The effect of anvil geometry and welding energy on microstructures in ultrasonic spot welds of AA6111-T4

The formation of ultrasonic spot welds of AA6111-T4 has been investigated using a single-transducer unidirectional wedge-reed welder. The evolution of weld microstructures and weld strength due to anvil cap geometry and welding energy was studied. The variations in lap-shear failure load and weld microstructures as a function of welding energy were only slightly influenced by the changes in the anvil cap geometry. Weld failure in lap-shear tensile tests occurs by interface fracture for low energy welds and by button formation for high energy welds. Initially, microwelds or weld islands several microns in diameter are generated presumably at asperities of the two pieces being joined. As the welding energy increases, the weld interface can change from a planar to a wavy morphology and the weld strength increases. Deformation wakes and bifurcation are ubiquitous in strong welds. Microporosity is observed at the periphery of growing weld islands and along the flow lines associated with the wavy deformation microstructures. Grain growth occurs inside the weld zone after isothermal annealing. However, the porous microstructure at the weld interface is not affected by isothermal annealing. Ultrasonic spot welding of AA6111-T4 aluminum was found to be insensitive to variations in anvil cap size and the knurl patterns investigated in this research.