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.