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%.