Interfacial phenomena during ultrasonic welding of ultra-low-carbon steel and pure Ti

Abstract Changes in bonding strength and interfacial formation during the ultrasonic welding (USW) of steel and Ti were investigated. It was found that when the temperature of the joint was increased above 600 °C during USW, the bonding strength increased drastically and resulted in base-metal fracture. Interfacial, fracture, compositional, and crystallographical analyses revealed that phase transformation in Ti from α (hexagonal close-packed: HCP) to β (body-centered cubic: BCC) led to improved deformability at elevated temperatures, which promotes extensive bonding formation through the elimination of gaps near the bonding interface. Through this process, joints with good bonding and strength can be obtained.