Microstructure and mechanical property optimization of NiTi shape memory alloys fabricated by wire arc additive manufacturing with post-processing laser shock peening

NiTi shape memory alloys (SMAs) are widely used in biomedical and aerospace fields. Wire arc additive manufacturing (WAAM) provides an efficient route for fabricating NiTi components, but the process is prone to defects such as coarse grains, residual stress accumulation, and insufficient fatigue performance. In this study, the as-built WAAM-fabricated NiTi SMA was solution-treated at 950 °C for 2 hours and aged at 450 °C for 2 hours, promoting the formation of Ni 4 Ti 3 precipitates and improving martensitic transformation behavior. Laser shock peening (LSP) was subsequently conducted with pulse duration of 15 ns, spot diameter of 2.5 mm and pulse energy of 5 J to further regulate the microstructure and optimize the mechanical properties. After LSP treatment, significant grain refinement was achieved, with the average grain size decreasing to 52.93 μm. Meanwhile, LSP promotes the formation of Ni 4 Ti 3 precipitates, increasing their volume fraction from 5.6% to 11.2% with a more uniform spatial distribution. As a result, the tensile strength from 549 MPa to 571 MPa. Furthermore, after 25 loading cycles under a maximum applied strain of 7%, the residual strain of the LSP-treated specimens decreases from 3.43% to 1.23%. While the energy dissipation capacity is effectively retained, indicating a significant improvement in the early-cycle stability of the alloy. These performance optimizations are attributed to the synergistic effects of compressive residual stress, high dislocation density, and refined microstructure induced by LSP. This study confirms that LSP is an effective approach to improving the comprehensive properties of NiTi SMAs fabricated by WAAM.