Mechanical properties and their relations to microstructural characteristics of high-speed friction stir-welded AA6005A-T6 aluminum hollow extrusions

The AA6005A-T6 aluminum hollow extrusions were friction stir welded at high welding speeds ranging from 1200 to 3200 mm/min, and the mechanical properties and their relations to microstructural characteristics of joints were investigated in detail. Defect-free joints were obtained at the welding speeds less than 2400 mm/min, and voids appeared at the lower part of the nugget zone (NZ) in the joint welded at higher welding speed. The tensile tests revealed that increasing the welding speed decreased the tensile strengths of nonstandard joints monotonously, but it had little influence on the tensile strengths of standard joints. The evolutions of grains, dislocations, and strengthening precipitates in various zones led to the existence of the softened region, which consisted of NZ, thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ). The increasing welding speed narrowed down the width of the softened region and enhanced the lowest microhardness value located in the HAZ. The fracture paths of nonstandard joints were all located in the HAZ on the retreating side (RS), while the standard joints fractured in the HAZ or HAZ/TMAZ interface on the RS or advancing side, which demonstrated that both the microstructures and structural characteristics influenced the actual fracture locations of nonstandard joints.