New insights on the laser powder bed fusion processing of a NiTi alloy and the role of dynamic restoration mechanisms

NiTi alloys are a remarkable class of materials capable of exhibiting unique behaviors such as shape memory effect and superelasticity. Laser powder bed fusion (L-PBF), as one of the most prominent additive manufacturing processes, has been recently used by many researchers to fabricate high-quality NiTi alloys with desired properties. In this research, those findings were exploited to utilize an optimum range of volumetric energy density to produce dense parts and offer new insights into the microstructure and properties of the fabricated parts. Although the development of high residual stresses has been frequently reported, the effects of those stresses on the microstructural evolution of NiTi parts during the process have been rarely discussed. By utilizing finite element and analytical approaches, it was predicted that the driving force for the activation of restoration mechanisms, namely dynamic recovery and recrystallization, is provided at elevated temperatures, and the temperature range in which those mechanisms occur was calculated. Detailed microstructural investigations confirmed the results, revealing columnar grains along the building direction with strong substructures caused by dynamic recovery, in addition to new recrystallized grains at boundaries of the melt pools and scanned tracks. It was concluded that dynamic recovery acts as the dominant restoration mechanism during L-PBF of NiTi alloys, affecting both the microstructure and mechanical properties of the final part. Additionally, the peak temperature of the melt pool is a paramount factor that, if increased, results in further progress of the dynamic recovery mechanisms.