Tailoring phase transformation temperatures and functional properties through laser-based powder bed fusion: A pathway to functionally graded NiTi shape memory alloys

Tailoring the phase transformation temperatures and functional properties of NiTi alloys is quite challenging through conventional approaches, since they are extremely sensitive to the compositional and microstructural changes. In this work, we show that besides the ability to produce complex structures, the laser-based powder bed fusion (PBF-LB/M) can also act as a metallurgical method to tailor the phase transformation temperatures and functional properties of NiTi shape memory alloys. A processing map linking densification behavior with martensite transformation temperatures (MTTs) is first established, defining a processing window that simultaneously achieves near-full density (>99.5%) and broad tailorable MTTs. For instance, austenite transformation peak temperature ( T _{p(B19′→B2)}) of the samples fabricated using the same powders could be tailored over a wide range of 72 K, between 244 and 316 K. Accordingly, a transition from shape memory to superelasticity is achieved at room temperature. In the meanwhile, the plateau stress from 159 to 416 MPa could be obtained by manipulating scanning speeds from 1000 to 2600 mm/s. Functionally graded (FG) NiTi samples, which show multiple stress plateaus and stepwise actuation, were successfully produced by spatially varying process parameters at different locations of the sample. The functional stability of the FG NiTi samples is further improved through post-process heat treatment, including solution (1273 K for 1 h) and aging (623 K for 30 min). Additionally, a honeycomb structure with three-stage shape recovery behavior was produced, highlighting the feasibility of PBF-LB/M for fabricating FG NiTi components with complex geometry.