Enhancing shape memory properties of 3D-printed NiTi alloys by laser powder bed fusion through microstructural control

Near-equiatomic NiTi alloys have great potential for application in aerospace and biomedical fields due to their exceptional shape memory performance and superelasticity. Generally, traditional technology involves a series of processes starting from casting to hot working processes, such as forging, heat treatment, and machining, with the limitation of long periods, high energy consumption, and finite product design. Laser powder bed fusion (L-PBF) technology offers a method for rapidly fabricating high-precision and complex components. However, Ni burnout and residual thermal stress induced by high-temperature melting and extremely high-temperature gradient can affect formability and performance during the L-PBF process. In addressing the current challenges, this study attempts to regulate the quality and microstructure to improve the shape memory properties of NiTi alloy components through a combination of L-PBF process parameters. Besides, the influencing mechanism of parameters on the microstructure of NiTi alloys is clarified. The evolution mechanism of NiTi alloy by varying different printed process parameters in the tensile process is comprehensively examined. Subsequently, a clear correlation among the microstructure, phase transformation behavior, and mechanical properties is established. The results indicate that the regulation and control of process parameters improve the shape memory property of NiTi alloy by about 50 %. The L-PBF NiTi sample (V3) processed under the conditions at a high scanning speed of 1400 mm/s, a laser power of 160 W, and a hatching space of 60μm, exhibit the highest relative density and the best shape memory properties. This improved shape memory property is essentially caused by the finer grain size and gradually decreasing B19' martensite. Geometric characteristics and temperature distribution within the melt pool are influenced by process parameters, profoundly changing grain morphology and size. The initial phase composition of L-PBF NiTi alloys is sensitive to the process parameters, affecting the deformation behavior. A novel revelation is that the high shape memory effect is intricately linked to the superior mechanical properties of the alloy. Scanning speed significantly impacts the phase transformation behavior and shape memory effect of L-PBF NiTi alloys among the three parameters.