Revealing the crack formation mechanism induced by intermetallic compounds in thick plate Al/Mg friction stir welded joints

Aluminum (Al)/magnesium (Mg) friction stir welded (FSW) joints are prone to forming brittle intermetallic compounds (IMCs), which are accompanied by the formation of microcracks. The existence of these microcracks is detrimental to the mechanical properties of the joint. However, the underlying cause between crack formation and IMCs is still unclear. With the increasement of the thickness of the base metal, the precipitation behavior of IMCs is more complex due to the thermal-mechanical gradient along the thickness direction. The transmission electron microscopy (TEM) is employed to characterize phase structure and the mismatch degree, and the electron backscatter diffraction (EBSD) is used to identify the phase type and strain gradient. The results indicate that the upper of stir zone precipitates columnar Al3Mg2 phase and non-equilibrium AlMg phase, and the middle and bottom exist micro-sized Al grains and nano-sized Al3Mg2 phase. The upper of the Mg side interface consists of the eutectic layer and Al3Mg2 layer, and the middle and bottom of the interface composes of Al3Mg2 layer and Al12Mg17 layer. The cracks only occur at the upper side of the stir zone and at the Al/Al3Mg2 interface. The crack formation at the upper of the stir zone is related to eutectic reaction (Al+Al3Mg2↔Liquid). Compared with the Al3Mg2/Al12Mg17 interface and Al12Mg17/Mg interface, the Al/Al3Mg2 interface has the largest mismatch degree (δ=2.27%) and strain gradient (0.7°), which is also located where the cracks are easy to form. Furthermore, the nanoindentation test certifies that the Al12Mg17 phase (0.41MPa·m¹/²) has higher fracture toughness than that of the Al3Mg2 phase (0.28MPa·m¹/²). Therefore, it is more necessary to control the thickness of Al3Mg2 phase for suppressing crack formation.