Influence of Welding Speed on Zigzag Line Feature and Tensile Property of a Friction-Stir-Welded Al-Zn-Mg Aluminum Alloy

The zigzag line has significant and complicated effects on the mechanical properties of friction-stir-welded aluminum alloys. The zigzag line feature and its affecting characteristic on joint property are largely related to welding speed. In this paper, an Al-Zn-Mg aluminum alloy was butt friction-stir-welded, and the effects of welding speed on zigzag line feature and joint property were studied in detail in order to reveal the intrinsic correlations between the critical process parameter and the weld quality and, consequently, to provide guidance for the process optimization. The results indicate that at high welding speed of 300 mm/min, the loose and dilute distribution of the oxide layer fragments induces the relatively ambiguous zigzag line feature; the joint does not fail through the zigzag line during tensile test and shows superior tensile properties. With decreasing welding speed to 175 and 50 mm/min, the zigzag line feature is not weakened by the severer tool stirring action, as widely reported by previous studies, but is strengthened gradually due to the enhancement of the collective flow and more concentrated redistribution of broken oxide particles, leading to joint failure at zigzag lines and the degradation of tensile properties of joints.