Strength and hardness of post-weld heat-treated thick section 7075 Al alloy friction stir welds

Abstract

Abstract In this work, 10 mm thick AA7075-T651 aluminum alloy plates were friction stir welded. The welds were heat-treated using two different heat treatment techniques. Firstly, a special solutionizing method called cyclic solution treatment (CST), involving a repeated heating between 400 and 480 °C for 15 minutes and water quenching. Secondly, CST followed by artificial aging at 130 °C for 24 h. The effect of CST and artificial aging on the mechanical behavior of the welds was studied by dint of hardness, tensile, and impact tests. Weld microstructures were examined by optical microscopy and transmission electron microscopy (TEM). CST enhances hardness across the joint and homogenizes the hardness distribution while marginally improving the yield strength of the weld. CST followed by aging at 130 °C for 24 h was found to result in a significant improvement in the hardness and yield strength of the welded joint. Grain size in the weld nugget is not considerably altered after heat treatment. TEM studies revealed that a partial dissolution of coarse precipitates (MgZn2) and the formation of metastable (MgAlCu) precipitates take place in the weld nugget after CST. In addition, CST followed by artificial aging causes a uniform distribution of stable MgAlCu precipitates in the weld nugget, which thus seems responsible for the enhancement in hardness and strength of the joint.