

Pre-alloyed powders with high sphericity, fluidity, and uniformity prepared by vacuum atomization were used as raw materials in this study. Cu-Al-Mn alloy samples with different laser powers were formed on a stainless-steel substrate without preheating. The microstructure and martensite type of the alloy were determined by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). X-ray diffraction (XRD) and differential scanning calorimetry (DSC) determined the alloy’s phase composition and transformation behavior. The functional properties of the alloys were determined using bending and tensile loading recovery experiments. The microhardness and mechanical properties were measured at room temperature. The local deformation behavior during the tensile process at room temperature was analyzed using a full-field strain analysis. The changes in density, phase composition, phase-transition temperature, mechanical properties, and functional properties caused by changes in laser power were studied.

The alloy mainly comprises 18R martensite and γ₁ phase at room temperature ( Figs. 4 and 8 ). With the increase of laser power, the molten pool temperature of the alloy increases, the cooling rate decreases, and the solidification path in the alloy changes, which corresponds to the rise in the γ₁ phase in the alloy (Fig. 4). However, the precipitation of γ₁ phase and the volatilization of Al (Table 2) inhibit its precipitation; therefore, the γ₁ phase decreases when the laser power increases from 325 to 375 W. All the alloy samples exhibit the phase-transformation behavior of P (austenite parent phase) ? M (martensite phase) during the heating/cooling process. With the increase in laser power, the intensity of the endothermic/exothermic peak of the DSC curve first increased and then decreased. The number of martensites involved in the phase transformation increased and then decreased (Fig. 6). The sample with 325 W exhibited the best one-way shape memory effect. The shape memory recovery rate exceeded 95% (Fig. 12). The increase of γ₁ phase and the solid solution of Mn make the microhardness of the alloy increase when the laser power increases from 175 W to 325 W (Fig. 9). The sample with a laser power of 175 W had a stress platform for stress-induced martensite reorientation almost parallel to the abscissa. With an increase in the laser power, the work-hardening rate increased. The yield strength of the sample decreased and then increased (Fig. 10). This was accompanied by a change in the deformation behavior from uneven to uniform (Fig. 11), corresponding to the residual strain of the former with an increase in pre-deformation, its growth rate gradually slowed. In contrast, the residual stress of the latter was nearly linear with the pre-deformation (Fig. 13).

The results show that the change in type and number of martensite is the main reason for the change in phase transformation behavior and properties of the alloy. With the increase of laser power from 175 W to 325 W, the order degree of the alloy increases, the volatilization of Al element increases, the content of γ1' and hardness increases, the intensity of DSC exothermic/endothermic peak increases, the one-way shape memory effect improves, the slope of the stress-induced martensite reorientation platform of the tensile curve at room temperature increases, and the deformation mode of uneven local deformation expansion of 175 W changes to uniform deformation of 375 W. A Cu-Al-Mn alloy with nearly full density (99.89% ), nearly 100% one-way shape memory effect and ~780 MPa tensile strength was prepared. By adjusting the forming parameters, a feasible method can be provided to control the phase-transition temperature and mechanical properties of Cu-Al-Mn shape memory alloy.

Weight reduction in aircraft systems is of great significance for reducing the consumption of aircraft fuel, and therefore has always been crucial in the development of aviation equipment. The weight of electromechanical aviation products in aircraft systems accounts for approximately 30% of the whole aircraft, and the number of electromechanical parts accounts for approximately 60%, and are characterized by large weights and numbers, as well as complexity. Currently, there are two main approaches for reducing weight: upgrading materials and design. Conventional structural optimization and applications of lightweight metal materials to reduce weight have reached the optimization limit and are gradually becoming unable to meet new lightweight demands. Therefore, there is an urgent need to develop new materials, structures, and technologies to realize lightweight designs and manufacture electromechanical aviation products. Metal laser additive manufacturing technology with integrated design and manufacturing capabilities of material-structure-performance-function can provide advanced methods for lightweight design and high-performance manufacturing of electromechanical aviation products, improving the innovative design and rapid manufacturing capability of electromechanical aviation products and promoting the rapid development of the electromechanical aviation industry.

With its ability for highly flexible manufacturing and precision forming, laser powder bed fusion (LPBF) technology will be increasingly used in the design and manufacture of electromechanical aviation products. MOOG was the first company to apply LPBF technology to manufacture complex hydraulic parts and has accumulated considerable experience in pore control and material performance verification during the printing process of hydraulic products. MOOG has developed a new actuator structure and the next generation of aviation motor manifolds based on LPBF (Fig. 6). Based on LPBF technology, Liebherr developed a titanium hydraulic integrated valve block, which is 35% lighter than the original product and was certified for its first flight on the Airbus A380 in 2017 (Fig. 8). Domin Fluid Power, a British manufacturer of fluid power systems, developed a direct-drive servo valve using LPBF technology (Fig. 8). Materials Solutions developed a high-performance aerospace titanium alloy hydraulic manifold using an additive manufacturing (AM) design (Fig. 8). The hydraulic manifold is precisely shaped based on LPBF technology. The hydraulic manifold can satisfy the requirements of the most stringent aerospace working conditions while ensuring high safety and reliability. In the field of hydraulic products, an integrated hydraulic steering gear shell was successfully developed by Nanjing Engineering Institute of Aircraft Systems based on LPBF (Fig. 9), which demonstrated that LPBF can effectively solve the problems of multiple oil paths, multiple process plugholes, and complex structures in traditional steering gears. Compared to the traditional steering gear structure, the 3D printed steering gear shell reduced weight by approximately 51%, flow resistance by approximately 48%, and the manufacturing cycle by approximately 70%, showing high product reliability. In the field of fuel products, an integrated air refueling fuel hood was successfully developed by Nanjing Engineering Institute of Aircraft Systems based on LPBF (Fig. 11). By using a pneumatic edge layout, integrated precision forming of the fuel hood was realized. Compared with those by traditional manufacturing methods, the AM fuel hood achieves integrated airfoil forming, which significantly improves the yield and reduces the component weight and production cycle by approximately 41% and 50%, respectively. In the field of environmental control products, an integrated ejector was successfully developed by Nanjing Engineering Institute of Aircraft Systems based on LPBF (Fig. 13). The ejector realized an integrated design of the main pipe and ejector nozzle structures, which could reduce the risk of the manufacturing process. Compared with that by the traditional manufacturing method, the weight of the ejector was reduced by approximately 64%, processing cycle was shortened by more than 50%, and the heat transfer performance was improved by 9.3%. Moreover, multistructure integrated forming was realized, and the reliability of the product was greatly increased.

AM technology can help realize the integrated design and manufacturing of materials, structures, performances, and functions, aiding in the development of smart and lightweight electromechanical aviation products. Currently, AM is applied in the manufacture of various types of electromechanical aviation products, demonstrating great application potential. In the future, digital twin-driven AM technology, hybrid additive and subtractive manufacturing technology, and multimaterial AM technology will be the key research areas for the development of aviation electromechanical products.

In order to explore the differences of selective laser melting (SLM) formation in three modes: single laser scanning, dual-laser low-power synchronous scanning (the power of two lasers becomes half of the power of the single laser, and the other parameters remain unchanged) and dual-laser high-speed synchronous scanning (the speed of two lasers becomes double of the speed of the single laser, and the other parameters remain unchanged), we investigated the feasibility of forming stainless steel with the dual-laser and the impact of the techniques on forming quality. The study provides experimental support and a theoretical basis for high-efficiency forming research of multi-galvanometer multi-laser selective melting equipment.

Self-developed dual-laser synchronous scanning equipment was used to form 316L stainless steel samples under the same energy density through single-laser and dual-laser synchronous scanning. The corrosion samples were used to observe the morphology and microstructure of the molten pool. Finally, according to the above results, appropriate process parameters were selected, tensile samples were prepared, the mechanical properties of the samples formed with three modes were compared, and the fracture morphology was observed.

As far as spatter is concerned, compared with a single laser, dual-laser low-power synchronous scanning method is beneficial for increasing the depth of the molten pool and increases the metallic jet. At the same time, in dual-laser high-speed synchronous scanning mode, owing to the increase in the laser beam energy and shortened interaction time between the laser and powder, a higher temperature gradient occurs, which aggravates the Marangoni effect. The high-temperature bottom of the molten pool flows to the low-temperature sidewalls and rear edges, and more liquid flows from the molten pool at a certain initial velocity, thereby increasing the droplet spatter (Fig. 6).

Compared with the single lase forming sampler, the density of dual-laser low-power synchronous scanning formed sample is more sensitive to the laser power and increases linearly with the increase in power. This is owing to the influence of the superposition of the double beams. When the laser power is low, the temperature of the molten pool is relatively low, resulting in poor fluidity, and the retained gas cannot escape the molten pool in time, resulting in internal pore defects, which reduces the relative density of the sample. For dual-laser high-speed synchronous scanning, the density of the sample is comparable to that of a single laser and the range of density variation is extremely small. This may be because the energy threshold required for shaping is achieved even at relatively low laser power.

Compared with the single laser mode, the molten pool distribution of dual-laser low-power synchronous scanning was more uniform, and the distance between the bottoms of the molten pools increases by approximately 25%, from 32 μm±5 μm to 40 μm±5 μm (Fig. 9). Because the laser spot has a Gaussian distribution, when the laser power is reduced, the circular area that can reach the melting point decreases, but the two laser beams are scanned synchronously, and most of the energy of the second beam spot acts directly on the molten pool instead of the powder; hence, the depth of the molten pool increases. However, for dual-laser high-speed synchronous scanning, with the increase of laser power, the molten pool becomes wide and deep from wide and shallow. Compared to the molten pool formed by a single laser, the width direction changed significantly, and the most intuitive change is that the overlap ratio exceeds 50%. The laser beam is in contact with the powder, and heat spreads in the horizontal and vertical directions. For dual-laser high-speed synchronous scanning forming in this experiment, when the power is low, it is easy to produce a wide and shallow molten pool. When the power is increased to 240 W, an excessively high power causes the current layer to combine with the molten pool of the previous layer, resulting in a larger molten pool.

1) The dual-laser synchronous scanning method can be used to form 316L stainless steel samples. The density of the samples is above 99%, the tensile strength can reach 720 MPa, and the elongation rate exceeds 40%, meeting application requirements.

2) Both single laser forming and dual-laser high-speed synchronous scanning forming, the interior of the grain is dominated by columnar subcrystals, but the width of the single laser subcrystal is 0.50 μm, while the latter is approximately 0.35 μm. Columnar, equiaxed, and elongated equiaxed subcrystals coexist under dual-laser low-power synchronous scanning mode.

3) Dual-laser high-speed synchronous scanning method can double the forming efficiency.

When metal components are manufactured by selective laser melting, a suspension structure cannot be directly formed in the unmelted powder layer. A suitable support structure must be added in time to dissipate the heat generated during the metal powder melting process and restrain the deformation of the overhanging characteristics. Therefore, the manufacturability and formation quality of the overhang structure are related to the added support structure. Various types of support structures, such as block, plane, tree, and cone supports, have been designed for overhanging structures, and considerable research has been conducted on support structure optimization, support strength, and easy removal. However, the type of performance that differs between support structures is not clear. In practice, this depends primarily on the experience of selecting a certain support structure. In this study, the effects of cone, block, and combination supports on the thermodynamics and formation quality of overhanging specimens are studied, and the selection principle of the support type is formulated; this can provide a technical reference for the selection of supporting structures.

First, the thermal elastic-plastic finite element method is used to simulate the additive manufacturing process of the cone, block, and combination support specimens. By quantitatively analyzing the temperature data at the end of the build process along vertical and horizontal paths, which are extracted from the model, the influence of the heat conduction characteristics of the different support structures on the temperature field of the specimen is revealed. Three types of supporting specimens made of 316L powder are prepared using the selective laser melting (SLM) method. The surface morphologies of the specimens after removing the support are analyzed using the laser scanning confocal microscope and scanning electron microscope. The effect of the supporting structure on the surface morphology is studied. The effects of the supporting structure on the microstructure and elemental distribution are studied using an optical microscope and electron probe. Finally, the microhardness of the formed specimens is measured using a microhardness tester to characterize their mechanical properties.

The geometry of the support structure affects the distribution of the temperature field in the overhanging specimen. The block support reduces the temperature gradient in the edge region (Fig. 6), thus reducing the specimen warpage caused by thermal strain. The block support reduces the peak temperature by 6.7%, temperature gradient by 14.05%, and temperature oscillation amplitude by 41.07% of the specimen, indicating that the block support structure has excellent cooling performance, whereas a higher cooling rate helps the overhang structure to form fine grains with good microstructural properties. In addition, it is found that the combination support specimen has the smallest warpage (0.29 mm) and surface roughness (70.804 μm) among the three types of specimens (Fig. 12). The excellent support strength and heat dissipation performance of the combination support improve the formation accuracy of the specimen. The block and combination support specimens have refined grain and reticulated alloying element phases such as Cr and Ni in the sub-grains. The dense metal structure and the brittle and hard reticular phases increase the microhardness to 234 HV (Fig. 17). However, the microhardness fluctuation of the block support specimen is the largest owing to the existence of many porosity defects, and its value is 22 HV. Therefore, a combination support structure with better comprehensive performance is preferred to improve the mechanical properties of the overhanging structure.

The support structure changes the thermomechanical evolution process of the overhang plate. The block support can significantly reduce the peak temperature, temperature oscillation amplitude, and edge temperature gradient of the specimen, concluding that the block support structure exhibits excellent cooling performance. The combination support can not only restrain the specimen's warpage by providing sufficient support strength but also improve the surface topography quality by reducing the powder clusters on the lower surface of the overhanging specimen. Therefore, choosing a combination support is conducive to the preparation of overhang structures with higher dimensional accuracy. The support structure affects the microstructure of the overhanging area of the specimen. The metal structure of the cone support specimen is coarse, uniform, and has fewer porosity defects that can be removed by setting a small machining allowance. Owing to the intense competition of metal grain growth in the block support specimen, the refined grains and the brittle and hard reticular phases formed in the overhang area significantly improve the microhardness. The combination support has the advantages of the other two types of support. Increasing the proportion of block structures in combination-type supports is beneficial to improve the mechanical properties of specimens, and increasing the proportion of cone structures is conducive to reduce material consumption.

Compared to conventional material processing techniques, selective laser melting (SLM) technology possesses several advantages, including rapid processing capabilities and the ability to fabricate complex parts. The increasing demand for lightweight aluminum matrix composites has imposed higher requirements on their preparation methods. Graphene has excellent mechanical properties and is an ideal reinforcement material for metal matrix composite. However, the uniform dispersion of graphene into metals remains a challenge, and the strengthening mechanism of graphene nano-platelets (GNPs) in SLM-fabricated GNPs/AlSi10Mg composite materials needs to be explored. This study aims to investigate the effect of varying GNPs mass fraction (0, 0.1%, 0.3%, and 0.5%) on the microstructure and mechanical properties of SLM-fabricated AlSi10Mg composites, to reveal the strengthening mechanism of GNPs-reinforced AlSi10Mg composites.

Different types of GNPs/AlSi10Mg composite powders were prepared using a QM-3SP4 ball mill. The ball-milling parameters were set as follows: a ball material ratio of 8∶1, speed of 230 r/min, and ball-milling time of 2 h. Different types of GNPs/AlSi10Mg composite materials were prepared using a laser melting forming equipment (AM400, Renishaw, UK). Tensile testing was conducted using a universal tensile-testing machine (SHIMADZU AG-X plus). Sample hardness was measured using a precision automatic turret digital microhardness tester (JMHVS-1000AT type). The crystallographic structure was analyzed using an electron backscatter diffusion (EBSD) equipment, integrated into a JEOL JSM-7800F field-emission scanning electron microscope. The microstructure and phase composition of the composite materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS).

EBSD inverse pole figure (IPF) maps of the longitudinal section of the AlSi10Mg alloy and 0.5%GNPs/AlSi10Mg composite show that the addition of graphene does not change the grain orientation but refine the grains (Fig.4). The EBSD grain boundary misorientation distribution in Fig.5 indicates that the addition of GNPs increases the number of nucleation sites and hinders grain growth, resulting in an increase in LAGBs inside the melt pool and a decrease in the toughness. The SEM and XRD results show that the microstructures of the matrix and composite are composed of fine grains, coarse grains, and heat-affected zones. The dark grey color represents the aluminum solid solution phase, while the light grey network structure represents the eutectic silicon phase. Tensile tests show that the ultimate tensile strength of the 0.1%GNPs/AlSi10Mg composite material is (383±4) MPa, with an elongation at break of 8.4%±0.14%, indicating a good ductility strength. As the amount of GNPs increases, the ultimate tensile strength (UTS) and yield strength (YS) of the GNPs/AlSi10Mg composite materials show a decreasing trend. With an increase in the GNPs content, the fracture morphology of the composite material (Fig.9) shows a series of small cleavage steps, a river-like pattern, small tearing edge undulations, and plastic fracture characteristics formed by ductile dimples, transitioning towards brittle straight fractures. The strengthening mechanism of the 0.1%GNPs/AlSi10Mg composite material formed by selective laser melting is mainly controlled by the synergistic effect of thermal mismatch and load transfer strengthening. The pinning effect of the GNPs on the dislocations provides additional interfaces that can hinder their movement. In the 0.1%GNPs/AlSi10Mg composite material, the interface bonding between the GNPs and matrix is strong, and the dislocation resistance is high, resulting in the best plasticity and toughness among the composite materials.

The preferred orientation of the SLM-fabricated AlSi10Mg alloy is〈100〉. The addition of GNPs does not change the preferred orientation of the GNPs/AlSi10Mg composite material, although it reduces the proportion of the large-angle grain boundaries in the composite material. The phase composition of all GNPs/AlSi10Mg composite materials investigated are α-Al and eutectic silicon phases. As the GNPs content in the composite material increases, the hardness also increased, reaching a maximum of 168 HV. However, an increase in the number of GNPs led to an increase in the number of defects in the composite material. The ultimate tensile strength, yield strength, and elongation of the 0.1%GNPs/AlSi10Mg composite materials are (417±4) MPa, (254±5) MPa, and 8.4%±0.14%, respectively. These values gradually decrease to (224±6) MPa, (150±3) MPa, and 4.0%±0.45%, respectively, for 0.5%GNPs/AlSi10Mg composite material. The strengthening of the 0.1%GNPs/AlSi10Mg composite material is mainly controlled by the synergistic effect of the thermal mismatch and load transfer strengthening.

2219 aluminum alloy is a high-strength aluminum alloy that can be strengthened by heat treatment. Owing to its excellent processability, it has become one of the most preferred structural materials in the aerospace field, and is commonly known as aviation aluminum. Laser welding, which is the most commonly used connection method for 2219 aluminum alloys, is characterized by a high welding efficiency, small heat-affected zone, and high power density. However, when a single-beam laser is used to weld the butt structure of a moderately thick plate, the penetration is too shallow, resulting in incomplete penetration. When the laser power is increased to solve the problem of incomplete penetration, large welding deformations and residual stresses caused by excessive heat input occur. Laser mirror welding synchronously operates on the plate structure through a symmetrical double-laser heat source to form a joint molten pool and keyhole, which can solve the problems of large welding deformation and welding residual stress caused by single-beam laser welding. Owing to the complex interaction between the double heat sources in the laser mirror welding process, the joint molten pool generated by the double heat sources has different cooling rates during the cooling process, which leads to uneven distribution in the microstructure. Presently, the research on the formation mechanism of the microstructural differences in laser mirror-welded joints under different welding parameters and their influence on the mechanical properties is still insufficient. Therefore, laser mirror welding experiments are performed on a 2219 aluminum alloy with a thickness of 6 mm in this study. The microstructural differences at distinct positions on the welded joint and at the same position under different welding heat inputs and their formation mechanisms are compared and analyzed. Subsequently, the difference in the mechanical properties caused by microstructural differences are explored to provide a reference for improving the quality of laser mirror welding using a moderately thick aluminum alloy plate.

In this study, 2219 aluminum alloy, which is a widely used and representative material in the aerospace field, is used. The dimensions of the welding material are 50 mm×100 mm×6 mm. The laser mirror welding experimental equipment (Fig. 1) used in this study includes two mirror symmetry laser welding heads, two high-precision robots, a gantry, a megawatt disc laser, and a splitter. The process parameter variables for this experiment are mainly the laser power (P) and welding speed (V), and the combined process parameter variables are shown in Table 2. A hardness test is performed on the laser-welded joints using a microhardness tester. The thickness of the sample used in this study is 6 mm, and an electronic universal tensile machine with a maximum tensile force of 10 kN is used for operation. Finally, the microstructures and chemical compositions of the welded joints are analyzed using scanning electron microscopy (SEM) and energy disperse spectroscopy.

A comparison of the cross-sectional areas of the weld seams under different welding heat inputs reveals that the area increases with an increase in the welding heat input (Fig. 2). When the welding heat input is 913.12 J·cm^-1, the typical regions of laser welded aluminum alloy such as the non-dendritic equiaxed zone (EQZ), columnar crystal zone, and equiaxed crystal zone, appear from the heat-affected zone to the center of the weld seam. Additionally, abundant equiaxed dendrites appear in the joint part of the weld seam, and the grain sizes of the equiaxed crystals on the left and right sides of the weld seam are similar (Fig. 4). With an increase in the welding heat input, the width of the columnar crystal zone in the upper fusion line area decreases from 127 μm to 87 μm (Figs. 5-6), and the width of columnar crystal zone in the lower fusion line area becomes much smaller than that in the upper fusion line area (Fig. 7). Additionally, the width of the columnar crystal zone and number of equiaxed dendrites in the weld center area increase, which indicates that the increase in heat input refines the grains in the weld center area.

An increase in the welding heat input increases the "isosceles " ratio and size of the joint weld area. The width of columnar crystal zone in the upper fusion line area reduces from 127 μm to 87 μm, and the columnar crystal zone in the lower fusion line area is narrower than that in the upper fusion line area. Moreover, the central areas of the weld seams under different welding heat inputs have the equiaxed dendrite structure. With an increase in the welding heat input, the number of equiaxed dendrites in the central area of the weld seam increases, proving that the increase in the heat input has a positive effect on grain refinement. When the welding heat input increases, the mechanical properties of the welded joints are significantly enhanced. The maximum microhardness of the weld seam is 74.9 HV, and the tensile strength increases from 188.39 MPa to 259.47 MPa. Moreover, increasing the welding heat input can result in the formation of a more reliable joint weld seam and the enhancement of the tensile properties of the welded joints. Equiaxed dimples with different sizes exist at the fracture site, and the second phase, which is rich in Cu, exists at the bottom of the dimple. With an increase in the welding heat input, the fracture mode changes from mixed fracture to ductile fracture, the number of dimples increases, and the size becomes larger and more uniform.

Although milling and planing are used in traditional machining of V-grooves, tool wear and passivation affect the machining efficiency and quality. Laser subtractive forming technology exhibits the characteristics of no tool wear, high processing efficiency, and flexible forming, and uses a high-power laser beam as a heat source to realize non-contact processing. Nevertheless, there are some limitations in traditional laser subtractive forming technology. For example, laser drilling and laser ablation technologies generally use a pulsed laser, whose aperture size is of a micron level. As a result, the processing efficiency is considerably reduced when the processing amount is large. Furthermore, it is difficult for traditional laser subtractive forming technology to process millimeter-deep V-grooves at one time. In this paper, a new method of laser subtractive forming of V-grooves is proposed. The Lorentz force is generated inside the molten pool by applying an electromagnetic composite field to drive the melt discharge and achieve the purpose of V-groove machining. Based on a certain machining accuracy, this method can process V-grooves with different angles and depths. Moreover, this method can avoid the problem of tool wear in traditional machining, and its material removal efficiency is much higher compared to traditional laser material removal technology. Therefore, this technology is expected to become a new high-efficiency and high-quality material subtractive forming method.

316L stainless steel is employed as the substrate in the present study. First, a 2 kW semiconductor laser is used to irradiate the surface of the substrate, and the Lorentz force formed by the steady electric and magnetic fields drives the melt to overflow upwards. Simultaneously, a gas nozzle is applied at the tail of the molten pool to separate the metal melt from the substrate. The spatters are collected by a device to ensure that the V-groove forming is safe for operation environment. Second, the three-dimensional profile of the V-groove is captured by VHX-5000 three-dimensional microscope and the groove's wall surface is analyzed by VK-X 1000 shape-measuring instrument to calculate the roughness of processed area. Furthermore, the influence of the magnetic induction intensity on the thickness of the remelting layer in the processed area is analyzed. The influence of laser parameters and electromagnetic field parameters on the morphology of the V-groove is evaluated. Finally, the flow behavior of the molten pool is observed using a high-speed camera, and the mechanism of the Lorentz force in the machining of the V-groove is discussed.

A high-quality and efficient V-groove processing method is established through comparative experiments (Fig. 2), which verifies the feasibility of the new laser subtractive forming technology. Owing to the increase in the Lorentz force, the melt is accelerated to overflow upward, resulting in an increase in the depth of the V-groove and decrease in the remelted layer thickness (Fig. 5, Fig. 6). Owing to the change in the heat input, a V-groove with an angle of 34.82-65.20° and depth of 1719-5667 μm is prepared (Fig. 8). It is found that the material removal efficiency increases with an increase in the laser power, and the material removal efficiency is 67.58 mm³/s at a laser power of 2000 W (Fig. 9). The target V-groove is processed, and according to the national standard, the angle tolerance level is precision f and the depth linear tolerance level is medium m (Fig. 10). The videos captured by the high-speed camera show that the applied Lorentz force causes the melt to overflow (Fig.12), and the material removal efficiency is significantly increased.

In V-groove forming with electromagnetic composite field, the Lorentz force drives the melt to flow upward under different laser heat inputs, a V-groove with an angle of 34.82°-65.20° and a depth of 1719-5667 μm is obtained on the basis of ensuring a certain machining accuracy. The increase in the Lorentz force further promotes the overflow of the molten pool, and the metal melt adhering to the side wall is reduced; the thickness of the remelting layer at the bottom of the V-groove is 42 μm. Based on the video captured by the high-speed camera, the material removal efficiency is found to increase significantly when the electromagnetic composite field is applied, and the center forms obvious pits and grooves. Moreover, the material removal efficiency is mainly related to the laser power and reaches 67.58 mm³/s at laser power of 2000 W. A molten pool is formed by irradiating the substrate with a laser heat source. When an electromagnetic compound field is applied, the Lorentz force causes the melt to overcome the surface tension and gravity, flow upward, and eventually escape from the substrate with multiple spatters, completing the processing of the V-groove in the area to be processed.

Lightweight cars have become an inevitable trend for coping with global warming. A large number of advanced high-strength steels (AHSSs) have been developed to improve lightweight, safety, and other aspects. Third-generation AHSSs, represented by QP980 steel, are suitable for a variety of automotive parts, such as cross members, stringers, B-pillar reinforcements, base beams, and bumper reinforcements. Welding is an important joining method used in the production of automotive structures. Therefore, it is necessary to develop advanced welding technology for QP980 steel.

A new welding method using laser-TIG hybrid welding technology and a low-melting-point welding wire was developed. The lap contact portion of the substrate was 10 mm without gaps. The laser beam acted vertically on the substrate, and the angle between the TIG welding wire and substrate or laser beam was 45°. The welding wire was placed 25° away from the substrate. Ar (99.99% purity) was used as a shielding gas. The TIG current was used in DC mode. A scanning electron microscope equipped with an energy spectrometer was used to observe the microstructures and fracture surface morphologies of the welded joints. An electron microhardness tester was used to determine the microhardness variations in the joint. The sample tensile parameters were tested three times using a DN300 universal testing machine at a constant speed of 1 mm/min.

For the same arc current, the contact angle θ is smaller for laser-TIG welded joints than for TIG welded joints, and molten metal spread length S and brazing zone length W are larger than those for TIG welded joints (Fig.4). The results showed that the wettability of the laser-TIG-welded joint was better than that of the TIG welded joint. The laser improved the arc energy utilization, and the melt-pool temperature was higher. The higher the temperature, the better the wettability of the molten metal. The cracks started from the brazing zone (Fig.10); therefore, the formation of the brazing zone was directly related to the mechanical properties of the joint. The analysis of the joint strengthening mechanism was as follows: the arc current increases from 100 A to 140 A, and the increase in W increased the bearing zone of the brazing zone and the total load of the joint. During the tensile process, the deflection of the joint causes the stress concentration in the brazing zone to crack more easily. A longer W also limits the deflection of the joint. Simultaneously, with an increase in the current, the melting amount of the substrate increased, and the strengthening phase (Fe-rich islands and Fe-rich particles) of the joint increased. Therefore, with the increase of the current, the ultimate tensile shear load of the joint increases. After the laser was introduced, the attraction and compression effects of the laser on the arc concentrated the arc energy and improved the wettability of the joint. Simultaneously, the melting amount of the substrate increased, and the strengthening phases in the fusion welding and brazing zones increased. Therefore, the performance of the laser-TIG hybrid welded joint was better than that of TIG welded joint under the same arc current. With an increase in current, the attraction and compression effects of the laser on the arc are weakened, and the length gap of W decreases under a larger current; thus, the gap in the joint performance decreases. Based on the above analysis, it is important to further improve the performance of the lap welded joint of QP980 steel by using laser-TIG control joint forming.

A new welding method for laser-TIG hybrid welding technology and a low-melting-point wire were developed to investigate the QP980 lap welded joint formation, organization, and strengthening mechanisms. Macroscopic morphology: the joint consists of the fusion welding, brazing, and heat-affected zones; owing to the addition of the laser, the contact angle of the laser-TIG hybrid welded joint is further reduced, and the length of the brazing zone is further increased. The microstructure of the fusion welding zone is dominated by a copper-based organization with Fe-rich islands and Fe-rich particles distributed among them, and the brazing zone is distributed with Fe-rich particles. The tensile shear load for both welding methods increase with the increase of arc current; the laser-TIG hybrid welded joint has a higher tensile shear load for the same arc current. Increasing the arc current and introducing a laser increase the length of the brazing zone and improve joint performance.

Transparent glass-ceramics have attracted widespread attention as a composite material with excellent performance, and potential applications require the welding of the heat-tempered glass. Femtosecond laser, as a promising tool for processing in recent years, has been widely used in the field of processing due to its characteristics of high peak power, small heat-affected zone caused in the material, high processing accuracy, and wide range of applicable materials. However, a great deal of research work has been done based on the welding of untempered glass, and the welding of tempered glass is rarely reported. There are two ways to temper glasses, one is chemical tempering, and the other is thermal tempering. The thermally tempered glass can buffer part of the external stress and inhibit the melt from filling glass gap, thus increasing the difficulty of glass welding. To our knowledge, the non-optical contact welding of Li₂O-Al₂O₃-SiO₂ (LAS) transparent glass-ceramics with zero thermal expansion coefficient has been studied by femtosecond laser pulses for the first time. This study has potential significance for the further expansion of the applications of LAS transparent glass-ceramics.

A femtosecond laser beam with a wavelength of 1030 nm is used for experimental studies of glass welding to reduce the cracking tendency during the glass welding process. A lens with a focal length of 100 mm is used to focus the femtosecond laser beam, and the temperature gradient of the focused spot is smoother, which reduces the residual stresses induced during welding process. In order to achieve a strong heat accumulation effect of the femtosecond laser pulse inside the LAS transparent glass-ceramics, the repetition rate is kept at 500 kHz. And the spots between scanning lines are guaranteed to have certain overlap. The morphology and the shear strength of the welding region of LAS transparent glass-ceramics welded by femtosecond laser with different energies are studied, and the transmittance of the welded glass is measured. The energy window of LAS transparent glass-ceramic welding is obtained by simultaneously changing the energy of laser beam and the deviation between focus and interface. The physical phase analysis of LAS transparent glass-ceramics after welding is carried out using X-ray diffraction pattern.

Femtosecond laser beam focused by a long focal length scanning galvanometer system produced long filamentary longitudinal modification regions inside LAS transparent glass-ceramics (Fig. 2). It was caused by the self-focusing effect of the femtosecond laser beam propagating inside the LAS transparent glass-ceramics, which was balanced by the self-scattering effect formed by beam diffraction and plasma generation. With the increase in pulse energy from 2.0 μJ to 4.5 μJ, the weld width increased first and then decreased, and the maximum weld width reached 10.7 μm (Fig. 4). The transmittance decrease (Fig. 6) in the welding process is mainly caused by two mechanisms: the change of light transmission inside the glass caused by the change of refractive index at low pulse energy (Fig. 7) and the scattering loss caused by micro-nano pores at high pulse energy. The displacement of the modification region caused by the increase of pulse energy was compensated by the deviation of the focus position, which enlarged the energy welding window of LAS transparent glass-ceramics and obtained higher shear strength (Fig. 8). When the pulse energy was 3.5 μJ and the deviation between focus and interface was -500 μm, the shear strength of welded LAS transparent glass-ceramics reached (30.41±1.54)MPa. The diffraction peaks of the LAS transparent glass-ceramics before and after femtosecond laser irradiating were consistent, and the intensity of the diffraction peaks did not change significantly (Fig. 9), indicating that no new crystalline phase was generated in the irradiated area of the LAS transparent glass-ceramics.

Non-optical contact single scanning welding of LAS transparent glass-ceramics is achieved with femtosecond laser pulse for the first time. With the increasing of pulse energy, the optical transmittance of LAS transparent glass-ceramics decreases. The modification areas gradually move in the direction to the laser source as the pulse energy increases, and the widths of the weld seams at the interface increase to 10.7 μm and then decrease. When the laser pulse width is 300 fs, wavelength is 1030 nm, and single pulse energy is 3.0 μJ, the effective welding of LAS transparent glass-ceramics at the interface under the 100 mm/s high-speed scanning is achieved by using the heat accumulation effect at the high repetition rate of 500 kHz, and the shear strength after welding is as high as 23.51 MPa. The transmittance is 3% higher than that of two stacked original LAS transparent glass-ceramics. The energy window of LAS transparent glass-ceramic welding is enlarged and higher shear strength is obtained by controlling the laser beam energy and the deviation between focus and interface. The X-ray diffraction shows that the LAS transparent glass-ceramics welding is achieved without generating new crystal phase. The welding process is mainly establishing a strong connection at the glass interfaces after the melting of the SiO₂ glass phase.

Due to uneven heating and cooling characteristics, parts accumulate thermal distortions during periodic expansion and contraction during laser deposition. Geometrical defects, such as flatness defects, melting collapse, warping and cracking, seriously affecting the parts’ size accuracy. In particular, the warping and cracking of large-scale structural parts can easily cause material wastage and limit the development of laser deposition manufacturing technology. Therefore, realizing the online detection of warping and cracking prediction is an urgent problem for laser deposition manufacturing. In the current research, scholars have utilized digital image correlation technology and laser displacement sensor to monitor the transient strain field, warping and instant cracking. The detection accuracy was also improved by combining it with a machine-learning algorithm. However, while the results show the laws of warping and cracking, the global detection of warping and prediction of cracking have yet to be achieved. This study aimed to realize warping detection and cracking prediction through the changing trend and variation of the warping angle during the deposition process and to improve the forming quality.

A warping detection and cracking prediction system were built based on a laser deposition manufacturing system and laser scanner to monitor the surface morphology of parts in real-time. A new algorithm for warping detection and cracking prediction was proposed based on the warping angle and was verified by experiments. First, the surface topography data of the current deposition layer were obtained using a laser scanner, and in-situ detection was realized through morphology reconstruction and hand-eye calibration. The point cloud data were then pre-processed by straight-through filtering, statistical filtering, and bounding box filtering to prepare for surface reconstruction. Next, the warping angle of the intersecting line was calculated using the rotary and parallel slices that vertically crossed the reconstructed surface. Simultaneously, warping threshold Q₀ and cracking threshold K₀ were set according to the part size and allowable distortion degree. Finally, warping was determined according to whether the warping angle exceeded the threshold Q₀, and cracking was predicted by the changing trend of the warping angle and the variation in five consecutive layers (K). Cracking may occur when the warping angle increases and the variation K exceeds the threshold K₀.

This study proposes a new warping distortion measurable indicator for laser deposition manufacturing (for the plane), that is warping angle (Fig.4), which can be used to achieve warping detection and cracking prediction through rotary and parallel slices (Fig.5). The software framework (Fig.2) includes three main modules: visualization, system control and data processing, the whole process can be visualized using the PCL point cloud library. The effectiveness of the warping detection algorithm (Fig.6) was experimentally verified. The experimental results showed that the algorithm could determine the direction prone to warping using rotary slices (Table 1) and conduct complete coverage detection on the surface of the parts (Fig.10) using parallel slices. When the warping angle of an intersecting line exceeds threshold Q₀, a locally accurate judgment is made (Table 2). The cracking prediction algorithm (Fig.8) was verified experimentally. By calculating the warping angle Q in the cracking influence area and the variation K in five consecutive layers (Table 3), the results for layer 51 indicate that cracking may occur. With continued deposition, three cracks appeared in layer 55. The cracking phenomenon of Ti65 components during the laser deposition manufacturing process was successfully predicted (Fig.11).

In this study, a new algorithm for warping detection and cracking prediction based on the changing trend and variation in the warping angle was verified experimentally. The two schemes of rotary and parallel slices have their own characteristics. The former requires fewer calculations and can be used to determine the direction of warping, and the latter has a more comprehensive and uniform detection range and can be used to determine the warping distribution. The warping angle of the intersection lines between the slices and the reconstructed surface was calculated, and warping detection was completed by comparing them with the warping threshold. The maximum warping angle of each layer was recorded, and the changing trend and warping angle in the cracking influence area were combined with the cracking threshold to complete the cracking prediction. The experimental results demonstrated that the proposed algorithm is reasonable, logical, and robust. It can detect warping quickly and effectively and predict the occurrence of cracking. The warping detection and cracking prediction system enhances the quality and process monitoring ability of laser deposition manufacturing and provides essential evidence for process optimization.

Al-Si-coated press-hardened steel is a type of ultra-high-strength steel having both lightweight and safety properties. Owing to the presence of the Al-Si coating, Al segregation and ferrite formation occur in the weld during conventional laser welding, which significantly deteriorates the mechanical properties of the welded joints. The inhibition of Al segregation and ferrite formation is key to improving the mechanical properties of welded joints. In this study, based on the stirred melt pool characteristics of an oscillating laser beam and dilution effect of the welding wire, oscillating laser welding with a filler wire was used to improve the overall weld formation and reduce the ferrite formation in the weld. Compared with conventional laser welding, after hot stamping, the average hardness, tensile strength, and elongation of the welded joints assembled with oscillating laser filler wire welding were increased to 471 HV, 1561 MPa and 3.1%, respectively, which were increased by 20.2%, 8.4%, and 61.5% compared with those of conventional laser-welded joints.

The effects of the oscillation frequency on the weld formation, microstructure, and mechanical properties of joints were investigated using oscillating laser welding with Al-Si-coated 22MnB5 steel filler wire. Oscillation laser welding process with filler wire was compared with conventional laser welding to investigate the suppression of ferrite in the weld. Optical microscopy and scanning electron microscopy were used to analyze the weld formation, microstructure, chemical composition, and fracture morphology of the joints. The weld hardness was measured using a hardness tester, and the mechanical properties of the joint after hot stamping were measured using a universal stretching machine and extensometer. The equilibrium phase diagram of the laser welding process was calculated using the Pandat software.

The absence of filler material during conventional laser welding resulted in a concave weld surface. Oscillating laser welding with filler wire improved the weld formation; subsequently, the weld seam was fuller (Fig. 4). Compared with conventional laser welding, oscillating laser welding process with filler wire significantly reduced the area ratio of coating in the weld and diluted the Al content in the weld. The Al mass fraction in the weld decreased from 2.16% to 1.38% (Table 4), and the primary δ-ferrite volume fraction in the weld pool decreased from 82% to 71% (Fig. 12). The formation of δ-ferrite could be significantly inhibited by the oscillating laser welding process with filler wire. After hot stamping, the content of α-ferrites in the weld decreased significantly with an increase in the oscillation frequency from 0 Hz to 200 Hz (Fig. 6). Compared with conventional laser-welded joints, the hardness, tensile strength, and elongation of the welded joint assembled with oscillating laser filler wire welding increased by 20.2%, 8.4%, and 61.5%, respectively.

In this study, 1.5 mm thick Al-Si coated 22MnB5 steel was welded using the oscillating laser with filler wire, and the resulting characteristics were compared to those obtained with conventional laser welding. The cross-sectional shape of the conventional laser welded seam was “Y” shaped, and the upper and lower surfaces were concave. The volume fraction of α-ferrite in the weld after hot stamping reached 36.73%, and the hardness, tensile strength, and elongation of the welded joint were 392 HV, 1440 MPa, and 1.92%, respectively. The shape of the weld was improved by using oscillating laser welding with filler wire. The content of Al in weld was diluted and content of δ-ferrite was reduced. When the oscillation frequency increased from 140 Hz to 200 Hz, the volume fraction of α-ferrite in the weld decreased from 30.14% to 11.51%. As the oscillation frequency continued to increase to 320 Hz, the fluctuation of α-ferrite content in the weld became not significant. After optimizing the parameters, the hardness, tensile strength, and elongation of the joint increased to 471 HV, 1561 MPa, and 3.1%, respectively. Because α-ferrite formed in the weld under both welding processes after hot stamping, the joints all fractured at the weld. The brittle zone of the fracture that occurred in the conventional laser-welded joint was the cleavage plane with the river pattern, and the ductile zone was small and shallow. The area ratio of the brittle zone was greater than 80%, which was typical of brittle fractures. Compared with that in the conventional laser welding, the area ratio of the ductility zone in the fracture of the welded joint in oscillating laser filler wire welding was more than 90%; accordingly, the ductility of the welded joint was improved.

The Inconel 690 alloy has been widely used to manufacture nuclear steam generator tubes of pressurized water reactors and components of boiling water reactors owing to its excellent performance. However, corrosion and wear lead to local component damage during long-term operation under a high-temperature environment. Underwater repair can significantly reduce the amount of radiation exposed during the maintenance work. Compared with traditional underwater arc welding, underwater laser welding has the advantages of high control accuracy, wide range of welding materials, accurate control of heat input, a small heat-affected zone, and low residual stress. However, laser welding conducted directly under water aggravates the problem of porosity. Therefore, in this study, a gas drainage hood is used to drain the water around both sides of a welding sample, in order to provide protection during the underwater local dry laser welding of the Inconel 690 alloy.

The underwater local dry laser welding of the Inconel 690 alloy with the gas-assisted drainage device is carried out. During the laser welding process, the entire gas-assisted drainage cover is fixed under the welding head by a self-designed fixture. When argon flows from the nozzle, water is discharged and a local dry chamber is formed. Argon simultaneously serves as a shielding gas. The back protection system of a clamp fills with argon and discharges water from the area to be welded, wherein argon provides protection during the laser welding process. The effects of heat input and defocusing amount on weld formation, welding defect generation, and mechanical properties of the Inconel 690 alloy joint are studied by a single factor control method. Moreover, the process parameters of underwater local dry laser welding are optimized. Finally, the feasibility of underwater welding is proven by comparing the microstructures and welding properties of the underwater and onshore welded joints prepared under the same process parameters.

As the heat input increases, the width of the weld increases; the width of the top area increases owing to the Marangoni flow of the molten pool and the eruption of metal vapor (Fig.4). The grain size of the weld decreases with the decrease in the heat input, causing the mechanical properties to increase (Fig.6). The crystal morphology of the weld changes from planar to cellular to dendritic (Fig.7). With the increase in the defocusing amount, the weld width changes. When the defocusing amount is 0 mm, the highest mechanical properties are achieved. The optimized parameters of 0.03 kJ/mm heat input and 0 mm defocusing amount are used for underwater and onshore welding. Significant element segregation occurs between the dendrites in the weld zone of the onshore welded joints. However, due to the rapid cooling effect induced by water, the element segregation between the dendrites of the underwater welded joints is improved, and the element segregation in grains is more serious (Fig. 14). The mechanical properties of the underwater welded joints are similar to those of the onshore welded joints, and the microhardnesses of the weld zones of the underwater welded joints are significantly higher than those of the onshore welded joints.

The underwater local dry laser welding of the Inconel 690 alloy is conducted using a gas-assisted drainage device. The effects of the heat input and defocusing amount on weld formation, cross-sectional geometry, butt joint defects, and mechanical properties are investigated to optimize the process parameters. The results show that the heat input has a considerable effect on the width of the weld, and the Marangoni flow of the molten pool and the eruption of metal vapor lead to the increase in the width of the top area. The crystal morphology of the weld changes from flat to dendritic. As the heat input during welding decreases, the grain size of the weld decreases, while the mechanical properties continue to increase. The width of the weld changes with the increase in the defocusing amount. When the defocusing amount is zero, the joint exhibits the highest mechanical properties. The joint microstructures obtained through underwater and onshore welding consist of cellular crystals, columnar crystals, and dendrites. Cr and Ni segregation occurs between the dendrites in the onshore welded joints. The water-induced rapid cooling during underwater welding helps to improve the segregation degree of alloying elements between the joint dendrites, but worsens the intracrystalline segregation of the alloying elements. Using the optimized welding process parameters, the tensile strength of the underwater welded joints is similar to that of the onshore welded joints, the impact toughness reaches 90% of the onshore welded joints, and the microhardness of the underwater welded joints is higher than that of the onshore welded joints.

With advancements in science and technology, welding technology has progressed from manual to automated and intelligent welding. The widely used weld tracking technology based on laser vision can improve the ability of welding robots to perceive their environments, with the added advantages of non-contact and high precision. However, in real-time weld tracking, the collected weld images are often severely affected by strongly reflected light, splash, and arc noise. Therefore, laser stripes are accurately and quickly extracted from images containing a large amount of noise, and then obtaining weld information from them is a prerequisite for high-quality welding. To improve the weld location accuracy and the image processing speed in the weld tracking process, this paper proposes a lightweight multi-task deep learning model that combines laser strip segmentation and weld feature point location. The model consists of an encoder and a decoder. The laser fringe segmentation subtask and the weld feature point location subtask share the encoder backbone network. The decoder includes a laser fringe segmentation branch and a weld feature point location branch based on differentiable space-to-numerical transformation (DSNT). The entire model is designed in a lightweight manner, and it simultaneously adopts relevant information between multiple subtasks to further improve the performance of each subtask. In summary, we expect that the designed deep learning model can achieve accurate and rapid acquisition of weld features during the welding process.

In order to improve the weld location accuracy and image processing speed in the weld tracking process, a lightweight multi-task deep learning model combining laser strip segmentation and weld feature point location is proposed. The proposed model adopts the parameter hard sharing mechanism in multi-task learning such that the model uses fewer parameters. Specifically, the model consists of an encoder and a decoder. The encoder completes the feature extraction of weld position and edge information, while the decoder implements the output of the laser stripe segmentation and feature point location subtasks. The encoder network adopts the concept of a more efficient bilateral segmentation network, including context and spatial paths. The context path realizes the extraction of high-level semantic features of the image, and the spatial path provides edge detail information. In addition, to make up for the loss of detailed information, the spatial path is supervised with detailed information. To utilize the information that the weld feature point is located on the laser fringe, multi-stage supervision is adopted to make the encoder structure learn the characteristics of laser fringes. Therefore, the structure of the laser fringe segmentation subtask in the decoder only contains a simple convolutional layer and an upper sampling layer, which can realize the output of laser fringes. The DSNT module is introduced into the feature point location subtask to realize the fusion of the Gaussian thermal diagram method and the fully connected layer method so that the model is completely differentiated and has the spatial generalization ability of the Gaussian heat map method.

The results of laser fringe segmentation on images disturbed by noise demonstrate that our model exhibits good segmentation accuracy, and the detail information supervision of low layers can further improve the segmentation accuracy (Fig. 8); in addition, our model achieves a good balance between accuracy and speed (Table 2). The location results of the weld feature points show that DSNT can accurately locate the feature points of the weld under different noise interference conditions (Fig. 9). Through an experiment where the output layer structure of the network was changed, we verified that compared with the Gaussian thermal diagram method and fully connected layer regression method, the DSNT method can achieve subpixel-level location with minimal errors (Fig. 10). By changing the output structure of the decoder, it is experimentally verified that the laser stripe segmentation subtask can improve the performance of the weld feature point location subtask (Fig. 11). Finally, the experimental results verified that, compared with various deep learning models, the proposed network model can complete the segmentation of laser stripes and the localization of feature points while maintaining the inference time (Table 3).

In this study, a multi-task learning model for laser fringe segmentation and weld feature point location is proposed for weld images with multiple noises. Using detailed contour information to supervise the characteristics of the lower layer can improve the segmentation performance of laser fringes. By changing the network layer of the feature point location part, the DSNT module exhibits a higher weld feature point location accuracy than the Gaussian heat map method and the fully connected layer regression method. The multi-task learning method improves the accuracy of the location of weld feature points. In addition, the inference time of the proposed network can meet the real-time requirements of image processing in weld tracking. In summary, our model can effectively handle all types of welding noise and complete the feature extraction of welds, demonstrating good application prospects in automated welding.

Ultrafast lasers possess desirable advantages in the field of precision processing owing to their short pulse widths and high-energy peak densities. However, single-focus ultrafast laser processing technology suffers from low processing efficiencies. Parallel processing by beam splitting using a spatial light modulator (SLM) is a solution for improving the efficiency of ultrafast laser precision processing. Improving the multibeam uniformity after beam splitting is a key issue in ultrafast laser parallel processing. Two factors deteriorate the quality of multiple beams: the approximations in beam-splitting algorithms, which leads to poor calculation accuracy, and the poor beam quality of the laser in optical paths. To address these issues, a feedback GS-GA algorithm based on real-time feedback is proposed in this study.

A spatial light modulator is used for phase-only modulation by loading computer-generated holograms (CGHs). The algorithms for beam splitting and shaping are dominated by the Gerchberg-Saxton (GS) algorithm and its derivatives. The GS algorithm is highly sensitive to the initial phase value of the light source, which directly affects the quality of the reconstructed light field. However, it is difficult to obtain a suitable initial value. In this study, real-time feedback based on GS and genetic algorithms is introduced in the process of hologram generation to improve the uniformity of beam splitting. In addition, a loaded Fresnel lens phase is used to separate the zero-order beam, avoiding the aberration caused by deviations from the optical axis of a reconstructed optical field. After several iterations of the proposed algorithm, a beam array with a high uniformity is designed.

Three beam splitting methods, including the GS algorithm, GS-GA algorithm, and feedback GS-GA algorithm, are investigated in terms of the beam splitting and processing effect. Owing to a system error in the optical path, the multibeam uniformity derived from the GS algorithm is less than 80% of the theoretical value. Compared with the GS algorithm, the uniformity of the GS-GA algorithm is improved; however, the performance of the GS-GA algorithm decreases significantly when the number of split beams increases. Due to the real-time feedback from the system (Table 4 and Fig.6), the uniformity of the multi-beam energy distribution obtained by the feedback GS-GA algorithm is superior to those of the other algorithms. Compared with the conventional GS-GA algorithm, the use of the feedback GS-GA algorithm significantly improves the consistency of both the hole depth and diameter by more than 12% in the resulting hole arrays. The consistent performance for the hole depth and diameter is improved by approximately 40% compared with that of the GS algorithm (Tables 5 and 6, and Fig.7).

An optimized feedback GS-GA algorithm is proposed for pre-compensation, and its effectiveness is experimentally demonstrated. Using the results from camera feedback as the individual fitness function in a genetic algorithm, this feedback algorithm utilizes the genetic algorithm to select, exchange, and eliminate the initial value population, thereby improving the beam uniformity by approximately 94% for graphic and array multibeams. Furthermore, a laser precision punching experiment is performed using the beam arrays obtained via this feedback algorithm, which shows large aperture and hole depth uniformities of over 90%. The method proposed in this paper can achieve high-uniformity shaping for any form of designed multibeam with minimal requirements on the accuracy of the optical path system. This method obtains a multibeam distribution with beam splitting uniformity close to 90% in a certain focal depth range by dynamic zooming. The proposed shaping strategy significantly improves the beam splitting uniformity, which paves the way for the application of high-quality ultrafast laser parallel processing. The time-consuming computation of the proposed method can be improved by optimizing the solution range of the algorithm and using high-performance computer hardware to speed up the iteration of the algorithm.

Femtosecond laser direct-writing plays a key role in optical waveguide device fabrication owing to its advantages of being maskless, true three-dimension, high precision, and flexibility. However, due to the lack of a mature theoretical model that can completely describe the interaction mechanism of the laser pulse with different materials, numerous experiments are required to determine the set of optimal processing parameters. Scanning electron microscopy or atomic force microscopy can provide high-resolution images of fabricated samples, but they generally need the sample to be destroyed, which is a time-consuming process. An optical microscope combined with a laser processing system can be used for in-process monitoring. However, the material to be processed needs to be transparent to illumination, and the resolution of the optical microscope is generally limited. Recently, methods based on optical coherence imaging have been proposed to obtain depth information in addition to a two-dimensional image. However, owing to the limited dynamic range detection, these methods cannot be used to measure the weak scattered signal induced by the small refractive index of the material in optical waveguide device processing. Therefore, in-process techniques for efficiently determining the processing parameters of femtosecond laser direct-writing optical waveguide devices are useful and significant.

The backward reflection signal induced by femtosecond laser direct-writing optical waveguide devices was used for the in-process monitoring of the key processing parameters. We developed coherence domain reflectometry (OCDR) (Fig. 1) with a large reflection dynamic range (-10 dB to -95 dB) and high accuracy (±1.0 dB) to measure the reflection signal. By directly writing micro-nano defects in the core of a single-mode optical fiber with different processing parameters, backward reflection signals were generated and subsequently detected by the OCDR connected to one port of the fiber (Fig. 2). The variation tendency of the measured reflection curve was used to determine the optimal processing parameters, and the reflection for a specific case was used to identify the type of change in the material (Fig. 5).

The influence of three key parameters, that is, the pulse energy, pulse frequency, and direct writing speed of the femtosecond laser, on the backward reflection signal in fiber micro-nano processing is studied using OCDR. By increasing the pulse energy (fixed pulse frequency of 1 kHz), the variation tendency of the measured reflectivity can be divided into four different regions A, B, C, and D (Fig. 4), where the abrupt change point (0.355 μJ) between A and B is identified as the pulse energy threshold, and B is the optimal region for optical waveguide device writing owing to its relatively low return loss and large reflectivity tuning range. A similar behavior is also observed when the pulse frequency is changed (fixed pulse energy 0.415 μJ) (Fig. 6). By scanning the pulse frequency at different pulse energies, the threshold points for different setups of the two key parameters are obtained (Fig. 7). The results show that the threshold energy gradually decreases with an increase in the pulse frequency. By the distributed monitoring of the reflection signal along the direct writing path, we find that the reflectivity decreases with an increase in the writing speed (Fig. 8), and the uniformity of the reflectivity curve can be improved by multiple writing (Fig. 9).

A distributed sensing technique based on OCDR for the in-process monitoring of direct-writing microstructures in optical waveguides with femtosecond pulses is proposed. The measured backward reflection signal is used to determine the optimal fabrication parameters. The influence of three key parameters, that is, pulse energy, pulse frequency, and direct writing speed, on the backward reflection signal in fiber micro-nano processing is studied by OCDR. The measured results show that the threshold conditions of the pulse energy and pulse frequency that cause material property changes can be determined quickly and accurately via abrupt changes in the reflection curve, and the type of material property change can also be identified according to the curve variation and reflectivity. In addition, the influence of the direct writing speed on the fabrication uniformity is determined by taking advantage of the distributed sensing ability of the OCDR. This work provides a non-destructive, efficient, and in-process solution for exploring the processing parameters of femtosecond laser direct-writing optical waveguide devices.

Laser damage experiment in the high-speed wind tunnel is an important method for studying the mechanism of high-speed targets exposed to laser irradiation. There is no substantive progress in the instantaneous ablative behavior of laser-irradiated surfaces owing to the high-temperature radiation coupled with factors such as laser radiation and high-speed wind tunnel environment interference. The conventional methods are used to obtained data, such as the final ablation morphology, ablation depth, or average mass ablation rate, after the experiment. However, the traditional methods cannot provide instantaneous and reliable failure evolution process or real-time experimental data. The temperature of the specimens under laser irradiation was extremely high. For instance, the temperature of ceramic-based composites can exceed 3000 ℃ under a high laser power density. The experimental data on the instantaneous ablative morphology of high-temperature targets exposed to laser irradiation and supersonic tangential airflow have not been reported until now. In the present study, we propose an in-situ observation technology suitable for obtaining the instantaneous laser-irradiated ablative morphology of different materials. The real-time ablative behaviors of the metals and composite materials under supersonic tangential airflow were captured. The ablative characteristics of the specimens were analyzed using image processing methods, and instantaneous ablative data were obtained.

Titanium alloy, nickel-based superalloy, ceramic-based C/SiC, and carbon fiber-reinforced polymer CFRP composites are studied in this paper. First, an in-situ observation platform suitable for laser-irradiated extreme high-temperature environments was established, which mainly composed of a high-speed camera, auxiliary lighting system, attenuating filter, and narrow band-pass filter. Subsequently, laser damage experiments were conducted in a supersonic wind tunnel. The experiment employed a supersonic wind tunnel facility at the State Key Laboratory of High-Temperature Gas Dynamics (LHD) of the Institute of Mechanics, Chinese Academy of Sciences. It operates on the oxygen-hydrogen combustion principle and can provide a free stream of Mach number 1.8-4.0 in the test section. It comprises heaters, nozzles, air supply systems, consoles, and a measurement system. In the experiments, the tangential supersonic airflow was set to Mach number 3.0. The total temperature and pressure of the gas flow were 815 K and 1850 kPa, respectively. Finally, the optical flow method was used to analyze the ablative characteristics and particle motion velocity of each material, and the instantaneous ablation rate was obtained using the PIV method combined with the structural characteristics of the composite material layup.

The burn-through behaviors of titanium alloy and nickel-based superalloy were obtained. The burn-through time under the coupled action of laser and tangential airflow are 1.32 s and 1.44 s, respectively. The final perforation diameters are 7.23 mm and 5.72 mm, respectively. The difference in the flow pattern and burn-through time is attributed to the instability of the melt surface. According to the Kelvin-Helmholtz theory, the mechanism of the burn-through behavior is mainly related to the surface tension and density of the material. Although the melting point of the titanium alloy TC4 (1670 ℃) is higher than that of the nickel-based superalloy GH625 (1340 ℃), the high-density nickel-based superalloy exhibits better resistance to laser breakdown under tangential airflow condition. For the C/SiC composite, the ablative evolution process of the microscopic structure and the formation and migration of silicon dioxide droplets in the edge region of the laser irradiation are clearly visible in the experimental images. The results show that the in-situ observation technology can also be used to observe the ablative behavior of composite materials. Different braided structures can influence the ablative behavior and ablation depth. The ablation depth of the 2D C/SiC composite was 1.13 mm, whereas that of the 3DN C/SiC composite was 1.23 mm. Compared with the 2D C/SiC composite, the 3DN C/SiC composite exhibits higher thermal conductivity in the thickness direction, resulting in a significantly higher temperature than that of the 2D C/SiC composite; therefore, its thermochemical ablation rate is also higher than that of 2D C/SiC. The instantaneous ablation depths of the CFRP were obtained using PIVlab. The results showed apparent nonlinear behavior. The laser ablation depth of a CFRP composite under supersonic tangential airflow is related to the laser power density and airflow velocity. The ablation depth is 0.36 mm when the laser power density is 1273 W/cm², and the airflow velocity is Mach number of 1.8. When the airflow velocity increases to Mach number of 3.0, the ablation depth increases to 0.47 mm. When the laser power density increased to 2546 W/cm², the ablation depth increased to 1.07 mm. These results indicate that the laser power density has a strong influence on the laser ablation depth.

In this study, an in-situ observation technology of laser-irradiated high-temperature is proposed, and the instantaneous ablative morphology of metals and composite materials exposed to laser and supersonic tangential airflow is obtained. Real-time ablative data were calculated using image processing methods. The flow of molten metals in the wake zone and the diffusive characteristics of the heat-affected zone were obtained using the Horn-Schunck optical flow method. The ablative behaviors of the composites were related to the braided structure of the reinforced phase. The mechanical ablation effect of the 2D C/SiC composite is mainly sheet-like ablation, whereas the behaviors of the 3DN C/SiC and CFRP composites are mainly fiber-by-layer ablation. The instantaneous ablation depths of the CFRP composites were obtained using PIV method. The results show that the in-situ observation technology proposed in this study has broad application prospects in extreme high-temperature engineering, especially in the study of laser damage effects.

The traditional manufacturing sector is going through a scientific and technical revolution as a result of the rapid advancement of laser technology. Many countries have made significant investments in the research and development of new manufacturing models to achieve high-quality, high-efficiency, flexible automation, integration, and intelligent production. Surface polishing is a crucial step in the manufacturing of industrial products, and it has numerous applications in the domains of aviation, aerospace, automotive, mold, precision manufacturing, semiconductors, and other fields. Additionally, surface polishing quality also directly affects the appearance, performance, and service life of products. Current surface polishing methods like manual/mechanical, chemical/electrochemical, bonnet, abrasive flow, ion beam, and vibration polishing all have their advantages and are appropriate for particular applications; however, they are rarely intelligent or non-polluting and seldom have high quality or efficiency. Therefore, it is imperative to develop cutting-edge and effective surface polishing technologies to meet the strategic goals of reducing carbon emissions and realizing carbon neutrality as well as to support the advancement of the manufacturing industry in an environment friendly manner.

Laser polishing is a novel surface polishing technology that has drawn much interest from researchers both domestically and internationally because it does not pollute, can process diverse objects, has steady polishing quality, and can be easily automated. During laser polishing, the laser melts and evaporates the surface layer of the polished material, driving the molten metal to valleys under the action of capillary or thermal capillary forces, resulting in a smooth surface. Laser polishing not only generates a smooth surface, but also improves the surface properties of the material. Furthermore, laser polishing is more suitable for industrial applications than traditional polishing technologies because it can be integrated to processes such as additive manufacturing, laser welding, and laser cleaning to achieve efficient and intelligent product manufacture. At present, laser polishing technology is still fledgling and several technical challenges remain. Therefore, this paper looks back at the development of laser polishing technology, summarizes the challenges that must be overcome, and analyzes the future development trend of laser polishing technology in anticipation of promoting the advancement and maturity of laser polishing technology.

Laser polishing can be categorized into two types thermal and cold polishing, according to physical and chemical changes, where the basic mechanism of thermal polishing is the melting of the surface material and the subsequent redistribution of the molten surface material. Thermal polishing can be specified as employing shallow surface melting or over melting mechanism based on the melting depth. The melting depth in the shallow surface melting mechanism is close to the surface’s maximum peak-to-valley vertical distance, whereas the melting depth in the surface over melting mechanism is greater than the surface’s maximum peak-to-valley vertical distance. The cold polishing process has an insignificant thermal effect and removes the rough surface by an ablation or photochemical mechanism, resulting in a smooth surface. Different surfaces are appropriate for various laser polishing mechanisms, which are deeply related to the material properties, laser characteristics, process parameters, and processing environment. According to the characteristics of the shallow surface melting mechanism and numerous research results (Table 1), the laser polished surface roughness can reach several tens of nanometers under the shallow surface melting mechanism; however, the surface roughness reduction rate is small (10%-60%); in contrast, it is difficult to obtain a smaller surface roughness under the over melting mechanism but a high surface roughness reduction rate (>80%) can be realized. Moreover, the shallow surface melting mechanism is most widely applied in pulsed laser polishing processes and is suitable for metal surfaces with small original surface roughness due to its limited effect on low-frequency feature removal. The over melting mechanism is more frequently used in continuous laser polishing processes, and its large melting depth makes it suitable for metal surfaces with rough original surfaces. The ablation and photochemical mechanism are commonly utilized in ultrashort pulsed laser and short wavelength laser polishing processes, where the thermal effect is minor and ideal for hard and brittle materials like glass, ceramics, and some materials with poor thermophysical properties.

Recently, researchers have reported cutting edge findings in continuous laser and pulsed laser polishing. They found that to achieve high quality and effective laser polishing, scratches over the whole spatial frequency range must be smoothed out, which necessitates the employment of multiple laser polishing mechanisms. Consequently, a novel process combining continuous and pulsed laser polishing has become one of the most active pursuits of laser polishing research, and this combined process can realize high surface roughness reduction rates along with small surface roughness (Fig.10). Moreover, to enhance the heat and mass transfer of the laser polishing melt pool and improve the quality of laser polishing, energy field-assisted laser polishing technologies are being rapidly developed, such as ultrasound and magnetic field. The laser polishing technology has been proven in typical applications like molds, medicine, and additive manufacturing. The Fraunhofer Institute and RWTH Aachen University in Germany have conducted extensive scientific research on laser polishing on a variety of material surfaces, including mold steels, titanium alloys, additive manufacturing workpieces, and glass (Fig.15).

This paper reviews the recent research progress of laser polishing technology, including mechanism, process, surface morphology characteristics, multiple laser polishing, and laser polishing applications, and presents the outlook of the development of laser polishing technology that can promote its applications. Currently, laser polishing technology with various laser source types for diverse objects and applications, including energy field assisted laser polishing and other laser processing technology integration, are being developed to solve the problems of parameter complexity, poor polishing quality, excessive thermal effects, low processing efficiency, poor performance, and others. We believe that with the efforts of most scientist and engineers in the field, laser polishing technology will grow and thrive and will be adopted in the manufacturing sector.

All types of marine organisms attach and grow on the surface of marine facilities and equipment during the course of their service in the marine environment. This results in fouling by marine organisms, accelerated corrosion of metal materials, failure of key parts of the marine equipment, and other problems that affect the normal operation of marine equipment. Therefore, the effective removal of marine biofilms from the surface of marine service materials has become a key breakthrough in the exploitation of marine resources. At present, effective cleaning methods for marine biofilms mainly include chemical removal with fungicides, mechanical removal with artificial eradication, cavitation water jet flushing, ionizing radiation, and ultrasonic adhesion prevention. However, to a certain extent, all these methods have drawbacks, such as a low cleaning efficiency, poor cleaning quality, environmental pollution, and uncontrollable damage to the substrate. Therefore, it is necessary to develop a green, efficient, and high-quality cleaning method to prevent biological fouling of the surfaces of marine service materials. As a green cleaning technology with significant potential for development in the 21st century, laser cleaning technology has the advantages of a high cleaning efficiency, high precision, high quality, and minimal damage. Owing to the uneven thickness, physical properties, and chemical composition of marine biofilm layers, as well as the special interface bonding properties between the organic membrane layer and the inorganic metal matrix, there are many new challenges in the laser cleaning of marine biofilm layers on metal surfaces in marine service environments. The mapping relationship among the laser-cleaning characteristics, laser-energy parameters, and cleaning quality requires further study.

In this study, a nanosecond pulsed laser was used to conduct laser-cleaning experiments on marine biofilm layers formed on the surface of 30Cr3 high-strength steel, which is commonly used in ocean engineering, after soaking in the Huanghai Sea. The surface morphologies of the substrate before and after laser cleaning were observed using an optical microscope and a laser confocal microscope. The surface roughness of the substrate before and after laser cleaning was measured, and the microscopic morphology of the substrate surface was observed using a scanning electron microscope. The composition and distribution of the elements on the substrate surface were analyzed using an energy spectrum analyzer before and after cleaning. The effects of different laser energy densities on the desorption behavior of marine biofilm coatings on high-strength steel surfaces during laser cleaning were observed and summarized using high-speed imaging equipments.

A marine biofilm on the surface of high-strength steel contains two components: an extracellular polymeric substance (EPS) layer composed of organic components and a hard attachment composed of limestone (Figs. 4-7). The laser with the energy density of 9.95 J/cm² has the best cleaning effect on the marine biofilm on the surface of high-strength steel, as it achieves a good removal of the EPS layer and hard attachments and causes little damage to the substrate. The laser with the high laser energy density of 11.05 J/cm² completely removes the marine biofilm, however, the thermal damage to the surface of the substrate is large. The cleaning effect of the laser with the low laser energy density of 7.74-5.53 J/cm² is relatively poor, and the cleaning effect decreases with a decrease in the laser energy density (Figs. 8-11). Following laser cleaning, the surface roughness of the substrate decreases with increasing laser energy density. For a laser energy density of 9.95 J/cm², the lowest surface roughness S_a=17.31 μm is reached, which is about 47.8% lower than that before cleaning, and corresponds to the best cleaning parameters described above. However, when the laser energy density is further increased, the substrate suffers thermal damage owing to excessive cleaning, resulting in a substantial increase in the surface roughness (Figs. 12-13). High-speed imaging observations reveal that only a thin EPS layer is removed from the surface by laser ablation at a low laser energy density during the process of laser cleaning of a marine biofilm layer. However, there is no obvious removal effect for hard surface attachments. At a higher laser energy density, the removal of the EPS layer and hard attachment is significant. The EPS layer is mainly removed by ablative decomposition and combustion, whereas the hard attachment mainly breaks off and flies off the surface through thermoelastic vibration (Figs. 14-17).

The components and surface states of the marine biofilm on the surface of high-strength steel soaked in the Huanghai Sea are complex and uneven, and the marine biofilm can be roughly divided into two components: an EPS layer with uneven thickness, mainly composed of organic components, and hard attachments, mainly composed of limestone. Under the premise of no damage to the substrate, the laser cleaning effect of the marine biofilm on the surface of high-strength steel improves with an increase in the laser energy density. The laser with the energy density of 9.95 J/cm² shows the best cleaning effect, with no residue of the marine biofilm left on the surface after cleaning. Following cleaning, the surface roughness is 17.31 μm, a 47.8% reduction from the initial roughness. The EPS layer is primarily cleaned using ablative decomposition and combustion, and the hard surface attachments are primarily cleaned using thermoelastic vibration.

The service environment of composite paint layer on civil aircraft skin is severe, making local and overall paint removal become necessary steps for routine maintenance of civil aircraft. Currently, manual grinding and chemical stripping are main technical means for paint removal of civil aircraft. Manual grinding is inefficient and easy to damage aircraft skin. Although chemical stripping has higher efficiency, the serious waste pollution can not meet increasingly stringent environmental protection requirements. Therefore, laser cleaning technology with low cost, high efficiency and minimal pollution has entered vision fields of researchers. However, the interaction and coupling effects among ablation, plasma shock (shielding), and thermal stress mechanisms during the cleaning have not been deeply understood at present. In this paper, effects of different laser overlap rates in y direction on cleaning quality are investigated using high-repetition-rate pulsed fiber laser cleaning system. By analyzing macro-micro morphological characteristics and composition changes of cleaned surface, mutual effect and coupling behaviors of different paint removal mechanisms and thermal degradation principle of residual paint layer are elucidated. We hope that the above researches can provide theoretical and experimental support for industrial application of laser-cleaned aluminum alloy composite paint layer.

In this paper, the laser cleaning experiment of composite paint layers of 2A12 aircraft aluminum alloy is performed using the high-repetition-rate pulsed fiber laser complete processing equipment under process conditions of four different laser overlap rates in y direction (η_y=0%, 20%, 40%, 60%). Firstly, optical microscope (OM) is used to observe the states of cleaned surfaces and sections. Then, morphological characteristics of macro- and micro-structures as well as compositions and element contents on cleaned surface are further investigated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Furthermore, Fourier transform infrared spectroscopy (FT-IR) is adopted to study the change of functional groups in acrylic-polyurethane composite paint layer. At last, the composition change and content of cleaned surface are analyzed by X-ray photoelectron spectroscopy (XPS).

Laser cleaning experiments are carried out with y-direction overlap rates as variables, in which composite paint layer systems exhibit various removal degrees. Overall, with the y-direction overlap rate increases, effects of laser plasma shock and shielding are weakened and the ablation and gasification of composite paint layer are strengthened (Figs.4 and 6). Obviously, there are ablation-plasma shock and shielding-ablation circular paint removal mechanisms during the cleaning process [Figs.6(a) and (b)]. Main components of the crater surface formed by ablation include thermal degradation product of paint layer, ash, fixed carbon, topcoat colorants, and functional oxide particles (Fig.7). By analyzing the action characteristics of different paint removal mechanisms, it is found that the residual paint layer appears delamination and fragmentation, which is mainly caused by plasma shock and thermal stress [Figs.6(a)-(d)]. The changes of functional groups and components in residual paint layer show that the free radicals in acrylic resin chain segments are replaced and rearranged (Fig.8 and Table 8), which is due to the thermal effect of ablation and plasma shock.

Laser paint removal experiments on 2A12 aluminum alloy surfaces are performed under different laser overlap rates in y direction. Results demonstrate that the topcoat is removed completely and the primer is removed partially when the overlap rate is 20%; the residual primer exists barely with the destroyed oxide film and exposed aluminum alloy substrate when the overlap rate is 40%; the paint layer is completely removed and the substrate is damaged when the overlap rate is 60%. The research shows that there are three paint removal mechanisms of ablation, plasma shock (shielding),and thermal stress during the paint removal, in which the ablation leads to the stripping and deposition of the β-type copper phthalocyanine in primer and functional oxidation particles in composite paint layer and plasma and thermal stress cause physical damage of the paint layer. The appearance of crater-spacing-crater cleaning feature on cleaned surface demonstrates that the paint removal mechanisms of ablation and plasma shock (shielding) occur alternatively. At the same time, the heat affected zone forms on the surface of residual paint layer, which induces thermal stress paint removal during the cleaning. The thermal degradation mechanism is revealed by analyzing the changes of functional groups and components in residual paint layer.

Magnesium alloys have been widely used in aerospace, ship transportation, automobile parts, electronic equipment, and other fields because of their high specific strength, good specific stiffness, good machinability, and castability. However, owing to the extremely low electrode potential of magnesium alloy, it is easily corroded in a humid environment. Therefore, its poor corrosion resistance is one of the problems that prevent magnesium alloy from being further promoted and used, and improving the corrosion resistance of magnesium alloys is extremely important. The traditional methods to improve the surface corrosion resistance of magnesium alloys are micro-arc oxidation, rapid solidification, organic coating, etc. The production of superhydrophobic surfaces on magnesium alloy substrates is a simple and efficient method that has been used in recent years. However, the fabrication of superhydrophobic surfaces is often accompanied by etching with chemical reagents and modification with low-surface-energy substances, which increases the complexity and cost of the process. In this study, we used nanosecond laser processing of AZ91D magnesium alloy surfaces to form laser surface texturing, create periodic pits and dotted projections, and then supplemented them with low-temperature heat treatment to successfully modulate the surface of magnesium alloy from hydrophilic to hydrophobic to superhydrophobic. We investigated the effect of laser-processing-induced transformation of surface wettability on the salt spray corrosion resistance of magnesium alloys.

Magnesium alloys were used for this study. First, regular micro- and nanostructures were created using a nanosecond laser on the surface of magnesium alloy. Subsequently, they were placed in an ethanol solution, cleaned with ultrasonic waves, and placed in an oven for one hour of heat treatment at 150 ℃. Next, static contact angle and rolling angle measurements were performed on individual specimens using a contact angle measuring instrument to determine the wettability of the different specimens. Second, scanning electron microscopy was used to photograph the surface morphology and test the surface elemental composition of the magnesium alloys with different wettabilities, and salt spray corrosion experiments were performed on the magnesium alloys with different wettabilities, with corrosion times of 6, 12, 20, and 30 h. In addition, the surface morphology of the specimens after 30 h of salt spray corrosion was photographed using a confocal microscope, and the surface height and average height of corrosion layer were measured to calculate the thickness of the corrosion layer, which was used as a criterion to determine the corrosion resistance.

The surface contact angles of the prepared samples with different wettabilities are clearly differentiated (Table 2), among which the contact angle of the superhydrophobic Cassie state surface is 153.33° and the rolling angle is 3.51°. The elemental composition of the surface of the superhydrophobic specimens of magnesium alloy was tested before and after heat treatment, and it was found that the elemental content of C had an effect on the surface wettability (Table 3), and salt spray corrosion experiments were performed on each sample for different periods. The corrosion resistance of the superhydrophobic surface improved significantly with the increase in corrosion time (Fig. 4). The confocal surface morphology of the samples with different wettabilities after 30 h corrosion was photographed. Then, the average height of the corrosion layer was measured in combination with the contour method to calculate the thickness of the corrosion layer on the surface of the magnesium alloy (Fig. 8). The corrosion layer of the superhydrophobic Cassie state was the thinnest, measuring 20 μm, whereas the corrosion layer thickness of the original sample at this time was over 158 μm.

In this study, a nanosecond laser is used to texture the surface of AZ91D magnesium alloy, supplemented by low-temperature heat treatment, to obtain surfaces with different wettability states. After heat treatment, a superhydrophobic surface with excellent performance was obtained, with a contact angle of 153.33° and a roll angle of 3.51°. The laser modulation of wettability on the surface of magnesium alloys, as well as the mechanism of corrosion resistance improvement, were investigated by testing and characterizing the surface morphology and composition, static contact angle, and corrosion resistance of magnesium alloys with different wettabilities. The following are the main conclusions:

(1) Laser process parameters have an important influence on surface topography; laser processing and subsequent low-temperature heat treatment contribute to an increase in surface C element content; and surface topography and surface C element content can regulate surface wettability.

(2) The surface corrosion effect is determined by the corrosion capacity of the salt spray, corrosion resistance of the surface, and residence period of the salt spray on the surface. Laser processing can induce the formation of oxide films on the surface of magnesium alloys, which helps to improve their resistance to salt spray corrosion. The corrosion resistance of the laser-woven hydrophilic surfaces is higher than that of the original surface. Cassie surfaces with a small rolling angle and a superhydrophobic state have the best resistance to salt spray corrosion because of the short residence period of salt droplets on them. The dense and intact surface of the superhydrophobic Wenzel state with a contact angle greater than 150° and a large roll is one of the most important reasons for the good corrosion resistance of the surface.

Laser shock peening (LSP) is an advanced surface modification technology that uses laser-induced shock waves as a direct source of force for the production of plastic deformations on the surface of metals. It can be used to regulate the surface quality of the material and improve its properties. Laser shock peening has several advantages over other surface modification techniques, such as high loading pressure, large influence range, and controllable impact area and loading pressure parameters, and can be easily automated. It has a wide range of applications in aerospace, automotive manufacturing, and marine engineering industries. The plastic deformation is the fundamental process for laser shock peening in the reconstruction of surface stress, improvement of surface morphology, and hardening of surface materials. This deformation is influenced by a complex nonlinear dynamic process that is dependent on various factors such as laser shock wave pressure decay, the dynamic yield strength of materials, plastic strain rate, internal dislocations, and microcracking of materials. In this study, the flow law of plastic deformation of the TC4 titanium alloy under the action of shock waves is analyzed by combining numerical simulations and experimental studies. The reliability of the numerical model is verified through experimental studies, and the volume changes of internal plastic deformations under different laser shock parameters are calculated based on the numerical model. Additionally, the influence of plastic flows on volume distribution, stress reconstruction, and internal grain distribution of the material is analyzed.

A TC4 titanium alloy is used in this study. A combination of numerical simulations and experiments is used to study the flow law of plastic deformation of the TC4 titanium alloy by laser shock peening. First, a numerical simulation model of the laser shock peening of a TC4 titanium alloy is established. The surface deformation and residual stress distributions in the numerical simulation are extracted and compared with the experimental results, and the numerical simulation model is validated. After verifying the validity of the numerical simulation model, the internal deformation distribution of the material is extracted from the numerical simulation model, and the internal deformation distribution data is processed. Subsequently, the deformation distribution data is fitted, and the fitted equations are calculated to evaluate the flow of plastic deformation by obtaining the volume change of each part after the LSP. In addition, the microstructures of the surface layers at different depths before and after the laser impact treatment are observed using a field-emission high-resolution transmission electron microscope (TEM), and particle size measurements and distribution statistics are processed.

The clouds obtained from the numerical simulation show that other plastic deformations occur beyond surface microindentation and superficial convex deformations after laser shock peening. The effects of the shock wave cause the volume of the microindentation deformation produced in the center of the impact area to shift to the surrounding area. This results in the formation of the superficial convex deformation at the edge of the spot impact, and the remaining volume is squeezed into the material, leading to internal convex deformations (Fig. 6). The distributions of the residual stress and deformations on the surface at a power density of 3.02 GW/cm² are consistent with the numerical simulation results (Fig. 10). As the volume in the residual compressive stress zone decreases, the volume in the residual tensile stress zone increases, and a part of the volume is transferred to the internal part of the material during the plastic deformation process (Fig. 13). After laser shock peening, a large number of dislocations, dislocation walls, dislocation entanglements, and other sub-structural defects appear inside the grains. Lamellar dislocation accumulation occurs in the lamellar organization in the microindentation deformation region, and high-density dislocations are formed in the convex deformation region (Fig. 14). Variations in the grain size are directly related to the intensity of the plastic deformation. The grain size is the smallest in the microindentation deformation region, which is directly loaded by the laser spot. The superficial convex deformation region exhibits a larger grain size compared to the microindentation deformation region, while only a slight refinement in grain size is observed in the internal convex deformation region (Fig. 15).

In this study, the microindentation deformation is formed in the center area of a spot, the convex deformation is formed in the edge area of the spot, and the residual compressive stress is generated in the surface layers of the microindentation and convex deformation areas. The 3D morphology test results of the TC4 titanium alloy surface show that the volumes of the microindentation and convex deformations are not equal, mainly because the laser shock wave induces the flow of plastic deformation in the surface metal towards the interior of the material, forming a convex deformation area under the microindentation deformation layer and a residual tensile stress layer in this area. Based on the results of numerical calculations, the volume of the overall plastic deformation is extracted and calculated. The results show that the sum of the internal convex deformation volume and superficial convex volume is approximately equal to the microindentation deformation volume. In the absence of phase changes, the overall plastic deformation adheres to the volume constancy law. The TEM images before and after laser shock and particle size distribution results show that flow of plastic deformation of the TC4 titanium alloy by laser shock peening directly affects the grain refinement mechanism in each region; the degree of grain refinement decreases successively in the microindentation deformation area, the superficial convex deformation area and the internal positive deformation area. When a multispot laser shock peening is performed, all spots work together to form microindentation deformations, and small convex deformations appear at the edge of the indentation surface. In the part without an overlap of spots and impact of shocks, convex deformations are formed.

We uses the high-energy heat source of a high-power laser (20 kW) to address the problems of surface oxidation, decarburization behavior, uneven surface hardness, high costs, and environmental pollution in the conventional quenching process of 55 steel surface. By promoting the application of high-power lasers and technological innovation, the study aims to meet practical working conditions for high efficiency, low cost, energy conservation, and environmental protection. The single-variable principle is employed to obtain laser remelting and laser quenching process parameters with different laser power ranges, and the effects of both on the surface morphology, microstructure, and hardness of 55 steel are investigated. The goal is to enhance the working surface properties of 55 steel, thereby increasing its service life and safety, and reducing the likelihood of engineering accidents.

In the study of laser quenching on the surface of 55 steel, we found that neither an excessively high laser power nor an excessively quick scanning speed could improve the surface properties of the steel, particularly under high-power laser quenching with fast scanning. The hardened surface expanded with increased laser power, and despite remelting, the hardness value remained relatively constant. Consequently, a comparative study of laser remelting and laser quenching was proposed. First, to investigate the hardening effect of laser remelting and laser quenching on the working surface of 55 steel, the variation of surface hardness in specimens with increasing laser power at different laser ranges was investigated using the single variable principle, and the surface morphology and boundary thermal diffusion of each specimen were analyzed. Second, the effect of scanning speed on the surface hardness of 55 steel was investigated at medium laser power ranges to further characterize the hardening differences between laser remelting and laser quenching on that material. To characterize the intrinsic hardening difference between these processes, XRD analysis was performed on both low and high-laser-power specimens to study the variation of each physical phase with laser power and to obtain the physical phase difference between remelted and quenched specimens. Finally, the microstructure morphology and hardness changes of the cross-section of the high-laser-power specimens were analyzed, while the EDS line scan of the cross-section of the 2.1 kW specimen was performed to compare the diffusion of elements in the remelted and hardened layers, and based on the above analysis, the intrinsic hardening mechanisms of laser remelting and laser quenching were summarized and analyzed.

By comparing the laser-quenched and laser-remelted specimens within different laser power ranges, we found that the surface hardness has a similar variation pattern. The laser-remelted specimens exhibit a better hardening effect and a significant increase in the cross-sectional hardening layer depth with the increase in the laser power range (Figs. 9 and 10). In addition, the XRD analysis reveals that the laser-remelted specimens had fewer unfused carbide phases than the laser-quenched specimens at different laser power ranges (Fig. 8), and this conclusion is further verified by the microstructure morphology (Fig. 7). Moreover, in the XRD spectra of medium and low laser powers, the peak level of the strongest peak of martensite decreases first and then increases, which indicates that similar lattice distortion occurs with the increase of power in different laser power ranges. In the microstructure comparison, we found that the laser-remelted specimens exhibit more uniform and dense martensite (Fig. 6), and the carbon elements in the remelted layer show stronger diffusion ability than other alloying elements (Fig. 5).

In this study, the effects of different laser power and scanning speeds on the surface hardening effect of 55 steel are investigated. Additionally, the surface hardening differences between remelted and quenched specimens are compared, and the intrinsic hardening mechanism is also analyzed. We found that laser remelting exhibits a superior hardening effect than laser quenching. The surface hardness range of laser-quenched specimens is 446-520 HV, the depth range of the hardened layer is 621-709 μm, with a maximum cross-sectional hardness of 720 HV. For laser-remelted specimens, the surface hardness range is 480-613 HV, the depth range of the hardened layer is 709-813 μm, and the maximum cross-sectional hardness is 755 HV. Carbide particles in the remelted specimens gradually dissolve and diffused with increased power, forming a smoother grain boundary structure. Notably, the remelted layer contains higher carbon atom content and achieves higher hardness, while the quenched specimen still has visible unfused carbide particles in the microstructure of the hardened layer, limiting the acquisition of its high-hardness hardened layer. In addition, the microstructure of both quenched and remelted samples consists of lamellar and slate-like martensite, residual austenite, and some unmelted carbides, but the remelted sample has a more uniform and denser microstructure, a higher content of martensite, and fewer unmelted carbides, and the microstructure is free of pores, cracks, and other defects, which obtains a better hardening effect and provides guidance for the study of surface laser hardening of 55 steel.

In recent decades, the global market demand for dental and orthopedic implants has grown steadily owing to an aging population, an increase in bad habits, and frequent accidents. Titanium alloys have a low specific gravity, high corrosion resistance, better compressive strength, and fracture toughness than other implant materials and are currently widely used in biomedical implant engineering. However, owing to the biological inertia of titanium alloys, long-term implantation may induce inflammation around the implant and affect its lifespan. To realize a tight connection between the implant and the surrounding tissue of the human body and form a stable osseointegration interface, the modification of the implant surface is very important. Among these surface properties, wettability mainly affects the type, quantity, and conformation of proteins deposited on the implant surface, and subsequently affects the cell response at the material interface. With deeper research on the surface modification of laser-irradiated materials, the coupling relationship between various surface properties after modification becomes complicated when considering the functional effect of hydrophilicity alone. However, the differences in the surface morphology make the relationship between wettability and cell biological behavior inconclusive. Therefore, it is necessary to systematically evaluate the wettability of surfaces with different structural features after femtosecond-laser texturing. The scanning path strategy and structural changes caused by laser parameters proposed in this study provide ideas for the design and construction of biomedical implant surface structures.

Ti6Al4V sheets with a thickness of 0.5 mm are prepared for the experimental study. Before processing, the samples are polished using a abrasive wheel, followed by ultrasonic cleaning in deionized water, acetone, and absolute ethanol for 15 min. Surface texturing is performed using a ytterbium fiber laser with a central wavelength of 1030 nm and a pulse duration of 300 fs. Sixteen samples are fabricated using two scanning path strategies to determine the effects of laser parameters on the surface structure and wettability. A scanning electron microscope (SEM) is used to observe the structural morphology of the processed sample surfaces. The cross-sectional morphology and surface roughness S_a of the samples are characterized using a confocal microscope. To evaluate the wetting properties, the static contact angles of the sample surfaces are measured using a video optical contact angle meter. The X-ray photoelectron spectroscopy (XPS) is used to detect the chemical compositions of the sample surfaces before and after femtosecond laser texturing.

In this study, the surface of titanium alloy is textured by changing the laser energy density and number of scans. With increasing laser energy density, the top of the microstructure gradually bulges, and the laser induced periodic surface structure (LIPSS) bifurcates and produces a large number of surface nanoparticles. When the number of scans increases, a thick recast layer appears on the edge of the microstructure, whereas the LIPSS structure does not disappear because of the low energy density (Fig. 3). The surface topography becomes more undulating as the laser energy density increases, which is consistent with the SEM image results (Fig. 4). The surface roughness S_a and microstructure height do not increase linearly with the energy density but tend to saturate (Fig. 5). Increasing the number of scans results in slight changes in the overall surface topography (Fig. 6). With an increase in the number of scans, the surface roughness and microstructure height exhibit continuously increasing trends (Fig. 7). After femtosecond laser texturing, the hydrophilicity of all surfaces improves, and the contact angle decreases with increasing energy density. The effect of the number of scans on the surface wettability is not significant at low energy density values, and the contact angle decreases slightly compared to that of the untextured samples. However, when both the laser energy density and number of scans are increased, the wetting behavior of the material surface changes drastically, and a superhydrophilic surface that is rapidly wetted by the droplet within 3 s is observed (Fig. 8). The XPS results show that the femtosecond-laser texturing changes the chemical composition of the titanium alloy surface. The carbon content decreases and the oxygen content increases significantly (Fig. 9). The further analysis of the Ti2p fine spectra shows that an oxidation reaction occurs when the femtosecond laser textures the surface of the titanium alloy, and a high laser energy density or number of scans promotes the conversion of elemental titanium to high-valence titanium oxide (Fig. 10).

Femtosecond laser irradiation of a Ti6Al4V surface can be used to effectively construct designed structures with different morphological and dimensional characteristics. Although both the laser energy density and number of scans contribute to an increase in the surface roughness of titanium alloys, there is a fundamental difference in terms of ablation. The energy density directly affects the degree of laser ablation on the surface, and the surface roughness is saturated by the plasma shielding at high energy density. The number of scans determines the height of the microstructure and the depth of the grooves; the higher the number of scans, the greater the surface roughness. Appropriate energy density values and numbers of scans help maintain a titanium alloy surface with regular microstructural features. Moreover, femtosecond laser processing significantly improves the hydrophilicity of the titanium alloy surface. A superhydrophilic surface with rapid droplet spreading is obtained at the laser energy density of 2.31 J/cm2 and the number of scans of 50, which may be attributed to the porous morphology with high roughness created by the increase in the laser energy density and number of scans. However, further research shows that the surface hydrophilicity of titanium alloys is not entirely dependent on the surface structural dimensions, and that the surface chemical composition changes caused by laser ablation also mediate the surface wetting process.

Laser processing has the advantages of controllable energy, no contact, high efficiency, and high precision. It has good application prospects in the processing of hard and brittle materials, such as SiC ceramics. In this study, a new multi-beam coupled laser processing platform was used to polish SiC ceramics by adjusting the bidirectional spot overlap rate (δ) and energy density (E_D) of the multicoupled laser. The effects of the multibeam coupled laser energy density and bidirectional spot overlap rate on the surface morphology, roughness, phase change, and element distribution of SiC ceramics were systematically investigated. The material removal mechanism of SiC ceramic laser processing was revealed. This study can provide a technical support for optimizing the laser polishing process of SiC ceramics.

First, a multibeam coupled nanosecond laser was used to polish SiC ceramics under different process parameters. Then, laser confocal microscopy, Raman spectroscopy, scanning electron microscopy, and other test methods were used to analyze the microstructure, phase change, and element distribution of SiC ceramics. The removal mechanism of SiC ceramic materials using the coupled laser and the effects of various parameters on surface quality were analyzed from multiple perspectives.

The multibeam coupled laser focal point shows a Gaussian-like energy distribution [Fig. 2(a)], with a broader and more uniform effective action surface on the material within the spot and a smaller heat-affected zone (Fig. 3). The polished surface shows a more pronounced color shift at the macroscopic level [Fig. 4(a)] when δ and E_D are low. The polished area is dull gray-black and gradually changes to a slightly brighter metallic color as δ and E_D increase. The polished surface microstructure (Fig. 5) appears relatively flat compared with the original surface microstructure [Fig. 4(b)], and the polished surface roughness decreases to 0.73 μm when E_D is 4.254 J/cm², δ is 75%, and the number of scans is two [Fig. 8(b)]. However, the occurrence of edge stacking attributed to many recast layers is not conducive to further improvement of the surface quality. The characteristic peak of the Si singlet appears in the Raman spectral curve of the polished surface recast layer (Fig. 9), indicating that a violent photochemical reaction accompanies polishing. Chemical bond breaking and reorganization occur, alleviating the problem of excess free carbon on the surface (Figs. 10 and 12) and introducing oxygen elements when δ and E_D are extremely high. This is owing to the extended temperature span of the laser action region, leading to thermal stress inside the surface material, which in turn, generates microcracks from the pore edge to the surrounding area (Fig. 11).

In this study, a new multi-beam coupled nanosecond laser was used to polish SiC ceramics. The effects of the laser bidirectional spot overlap rate and energy density on the macroscopic and microscopic morphologies, roughness, phase, and element evolution of the polished surface were investigated. The results show that the violent photochemical reaction significantly alleviates the problem of excessive surface free carbon when δ and E_D are high, the surface morphology of the material becomes smooth, and the surface roughness decreases. However, this results in the formation of many recast layers on the surface when E_D is very high, which is not conducive to further reduction of surface roughness.

Zirconia ceramics are used extensively as dental restorative materials because of their excellent mechanical properties and biocompatibility. However, their surface functionalities, which include anti-bacterial and anti-corrosion properties, still require further improvement. Inspired by nature, superhydrophobic surfaces with micro/nanostructures and a low surface energy have received considerable attention for their outstanding self-cleaning, anti-bacterial, and anti-corrosion properties. In recent years, laser surface texturing has been demonstrated as an effective method for fabricating superhydrophobic zirconia ceramic surfaces. However, post-process treatment methods, including long-term storage in air, heat treatment, and silane reagent immersion, are either time-consuming or toxic. There is therefore a need to develop a time-efficient, low-cost, and ecological laser-based technique for fabricating superhydrophobic zirconia ceramic surfaces.

Commercially available zirconia ceramic (Y-TZP), a zirconia-toughened ceramic prepared with yttrium oxide (Y₂O₃) as the stabilizer with excellent mechanical properties and biocompatibility, was used as the experimental material. Laser surface-texturing experiments employed a laser marking machine equipped with a 355 nm UV laser source (Fig. 1). Upon laser surface texturing, the laser-textured zirconia ceramic surface immediately became superhydrophilic. To achieve the wettability transition, a mixture of 25 μL dimethyl silicone oil (volume fraction 0.4%) and isopropyl alcohol (volume fraction 99.6%) was dripped onto the surface of the laser-textured zirconia ceramic sample. The sample was then placed onto a 200 ℃ hot plate for 10 min. For surface characterizations, the surface topography and chemical composition of the laser-textured zirconia ceramic surface were first examined using confocal laser-scanning microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Then, the wettability of the laser-textured zirconia ceramic surface was evaluated using a contact-angle goniometer equipped with a high-resolution CMOS camera. The long-term durability and self-cleaning properties of the superhydrophobic zirconia ceramic surface were characterized by tape peeling and self-cleaning experiments.

Careful experimental investigations and analysis revealed several key findings: (1) Laser surface texturing induced periodically arrayed surface micro/nanostructures on the zirconia ceramic substrates. By regulating the laser processing parameters, periodic columnar structures or microgrooves with different densities were fabricated (Fig. 2), indicating that the laser-induced surface structure can be well controlled. (2) The surface chemistry analysis show that the laser texturing process oxidized the zirconia ceramic surface while also forming a periodic surface structure. After the mixed solution of silicone oil and isopropyl alcohol dripping and heat treatment, an increase in the carbon and silicon contents was detectable. This result indicates that hydrophobic functional groups, including —CH₂—, —CH₃, and CC, as well as a silicon-based thin film should have been absorbed and deposited onto the laser-textured zirconia ceramic surface, rendering the laser-textured zirconia ceramic surface superhydrophobic after the treatment (Fig. 4). (3) The measured contact angle shows that the untreated zirconia ceramic surface was intrinsically hydrophilic (contact angle 80.4°±2.4°). Immediately upon laser texturing, the zirconia ceramic surfaces became superhydrophilic with a saturated Wenzel regime (contact angle 0°). After mixed solution of silicone oil and isopropyl alcohol dripping and heat treatment, all the laser-textured zirconia surfaces turned superhydrophobic (Fig. 5) with a contact angle of 153.8°±1.2°. (4) The wettability and adhesion of the zirconia ceramic surface can be adjusted by controlling the laser parameters (Fig. 7). For scanning speeds in the range 10-200 mm/s, micro/nanostructures with different densities can be induced by laser surface texturing. At low scanning speeds, a highly adhesive superhydrophobic surface can be prepared using the hybrid process (laser processing+silicone oil modification+heat treatment). As the scanning speed increases, the zirconia ceramic surface displays superhydrophobicity with low adhesion. (5) Long-term storage in air, tape peeling tests, and self-cleaning experiments indicate that a superhydrophobic zirconia ceramic surface prepared with 50 mm/s scanning speed and 100 μm line spacing exhibits excellent stability, durability, and self-cleaning properties (Figs. 9 and 10).

This study developed a convenient and efficient laser-based surface-texturing method to modulate and control the surface functionalities of zirconia ceramic. Laser texturing generated periodic surface structures and oxidized the zirconia ceramic surface, while the subsequent mixed solution of silicone oil and isopropyl alcohol dripping and heat treatment accelerated the absorption of hydrophobic airborne organic compounds and deposited a silicon-based thin film on the laser-textured zirconia ceramic surface. Careful experimental validation and analyses reveal that the surface structure, chemical composition, and wettability can be well controlled and regulated using this method. Furthermore, laser parameters significantly affect the wettability of zirconia ceramic surface. The laser scanning speed and line spacing must be controlled within a certain range to ensure the superhydrophobicity and adhesion of the zirconia ceramic surface. The fabricated surface also displays good self-cleaning performance, stability in air, and resilience to tape peeling. This method will provide a feasible and highly efficient solution for regulating and controlling the surface functionalities of zirconia ceramic, opening the way for more practical and important applications.