Gap tolerance allowance and robotic operational window for friction stir butt welding of AA6061

In this work, it was determined that with increasing weld pitch, the occurrence of a “lazy S” defect in the weld nugget of friction stir welded (FSWed) AA6061 became increasingly pronounced, though its impact on the bend performance of the weld was negligible. For a fixed weld pitch of 0.48, the effect of gap, i.e. the spacing between two sheets at the butt joint interface, on the joint quality of AA6061 was evaluated in terms of the welding defects, microstructure, hardness and bend performance. Fully penetrated welds without metallurgical defects such as wormholes were obtained up to a joint gap of 0.5 mm. Though the overall microhardness and bend performance of the welds remained unaffected until a joint gap of 0.8 mm, the decrease in the forge force during FSW beyond a joint gap value of 0.5 mm may represent a more critical limit in regards to the industrial application of the process; this is especially important when applying force control during processing to ensure a constant shoulder penetration in the material for addressing practical considerations, such as thickness variations in the assembly, clamping distortions and tool wear. Based on these results and using force amplitudes recorded during the welding experiments, a robotic scenario was synthesized with an appropriate operational window for continuous-path friction stir butt welding (FSBW) of 3.18-mm-thick sheets clamped to a 1 m × 1 m horizontal welding table. An appropriate industrial robot model was selected and the associated geometric workcell layout was developed for this application. This scenario was implemented in a physical prototype and used to successfully produce 1-m-long FSWed assemblies that exhibited good tensile mechanical performance.