Tailoring the superelasticity of NiTi alloy fabricated by directed energy deposition through the variation of residual stress

Abstract

The dynamic thermal cycle of the directed energy deposition process inevitably leads to considerable residual stress within the as-deposited NiTi components. The high residual stresses in the as-deposited NiTi components were mostly removed by stress-relief annealing at 200 °C for 3 h without detectable change of its chemical composition and microstructure. The incremental hole drilling method was used to measure the residual stresses in samples of both the as-deposited and stress-relief annealed variants. Meanwhile, the changes in residual martensite and dislocation density during cycling were examined by quasi-in-situ EBSD observations. The results indicate that the stability of the cyclic process is jointly influenced by the residual stress and dislocation. When the residual stress was 1.6 %, the superelasticity of the specimens increased from 69.81 % to 95.72 %; After releasing the residual stress at a strain of 2.5 %, the superelasticity of the specimens increased from 63.09 % to 73.70 %. This study provides useful guidance for improving the superelasticity of NiTi alloys fabricated by directed energy deposition.