Micro selective laser melting of NiTi shape memory alloy : defects, microstructures and thermal/mechanical properties

Abstract

The use of micro selective laser melting (μ-SLM) enables product miniaturization, which is one of the megatrends in the metal processing industry and increasingly find its applications in biomedical and electronics industries. Among these, NiTi shape memory alloy (SMA) shows a great promise in functional micro-scaled components such as stent. There are inevitably some imperfections in SLM, but the imperfection formation in μ-SLM may not be the same as that in the conventional SLM. This work studies the imperfections in μ-SLM produced NiTi samples, with focus on defects, microstructure and thermal/mechanical behaviors. The effects of substrate material, laser-related process parameter and scanning strategy on defects such as porosity and cracks were analyzed, and a process window for “Scanning speed – Hatch spacing” was determined. Transformation peak was hardly detected in thermal behavior of as-printed and post heat-treated μ-SLM NiTi, resulting from microstructure inhomogeneity, Ti-rich impurity phases TiC1−xNx/Ti4Ni2Ox and precipitate Ti2Ni, which were introduced by powder preparation, μ-SLM or post heat treatment. The as-printed NiTi shows higher compressive strength and fracture strain than the post heat-treated samples, reaching 2796.57 MPa and 27.80% on average, respectively, but the plateau stress-strain stage is indistinguishable due to inhomogeneous and localized stress-induced martensite transformation. The brittle Ti2Ni phase was introduced in post heat treatment, leading to inhomogeneous microstructure and lower ductility. The underlying mechanisms revealed in these imperfections could serve as a guideline for defect control, process optimization, as well as post heat treatment methods for μ-SLM of NiTi alloy.