Effect of pre-mixed powders on the microstructure and superelasticity of laser directed energy deposited NiTiCu shape memory alloy

Abstract

Laser directed energy deposition (DED), as an advanced additive manufacturing process, was employed to the fabrication of NiTi shape memory alloys using pre-mixed powders. In this work, an innovative method of ultrasonic dispersion assisted powder preparation is proposed. The impact of two mixing powder techniques, namely, mechanical stirring combined with ultrasonic dispersion (MSUD) and ball milling combined with ultrasonic dispersion (BMUD), on the microstructure, phase transformation behavior and superelasticity of the NiTiCu alloy by the DED process has been systematically studied. The result shows a homogeneous NiTiCu powder mixture is achieved by the BMUD process. Microstructure analysis indicates the as-deposited NiTiCu alloy with MSUD exhibits columnar grain structure, containing a large amount of dendrite structures, while equiaxed grain structure is formed in the BMUD process. The as-deposited NiTiCu alloy using BMUD process exhibits a more stable superelastic response during cycling loading compared with the MSUD process. The effect of two pre-mixed processed on the superelasticity of the deposited NiTiCu alloy is also discussed.