Effect of post-weld heat-treatment on the stress-corrosion cracking behaviour of laser-welded shape memory NiTi wires in Hanks' solution

In this study, the stress-corrosion cracking (SCC) behaviour of laser-welded NiTi wires before and after post-weld heat-treatment (PWHT) was investigated. The samples were subjected to slow strain rate testing (SSRT) under tensile loading in Hanks' solution at 37.5 °C (or 310.5 K) at a constant anodic potential (200 mV_SCE). The current density of the samples during the SSRT was captured by a potentiostat, and used as an indicator to determine the susceptibility to SCC. Fractography was analyzed using scanning-electron microscopy (SEM). The experimental results showed that the laser-welded sample after PWHT was immune to the SCC as evidenced by the stable current density throughout the SSRT. This is attributed to the precipitation of fine and coherent nano-sized Ni₄Ti₃ precipitates in the welded regions (weld zone, WZ and heat-affected zone, HAZ) after PWHT, resulting in (i) enrichment of TiO₂ content in the passive film and (ii) higher resistance against the local plastic deformation in the welded regions.