Effect of reduced or zero pin length and anvil insulation on friction stir spot welding thin gauge 6111 automotive sheet

Friction stir spot welding (FSSW) is a relatively new process, which has not yet been fully optimised. The aim of the work presented was to investigate the influence of pin length and an insulating anvil on FSSW thin (0.9 mm) 6111-T4 aluminium automotive closure panels. A pinless, or 'zero pin length', tool was also tested. With a normal pin tool and a steel anvil the optimum pin length was found to be considerably shorter than conventionally used, being in the range 0.7-1 mm, as opposed to similar to 1.4 mm. The insulated anvil increased the peak temperature in the bottom sheet by 45 degrees C, but there was a 15% reduction in lap shear strength when used with a conventional pin tool. In the case of the pinless tool, successful welds were produced with comparable strengths to the highest values measured with the optimum conventional tool without a retained weld keyhole or top sheet thinning (hooking).