•Ultrasonic welding of Cu-Ti with Al interlayer was studied.•Interfacial strength evolved from plastic flow and mechanical interlocking.•Large plastic strain caused interface-adjacent cracks, reducing strength.•Introduced Al interlayer prevented base metal damage, improving weld quality.•Al interlayer increased interfacial strength by 26% via enhanced frictional heat. This study investigated the ultrasonic welding of Cu-Ti joints, revealing that the interfacial strength originated from mechanical interlocking. However, strength was diminished owing to interface-adjacent crack initiation due to excessive plastic strain. Introducing an Al interlayer prevented base metal damage with more frictional heat, and enhanced interfacial strength by 26%.