High strength welding of Ti to stainless steel by spot impact: microstructure and weld performance

Vaporizing foil actuator (VFA) spot welding, a type of spot impact welding, was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS). The interfacial microstructures and fracture surfaces were characterized using scanning electron microscopy (SEM). Lap shear tests that strained the samples to failure with digital imaging correlation (DIC) were conducted to study the mechanical performance of these welds. A mesh-insensitive structural stress method was used to understand the stress distribution and model the failure modes of VFA welds in ABAQUS. Despite experimental scatter in this developing joining method, most samples failed through the base metal, but multiple failure modes coexisted, including interface failure. These failure modes were used to classify the results. The failure process can be best understood through the lens of the spatial variation that is natural in this type of weld. The center is naturally unwelded, and there is an annulus of high strength material surrounding this unwelded zone that has a wavy morphology. The mesh-insensitive structural stress method could naturally provide a link between the joint structure and the mechanical properties of the spot impact welds. This could show that despite varied failure modes and nugget strength, strength itself is not usually affected adversely by the size of the central unbonded zone.