Effects of rotational speed on microstructure and mechanical properties of dissimilar AA6061 and AA7075 friction stir welding joints

This study takes AA6061 and AA7075 dissimilar friction stir welded joints as the research object, and explores the influence of rotational speed on the organization and mechanical properties of joints. The results show that increasing the rotation speed of the stirring tool can promote the mixing of joint materials, and the low rotational speed will lead to insufficient heat input, limited mixing of joint materials, and the bottom of the weld will be prone to kissing defects affecting the joint properties. Increasing the rotational speed, the joint material mixing is sufficient, and the nugget zone (NZ) forms "onion ring" morphology. The NZ undergoes dynamic recrystallization during welding, forming fine equiaxial crystals. Increasing the rotational speed increases the grain size and reduces the strengthening effect of grain boundaries. At a rotational speed of 1200 rpm, kissing defects at the bottom of the weld resulted in deterioration of the joint properties and fracture in the NZ during tensile testing. Increase the rotational speed to 1800 rpm-3000 rpm, which can avoid kissing defects, tensile fracture location and the hardness of the minimum value coincide with the AA6061 side of the heat-affected zone (HAZ), this rotational speed range, increasing the rotational speed will be accompanied by a reduction in the strength of the joint, the highest strength factor is 71.1 % (1800 rpm).