Laser beam energy dependence of martensitic transformation in SLM fabricated NiTi shape memory alloy

In this study, a near equiatomic NiTi alloy was fabricated by selective laser melting method (SLM). The effects of laser power and scanning speed on the tensile properties, shape memory properties and microstructure were comparatively investigated. Since the shape memory NiTi alloy can be characterized by its martensitic start phase transformation (M-s temperature) and by its critical stress inducing martensitic transformation (sigma(c)), the evolution of these two parameters was investigated as the function of the delivered laser beam energy by differential scanning calorimetry and tensile tests. It was observed that the increase of the scanning speed under a certain laser power and the increase of the laser power under a certain laser beam energy density promote an increase of the critical stress (sigma(c)) and a decrease of the M-s temperature. Consequently, high laser beam energy suppresses the formation of the martensitic B19' phase and therefore stabilizes the austenitic B2 phase. XRD and TEM observations confirm the dependence of the B2 and B19' phase formation with the processing parameters. On the other hand, the relationship between the M-s temperature and the critical stress sigma(c) was also plotted in good accordance with the Clausius-Clapeyron equation. Two new coefficients called Energy Dependence Coefficient of martensitic transformation Temperature (EDCT) and Energy Dependence Coefficient of critical Stress (EDCS) were defined and calculated to describe the laser beam energy dependence of the martensitic phase transformation. These two new thermodynamic coefficients are thus very suitable to establish a link between the machine parameters (through the delivered laser beam energy) and the nature of the material (through the martensitic transformation) in the NiTi alloy fabricated by SLM.