Formation Mechanism of Thicker Intermetallic Compounds in Friction Stir Weld Joints of Dissimilar AA2024/AZ31B Alloys

The hybrid structures of AA2024 aluminum alloy and AZ31B magnesium alloy have the advantages of being lightweight, having high specific strength, etc., which are of great application potentials in the aerospace industry. It is a key problem to realize the high-quality welding of these two dissimilar alloys. In this study, the friction stir welding (FSW) tests of AA2024 aluminum alloy and AZ31B magnesium alloy plates of thickness 3 mm were carried out. The intermetallic compounds (IMCs) at the bonding interface were characterized by scanning electron microscope, electron probe, and transmission electron microscope. It was found that the IMCs at the bonding interface in weld nugget zones of dissimilar AA2024/AZ31B FSW has a double-layer structure and a much larger thickness. During the welding process of AA2024/AZ31B, when the boundary of magnesium grains bulges and nucleates, the aluminum atoms diffuse into the magnesium grains, and the γ phase (Al12Mg17) nucleates at the bonding interface. The β phase (Al3Mg2) then precipitates at the grain boundary of the γ phase and preferentially grows into γ phase grains. The continuous grain growth to the aluminum side makes the copper contained in AA2024 aluminum alloy concentrate on the side of β phase, which reduces the nucleation work of recrystallization and phase transformation, and further promotes the nucleation and growth of IMCs grains. This is the main reason for the thicker IMCs in the FSW weld of dissimilar AA2024/AZ31B alloys.