High strength welding of NiTi and stainless steel by impact: Process, structure and properties

Abstract

High strength solid-state welds are demonstrated between NiTi shape memory alloy (SMA) and 436 stainless steel (SS) through Vaporizing Foil Actuator Welding (VFAW) method. This process uses the rapid ablation of a foil in a lab-scale process that is similar to explosive welding. The welding process characteristics including collision speed and angle were estimated using an optical method, Photon Doppler Velocimetry (PDV). The local microstructure had strong spatial variation along the weld. The first area to impact, the center of the structure does not weld, as impact is at a normal inclination. This center region has the highest collision speed, but lowest angle; moving outward collision speed decreases and impact angle increases. This causes a progression of varied weld morphologies. Transmission Electron Microscopy shows that some regions that locally melted solidified as amorphous zones. This corresponds to the Thermo-Calc calculations which also predict a high glass forming potential in the resulting multi-component alloy. While the local structure is heterogeneous from varied features across the entire span of the weld (over millimeters) to thin amorphous layers, they exhibit much higher joint efficiency than traditional welding technologies. This work sets a foundation to design processes to harness this high joint strength available through impact welding with other weld morphologies.