Laser power modulated microstructure evolution, phase transformation and mechanical properties in NiTi fabricated by laser powder bed fusion

In this study, two sets of optimized laser powder bed fusion (LPBF) additive manufacturing parameters with similar energy density but different laser powers (HP: high laser power with high scanning speed, LP: low laser power with low scanning speed) were used to produce fully dense and crack-free NiTi samples. The microstructure, phase transformation and mechanical properties of the LPBF fabricated HP and LP NiTi were investigated using scanning electron microscopy (SEM), differential scanning calorimeter (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA). The results showed that the HP and LP NiTi samples had different microstructures, phase transformation temperatures and mechanical properties. It was found that the HP NiTi samples predominantly contained austenite at room temperature and exhibited lower phase transformation temperatures. In contrast, the LP NiTi samples contained a large amount of martensite and had larger thermal memory recovery and better damping capacity. Additionally, the microstructure, phase transformation temperatures and mechanical properties were found to vary at different locations along the building direction in both HP and LP NiTi. This study implies that by manipulating the LPBF processing parameters, in particular the laser power, the phase transformation, microstructure and dynamic mechanical properties of the LPBF fabricated NiTi can be controlled to target different applications.