Revealing the effect of layer-varying deposition strategy on laser-directed energy deposition bionic gradient NiTi alloy

In this work, a novel deposition strategy, layer-varying deposition strategy (LVDS), to fabricate bionic gradient NiTi alloys using laser-directed energy deposition is proposed. The strategy includes different process parameters (the laser power and scanning speed), the scanning paths (alternative optimised spiral pattern, x-axis rotation pattern, and arc alternative pattern) and interlayer interval time. The effect of LVDS on the printability, microstructure, phase transformation behaviour, mechanical properties and superelasticity is investigated. The results demonstrated that the porosity of upper, middle and bottom are 0.156%, 0.569% and 0.262%, respectively. Additionally, the element composition tests discover mutation points under various regions in the NiTi to determine the feasibility of the bionic gradient using LVDS. To evaluate the effect of the LVDS on Ni-loss, simplified mathematical models are constructed. The importance of the index that influences the Ni-loss: Etot (including the laser power, scanning speed and overlap rate) > synergistic effect of interlayer interval time and scanning path > scanning path. The recovery ratio of bionic gradient NiTi alloy, upper, middle and bottom after the 1st cycles possess 87.4%, 85.1%, 80.9%, 82.5%, respectively. This work provided insights and guidelines for additive Manufacturing process development and optimisation.