This work aims to investigate the ultrasonic weldability of Al ribbon (width 2.0 mm × thickness 0.2 mm) to Cu sheet (thickness 1.0 mm) using a WC tool of 2.0 mm × 1.0 mm (sonotrode tip size) and to understand joint formation process by examining joint microstructures and fracture behavior for different bonding times. For the selected conditions of 20 W power, 30 N clamping force, 0.1–0.8 s bonding time, it was found that sound lap joints could be readily obtained when the bonding time reached and exceeded 0.4 s, which fractured within Al ribbon, but not along interface, owing to the formation of dense and thin alloying layer of 2∼3 µm thickness with a continuous composition gradient. When such bonding has been established, the actual slipping motion shifted upwards to the top of Al ribbon. On the other hand, both microstructure and fracture surface observations indicated that in the early stage, localized adhesion occurred accompanied by the detachment of just adhered Al part from remaining Al ribbon body, leading to a cracking (called secondary interface) within weak Al ribbon. Thus, USW of Al ribbon to Cu sheet was achieved through a series of slipping at three kinds of transient interfaces: localized adhesion at original interface and alloying of the adhered Al together with detachment within Al ribbon (forming the secondary interface); re-bonding at the secondary interface together with further bonding at original interface under resisted slipping and high frictional coefficient as a result of surface roughening by previous isolated compound formation at original interface; and final upwards shifting of slipping to the third interface between top surface of Al ribbon and tool end.