Functional Properties of a Ni-rich Ni–Ti–Hf Shape Memory Alloy Fabricated via Laser Beam Powder Bed Fusion—Impact of Porosity and Precipitation Characteristics on the Thermal Hysteresis

Abstract

AbstractAdditive manufacturing (AM) is very promising for the fabrication of complex parts made from shape memory alloys (SMAs). In the present study, a Ni–Ti–Hf shape memory alloy has been processed by laser beam powder bed fusion of metals (PBF-LB/M). Employing different sets of processing parameters, i.e., a variation of scanning speed, specimens characterized by various microstructures and porosities were obtained. Microstructural analysis revealed that processing of Ni–Ti–Hf SMAs at a low energy level promotes the formation of a fine-grained microstructure with numerous lack of fusion defects. Transmission electron microscopy (TEM) studies revealed that H-phase precipitates are present in any case, leading to an increase in the Ms-temperature and, thus, a phase transformation at room temperature. The thermal stability of the phase transformation behavior in different Ni–Ti–Hf as-built conditions was studied using differential scanning calorimetry (DSC). DSC analysis showed that specimens with a higher defect density are characterized by smaller thermal hysteresis.