Influence of processing parameter on phase transformation and superelastic recovery strain of laser solid forming NiTi alloy

Abstract

NiTi alloys with superelasticity and shape memory effects are currently widely used because of its excellent deformation recovery ability, mechanical strength, and biocompatibility. The Ni 50.8 Ti 49.2 alloy was manufactured by using laser solid forming (LSF) technology with pre-alloyed powder in this work. The microstructure, phase transformation and shape recovery of the deposited alloy were influenced by the parameters used in depositing process. The as-deposited Ni 50.8 Ti 49.2 alloy formed by LSF is columnar crystals epitaxially grown along the deposition direction, with a small amount of equiaxed crystals appear between the deposition layers. Two-step/multi-step martensitic transformation occurs in the deposited Ni 50.8 Ti 49.2 alloy. The martensitic transformation temperature slightly changes with the deposition height, whereas the R transformation temperature basically does not change with the deposition height. In the test of the mechanical properties, sample with a laser power of 1700 W and a scanning rate of 680 mm/min has a compression superelastic recovery as high as 9.5%. Moreover, Ti 2 Ni precipitated phase and R phase appeared in all samples, but only the precipitated Ti 2 Ni and R phase in this sample had a certain orientation relationship with the matrix. Shape memory characteristics was also tested by compression deformation, and the maximum recovery strain of shape memory effect is 6.1%, which is obtained in the sample deposited with 1500 W laser power and 480 mm/min scanning rate. • Columnar crystals grow along the deposition direction in LSF Ni 50.8 Ti 49.2 alloy. • Superelastic recovery as high as 9.5% was obtained in LSF Ni 50.8 Ti 49.2 alloy. • Orientation relationship of precipitation with matrix leads to high recovery rate.