Energy Generation and Stir Zone Dimensions in Friction Stir Spot Welds

Energy generation and utilization during friction stir spot welding of Al 6061-T6 and AM50 sheet materials are investigated. The dimensions of the stir zones during plunge testing are largely unchanged when the tool rotational speed increases from 1500 RPM to 3000 RPM (for a plunge rate of 1 mm/s) and when the rate of tool penetration increases from 1 mm/s to 10 mm/s (for a tool rotational speed of 3000 RPM). The energy resulting from tool rotation is also unaffected when higher tool rotational speeds are applied.

The rotating pin accounts for around 70% and 66% of the energy generated when 6.3 mm thick Al 6061-T6 and AM50 sheet materials are spot welded without the application of a dwell period. In direct contrast, the contribution made by the tool shoulder increases to around 48% (Al 6061-T6) and to 65% (AM50) when a four second long dwell period is incorporated during spot welding of 6.3 mm thick sheets. The increased contribution made by the tool shoulder is due to the tool shoulder remaining in contact with stir zone material for a much longer period during the friction stir spot welding operation.

Only a small percentage of the total energy generated during tool rotation (about 4%) is required for stir zone formation during plunge testing of Al 6061-T6 and AM50 sheets. The remainder of the energy generated by tool rotation dissipates in the sheets being welded, in the tool assembly, anvil support, and clamp and surrounding atmosphere. The presence of a thread on the rotating tool has negligible influence on the amount of energy generated during spot welding.