Effects of Laser Scanning Speed on Microstructure and Properties of Ti- Ni Shape Memory Alloy

Selective laser melting (SLM) was used to fabricate Ti-Ni shape memory alloys. The effects of scanning speed on internal defects, molten pool morphologies, phase transformation behavior and mechanical properties of the alloy were studied using OM, SEM, XRD, and room-temperature compression. The results show that with the increase of laser scanning speed, the internal defect morphology changes from a nearly regular spherical shape to an irregular one when other printing parameters of SLM remain unchanged. For instance, the width and continuity of the molten pool morphology decrease with the increase of laser scanning speed. Besides, the increasing laser scanning speed initiates that the phase composition of Ni-rich Ti-Ni shape memory alloy changes, B19' martensite content decreases, and B2 austenite content increases. And the transformation temperature of the formed Ni-rich Ti-Ni shape memory alloy decreases at first and then increases with the increase of laser scanning speed, but there is a single transformation peak during the process of temperature rise and fall. At 900 mm/s of the scanning speed, the relative density of the alloy reaches 98.5%, yielding the minimum internal defects. Simultaneously, the sample has a maximum compressive strength of 3120 MPa and a compressive strain of 41%, which is the best printing parameter to produce exceptional properties.