Refill friction stir spot welding was employed to produce 6061-T6 aluminum alloy joints with different sleeve plunge depths. The interface characteristics of joint-line remnant and hook are investigated by optical and scanning electron microscopy. The joint-line remnant consists of primary bonding region and secondary bonding region, and two types of hook can be identified as downward hook and upward hook. Tensile shear results demonstrate that joint-line remnant and hook make interaction effects on tensile shear properties. The optimal joint is achieved when sleeve plunge depth was 2.0 mm with the corresponding failure load of 8673.4 N. Three different types of fracture mode are exhibited in joints produced at different sleeve plunge depths, which are closely related with the morphology of interface characteristics.
Three sleeve structures (conventional sleeve, threaded sleeve, grooved sleeve) were used for refill friction stir spot welding (RFSSW) of 2A12-T42 aluminium alloy to investigate the effect of sleeve geometry on hook formation and mechanical properties of the joints. The high-pressure area generated during the plunge stage was responsible for the hook formation. Threads and grooves sleeves not only enhanced the mixing effect but inhibited the hook formation by breaking up the alclad layer or promoting the downward material flow, respectively. The welds using a grooved sleeve had the lowest hook height and highest tensile-shear failure load. As the hook height decreased, the upward tendency of crack propagation was suppressed, which confined the propagation paths within the regions of higher hardness.
In the present study, 8 mm-thick 5251 aluminum alloy was self-reacting friction stir welded (SRFSW) employing an optimized friction stir tool to analyze the effect of welding speed from 150 to 450 mm/min on the microstructure and mechanical properties at a constant rotation speed of 400 rpm. The results indicated that high-quality surface finish and defect-free joints were successfully obtained under suitable process parameters. The microhardness distribution profiles on the transverse section of joint exhibited a typical “W” pattern. The lowest hardness values located at the heat-affected zone (HAZ) and the width of the softened region decreased with increasing welding speed. The tensile strength significantly decreased due to the void defect, which showed mixed fracture characteristics induced by the decreasing welding speed. The average tensile strength and elongation achieved by the SRFSW process were 242.61 MPa and 8.3% with optimal welding conditions, and the fracture surface exhibited a typical toughness fracture mode.
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