Material Interactions in a Novel Pinless Tool Approach to Friction Stir Spot Welding Thin Aluminum Sheet
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des with the formation of a hook that becomes noticeable with this tool type in the 2.5-second welds (Figures 7(b) and 8). With the longer wiper tool, the weld strength also falls off more rapidly than with the short wiper tool (Figure 2(c)). This is probably a direct result of the more prominent hook formed by the greater depth of top sheet penetration, which occurs as a result of the larger wiper area on this tool’s surface.
V.
CONCLUSIONS
It has been shown that friction stir spot welds can be produced in a thin, ~1-mm thick aluminum automotive sheet with a high lap shear strength of ~3.5 kN and a weld cycle time of less than 1 second, using a pinless tool. The welds leave a minimal surface impression on the sheet (shoulder plunge 0.2 mm) with no keyhole. Furthermore, the optimized welds failed by tear-out of the weld area around the edge of the weld foot print and consequently had high failure energies. X-ray tomography has shown that the optimized welds retain few defects. Material flow experiments have been used to understand the stages of weld formation and relationship to the tool surface features. Despite the ‘‘simple’’ nature of a FSSW process without a pin, material flow in the weld zone was found to be surprisingly complex, which results partly from the highly transient conditions. Controlling the flow pattern in the weld is clearly important and strongly influenced by features on the tool surface. In general, welding was more rapid and successful with fluted features machined on the tool surface. With profiled tools, a high depth of penetration of the deformation zone and a strong intercalated interface was observed to form for surprisingly short weld times. Both the scroll and wiper features on the tool tend to drive material toward the middle of the weld at the top surface, which displaced bottom sheet downward at the center of the weld, increasing the depth of penetration of the deformation zone and assisting weld formation. However, this resulted in formation of a ‘‘hook’’ with longer weld times. Overall, it was found that to produce a rapid highstrength FSSW joint with a pinless tool, a balance must thus be kept between the tool coupling too strongly with the top sheet, before the temperature has risen sufficiently for the material to soften enough to prevent cracking, and ensuring sufficient radial flow, to obtain great enough penetration of the deformation zone to achieve bonding. In the optimized welds, where failure is dominated by weld pullout, joint strength was controlled by the strength of the weakest path through the top sheet near the edge of the tool impression. This results in a loss of strength with longer weld dwell times because of the progressively more severe hooking behavior, which is dependent on the tool design. The rapid thermal cycle of the optimized welds led to little thermal damage to the parent sheet properties. Indeed, the weld zone was found to be harder than the parent material as a result of near full-strength recovery VOLUME 42A, MAY 2011—1281
to parent properties from postwel
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