Tuning the functional properties by laser powder bed fusion with partitioned repetitive laser scanning: Toward editable 4D printing of NiTi alloys

NiTi shape-memory alloys (SMAs) are promising materials for smart structures by 4D printing. However, most of the researches related to 4D printed NiTi alloys remain at the conceptual level. In this work, NiTi SMAs were 4D printed using laser powder bed fusion (LPBF) combined with a partitioned repetitive laser scanning (PRS) strategy to tune the phase transformation behavior and mechanical properties. A novel fabrication strategy is proposed to realize the editable 4D printing of NiTi SMAs. The intrinsic influencing mechanism was found that a lower laser power in the remelting stage would act as a thermal treatment, promoting the transformation of the B2 to B19′ phase. A higher repetitive laser power would not only remelt the solidified track to fill the previously formed micropore defects but also refine the grains. The mechanical properties of NiTi alloys are finally enhanced. Different sub-regions under various energy inputs exhibit distinct phase transformation behaviors and mechanical properties. The final 4D-printed NiTi alloy exhibits gradually graded transformation behaviors, with a tensile strength of 450 MPa and an elongation of 7.4 %. It is believed that this work could facilitate the development of NiTi alloys for 4D printing.