Recent studies have shown that Ti6Al7Nb alloy, which is prepared to improve biocompatibility, exhibits mechanical properties comparable to Ti6Al4V alloy while demonstrating superior corrosion resistance, ductility, biological compatibility, and bioactivity. It is considered an ideal biomaterial for medical titanium alloys. However, for components with complex geometries or medical implants, traditional manufacturing methods require significant time and labor costs. To overcome these manufacturing barriers, the use of laser powder bed fusion (LPBF) technology for producing metal components and implants has emerged as a solution. However, determining the optimal processing window for LPBF is challenging owing to the multiple parameters affecting the quality of the products. Additionally, during the LPBF process, excessive cooling rates can lead to rapid solidification of the melt pool, resulting in the formation of fine grains and supersaturation. In the case of LPBF-formed Ti6Al7Nb alloy, the presence of ultrafine needle-like α' martensitic structures leads to a high yield strength but low ductility because of the formation of α' martensite. While the strength of the material is guaranteed, there may be shortcomings in plasticity. Therefore, post-processing heat treatment of the formed components is essential. This study aims to adjust the key parameters of LPBF to obtain the optimal processing window and conduct heat treatment on specimens formed using the optimal processing parameters. The influence of the main parameters of LPBF on the quality of the formed products is investigated, and the effect of different treatment methods on the microstructure and mechanical properties of the specimens is analyzed.

In this study, Ti6Al7Nb powder with a particle size distribution of 25?65 μm was used to prepare alloys with different forming parameters. Five sets of parameters for laser power and four sets each for hatch spacing and scanning speed were established, resulting in a total of 80 experiments conducted using an orthogonal experimental design. The optimal processing parameters were determined through a phase analysis of the formed alloys (Fig. 3 and Table 3). Subsequently, the specimens formed using the optimal parameters were subjected to heat treatment using the three solution treatment temperatures shown in Table 2. The microstructure variations were studied via optical microscopy, scanning electron microscopy, and X-ray diffraction analysis. Tensile tests were performed to obtain the mechanical properties of the specimens subjected to different treatments, and fractographic analysis was also conducted. The optimal heat treatment regimen was derived through these methods.

In the LPBF-formed alloy specimens, columnar β-phase crystals grow along the formation direction, whereas needle-like α' martensite phases precipitate at a 45° angle to the formation direction, as illustrated in Fig. 8. The material exhibits high strength but low ductility, with the tensile fracture surface mainly characterized by shallow dimples and transgranular cracking features [Figs. 13(a)?(d)]. This behavior is attributed to the significant differences in thermal gradient present in the LPBF process. Under 850 ℃ solid solution treatment, the strength of the specimens decreases, whereas the ductility significantly improves, surpassing ASTM standards. The tensile fracture surface exhibits pronounced necking, primarily due to the decomposition of the needle-like α' phase into α+β phases during high-temperature heat treatment, although the decomposition is not complete. In contrast, 950 ℃ solution treatment results in the dispersion and pronounced orientation of needle-like α' martensitic phases and an increase in the precipitation of β phases within the grain interiors, with secondary α phases isolated by the organization of the β phase. This structural arrangement leads to lower strength and ductility in the S2 specimen compared to the S1 specimen, albeit with a slight increase in surface hardness. Heat treatment at 1050 ℃ within the α+β dual-phase region results in a typical Widmanst?tten microstructure. The β phase undergoes β→α+α' transformation at high temperatures, leading to a significant increase in material hardness and strength, but with reduced ductility compared to the previous two heat treatments (Figs. 11 and 12). The tensile fracture surfaces display extensive tearing ridges and river-like patterns. After aging treatment, the microstructure of the solid solution treated specimens undergoes minimal change, but both the strength and ductility exhibit improvement, particularly in terms of yield strength. The enhanced strength is attributed to the further decomposition of the metastable α' phase, the increased content of dispersed α+β phases, and the strengthening effects of fine grains, as corroborated by Fig.s 6 and 7. An integrated analysis of the experimental heat treatment regimens indicates that a combination of the 850 ℃×0.5 h / air cooling (solid solution treatment) and 550 ℃×0.5 h /air cooling (aging treatment) can achieve an optimal strength?ductility balance for LPBF-formed Ti6Al7Nb alloy specimens.

The Ti6Al7Nb alloy was successfully shaped using LPBF technology, and by controlling the main forming parameters, the optimal alloy forming quality was achieved. The microstructure and mechanical properties of the alloy were adjusted to meet medical standards through solid solution and aging treatment. The research results indicate that the best formation quality of the specimens is achieved under a laser power of 300 W, a hatch spacing of 0.12 mm, and a scanning speed of 1150 mm/s. The comprehensive results of all heat treatment strategies suggest that a solid solution treatment temperature selected within the mid-section of the α+β dual-phase region is most suitable. At this temperature, the needle-like α' martensitic phase decomposes into the α+β phase and distributes uniformly throughout the alloy. With an increase in the solid solution treatment temperature, although the complete decomposition of the needle-like α' martensitic phase is more pronounced, higher undercooling can lead to the transformation of the β phase into the α and α' phases. Following the aging treatment, the overall strength of the alloy is enhanced owing to the recrystallization and decomposition of the α' phase.