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, 0.10.8 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 2³3 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.

In-situ observation was performed on a transparent silica substrate during ultrasonic Al ribbon bonding, using a high-speed video camera with differing frame rates, 10 4 fps and 10 3 fps, to clarify the adhesion behavior. The bonding process was observed as follows. Initially, friction slip occurred, producing multiple island streaks in the direction parallel to the ultrasonic vibration. The island streaks were formed as a scratch, due to surface waviness of the Al ribbon. Momentarily, a belt-shaped bond zone was formed at the center, normally due to the ultrasonic vibration. The island streaks could be clearly observed at 10 4 fps. However, the central belt zone was unclear and appeared translucent at 10 4 fps; although it was clear when observed at 10 3 fps. The island streaks were unclear at 10 3 fps. The positional relation of the island streaks and the central belt zone was confirmed from in-situ observation results of a twist and peel test of Al ribbon bonded to silica substrate. The central belt zone was between the island streaks and the silica substrate.