Abstract
The morphology of joint surface revealed that a narrow process window exists for acquiring multilayer defect-free joints with micro friction stir welding (μFSW). The optimal welding condition for realization of the maximum tensile load was predicted: a rotation rate of 20,000 rpm, welding speed of 3 mm/s, plunge depth of 0.85 mm, and use of a stepped pin. Significant grain refinements and material hardening in the weld zone were observed in multilayer defect-free joints, with a decrease in the bonding length between adjacent layers from the top layer to the bottom layer. A more distinct thermomechanically affected zone was found on the retreating side than on the advancing side. Peel tests revealed different maximum peel loads and fracture modes for different pairs of adjacent layers, indicating nonuniform bonding strengths in different layers; and the strengths were dependent on the tool geometry, layer position, and welding position (advancing or retreating side).
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