Effect of post-heat-treatment on thermal and physical characteristics of NiTi tubes produced via conventional drawing and laser powder bed fusion

Abstract

NiTi is one the metal alloys exhibiting shape memory and superelasticity which makes it suitable for a wide range of engineering and biomedical applications. Nickel-rich NiTi requires heat treatment and cold work as post-processing steps to enable appropriate phase production for final applications. In the current work, NiTi tubes (50.8%Ni-49.2%Ti) produced using Laser Powder Bed Fusion (PBF-LB) and conventional drawing were heat treated at a constant temperature of 500 °C for 10, 30, and 60 min. Differential scanning calorimetry results indicate that the austenite finish temperatures for the tubes were set at 25.4 °C and 26.62 °C after 60 min of heat treatment for the PBF-LB and conventionally drawn tubes respectively. The enthalpy of phase transformation for PBF-LB tubes increased from 2.89 J/g to 17.14 J/g while for conventionally drawn tubes it increased from 5.41 J/g to 16.08 J/g. The EDX measurements on as-built and heat-treated tubes indicated no loss of nickel during the heat-treatment process. The thermal expansion results show an unstable evolution of CTEs in as-built NiTi tubes which were stabilized via heat-treated with resulting CTE ≈11.4 × 10−6/ºC. The hardness decreased in the heat-treated tubes as compared to the as-built tubes. The hardness decreased by 12.6% for PBF-LB and 12.4% for the conventionally drawn tubes respectively.

Keywords

• Nitinol
• Heat treatment
• Superelasticity
• Phase transformation
• Thermal expansion