Abstract:
An investigation was conducted to develop low Z-force (normal/forge load) friction stir spot welds (FSSWs) using conventional tooling and process development approaches. Low Z-forces can be achieved by studying the relationship between pin tool features, geometries, processing parameters, and resultant strength of coupons produced by friction stir spot welding (FSSW). The effects of geometrical and feature changes of pin tool designs—including shoulder diameters, shoulder features, probe diameters, probe shapes, and probe features—on the joint properties of 0.040-inch-thick bare 2024-T3 aluminum alloy were evaluated. Welding tools included Psi™, Counterflow™, Modified Trivex™, and V-flute™ pin tools. A Box-Behnken design of experiments (DOE) approach was used to investigate the effects of three process parameters: spindle speed, Z-force (forge load), and travel speed. The goal of the investigation was to maintain the ultimate tensile load (UTL) in unguided lap shear coupons tested in tension while reducing the Z-force required for producing a sound joint. This goal was achieved on a specially built MTS Systems Corporation ISTIR PDS FSW gantry system. In addition to single-spot unguided lap shear tests, the performance of low Z-force FSSW joints was evaluated by optical metallographic cross-section analyses, which were then correlated with process parameters, UTL, and pin tool designs. The maximum Z-force spikes encountered during the initial plunge were reduced by an order of magnitude, and the Z-force processing loads were reduced by half for Octaspot™ swept FSSW, most effectively by controlling the plunge rate under force control. Additional reductions in Z-force were achieved by refining the conventional FSSW tool shoulder and probe designs. Therefore, it was demonstrated that weld forces can be reduced to the point where it would be feasible to perform robotic low Z-force FSSW for at least some applications.
