Material flow during refill friction stir spot welded dissimilar Al alloys using a grooved tool

Material flow, intermixing and liquation cracking during refill friction stir spot welded (refill FSSW) dissimilar Al alloys were first investigated using a grooved tool. The results revealed that the weld was fabricated by a back-extrusion and forging process, and the material flow in the vertical direction created a mechanical inter-locking joining mechanism. During the sleeve plunging period, a constant thickness of dynamic recrystallization layer of Al 6022 sheet material was trapped underneath the sleeve bottom, and a drop region was formed at the sleeve outer periphery. Dissimilar intermixing was formed during the retraction period, in which the grooved sleeve facilitated the formation of intermixed microstructure comprising intermingled dissimilar lamellae adjacent to the rotating sleeve. Cracking caused by local melting of ŋ (MgZn₂) phase particles was formed in the stir zone, which disappeared after welding process. The Al 6022 sheet material was significantly softened, since the density of needle shaped Mg₂Si precipitate particles was relatively lower compared to that of the substrate material.