Diamond Polishing Based on Laser Composite Technology
Zhiyan Zhao, Yusen Feng, Ziyi Luo, Detao Cai, Yafei Xue, Zhongqiang Wang, and Yanhao Yu
ObjectiveDiamond is a wide-bandgap semiconductor material with several excellent physical and chemical properties. It has an ultrawide bandgap of 5.5 eV, which is significantly higher than those of GaN, SiC, and other wide-bandgap semiconductor materials. In addition, it has a low dielectric constant, low friction coefficient, high carrier mobility, high electron drift speed, and high thermal conductivity. These unique properties make diamonds have an important application value in optics and microelectronics. Because of the high hardness of diamonds, the traditional mechanical polishing method, which yields low polishing speeds and has high costs, cannot achieve an ideal effect. Ion-beam etching is a highly efficient noncontact surface-polishing method for super-hard and brittle substrates. However, it is unsuitable for industrial production because of its high cost. Laser processing is a noncontact processing technique that can handle curved surfaces. It has high processing efficiency and can achieve high-quality processing of various hard materials. Therefore, laser polishing can be used to polish the diamond film using the high energy of the laser to ablate the edges of the diamond particles. It can reduce the surface roughness and flatten the film, but typically induces a surface microstructure or nanostructure on the film surface and introduces a graphite layer. Although several polishing methods for diamond films have been developed, they have limitations, and it is difficult to satisfy the increasing application requirements. To solve these problems, we propose a composite polishing method that uses laser polishing combined with ion-beam etching. By further optimizing the polishing process parameters, a diamond surface without a modified layer is obtained, and the roughness is reduced. The results of this study provide technical support for diamond micromachining and related microdevice preparation.MethodsThe research object of this study is a diamond film prepared via chemical vapor deposition (CVD). The CVD diamond film was first prepolished using a femtosecond laser. The incidence angle of the laser was varied, and the diamond surface was initially polished by controlling the femtosecond laser output power and exposure time. The three-dimensional (3D) surface morphology and roughness of the diamond films were characterized and analyzed using 3D laser microscopy. Next, the power parameters of the nanosecond laser were controlled, and fine polishing was performed. The effect of nanosecond laser machining on the surface roughness of the films was assessed. Subsequently, the effect of the ion-beam etching time on the roughness of the CVD diamond was analyzed. The morphology of the polished diamond films was observed using cold-field emission scanning electron microscopy. The Raman scattering spectra of the samples were measured using Raman spectrometry to analyze the changes in the graphite layers during different polishing processes.Results and DiscussionsAfter femtosecond+nanosecond machining and ion-beam etching, the roughness of diamond surface decreases significantly, from 4 μm without etching to 0.47 μm after etching. In addition, the graphite layer formed by the thermal effect during laser processing can be effectively removed, and the diamond surface can be polished without modification and with high smoothness.1. Using the femtosecond+nanosecond polishing method to polish the surface of diamond film can effectively reduce the surface roughness and produce a smooth surface.2. Laser-polished diamond is typically converted into graphite because of the thermal effect that accumulates on the diamond film surface, which ablates the film surface and forms a graphite layer on the surface. By bombarding the laser-polished surface structure with an ion-beam, graphitization can be effectively eliminated, and an unmodified layer can be formed.3. The use of field mirrors to polish diamond films can result in efficient large-area processing. With shortened scanning and ion-beam etching time, rapid preparation can be achieved, creating conditions for the industrial application of diamond films.ConclusionsIn this study, the effect of femtosecond+nanosecond+ion-beam polishing on the roughness of CVD diamond films was investigated. Ideal surface roughness can be achieved by selecting suitable laser processing and ion-beam polishing parameters. The composite polishing technology of laser polishing and ion-beam etching can effectively polish CVD diamond films. By controlling the femtosecond laser output power and exposure time and varying the laser incident angle, rough polishing of the diamond surface can reduce the roughness and formation of the graphite layer. Nanosecond processing can be fine-processed after applying femtosecond rough processing, however, owing to the thermal effect, a graphite layer is formed during processing. Finally, the graphite layer is effectively removed via ion-beam etching. High-quality polishing is achieved without modifying the layers. Compared with the roughness of approximately 4 μm before polishing, the surface roughness of the composite polished diamond film decreases significantly, with the minimum value reaching 0.47 μm. The proposed method polishes diamond surfaces and provides support for the micromachining and fabrication of micro-optical components on diamond surfaces.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602210 (2024)
  • DOI:10.3788/CJL231148
Causes of Defects in Selective Laser Melting of AlSi10Mg
Shuguang Yao, Yunhui Dong, Xianglong Li, and Minhan Xie
ObjectiveThe parameters for selective laser melting (SLM) directly affect the morphology and microstructure of the melt-forming process, which in turn affect the mechanical properties of the formed structure. Metal powder rapidly heats up and melts under high-speed laser irradiation, forming a metal molten pool. The complex heat and material exchange processes inside and outside the molten pool are difficult to detect in real-time using monitoring instruments. To address the defects generated during the SLM forming process of an AlSi10Mg alloy, this study employed experimental and numerical simulation methods to investigate the effects of forming parameters such as laser power and scanning speed on the morphology of single- and double-channel of the AlSi10Mg alloy.MethodsFS271M laser selective melting equipment was used for single- and double-channel SLM forming of the AlSi10Mg powder. The aluminum substrate was preheated to 130 ℃, the forming cavity was filled with high-purity argon gas as a protective gas, and the oxygen volume fraction was controlled to be less than 0.15%. Table 2 lists the forming parameters. The melt length was set to 20 mm. To facilitate subsequent observation, 1 mm spacing was set for single-melt scanning, and the forming process was repeated five times. After forming and cooling, the morphology of the melt was observed and analyzed using an AM7031MT digital microscope. In addition, Flow-3D v11.1 software was used to simulate the single-channel laser selective melting forming process. A numerical simulation was conducted to investigate the physical effects and phenomena such as thermal radiation, heat conduction, solid-liquid phase transition, molten pool evaporation, gravity, surface tension, and the Marangoni effect derived from the SLM process.Results and DiscussionsUnder different scanning speeds using a laser power of 300 W, the overall continuity of the formed melt is good, no obvious spheroidization is observed, and the degree of overlap is high. As the laser-scanning speed decreases, the width of the melt gradually increases, and a clear ripple morphology is generated at a scanning speed of 700 mm/s. When a 100 W power laser is used for melt forming, the discontinuity and spheroidization of the melt are more severe. The width of the laser heat-affected zone decreases with an increase in the laser scanning speed. The lower the scanning speed, the more obvious is the degree of oxidation and blackening of the powder molten pool. The oxidation effect of the AlSi10Mg powder during processing is a major reason for the low density of the formed structural components. In practical experiments and production, the first-layer premelting method can be adopted to consume as much residual oxygen in the cavity as possible, reducing negative oxidation effects during the molding process. Under the action of a low scanning speed and high energy density laser, the spattering and airflow of the molten pool become more intense, making it easier to produce small-particle spheroidization defects on the forming plane. The keyhole depth generated by the metal molten pool under steam recoil pressure can reach 100 μm. As the laser moves, the molten pool rapidly cools and solidifies due to the high thermal conductivity of the aluminum alloy materials. If the keyhole is not completely filled by the molten pool fluid, pore defects form. Therefore, avoiding keyhole generation while ensuring the continuity of the melt path is necessary. The discontinuity of the melt path is mainly caused by insufficient melting of the powder layer. Reducing the thickness of the powder layer can improve the discontinuity caused by insufficient energy. However, the selection of SLM forming parameters should consider the product-forming efficiency while ensuring the quality of structure forming. Reducing the thickness of the powder layer prolongs the structure-forming time and affects the forming efficiency, and increasing the preheating temperature reduces the energy required for melting. To investigate the effects of the preheating temperature on the morphology of the formed channel, a laser power of 100 W and scanning speed of 800 mm/s were selected as scanning process parameters, and the preset environmental temperature T0 was gradually increased for calculation. At T0=500 K, the discontinuity phenomenon in the forming area is eliminated.ConclusionsThis study investigated the single-layer melt forming of AlSi10Mg powder material through experimental and numerical simulation methods. It was found that the surface tension and melt recoil pressure play crucial roles in the evolution and motion of the molten pool. Even when high-purity argon gas is used as the protective gas for the experiment, because of the oxidizability of the AlSi10Mg material, residual oxygen still affects the quality of the melt forming. Therefore, the oxygen content in the forming cavity should be minimized as much as possible prior to forming. Because the AlSi10Mg alloy powder has a weak laser absorption ability, the energy absorption rate was set to 12% in this study. For a given powder bed with a thickness of 50 μm, a mobile laser beam with a linear energy density of 200 J/m is required to completely melt the powder layer. Under low-power 100 W laser scanning, because of the low energy density of the laser, the melt channel is prone to discontinuity and large-scale spheroidization. Increasing the input energy density by reducing the scanning speed does not effectively solve the problem of uneven melt channels. Obtaining a smoother filling in the keyhole formed under low-speed scanning is difficult, which reduces the quality of the melt channel formation. By increasing the preheating temperature, the laser line energy density required for melting can be reduced, and the morphology of the melt formed at low power can be improved.
  • Aug. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 16, 1602304 (2024)
  • DOI:10.3788/CJL231057
Solidification Microstructure Volume of Fluid Phase Field Model for Laser Welding Nickel‑Based Alloys
Yichen Li, Lei Wang, He Li, Yong Peng, Runhuan Cai, and Kehong Wang
ObjectiveLaser utilization as a heat source to connect nickel-based superalloys has been applied in aviation, aerospace, weapons manufacturing and other fields. Solidification behavior of the laser welding molten pool of nickel-based alloys, including the nucleation, growth, collision and movement of grains, and the diffusion, enrichment and segregation of alloying elements, directly affect nickel-based alloy laser weld performance. Therefore, in-depth research of the solidification behavior of the nickel-based alloy laser welding molten pool is of paramount significance, particularly for laser welding process optimization, welding defect formation control, and the improvement of laser weld mechanical properties.MethodsThe macroscopic heat and mass transfer coupling model alongside microstructure evolution is utilized to quantitatively simulate the macroscopic heat and mass transfer, and the microstructure evolution of a laser welded IN718 alloy. The fluid volume (VOF) method is used to simulate the molten pool morphology and temperature field distribution of the macroscopic heat transfer process, and the temperature field distribution replaces the solidification parameter variables relating to the phase field control equation and is brought into the phase field model for microstructure evolution process simulation.Results and DiscussionsThe simulated and experimental microstructures are shown in Fig.6, where it can be observed that all tissues are columnar crystal structures with a consistent morphology. As shown in Fig.7(a), the solidification velocity R and temperature gradient G at different positions were extracted in the macroscopic simulation results. The solidification velocity R gradually decreased from top to bottom with the molten pool boundary, and the temperature gradient G gradually increased. The cooling rate G·R showed a decreasing trend, the dendrite spacing gradually increased, and the results are shown in Fig.7(b). Compared with the Hunt and Kurz et al. numerical models, the phase field model calculated results are more accurate and close to the experimental results, showing consistent regularity, as shown in Fig.8. The distribution of Nb elements perpendicular to the growth direction of columnar crystals in the simulation results is shown in Fig.9 (b), this shows obvious periodic changes, and the change period is closely related to the phase morphology, because the IN718 solute partition coefficient is less than 1. Solute elements tend to be segregated and enriched at the dendrite gap position. In the simulation results, the mass fraction of Nb elements inside the solid-phase dendrite is significantly reduced, and the lowest mass fraction occurs at the columnar center owing to segregation caused by solute redistribution during solidification. The mass fraction of Nb element increased significantly at the liquid phase position of columnar crystal gap, and the mass fraction of Nb element in the position was relatively higher than that closer to the bottom of the columnar crystal. From Fig.10, the Laves phase enriched by the Nb element precipitated from the IN718 molten pool after solidification is distributed in the γ phase matrix in the shape of droplets. The Laves phase morphology and distribution are approximately consistent with the simulated distribution of Nb elements. The SEM results are shown in Fig.11(b), the EDS spot scan analysis is performed on the illustrated position, with the test results shown in Figs.11(a) and (c). The mass fraction of Nb element in the matrix γ is approximately 4.7%. The mass fraction of Nb element in the Laves phase is approximately 9.8%, and the Nb element mass fraction in Laves phase is significantly higher than that in matrix γ. This proves the accuracy and reliability of the simulation results.ConclusionsThe results demonstrate that the simulated microstructure grows in a columnar crystal structure. The solidification rate R of the molten pool gradually decreases from top to bottom, the temperature gradient G gradually increases, the cooling rate G*R decreases continuously, and the primary dendrite arm spacing increases with the decrease in cooling rate, from 4.52 to 7.12 μm, which is consistent with the experimental results. The mass fraction of Nb elements in the columnar crystal spacing increased significantly, and this mass fraction was relatively higher near the bottom of the columnar crystals in the liquid phase. The Nb elements are finally distributed in the shape of droplets and approximately consistent with the Laves phase in morphology and distribution, which is also consistent with the experimental results. The microstructure transformation process and elemental segregation behavior of IN718 in laser welding are examined, and the solidification theory of the laser welding molten pool of nickel-based alloys is enriched. Finally, this research provides a foundation for a numerical solution of the defect formation process for laser welding IN718 cracks and pores.
  • Jun. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 12, 1202102 (2024)
  • DOI:10.3788/CJL230905
Effects of High-Power Laser on Droplet Transfer and Weld Formation in Different Types of Gas Metal Arc Welding
Yafeng Zheng, Hechao Wang, Haojie Zhang, Qunli Zhang, Liang Wang, Huaxia Zhang, Rangda Wu, and Jianhua Yao
ObjectiveCompared with laser welding and arc welding, laser-arc hybrid welding not only inherits the advantages of laser welding and arc welding but also makes up for respective shortcomings. Thus, it is an advanced welding process method with great application prospects. With the continuous development of laser technologies, laser power has exceeded 10 kW or even higher. Therefore, in order to make the development of lasers well meet the need of actual industrial production, the basic theoretical research on high-power laser-arc hybrid welding has been a hot spot in the academic community in recent years. Researchers have carried out a lot of research on the interaction mechanism between laser and arc. However, the laser power involved was mostly below 5 kW. There are few reports on the mechanism regarding the effect of a high-power (higher than 5 kW) laser on the droplet transfer in laser-arc hybrid welding. Therefore, in this study, a high-power (7.5 kW) laser is introduced into the different modes of arc [standard metal active-gas(MAG), cold metal transfer (CMT), and pulsed arc] welding process, and its effects on droplet transition, weld forming and welding efficiency are compared and studied by using high-speed camera, optical microscope, etc.MethodsIn this study, a high-power laser-arc hybrid welding platform was built, which mainly consisted of a continuous fiber laser, a welding system, a manipulator arm, and a high-speed camera system. The high-power laser-arc hybrid welding experiments were carried out on 10 mm thick Q345 steel, and the laser used in the test was a fiber laser (maximum output power of 12 kW), with an output laser wavelength of (1080±10) nm and a focused spot diameter of 0.2 mm. Before the welding test, an angle grinder was first used to grind the surface to be welded, and then the ground surface was cleaned with alcohol. The arc-guided laser-arc welding was chosen for obtaining a stable droplet transition process. In order to further understand the influence of a high-power laser on droplet transition in different modes of arc welding, the laser was coupled with three different arc modes (standard MAG, CMT and pulsed arc). The welding shielding gas used in the welding process was the Ar and CO2 mixture with a flow rate of 20 L/min. During the welding process, a high-speed camera was used to track and monitor the droplet transition behavior with a frame rate of 10000 frame/s. In order to obtain a clear droplet transition image, an infrared filter was added to the camera lens before the experiment began. Image pro plus software was used to process the pictures taken by the high-speed camera, and the droplet transition mode and the number of droplet transitions within 500 ms under each parameter were counted, so as to calculate the droplet transition frequency within 1 s. After welding, the forward and cross-sectional morphologies of the weld were observed by optical microscope.Results and DiscussionsThe high-power laser has a significant effect on the droplet transition mode of arc welding in different arc modes. During standard MAG welding, the high-power laser attracts and compresses the arc, resulting in a significant reduction in arc length. Meanwhile, metal vapor and plasma ejected from the keyhole reduce the droplet transition frequency (Figs. 6 and 7). In the case of CMT welding, the high-power laser extends the single short-circuit transition period, and the resulting molten pool oscillation reduces the stability of the short-circuit transition (Fig. 8). Regarding the pulsed arc welding process, the high-power laser increases the melting rate of the welding wire. In the meantime, the droplet transition mode changes from the droplet transition to the jet transition, and the droplet transition frequency is significantly increased. The air flow at the key hole hinders the droplet transition, so that the droplet transits to the side of the molten pool (Figs. 11 and 12). Compared with that during arc welding, the weld melting width increases during laser-standard MAG and laser-pulsed arc hybrid welding, while no obvious change in weld width is observed in the case of laser-CMT hybrid welding. The residual height of welds in laser-standard MAG and laser-CMT hybrid welding decreases significantly, while the residual height of welds in laser-pulsed arc hybrid welding increases slightly. This is attributed to different degrees of influence of the laser on the droplet diameter and transition frequency in three different modes of arc welding. Furthermore, the melting energy increment value (ψ) of laser-arc interaction varies under different hybrid welding conditions, among which laser-pulse arc welding has the highest ψ value (36%), followed by laser- standard MAG welding (19%), while laser-CMT welding has the smallest ψ value (-12%).ConclusionsIn this study, the effects of laser (7.5 kW power) on droplet transition and weld formation in different modes of arc welding were investigated. The results reveal that the addition of laser has a significant influence on the droplet transition in standard MAG, CMT and pulsed arc welding processes. During standard MAG welding, the high-power laser attracts and compresses the arc, resulting in a significant reduction in arc length, and the metal vapor and plasma ejected from the keyhole reduce the droplet transition frequency. In the CMT welding process, the laser extends the single short-circuit transition cycle, and the melt pool oscillation caused by the high-power laser reduces the stability of the short-circuit transition. Regarding the pulsed arc welding process, the addition of a high-power laser increases the melting rate of welding wires. The droplet transition mode changes from the droplet transition to the jet transition, and the droplet transition frequency increases. Meanwhile, the air flow at the key hole hinders the droplet transition, so that the droplet transits to the side of the molten pool. Compared with arc welding, the weld melting width increases during laser-standard MAG and laser-pulsed arc welding, while no obvious changes in weld width are observed in the case of laser-CMT hybrid welding. The residual height of the welds in laser-standard MAG and laser-CMT hybrid welding decreases significantly, while the residual height of welds in laser-pulsed arc hybrid welding increases slightly. The melting energy increment values of the interaction between laser and arc under three arc modes are: laser-pulsed arc hybrid welding (36%), laser-standard MAG hybrid welding (19%), and laser-CMT hybrid welding (-12%).
  • Jun. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 12, 1202107 (2024)
  • DOI:10.3788/CJL230766
Process Parameter Optimization and Microstructure and Property Investigation in Laser Cladding of Ti60 Alloy
Zongfu Shu, Chunping Huang, Yaozu Zhang, and Fenggang Liu
ObjectiveTi60 is a near-α titanium alloy with good high-temperature performance that has been identified as an important candidate material for aero-engine compressor blades and integral blades. However, when high-temperature titanium alloys are fabricated using traditional processing technology, it has the disadvantages of difficult formation, low material utilization, and high cost. Laser cladding technology uses a laser with high energy density to melt the powder preset on the surface of the substrate , so as to obtain the expected performance of the cladding layer. There are many parameters of the laser cladding process that have significant influence on the forming quality. At the same time, complex thermal cycling in the laser cladding process leads to differences in the grain size, morphology, and size of the precipitated phase, which makes the differences in the mechanical properties of the laser cladding significant. Therefore, this paper mainly studies the effect of the process parameters on the forming quality of laser cladded Ti60 alloy, and the microstructure evolution and tensile properties of laser cladded Ti60 alloy are analyzed to lay a theoretical foundation for the application of laser cladded high-temperature titanium alloy components in the aerospace field.MethodsThe material selected in this experiment is Ti60 powder with a particle size of 50?150 μm, prepared using the plasma rotating electrode process (PREP). TC4 titanium alloy is used as the substrate, and the laser cladding system is used as the laser cladding experiment system. The section of the laser cladded sample along the thickness direction of the cladding layer is machined via electric discharge wire cutting into a flake sample with a thickness of 5 mm for the metallographic sample. The Kroll reagent is then used for etching, and finally, the microstructure is observed using a metallographic microscope and field emission scanning electron microscope (SEM). A field emission transmission electron microscope (TEM) is used to analyze the precipitated phase of the cladding specimen. A microhardness tester is used to test the Vickers hardness of the Ti60 cladded sample from top to bottom. The tensile experiment is performed on the high-temperature tensile test machine at room temperature, 300 ℃, and 600 ℃, with a tensile speed of 1.0 mm/min. The tensile fracture is observed, and the fracture morphology and fracture mode are analyzed.Results and DiscussionsThe influence of different factors on the size of the laser cladding layer is analyzed according to the shape and size of the cladding layer measured by the image scanner. When the width of the molten pool is large, the cladding efficiency can be effectively improved, the material utilization rate can be improved, and the cost can be reduced. The thickness of the cladding layer has a significant influence on deposition along the height of the cladding layer (Fig. 5). The microstructure at the top region of the cladded sample is the thin layer of equiaxed grain, and its grain size gradually increases with increasing laser power. In the central region of the sample, the original β grains can be observed growing in the deposition direction along the epitaxial columnar pattern across multiple cladding layers. Moreover, the larger the laser power, the coarser the columnar grains and the microstructure inside the grains (Fig. 6). The sample of the laser cladded Ti60 block is mainly composed of a netted basket of lath α and interlath β phases. There are white Ti5Si3 phases with different shapes on the slat α, and the content of the Ti5Si3 phase gradually decreases from the bottom to the top (Fig. 9). At room temperature, the tensile strength and yield strength of the laser cladded Ti60 samples are 1128 MPa and 1035 MPa, respectively, and the elongation and section shrinkage are 8.8% and 14.4%, respectively. At 300 ℃, the tensile strength and yield strength are 932 MPa and 796 MPa, respectively, and at 600 ℃, the tensile strength and yield strength are 739 MPa and 627 MPa, respectively (Fig. 12).ConclusionsThe microstructure at the bottom and top regions of the laser cladded Ti60 alloy sample is composed of β equiaxed grains, and the middle region is composed of β columnar crystals. Its size gradually increases with increasing laser power. The microstructure is mainly composed of lath α and interlath β phases, and there is a large amount of the white precipitated phase in the lath α phase. With an increase in laser power, the microstructure changes from a net basket structure to a Weisberg structure. The micro-hardness distribution of the bulk sample is uniform, and its hardness value fluctuates in the range 420?440 HV. The tensile strength of laser cladded Ti60 alloy at room temperature is 1128 MPa, and the elongation and section shrinkage after fracture are 8.8% and 14.4%, respectively. When the temperature is 300 ℃ and 600 ℃, the tensile strength is 932 MPa and 739 MPa, respectively.
  • Jun. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 12, 1202202 (2024)
  • DOI:10.3788/CJL231003
Scaling Law for Thermo‑Mechanical Responses of Metal Plate Subjected to Laser Irradiation Under High‑Speed Airflow Condition
Yue Cui, Ruixing Wang, Te Ma, Wu Yuan, Hongwei Song, and Chenguang Huang
ObjectiveLaser technology is extensively used in various fields, including additive manufacturing, removal processing, and laser weaponry. This technology has the potential to revolutionize battlefield dynamics through defensive and offensive applications. Research on laser irradiation under high-speed airflow provides a theoretical basis for efficient damage strategies and the protection of aircraft, this is crucial for deploying military laser systems. However, conducting real-scale model tests for large-scale engineering structures is challenging due to equipment limitations and testing environments. Additionally, wind tunnel tests with real-scale models are prohibitively expensive and time-consuming, preventing extensive testing. Consequently, scaled-model tests are often relied upon for regularity studies. Therefore, establishing a similarity relationship in the thermomechanical responses between real and scaled models under laser irradiation and high-speed airflow is a practical approach. Significant efforts have been made to understand the similarity theory of laser-induced thermomechanical behavior under static air conditions. Nonetheless, due to the complex fluid-thermal-structural interactions in high-speed airflow, the similarity criteria for thermomechanical responses in an airflow environment significantly differ from those in static air. In this study, we propose new similarity criteria and scaling laws suitable for the thermomechanical responses of a metal plate subjected to high-speed airflow and laser irradiation.MethodsTo clarify the similarity relation of thermomechanical responses for metal plates under coupling conditions, the effects of the tangential airflow were equivalently converted to the structural force and thermal load boundary conditions using the approximate equivalence method, and the dimensionless governing equations of the coupling problem were established. Thus, combined with the analysis of dominant factors, the similarity criteria and scaling laws suitable for the thermomechanical responses of the metal plate under the combined action of a high-speed airflow and laser were determined. According to the similarity criteria, there is a contradiction in the similarity relationship between the thermal boundary condition and force boundary condition under the fluid-thermal-structural coupling effects, which cannot be satisfied simultaneously. Considering that the thermal stress induced by the temperature gradient is much greater than the mechanical stress due to the aerodynamic force under the combined action of high-speed airflow and laser irradiation, this study focused on the similarity of aerodynamic heat transfer, ignored that of the aerodynamic force, and established the corresponding scaling law. Then, a fluid-thermal-structural coupling numerical example of a metal plate irradiated by a high-power laser under tangential flow was conducted to verify the scaling law under different scale ratios and Mach numbers.Results and DiscussionsThe similarity criteria and scaling laws for the fluid-thermal-structural coupling analysis of the metal plate subjected to laser irradiation and high-speed airflow are presented in Tables 1 and 2, respectively. A numerical example of the fluid-thermal-structural coupling of a metal plate irradiated by a high-power laser under a tangential flow is conducted to verify the scaling law. The results show that under different scale ratios and Mach numbers, the predicted response errors between the scaled and original models are within 1%, which proves the reliability and accuracy of the scaling law. Simultaneously, with the increase in scale ratios or Mach numbers, the aerodynamic heat transfer effect is enhanced, making the thermal-mechanical response difference between the scaled model and real model more obvious when the aerodynamic similarity criteria are not considered.ConclusionsIn this study, similarity criteria and scaling laws suitable for the thermomechanical responses of a metal plate under the combined action of a high-speed airflow and laser are determined. Several numerical examples are conducted and compared to verify the proposed similarity criteria and scaling laws. The main conclusions are as follows: (1) Using the approximate equivalence method and analysis of dominant factors, the effects of the tangential airflow are equivalently converted to structural force and thermal load boundary conditions, and the similarity criteria and scaling laws are determined. Considering that the thermal stress induced by the temperature gradient is significantly greater than the mechanical stress caused by the aerodynamic force, this study focusses on the similarity of the aerodynamic heat transfer and ignores the similarity of the aerodynamic force. (2) A fluid-thermal-structural coupling numerical example of a metal plate irradiated by a high-power laser under tangential flow was conducted to verify the scaling laws under different scale ratios and Mach numbers. The results show that the predicted response errors between the scaled and original models are within 1%, which proves the reliability and accuracy of the scaling laws. (3) However, the scope of application of the proposed similarity criteria should be emphasized in the following aspects: the similarity criteria are applicable for calorically perfect gases. For hypersonic flows, complex chemical reactions occur at high temperatures, and the similarity criteria are no longer applicable. The similarity criteria are applicable for the plate flow condition. However, for the non-plate flow, such as the flow around a blunt-nosed bodies, the similarity criteria are no longer applicable. The similarity criteria are applicable to the thermal-mechanical responses of the metal structure before melting. When melting is involved, similarity criteria are no longer applicable.
  • Jun. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 12, 1202103 (2024)
  • DOI:10.3788/CJL231077
Research on Laser Cleaning Process of Paint Layer on Carbon Fiber Composite Aircraft Skin
Junyi Gu, Wenqin Li, Xuan Su, Jie Xu, and Bin Guo
ObjectiveResin-based composite material (CFRP) surface coatings have always faced the risk of morphological damage and substrate overheating during laser cleaning. The key for solving these problems lies in the need for sufficient process experiments to establish a reliable relationship between the cleaning parameters and characteristics. Cleaning depth (H), surface roughness (Sa), and cleaning temperature (T) are the three most important cleaning indicators. H represents cleaning efficiency and effectiveness, Sa is related to the quality of re-coating, and T reflects the trend of thermal damage. Therefore, this study uses an infrared nanosecond laser to remove paint from a CFRP surface and uses laser power (P), scanning speed (V), overlap rate (η), and repetition frequency ( f ) as variables to study and statistically analyze the H, Sa, and T of the samples. Infrared thermography and high-speed imaging techniques are used to observe the temperature response of the samples, the state of the plume, and the dynamic behavior of the paint layer to determine the cleaning mechanism of the paint layer. This study is expected to provide a basic reference for improving the efficiency of laser paint removal and the quality of respraying and reducing thermal damage to CFRP substrates.Methods Four controllable parameters are usedlaser power, scanning speed, repetition frequency , and overlap rate. Five levels are designed under each group of parameters to form an L25 orthogonal matrix. Then, a laser cleaning experiment is conducted to obtain 25 sets of samples ranging from No.1 to No.25. After the cleaning procedure is completed, the macroscopic and microscopic morphologies of the cleaned samples are observed. At the same time, the paint cleaning depth and sample surface roughness are measured via a laser confocal microscope. Finally, the obtained experimental data are analyzed using the analysis of the variance (ANOVA) and signal to noise ratio (S/N) methods. In addition, an infrared thermographic camera is used to record the temperature response of the experimental samples during the cleaning process, and a high-speed camera is used to capture the dynamic behavior of the samples.Results and DiscussionsA signal-to-noise ratio analysis is performed on the cleaning depth, surface roughness, and cleaning temperature using the expected large, large, and small characteristics, respectively. The analysis results (Table 4) indicate that for the cleaning depth, the influencing factors are ranked from high to low by weight, namely, lap rate, laser power, scanning speed, and repetition frequency. For surface roughness and cleaning temperature, the influencing factors are ranked from high to low by weight, namely, lap rate, scanning speed, laser power, and repetition frequency. The ANOVA results (Table 5) indicate that for cleaning depth, roughness, and cleaning temperature, the critical probability (P') values of the overlap rate, scanning speed, and laser power are all less than 0.05. Therefore, at a 95% confidence level, the overlap rate, scanning speed, and laser power have statistically significant effects on cleaning depth, roughness, and cleaning temperature. In contrast, the contribution rate of repetition frequency is relatively low, with a P' value greater than 0.05, making it a less important process parameter. The detection results (Fig. 8) by the infrared thermal imager indicate that the laser cleaning process causes two high-temperature areas. The first is where the laser acts on the substrate. The minimum cleaning temperature in this area is 244 ℃, and the maximum cleaning temperature is 590.4 ℃. The other high-temperature region is the high-temperature plume region above the sample. The high-speed camera monitoring results (Fig. 11) indicate that the paint layer undergoes drastic changes due to the action of the laser, the most obvious being the generation of bright plasma and the formation of a plume perpendicular to the sample. A large number of turbid particles are observed inside the plume.ConclusionsThis study focuses on the influence of process parameters on the laser cleaning of paint layer on the CFRP aircraft skin. For the cleaning depth, surface roughness, and cleaning temperature, the overlap rate is the most significant influencing parameter, with contribution rates of 50.51%, 59.07%, and 69.09%, respectively. A lower overlap rate is not conducive to the uniform removal of paint, and an increase in the overlap rate will significantly increase the temperature of the substrate. Laser power and scanning speed also have a significant influence on cleaning depth, surface roughness, and cleaning temperature, whereas repetition frequency has no significant effect. The removal of paint is mainly based on the thermal erosion mechanism. During the cleaning process, the surface temperature of the paint layer rapidly increases to the decomposition temperature and the paint transforms into small particles and gases, forming a high-temperature plume above the sample. The above results will provide a reference for improving laser paint removal efficiency and respraying quality and reducing substrate thermal damage.
  • Jun. 25, 2024
  • Chinese Journal of Lasers
  • Vol. 51, Issue 12, 1202201 (2024)
  • DOI:10.3788/CJL230927