Microstructure and mechanical properties of friction stir welding joint during post weld heat treatment with different zigzag lines

Abstract

Zigzag line is a common defect in friction stir welding (FSW) joint. The formation mechanism of the zigzag line in Al–Cu alloy FSW joint and its influence on the microstructure and mechanical properties during post weld heat treatment (PWHT) were studied by scanning electron microscopy (SEM), microhardness and tensile tests. It is found that the occurrence of zigzag line for PWHT joint is determined by PWHT process which in nature depends on residual stress and thermal stress of FSW joint. The optimization of PWHT process to reduce the residual and thermal stress can trigger for the deterioration of mechanical properties of PWHT joints with zigzag line. No obvious decrease of tensile properties is observed for T6-450 and T6-495 joints although zigzag line appears in the weld. PWHT determines the sizes of zigzag line cracks and consequently determines the fracture location and characteristics of FSW joint.