Nickel-based single-crystal superalloys are widely used in the manufacture of single-crystal turbine blades in the combustion chambers of aircraft engines owing to their excellent high-temperature mechanical properties. However, these components often suffer from severe damage, such as edge erosion, cracking, and pitting, owing to the harsh operating conditions, including high temperatures and pressures, requiring repairs to extend their service life. Laser additive manufacturing technology has garnered significant attention for repairing nickel-based single-crystal superalloys owing to its unique advantages, including controllable heat input, the ability to fabricate complex structures, and reparability. However, the appearance of grain defects during the repair process may lead to serious failure phenomena, such as crack propagation and component fracture, during component operation, thereby posing potential risks to the safety and reliability of aircraft engines. Therefore, this study selected a DD6 second-generation nickel-based single-crystal superalloy as the substrate material and used lasers with powers of 1200 and 1500 W to re-melt the substrate, revealing the mechanisms of stray grain formation at the fusion line, top of the molten pool, and at the intersection of the dendrites, thereby providing a theoretical basis and technical support for the laser additive repair of nickel-based single-crystal superalloys.

This study employed a DD6 nickel-based single-crystal high-temperature alloy prepared via directional solidification as an experimental substrate material. Subsequently, a laser cladding additive manufacturing device equipped with a 2 kW German Rofin fiber-coupled semiconductor laser was utilized. The process parameters included laser powers of 1200 and 1500 W, a scanning rate of 3 mm/s, and a spot diameter of 4 mm for the single-track remelting experiments on the substrate (001) crystal surface. To prevent sample oxidation during remelting, argon gas was used as a protective gas at a flow rate of 10 L/min. The dendritic morphologies of the substrate and remelted region were observed using an OLYMPUS DSX510 optical microscope. Simultaneously, scanning electron microscopy and X-ray energy-dispersive spectroscopy were employed for detailed analysis of the molten pool morphology and elemental distribution. Electron backscatter diffraction (EBSD) analysis was performed to further investigate the crystallographic properties of the specimens. The sample tilt angle was set to 70°, with a scan step size of 3 μm, using nickel as the calibration phase. HKL Channel 5 post-processing software was employed for texture and orientation deviation analysis of the samples.

The results show that, after laser remelting, the molten pool could be divided into four regions ([001], [100], [010], and [01¯0]) based on the growth direction of the grains (Fig.3), and the primary dendrite spacing increased with increasing laser power. From the crystallographic texture characteristics of the molten pool (Fig.5), it is evident that carbides mainly occurred at the fusion line, top of the molten pool, and intersection of the turning dendrites. Carbides at the fusion line occurred owing to the local collapse of the rough growth interface, causing a deviation in the crystal growth direction. This also increased the diffusion rate of the solute atoms and enhanced the degree of undercooling at the solidification interface, providing heterogeneous nucleation sites for grain nucleation, thereby inducing carbide formation. The carbides at the top of the molten pool were caused by the columnar-to-equiaxed transition, where the numerical simulation showed that the temperature gradient gradually decreased and the solidification rate increased from the bottom to the top of the molten pool (Fig.11), promoting the columnar-to-equiaxed transition. Carbides at the intersection of turning dendrites occur because of collisions resulting from the lower temperature gradient at this location compared with adjacent areas and changes in the direction of the temperature gradient. The thermal stress numerical simulation results (Fig.12) indicate that a low laser power input effectively increases the temperature gradient and reduces the residual stress level, which is beneficial for inhibiting carbide formation in single-crystal repairs.

This article utilized two distinct laser powers, 1200 and 1500 W, to conduct single-track laser remelting on DD6 nickel-based single-crystal superalloys, in conjunction with numerical simulations, to investigate the mechanisms behind stray grain formation in various regions within the molten pool. The principal findings are as follows:

1) Stray grains predominantly emerged in three areas: the fusion line of the molten pool, top of the molten pool, and convergence point of diverging dendrites.

2) At a laser power of 1500 W, there was an increased quantity and larger size of the stray grains. No stray grains were observed at the smooth interface of the fusion line, whereas carbides formed at rough interfaces could lead to the collapse of the solid-liquid interface, causing a deviation in the dendritic growth direction and inducing stray grain formation.

3) The development of stray grains at the apex of the molten pool was closely related to the temperature gradient of the molten pool and movement rate of the solid-liquid solidification interface. The minimal temperature gradient at the summit of the molten pool enhanced the likelihood of a columnar-to-equiaxed transition, fostering the generation of stray grains. Furthermore, it was discovered that a reduced laser power could increase the temperature gradient and suppress the formation of stray grains.

4) The genesis of stray grains at the confluence of diverging dendrites was attributed to collisions caused by the temperature gradient at the dendrite junction, which was lower than that in the surrounding areas, coupled with a shift in the direction of the temperature gradient.

To date, laser powder bed fusion (LPBF) is considered as the most advanced metal additive manufacturing technology. It has been widely adopted for the production of critical metal components in aerospace and healthcare industries. However, realizing quality stability and consistency is challenging because of the coupled effects of various factors during LPBF. The powder-spreading quality is a crucial characteristic of LPBF process monitoring. Defects during powder spreading can introduce defects into the formed components. In recent years, the application of computer vision in powder-spreading defect detection has shown promising results. However, the limited availability of annotated data constrains its performance. Large vision models, such as the segment anything model (SAM), exhibit remarkable generalization capabilities owing to pre-training on an extremely large dataset. This allows its transfer to various downstream tasks with minimal training data. However, owing to the lack of defect knowledge, absence of category information, and dependence on manual prompts, SAM cannot be directly applied to powder-spreading defect segmentation. This study addresses the requirements for powder-spreading defect segmentation by improving SAM, achieving excellent defect segmentation performance with minimal training samples, and exploring the potential application of large vision models in monitoring the additive manufacturing process.

In this study, the powder-spreading defect segment anything model (PSAM), based on SAM, was introduced. The overall structure of PSAM was similar to that of SAM, which consisted of an image encoder, an auto-prompt generator, and a mask decoder. Compared to the original SAM, PSAM incorporated the following improvements: To address the issue of knowledge transfer concerning SAM's pre-trained parameters, four Adapter modules were introduced into the SAM image encoder structure. These Adapter modules enabled efficient adjustment of image feature encoding. They were inserted behind the multi-head attention layer in the transformer module and comprised linear and convolutional layers. To satisfy the requirements for category information in the powder-spreading segmentation task, PSAM utilized an improved mask decoder. This decoder outputted segmentation masks that were equal to the number of categories in a single pass. Each output corresponded to a specific category-segmentation result. These outputs were then integrated to obtain a classification output. To overcome the challenges of manual prompting in industrial settings, an autoprompt generator was designed. This generator is a fully convolutional neural network with residual connections that extracts features from input images and generates prompt embeddings that can be used by a mask decoder. A combination of cross-entropy loss, focal loss, and Dice loss was used as the final loss function. Specifically, mean intersection over union (mIoU) was employed as the evaluation metric.

This study utilizes an off-axis industrial camera to acquire powder-spreading images during the formation of several components. A subset of these images is selected for pixel-level annotation and is categorized into six classes: background, super-elevation, incomplete, hopping, streaking, and lattice. The images and their corresponding labels are organized into a dataset and are divided according to certain proportions. The model is trained using the training set, and even with only 50 training images, PSAM exhibits excellent segmentation performance. The evaluation of PSAM using the test set yields an mIoU of 65.02%, representing an improvement of 8.51 percentage points over Deeplab v3 and 5.31 percentage points over U-Net (Table 1). The limited amount of data restricts the ability of the model to perform deeper feature learning, thus hindering its capacity for richer feature representation. However, the pretrained SAM possesses strong image-feature extraction capabilities. Therefore, excellent defect segmentation performance can be realized without extensive data training. Ablation experiments are conducted to evaluate the proposed improvements. The results indicate that the introduction of the Adapter module successfully transfers feature representation capabilities, and the automatic prompting effectively guides the mask decoder's output. Compared with the original SAM, PSAM realizes an mIoU improvement of 11.33 percentage points (Table 2).

The design of PSAM based on SAM realizes excellent segmentation performance even with a small amount of training data. Firstly, the model achieves transferring image encoder feature extraction capabilities from natural images to powder- spreading images by introducing the Adapter modules. Second, the mask decoder is modified to output the category masks. Finally, by incorporating an auto-prompt generator to encode the input images, automatic generation of visual prompt embeddings is achieved. Large artificial intelligence (AI) models are rapidly advancing. However, owing to the unique characteristics and complex operating conditions of industrial settings, these models still face challenges in practical applications in industrial scenarios. This study provides a preliminary exploration of the application of large vision models for powder-spreading defect detection in additive manufacturing. However, the full potential of large vision models has yet to be fully realized. The outstanding feature extraction, zero-shot generalization, and multimodal knowledge fusion capabilities of these large models can provide new solutions and approaches for additive manufacturing process monitoring, which are worth further exploration in future research.

As the main component of the entire precision optical systems for aerospace, the dimensional stability of the lens tube has an undeniable impact on the overall accuracy and performance of space optical systems. However, the complex thermal environment in which the satellite operates for a long time, such as direct solar radiation or the Earth's infrared radiation, can significantly affect the detection accuracy of the optical system. Therefore, Invar alloy 4J36, with excellent dimensional stability, can be selected to manufacture structural components of lens tubes. Due to its unique "Invar effect", the Invar alloy 4J36 exhibits an extremely low coefficient of thermal expansion at its Curie temperature (230 ℃) , and it can be effectively used in the manufacturing of space optical devices. However, owing to the high hardness and poor machining performance of 4J36, traditional manufacturing methods, such as turning and milling, require long processing time periods and result in serious material waste. Selective laser melting (SLM) is an additive manufacturing technology that has the advantages of high design freedom, a short production cycle, and wide applicability. Furthermore, it can be used to manufacture structurally complex products at a faster rate when compared to traditional manufacturing methods. To date, the SLM additive manufacturing process for Invar alloy lens tubes is not mature. Through a series of adjustments to the powder quality, process parameters, 3D models, and residual stress, we hope to obtain high-quality structural components of lens tubes that satisfy usage requirements. This can aid in further exploration of the universe.

SLM was used to shape an Invar alloy lens tube, and the Invar alloy powder material was selected according to the standard set at the beginning of this study to ensure that the microstructure of the finished product exhibits no obvious defects. The essence of SLM technology is the direct interaction between laser and powder. Hence, to explore the optimal laser selective melting process parameters and optimize the quality of Invar alloy tube products, single factor experiments were conducted with laser scanning spacing and scanning speed ranging from 0.08 mm to 0.11 mm and 900 mm/s to 1200 mm/s, respectively. The goal was to obtain different laser energy densities. Subsequently, the surface of the test block was cleaned and polished, its surface microstructure was observed via a metallographic microscope, and its mechanical properties were characterized using an electronic universal testing machine. Additionally, we optimized the structure of the three-dimensional model of the lens tube, calculated its overall stress and strain through simulations, and compared the simulation results with those of the original model to analyze the optimization. Subsequently, heat treatment was performed on the lens tube to relieve its residual stress, and the printing accuracy and residual stresses at specific points were characterized via X-ray scanning. Finally, the thermal expansion coefficient of the lens tube after heat treatment (530 ℃±10 ℃, 1 h) was tested using a thermal expansion instrument to evaluate its structural stability.

After standardized screening, the loose density of the Invar alloy powder can reach 4.7 g/cm³, and the powder sphericity can reach up to 0.89. Additionally, the smooth flowability of the powder surface is significantly improved to 14.7 s/50 g, and no powder accumulation occurs during the additive manufacturing process using this powder material. The surface of the product was smooth and crack-free (Fig. 3). Experimental results show that the optimal scanning spacing for SLM additive manufacturing is 0.09 mm, and the surface microstructure is smooth without obvious defects such as keyholes and lack of fusion (Fig. 4); the optimal scanning speed is 900 mm/s. Simultaneously, the surface microstructure is complete and smooth, without cracks (Fig. 5), with a tensile strength of 482 MPa and yield strength of 388 MPa. They exhibit excellent mechanical properties (Table 5). After topology optimization, the service strain of the lens tube structure at the same point under the same specifications is only 0.09 mm(Fig. 6). Furthermore, when compared to traditional models, it can save materials and improve efficiency. After the stress-relief heat treatment, there is no evident defects inside the lens tube(Fig. 10), and the maximum residual stress is only 13% of its yield stress. The thermal expansion coefficient of the lens tube is 1.9×10^-6 K^-1 (Table 6), which satisfies the requirements of high dimensional stability.

This study successfully realizes high-quality manufacturing of Invar alloy lens tube using SLM additive manufacturing technology. First, by establishing physical and chemical specifications for the powder materials, macroscopic defects, such as cracks and inclusions, in the lens tube are avoided in the initial stages of the experiment. The SLM process parameters are optimized. The optimal process is determined and corresponds to a scanning spacing of 0.09 mm and scanning speed of 900 mm/s. Simultaneously, the surface microstructure is observed as smooth and free of defects such as cracks and lack of fusion. The best mechanical properties are obtained using this process. The best mechanical properties correspond to a tensile strength of 482 MPa, a yield strength of 388 MPa, an elongation of 29%, and a shrinkage rate of 73%. Topological optimization is performed using the original 3D model of the lens tube. After optimizing the structure, the overall stress concentration of the product under service conditions is significantly reduced, and the maximum deformation degree of the product is only 0.09 mm. Additionally, the structure adopts self-supporting formation, which effectively saves powder materials. Finally, the lens tube is subjected to post-treatment to eliminate residual stresses. The maximum residual stress inside the product after the heat treatment is 60 MPa, which is only 13% of its yield stress. Simultaneously, the lens tube exhibits an extremely low coefficient of thermal expansion (1.9×10^-6 K^-1), which satisfies the requirement of high structural stability of space optical lenses in complex thermal environments.

For the extreme complex working conditions in the aerospace field, Ti6Al4V/NiTi heterogeneous functional structure can give full play to the advantages of its high specific strength, corrosion resistance and other material properties while realizing the functional requirements such as intelligent deformation. However, the two alloys have significant differences in melting point, coefficient of thermal expansion, thermal conductivity and specific heat capacity, leading to the challenge in the high-quality preparation of Ti6Al4V/NiTi alloy heterostructures. On the one hand, the brittle intermetallic compounds (such NiTi₂, Ni₃Ti and Al₃Ti) generated during the forming process, induce a decrease in the interfacial bonding strength, bring on a potentially high cracking tendency during the forming process. On the other hand, cracks are sprouted in the laser deposited formed parts due to the high temperature gradient during the deposition process and the accumulation of thermal stresses caused by rapid solidification, thus restricting the metallurgical bonding between the interfaces of heterogeneous material structures. In this study, Ti6Al4V/NiTi heterogeneous materials were successfully prepared using in-situ gradient additive technology for heterogeneous materials. We hope that this study will lay the foundation for the practical application of aerospace-oriented Ti6Al4V/NiTi heterogeneous functional materials on complex structural components.

Ti6Al4V and NiTi alloy powders were used in this study. Firstly, in-situ preparation of 11 thin-walled Ti6Al4V/NiTi alloys with different mass fraction ratios was carried out in an oxygen-enriched environment using in-situ gradient additive technology for heterogeneous materials. Secondly, the microstructures and phase compositions of the composites with 11 compositional ratios were analyzed and characterized by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). On this basis, actual characterization results of the elemental content of the 11 component ratios were compared with the compositional design results. Then, Ti6Al4V/NiTi heterogeneous materials were prepared by combining gradient transition composition design and substrate thermal management. And the metallurgical bonding properties between the interfaces of different gradient regions as well as the elemental species and contents were characterized by scanning electron microscope (SEM) observations and EDS analyses. Finally, microhardness tests were performed on the prepared Ti6Al4V/NiTi heterogeneous materials to characterize their mechanical properties.

For the 11 kinds of Ti6Al4V/NiTi alloys with different mass fraction ratios, the XRD analysis results show that the phase compositions from 100% Ti6Al4V to 100% NiTi are in the following order: α-Ti+β-Ti→α-Ti+NiTi₂→NiTi₂→NiTi₂+NiTi [see Fig. 3(a)]. With the increase of NiTi alloy powder content, the Ti elemental mass fraction changed from 90.7% to 46.5% and the Ni elemental mass fraction increased from 0.1% to 53.3% (Fig. 5). The compositional design is in good agreement with the actual results. SEM and EDS analysis results show that the Ti6Al4V/NiTi heterogeneous materials prepared after component gradient optimization have good metallurgical bonding between the gradient layer interfaces (Table 2). With the gradual increase of NiTi component, the phase composition from Ti6Al4V zone to NiTi zone evolves as α-Ti+β-Ti→α-Ti+NiTi₂→NiTi₂→NiTi₂+NiTi→NiTi→NiTi+Ni₃Ti (Table 2). The average microhardness in the gradient transition zone varied from 343 HV±13 HV in the Ti6Al4V zone to 275 HV±10 HV in the NiTi zone; whereas, the precipitation of NiTi₂ reinforced phase resulted in the highest hardness value of 576 HV±5 HV in the 40% Ti6Al4V+60% NiTi zone (Fig. 8).

In this study, preparation of Ti6Al4V/NiTi alloys with different mass fraction ratios was firstly carried out in an oxygen-enriched environment by employing an in-situ gradient additive technology for heterogeneous materials. Microstructure evolution and phase composition of the composites with 11 compositional ratios were also analyzed. EDS spectroscopy results show a good agreement between the compositional design and the actual characterization, thus proving the feasibility of the Ti6Al4V/NiTi heterogeneous alloy powder synchronous conveying method proposed in this paper. Then, the integrated deposition and forming of Ti6Al4V/NiTi heterogeneous materials with the optimized component gradient transitions was finally achieved by proposing an isoenergetic energy density forming method and thermal management of the substrate at 400 ℃ to reduce the content of brittle intermetallic compounds as well as to lower the thermal stresses. Metallographic observations show good metallurgical bonding between the interfaces in the different gradient regions. Thermal management of the substrate at 400 ℃ helps to reduce the cracking tendency of the Ti6Al4V/NiTi heterogeneous alloy. Our study shows that integrated deposition and forming of Ti6Al4V/NiTi heterogeneous materials can be carried out by rational gradient composition design combined with temperature regulation of the forming process. Purpose of this study is to lay a foundation for the practical application of Ti6Al4V/NiTi heterogeneous functional materials on complex structural parts.

Morphing aircrafts can change their shape according to different flying environment and conditions, which makes their aerodynamic efficiency much better than traditional aircrafts. In order to achieve multi-dimensional deformation, mechanical metamaterials that exhibit designable morphing capability have been widely studied. Particularly, structures with coupled tension‒tsist characteristics are necessary in case that attacking angle should be changed to adjust the aerodynamic load distribution on the wing surface. Therefore, this study proposes a novel metamaterial structure that can exhibit coupled tension‒twist deformation, which significantly increases the twisting angle of a cross section under axial loading. The methodology of this study can provide valuable guideline for the future design of morphing aircrafts.

Models of the metamaterial cell structure were built using beam elements. Two types of beam structures with different coupled tension‒twist properties were designed by cell stacking. The stiffness and coupled tension‒shear deformation of the cells were studied by finite element analysis (FEA). After the cells were stacked, the coupled tension‒shear deformation of the cells transformed into coupled tension‒twist deformation of the beam structures. The deformation capabilities of the beams and related parameters were then investigated. Finally, samples of different lengths of two types of beams were prepared by selective laser sintering (SLS) of PA12 material for experimental verification. Samples were loaded by hanging weights on the free end, and the other end was fixed by an industrial bench vice. The twist angle was measured indirectly using a laser sensor.

The beams were designed with the ability to exhibit coupled tension‒twist deformation with a twist angle higher than 15°. Results show that the twist angle of the four cells combination cantilever beam is significantly greater than that of the two cells combination cantilever beam. Under a tensile load of 46.69 N, the twist angles of the aforementioned beams are 0.667° and 0.479°, respectively, with the results being consistent with the FEA. In addition, weights of the four and two cells combination cantilever beams are 319.94 and 311.32 g, respectively. This means that with 2.77% greater weight, the value of the coupled tension‒twist parameter increases by 42.97%. The twist angle for the cantilever beams is shown to increase linearly with the number of stacked cells, which enables a larger twisting angle if needed.

In this study, a novel mechanical metastructure with coupled tension‒twist deformation capability is proposed. The metastructures can transform the coupled tension‒shear deformation of unit cells into coupled tension‒twist deformation of beams by cell stacking, which can significantly improve section twist angle under axial loads. The proposed designing method is verified by finite element modeling and experimental testing of beam samples.

GH4169 superalloys are widely used in aerospace engines and other high-temperature components. Powder recycling of the GH4169 alloy during selective laser melting (SLM) can significantly reduce the preparation cost and shorten production cycles. However, the components formed by the SLM using recycled alloy powders exhibit differences in microstructural defects and performance behavior because of changes in the size distribution, shape, uniformity, and composition of the powders. This study investigates the effects of the microstructure, defects, and particle size distribution of GH4169 alloy powders after different recycling times on the microstructure, phase distribution, tensile behavior, and deformation mechanism of formed parts in a heat-treated state.

The GH4169 alloy powder prepared via argon atomization is used in the SLM forming process. The powder is printed and reused for 0‒13 times without adding the newly prepared powder. The large-sized inclusions and support residues are removed by using a 100 μm powder sieve. The specimens are defined as 0th, 6th, 10th, and 13th specimens, according to the number of times the powder is recycled, as shown in Table 1. The 0th, 6th, 10th, and 13th specimens are heat-treated after SLM formation, using the heat-treatment schedule shown in Table 2. Finally, after sample preparation and polishing, scanning electron microscope (SEM) and transmission electron microscope (TEM) photographic analyses are performed.

After multiple powder recyclings, the powder still exhibits good overall degree of sphericity, but the powder morphology changes with an increase in the usage time. The number of defective powders, such as satellite powder and irregular particles, is relatively small among powders with fewer recycling times (0 and 6), as indicated by the powder particles marked by the dashed circle in Fig.1. However, as the recycling time gradually increases, the number of satellite balls in the powder significantly increases in the 10th and 13th samples. Some particles even have 2 or 3 layers of irregular powder coated on their surfaces, which results in an increase in the powder surface roughness and a decrease in flowability, thereby leading to the formation of unmelted pores and micropores in the heat-treated samples (Fig.4). After treatment, the nanosized γ″ and γ′ strengthening phases as well as residual Laves phases exist in the matrix. Moreover, nanosized δ phases and carbides exist at the grain boundaries. As the powder recycling time increases, there is a slight decrease in both the strength and plasticity of the alloy. Each reaches its lowest value in the 13th sample (Table 4) at room temperature (RT) and 650 ℃, which is mainly attributable to the increase in the content of pore defects in the alloy. However, in the 6th sample, the performance reaches its peak, with an ultimate tensile strength (UTS) of 1430.00 MPa, yield strength (YS) of 1318.70 MPa, and elongation of 22.00% at RT. At 650 ℃, the performance has a UTS of 1205.00 MPa, YS of 1130.00 MPa, and elongation of 24.00%. The tensile fracture mode of all specimens at RT is a mixture of cleavage fracture and microporous aggregation fracture, and microporous aggregation fracture is observed at 650 ℃. After powder recycling, the content of porosity and crack defects significantly increases, especially at 650 ℃, where micropores can directly merge to form cracks and thereby damage the properties of the alloy.

In this study, the average particle size of the powder increases, the surface roughness increases, and the fluidity decreases after powder recycling, resulting in pore defects in the heat-treated specimens and leading to the impairment of mechanical properties of the alloys. However, the fracture mode and deformation mechanism are unaffected. The tensile deformation mechanisms of the alloy at the two selected temperatures are the nanoscale δ phase, carbides, Laves phase, and γ″/γ′ hinder dislocation movement. At 650 ℃, micro-twinning appears, synergistically strengthening the strength and plasticity. The main sources of strengthening and toughening are the precipitation strengthening, dislocation strengthening, and fine grain strengthening.

Laser powder bed fusion (LPBF) is a highly promising technique that affords significant advantages in mitigating the high costs and lengthy procedures associated with manufacturing precise and complex components in the aerospace industry. However, the printing process encounters uncontrollable issues, such as fluctuations in laser energy, unstable airflow, and damage to the recoater. These issues can lead to uneven powder spreading thickness, causing deposition defects that critically impact part quality. To improve the formation quality, the deposition defects caused by abnormal powder thickness must be monitored. Despite rapid advancements in online monitoring technologies, the complexity of signal data and its unclear correlation to actual part defects present significant challenges. Establishing the relationship between the deposition defect and monitoring signal for different powder thicknesses is necessary to address the issues related to powder spreading anomalies. Moreover, developing rapid and effective diagnostic methods is crucial to providing a foundation for the feedback control of defects. This study demonstrates the use of an online monitoring system that integrates proprietary photodiodes and high-speed cameras to collect and analyze data across various powder thicknesses. We establish a foundation for the online monitoring and real-time diagnostics of defects by investigating the evolution patterns of part surface quality and internal defects.

In the experimental study, substrates pre-treated with milling are customized with designs of grooves with different depths ranging from 30 μm to 300 μm in 30-μm steps (Fig.4). The powder is spread across these grooves, and a recoater is used to ensure that each groove reaches the designated thickness. Single-layer laser exposure is performed in different areas using different laser parameters (Table 1). The light intensity and melt pool area are monitored online at a frequency of 10 kHz during the printing process by using three off-axis photodiodes and a coaxial high-speed camera. After printing, the surface morphology and internal defects of the samples are characterized using confocal laser scanning microscope and scanning electron microscope. The impact of powder thickness on deposition defects is investigated by integrating online monitoring signals with offline material characterization data.

The mapping images of light intensity and melt pool area distribution (Fig.5) reveal that, as powder thickness increases under the same process parameters, the light intensity gradually decreases, and the melt pool area increases. Additionally, under the same powder-layer thickness, the average intensity of the melt pool decreases with decreasing energy density (Fig.6). The surface roughness increases with powder thickness (Fig.8). For instance, with the laser power at 200 W and scanning speed at 1000 mm/s, the surface roughness increases from 4.16 μm to 117.86 μm as the powder thickness increases from 30 μm to 300 μm. The surface morphology (Fig.9) and internal porosity defects (Fig.10) indicate the following three stages of the melt pool with increasing powder thickness: 1) smooth surface with clear melt tracks and uniform melt pool depth; 2) continuous melt tracks on the surface but large fluctuations in melt pool size (width, height, and depth); 3) melt track discontinuities with the emergence of large balling defects over 150 μm, leading to porosity between melt tracks and at the bottom. Monitoring signals under different powder thicknesses can be categorized into three corresponding stages based on melt pool conditions. The balling state requires the device to immediately detect anomalies and quickly respond in subsequent layers. Receiver operating characteristic (ROC) curve analysis shows that selecting 0.21 V as the threshold for low values and 7.14% as the threshold percentage yields a model with a good capability to identify deposition defects (Figs.12 and 13).

In summary, printing characteristics under different powder thicknesses during the LPBF process are investigated. When the powder thickness increases in LPBF, the surface quality worsens, and internal porosity defects occur. When the powder thickness exceeds 90 μm, large balling defects can exceed 150 μm in size. The relationship between the light intensity collected by photodiodes and melt pool area captured by high-speed cameras is analyzed. The monitoring of the melt pool light signal is highly sensitive to deposition defects due to powder spreading anomalies. As the powder-layer thickness increases, the light intensity decreases, while the melt pool area increases. As the thickness increases from 30 μm to 300 μm, the average intensity of the melt pool decreases from 0.6 V to 0.2 V. On the basis of these results, a novel diagnostic method for deposition defects is devised by employing threshold percentages derived from optical monitoring signals. When the proportion of light intensity values less than 0.21 V exceeds 7.14%, it can be diagnosed as abnormal powder spreading with a true positive rate of 97.22%.

Additive manufacturing can be used to construct complex structures and facilitate the design of an overall structure by adding materials layer-by-layer to form parts. Additive manufacturing technology has been widely used in the automotive, electronics, aerospace, and medical fields and plays a crucial role.

However, during the additive manufacturing process, parts with overhangs are often encountered and cannot be successfully printed without considering the overhangs. For traditional 2.5-axis 3D printers, two methods are used to solve the problem of overhanging structures that cannot be printed. One method involves adding support structures below the area with the overhanging structures, and the other requires achieving self-support of the structures through structural optimization. Adding support structures can prevent warping and reduce the structural deformation of a part. However, this method increases the production time and material costs. In addition, further postprocessing is required to remove unwanted support structures, which is time-consuming and affects the surface accuracy of the part. Therefore, it is important to achieve self-support of a printed part to reduce the material cost, printing time, and postprocessing time.

We summarize the research progress in structural self-supporting design for additive manufacturing. First, the principle of the structural self-supporting design of additive manufacturing is summarized, and the research progress in the self-supporting design of the overall structure of additive manufacturing parts and the self-supporting design of additive manufacturing infill structures are reviewed. Based on different structural optimization methods, it is further divided into structural self-supporting design using continuum structural topology optimization, discrete structural topology optimization, and shape optimization. Next, the advantages and disadvantages of each method are analyzed. Finally, solutions to improve computing efficiency and structural performance are discussed, along with future application scenarios and research priorities.

Additive manufacturing of structural self-supporting designs is critical for saving printing time and material, but it has not been systematically reviewed. This paper first summarizes the structural self-supporting design principle of additive manufacturing and reviews the research progress of the self-supporting design of the overall structure of the part, which is divided into three parts: research progress in structural self-supporting design based on continuum structure topology optimization, discrete structural topology optimization, and shape optimization. Previous studies were mainly based on continuum structure topology optimization, and the research progress in structural self-supporting design based on continuum structure topology optimization is presented in four parts: research progress in structural self-supporting design using the SIMP method and its improved version, the level set method, the BESO method, and feature-driven optimization. Subsequently, the research progress in the self-supporting design of additive manufacturing infill structures is reviewed. Finally, self-supporting designs of additive manufacturing structures are summarized and discussed. The structural self-supporting design of additive manufacturing is still in its infancy, and the following prospects are proposed to further develop this field.

(1) Perform 3D case extensions. Despite the rapid development of structural self-supporting design, the proposed method is still in its infancy and has been mainly applied to 2D cases based on the “rule of thumb” of printable overhang angles. Therefore, the extension to 3D cases still requires further investigation.

(2) Improve the computational efficiency of sensitivity. Previous studies were mainly based on continuum structure topology optimization, and topology optimization design has problems, such as large design variables, which often leads to high computational costs owing to the excessive number of elements in the sensitivity calculation design. Therefore, it is necessary to improve the sensitivity calculation method and increase calculation efficiency.

(3) Comprehensive consideration of the overhang feature constraints, printing direction, and topological layout. Compared with considering only the overhang angle constraint, a comprehensive consideration can further reduce the loss of structural performance. Moreover, the threshold value of the overhang angle often depends on the direction of printing. Therefore, in future research, the integrated consideration of printing direction and topological layout should be the focus.

(4) Combine self-support with other structural properties. During the melting and solidification of metallic materials printed by additive manufacturing, residual stresses and deformations are typically induced, resulting in printing failure or a decrease in strength and dimensional accuracy. Therefore, considering a self-supporting design that considers the residual stress and deformation of the structure is an important direction for future development. In addition, lightweight design is required in the aerospace field and should be considered in combination with light weight during the self-supporting design process.

Laser additive manufacturing is a technology that utilizes a laser beam to melt and mold powders layer by layer based on a 3D model. It has made outstanding progress in the molding of metallic structural materials such as large and complex structural parts in the aerospace industry. Laser additive manufacturing has also achieved remarkable progress in the fabrication of metallic functional materials. Shape memory alloys are a type of metallic functional materials that exhibit shape memory, superelasticity, and elastocaloric effects. Through design and optimization of the process strategy, shape memory alloys with excellent functional properties and complex shapes could be fabricated by laser additive manufacturing. Laser additive manufacturing offers an effective method to research metallic functional materials with outstanding performance that can meet the application requirements.

In this paper, we systematically summarize the research on laser additive manufacturing of metallic functional materials and their characterization by in-situ synchrotron radiation. We further introduce the research progress on laser additive manufacturing of high-performance shape memory alloys as well as the latest progress of metal L-PBF and L-DED technologies for synchrotron radiation-based in-situ X-ray diffraction (XRD) research. In the first part of this paper, the dominant types of laser additive manufacturing and their basic principles are introduced. On this basis, the relationship between the functional properties of shape memory alloys and the parameters of the process strategy is revealed. This relationship offers a guideline for how to fabricate a shape memory alloy with targeted properties. In the next part, the research progress on high-density shape memory alloys fabricated through laser additive manufacturing is introduced. The guidance of results predicted by computer is convenient for selecting the combinations of parameters that could be used to fabricate shape memory alloys with high density. The final part presents the research progress on synchrotron radiation-based in-situ X-ray characterization in the laser additive manufacturing process. This part introduces the characterization platform and typical applications of in-situ XRD in the laser additive manufacturing process of metallic materials. We describe some scenarios involving the phase transition dynamics measurement and in-situ characterization methods of single crystals in additive manufacturing. We also present the future development trends.

The molten pool in L-PBF and L-DED metal additive manufacturing processes has the characteristics of non-equilibrium and rapid solidification, and the microstructure of metallic functional materials can be controlled by adjusting the parameters of these processes. The additive manufacturing process may produce micro-defects such as keyholes and lack of melting, and it also tends to form columnar crystals with a certain orientation. Based on the Eager-Tsai model, a fabrication-quality distribution map can be predicted, with the parameters as the coordinates. On this basis, the process strategy can be adjusted to obtain a columnar crystal alloy with high orientation and high quality, and the mechanical properties can be further optimized. Synchrotron radiation-based in-situ XRD can effectively characterize the phase transition dynamics, texture evolution, and grain size changes in the additive manufacturing process, which provides insights into the control of the process parameters in additive manufacturing of metallic functional materials. The application of synchrotron radiation-based in-situ XRD can provide a key reference for additive manufacturing in terms of improving the functional characteristics and optimizing the component quality. By delving deeper into the microscopic evolution of the additive manufacturing process, researchers can better understand the properties of metallic materials, so that they can precisely manipulate the process parameters to achieve precise controlling of metal functional materials.

Laser additive manufacturing technology merges design and production, incorporating crucial elements such as materials, structure, process, and performance. This integration offers an efficient and cost-effective way to create prototypes and test new designs. It plays a vital role in manufacturing and repairing complex parts comprising nickel-based superalloys. However, this technology faces challenges with traditional high-strength nickel-based superalloys. The differences in composition and strengthening mechanisms, along with the rapid solidification and phase transitions unique to laser additive manufacturing, can lead to issues. The high alloying degree causes a wide solidification temperature range, while the abundance of intermetallic compounds leads to varying strength and ductility at high temperatures. This in turn increases the risk of microcrack defects. These defects can degrade the quality and mechanical properties of γ′ phase strengthened nickel-based superalloys produced through this method. Therefore, understanding the characteristics, formation mechanisms, and influencing factors of cracks, as well as recognizing the crack control methods and related achievements, can lay a theoretical foundation for exploring universal crack resistance pathway and composition design of superalloy matching additive forming characteristics.

This paper offers an in-depth exploration of various crack types in γ′ phase strengthened nickel-based superalloys used in laser additive manufacturing, including the morphology and mechanisms of solidification cracks (Fig.2), liquation cracks (Fig.3), ductility-dip cracks (Fig.5), and strain aging cracks (Fig.6). It elucidates the connections between the solid phase fraction and index for solidification cracking susceptibility, the differential scanning calorimetry curve and liquation sensitivity, the relationship between alloy ductility and the temperature range for ductility dip, as well as the link between γ′ phase forming elements and the risk of strain aging cracking. The discussion includes common strategies for enhancing crack resistance, such as modifying the composition to alter solidification characteristics and minimize or eliminate the formation of low-melting-point phases (Fig.8), introducing second-phase particles to encourage the shift from columnar to equiaxed crystal growth, thereby altering the residual stress state (Fig.9), and optimizing laser processing parameters to directly improve microstructure and forming quality (Fig.10). Furthermore, post-treatment methods significantly contribute to reducing cracking tendencies and enhancing the mechanical properties of superalloys. The ultimate approach to addressing the cracking issue involves developing nickel-based superalloys with specific compositions tailored for laser additive manufacturing. Recent successes in designing crack-free new alloys have leveraged tools such as thermodynamic calculations (Fig.11), machine learning (Fig.12), the cluster structure model (Fig.13 and Table 2), and the multi-principle-element concept (Fig.14). The shift from empirical to scientific and rational design in material research is being advanced by the use of phase diagram calculations for alloy design, supported by reliable thermodynamic databases. Machine learning facilitates the rapid development of mathematical models that quantitatively link material composition, processes, structure, and properties, enabling precise screening of target materials. The cluster structure model offers insights into how alloy elements’ type and amount affect formability. Meanwhile, the multi-principle-element concept emerges as an efficient strategy for simultaneously enhancing crack resistance and the strength-ductility balance. In summary, this paper’s overview of advancements in crack control and composition design for γ′ phase strengthened nickel-based superalloys in laser additive manufacturing offers practical insights for the future creation of printable, high-temperature, high-strength nickel-based superalloys and their components (Fig.15).

Significant progress has been made in controlling cracks in γ′ phase strengthened nickel-based superalloys for laser additive manufacturing, laying a theoretical and methodological foundation for creating crack-free superalloys through laser processing. Despite these advancements, developing precipitation-strengthened nickel-based superalloys and their components that maintain high-density forming, along with stable microstructure and performance in high-temperature environments, remains challenging. Future research should focus on several key areas. First, it is crucial to understand the fundamental differences in cracking mechanisms between different alloys. Establishing a clear link between the types and contents of γ′ strengthening elements, their interactions, and their impact on crack sensitivity will aid in developing universal crack prevention and control strategies for similar alloys. Second, it is vital to develop swift design criteria for alloy compositions that align with desired performance and printability, establishing a distinct system of γ′ phase strengthened nickel-based superalloys tailored for laser additive manufacturing. Third, enhancing the understanding of the alloys’ resistance to creep, fatigue, corrosion, thermal shocks, and the long-term stability of their microstructure and performance at high temperatures will further promote their adoption in critical sectors such as aerospace and nuclear power, among others. Finally, achieving mold-free manufacturing of crack-free nickel-based single crystal superalloys with superior overall performance, alongside the production of large, precise, and complex structural components, is essential. This advancement aims to fulfill the demanding conditions of aircraft engines operating at higher temperatures and in more severe environments.