Time-varying deposition strategy for comprehensive properties improvement of bionic bidirectional gradient NiTi alloy fabricated by laser directed energy deposition

Inspired by the property mechanism principles of C. squamiferum (impact resistance) and bamboo (energy absorption), a bionic bidirectional gradient NiTi alloy was developed and fabricated by the innovative time-varying deposition strategy (TVDS), including different process parameters (the laser power and scanning speed), different scanning paths (alternative optimised spiral pattern and x-axis rotation pattern) and interlayer interval time. This study explores how TVDS influences the printability, microstructure evolution, phase transformation behaviour, mechanical properties, and superelasticity response. However, the element distribution tests discover mutation points under various regions in the GD-X-GD-Z (V1) and GD-X-LD (H1) planes to determine the feasibility of the bionic bidirectional gradient using TVDS. The recovery ratios along the H1 and V1 planes after 1st and 10th cycles are 90.7% and 88.4%, 82.6% and 67.5%, respectively. The numerical modelling framework is established to elucidate the thermal fields generated by TVDS and provide support for the experimental data.