Aluminium foil-aided ultrasonic welding of NiTi and 304 stainless steel: a study of mechanical properties and interface microstructure

The achievement of an effective connection between superelastic NiTi and 304 stainless steel (304 SS) using ultrasonic welding with varying welding energy inputs has been studied. It has been observed that the addition of aluminium foil serves to a tight joint, which displays two kinds of interfaces where no defects were observed. With the increase in input energy, the peak load exhibits an initial growth phase followed by a decrement phase, and the interfacial heat production intensifies gradually, leading to subsequent interfacial oxidation. When the input energy is 750 J, the joint peak load reaches its maximum at 890 N. The fracture pattern of the joint exhibited both interfacial fracture and edge fracture. In high welding energy situations, Fe4Al13 (a brittle phase) is produced, which deleteriously impact on the joint's performance. Conversely, low welding energy results in the absence of any brittle phase formation.