Structure-property characteristics of Al-Cu joint formed by high-rotation-speed friction stir lap welding without tool penetration into lower Cu sheet

In order to explore a novel approach to weaken the tool/workpiece interaction during friction stir lap welding of dissimilar materials, Al and Cu plates with 3 mm thickness are successfully friction stir lap welded without tool penetration into the lower hard Cu plate at high rotation speeds. Large-size Al-Cu coexistence zones composed of the composite-like structures and layered structures are formed at the Al-Cu interfaces for 2500 and 3000 rpm, where intermetallic phases Al2Cu, AlCu and Al4Cu9 are identified. Increasing rotation speeds to 3500 and 4000 rpm leads to the formation of thin and uniformly distributed layered structures featured by indiscernible intermetallic compounds at the Al-Cu interfaces, and significant hook-like structures are extended from the lower Cu sheets into the upper Al sheets. All joints fail along the Al-Cu interfaces during tensile shear test. The lap joints fabricated at 2500 and 3000 rpm present higher fracture loads than those produced at 3500 and 4000 rpm, owing to the more sufficient material mixing and the induced larger Al-Cu bonding areas under the pin stirring effects.