Investigation on variations in hardness and microstructure of in-process cooled 7075 aluminum alloy friction stir welds

Abstract

Abstract AA7075-T651 plates were friction stir welded using in-process cooling with the intention of investigating variations in microstructure and hardness across the joint in through thickness direction. The variations were studied using optical microscopy, transmission electron microscopy (TEM) and hardness tests. Water cooled welds exhibit better performance in terms of hardness compared to air cooled welds. However, both welds were found to display considerable reduction in the hardness in the heat-affected zone (HAZ). Furthermore, hardness across the joint enhances as the survey proceeds away from the surface of the specimen to the root of the weld. The weld nuggets in both welds reveal fine recrystallized grains and the grain size was found to dwindle from surface to root of the joint along the thickness of the weld. TEM studies disclose that the weld nugget suffers from dissolution of precipitates and the HAZ consists of precipitate-free zones, which are considered accountable for lower hardness in the HAZ. It is concluded that defect-free welds can be made with thick plates of AA7075-T651 using in-process cooling and furthermore, water cooled welds exhibit better hardness values.