Influence of laser powder bed fusion and ageing heat treatment parameters on the phase structure and physical behavior of Ni-rich nitinol parts

This paper examines the effect of ageing temperatures (400, 500, & 600 °C) and times (30, 60, & 90 min) on Ni-rich NiTi samples produced via the Laser Powder Bed Fusion (L-PBF) technique. Variations in the process parameters of the L-PBF which were examined included laser power at 150W and 180W, and the scanning strategy at 500 mm/s and 1000 mm/s. Ageing heat treatment enabled setting the functional properties of phase change temperatures, stress-strain response, and phase change enthalpies. Samples aged at 400 °C were found to have much higher austenitic finish temperature (40–60 °C) whereas those aged at 600 °C had near-zero or sub-zero Af temperatures. This was due to the lower nickel concentration resulting from the higher heat treatment temperature. Samples heat-treated at 500 °C exhibited Af temperature which was near to the room temperature and showed co-existence of both austenitic and martensitic phases. It was also found that the crystallite size increased from 6 to 22 nm for an ageing temperature of 400 °C, to between 38 and 48 nm for an ageing temperature of 600 °C. The highest microhardness value of 380HV was recorded from the lower heat treatment temperature and the lowest microhardness of 270HV was recorded for samples heat-treated at 600 °C. This decrease in the microhardness values is attributed to the increased crystallite size of the aged samples at higher temperatures. This work identifies for the first time the novel ability to control NiTi phase change temperature and mechanical properties using ageing heat treatments which is critical for the materials application for personalised stents. Higher heat treatment temperatures were found to lead to larger crystallite sizes.