Effect of Cooling Rate on the Microstructure and Strength of Friction Stir Welded Joints in a High-Strength AA7075 Alloy

The application of arc, plasma, laser, or electron beam welding methods to high-strength heat-treatable aluminum alloys, such as AA7075 belonging to a Al-Zn-Cu-Mg system, makes little sense because of the formation of weak heat-affected zones and hot cracks. Attempts were made to produce strong joints in these alloys using friction stir welding, which is a low heat input method. However, there are still problems with overaging and the formation of weak thermomechanically affected zones. In this research, we employ liquid flow cooling of the weld zone to eliminate overaging. It is found that the microhardness in the stir zone and the joint strength are maximum due to cooling at a medium liquid flow rate of ~4–6 l/min. All samples were subjected to artificial aging to eliminate weak thermomechanically affected zones, which further increased the microhardness and strength of joints. Structural studies reveal that the contribution from the dispersion strengthening mechanism under internal aging, which can be further enhanced by artificial aging, is maximum at medium cooling rates.