Controlling the strengthening precipitate (β'') in friction stir processed precipitation hardening aluminum alloys.

The objective of this work is to identify a primary process response that can be correlated to the mechanical properties of friction stir welded precipitation hardening aluminum alloys. The effects of welding parameters on mechanical properties and post-weld microstructure have been investigated. However, a robust relationship between welding parameters and mechanical properties has been elusive. In this study, butt-welds were produced by friction stir welding aluminum alloy 6061-T6, over a wide range of process parameters. During the welding process, two tool-weld interface temperatures were measured, at the outer edge of the shoulder and the side of the probe, using thermocouples mounted in the friction stir tool. In addition, the temperature at the interface between the workpiece and backing plate was measured using a stationary thermocouple. A strong correlation was found between the temperature gradient between the shoulder and the probe (measured by the tool-mounted thermocouples) and cooling rates measured by the stationary thermocouple located at the interface between the workpiece and backing plate. The cooling rate in the weld zone has been shown to have a strong influence on the weld strength for heat treatable aluminum alloys. The results of this research indicate that the measurable temperature gradient along the length of the friction stir tool can be used to determine, hence control, the weld strength.