Size effect on the microstructure, phase transformation behavior, and mechanical properties of NiTi shape memory alloys fabricated by laser powder bed fusion

In this work, NiTi samples with different thicknesses (0.15–1.00 mm) were fabricated by laser powder bed fusion (LPBF) under variable scanning speeds (500–1200 mm s–1). The densification behavior, phase transformation behavior, and mechanical properties of the sample with different thicknesses are studied. The results indicate a strong size effect in the LPBF-fabricated NiTi alloy. The decrease of the sample thickness results in (i) the increase of porosity, (ii) the decrease of the number of adhered NiTi powder particles at the surface, (iii) the monotonous decrease of the martensitic transformation temperatures (MTTs), and (iv) the decrease of the shape recovery temperature. The influence of sample thickness on the melt-pool behavior, and thus the microstructure and performance of NiTi alloys are discussed. It is suggested that the melt-pool is deeper and narrower in the thin samples than in the thick samples. We conclude that, apart from the LPBF process conditions, the sample dimensions have also to be considered to fabricate NiTi structures with predictable properties.