Strengthening of Al 6061 alloy by high-pressure torsion through grain refinement and aging

The concurrent strengthening process by high-pressure torsion (HPT) and fine precipitation hardening of an Al 6061 alloy has been studied. The HPT was conducted on disks of the alloys at room temperature with an applied pressure of 6 GPa for 1 and 5 turns with a rotation speed of 1 rpm. It is shown that the HPT processing leads to microstructural refinement with an average grain size of ~200 nm and to an increase in hardness up to a saturation after 5 turns. Aging treatment was performed at temperatures of 373 and 423 K for a maximum period up to 256 hours. The hardness at the saturation level remains the same during aging at 373 K. When aged at 423 K, the hardness decreases from the beginning of the aging. This study thus suggests that aging of the HPT-processed sample may not be effective to strengthen the alloy if it is carried out at higher temperatures. It is suggested that simultaneous strengthening may be achieved through reduced straining and aging at reduced temperatures.