Experimental investigation of material flow behavior during refill friction stir spot welding

The quality of the welding or defect repair of metals using refill friction stir spot welding (RFSSW) is fundamentally influenced by material flow. The welding community is still lack of comprehensive understanding of the flow pattern during the RFSSW process since numerical modeling usually has its own compromises, while current experimental observations of such solid-phase material-flow are lack of precision and suffer from limited visualization. A novel experimental material-flow observation method is proposed to chareactrize the dynamic flow behavior by mixing a few another alloy with the same matrix into the welding zone as a tracing of the flow pattern. Further, the process was combined with the emergent stops at different stages of the RFSSW for ease of analysing the dynamic vertial flow pattern. Meanwhile the deviation angles of the shear textures formed in the welding process were measured and calculated for determing the flow orintation in the horizental planes. During the plunging stage, the flow-climbing zone and flow-expansion zone were observed, with the initial joint connection occurring at their intersections. The threaded sleeve formed an outer stirring zone. A fan-shaped expansion zone developed between the flow-climbing zone and flow-expansion zone during tool dwelling, which reduced the width of the bonding ligament. When the threaded sleeve rotated in a beneficial direction, the generated axial thrust mitigated the incomplete-refill defects. The horizontal material flow in the plunging stage gradually aligned with the ideal shear direction from top to bottom, and in the refilling stage, the flow aligned more closely from bottom to top. Overall, the RFSSW material-flow patterns at different stages were revealed, which could provide explanations for formations and fracture mechanisms of three distinct hook defects. The proposed experimental approach for analyzing the solid-state material flow in this study can be applied to investigations of similar solid-phase processes and help to optimize the tool design.