In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of al-clad 2A12-T4 aluminum alloy during refill friction stir spot welding. There are two material flow components, i.e., the inward- or outward-directed spiral flow on the horizontal plane and the upward- or downward-directed flow on the vertical plane. In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer. In the refill stage, this is process reversed. As such, there is no obvious vertical plasticized metal flow between adjacent layers. Welding leads to the coarsening of S (Al₂CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone. Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.

在这项研究中，我们使用了停止作用技术来实验研究铝包层2A12-T4铝合金在填充摩擦搅拌点焊过程中的材料流动和微观组织演变。有两个物料流分量，即，在水平面上的向内或向外螺旋流和在垂直面上的向上或向下螺旋流。在下降阶段，增塑金属流入型腔的流动类似于堆叠的流动，从而上层被下层向上推动。在重新装填阶段，此过程相反。因此，相邻层之间没有明显的垂直增塑金属流。焊接导致S（Al ₂热机械影响区中的CuMg）和搅拌区中S的减少。连续的动态重结晶会在搅拌区形成细等轴晶粒，但由于变形速率较低且S沉淀在位错和亚晶界上的钉扎作用，该过程在热机械影响区变得困难。这导致该区域中大部分的低角度晶界。