Ni-loss compensation and thermomechanical property recovery of 3D printed NiTi alloys by pre-coating Ni on NiTi powder

The approach of pre-coating Ni on NiTi powder was adopted as an effective route to address the thermomechanical property degradation of NiTi alloy caused by Ni-loss during the printing process of laser powder bed fusion. The electroless plating process of Ni on NiTi powder was initiated with the removal of the outermost TiO2 layer by alkaline solution and the exposure of Ni catalytic nucleation site. The newly exposed Ni was an active catalyst for coating Ni and eventually triggered the deposition of Ni on NiTi powder via self-catalytic reaction. The intrinsic homogeneity of Ni, as a major alloying element in NiTi powder, ensured high uniformity of Ni-coating on the surface of the powder and good sphericity, necessary for good printability. Furthermore, Ni-coating increased laser absorbability, and a higher content of Ni compared to the theoretical value was thereby required to compensate for the Ni-loss. Simultaneously, densification was enhanced even at lower laser energy density on using Ni-coated powder. During the printing process, ultrafine Ni-coating was rapidly homogenized such that the thermomechanical property was completely recovered. As a consequence, it is underscored that Ni-loss can be addressed by compensating Ni on the original powder prior to the printing process, providing a new pathway toward printing of high-performance shape memory alloys.