Recrystallization and Strengthening Mechanism in Friction-Stir-Processed Al Powder Compacts

Green Al powder compacts were subjected to densification by friction stir processing (FSP). Prior to FSP, the Al powder was cold compacted at three different pressures of 50, 200 and 380 MPa to achieve green densities of 2.1, 2.3 and 2.5 g/cm(3), respectively. After FSP density increased to 2.7 g/cm(3) (100% densification), green density had effect on microstructure and properties obtained after FSP. The size and shape of the nugget area was found to depend on compaction pressure though the FSP parameters were unchanged. After FSP, the sample compacted at highest compaction pressure showed fine and recrystallized grains, whereas the sample compacted at lowest compaction pressure showed elongated grains with higher dislocation density. An extensive microstructural investigation suggested that grain refinement occurred by continuous dynamic recrystallization. Strengthening in sample with fine, recrystallized microstructure was dominated by grain boundary strengthening mechanism, whereas, in unrecrystallized microstructure, strengthening was due to strain hardening mechanism.