FRICTION STIR PROCESSING OF GREEN Al POWDER COMPACT

Abstract

Detailed study on microstructural evolution of friction stir processed (FSPed) commercially pure Al powder compact was carried out in this work. Initial study was conducted to correlate compaction pressure, microstructure and mechanical properties obtained after FSP of green Al powder compacts. Further, bulk texture and recrystallization mechanisms were analysed to understand the microstructural evolution during FSP. Several FSP parameters were employed on the green Al powder compacts to vary the strain, strain rate and temperature which facilitated the understanding the microstructural evolution. Finally, the thermal stability of the FSPed samples were studied after annealing treatment. In the first study, the effect of compaction pressure on the microstructural and mechanical properties of the green Al powder compacts after FSP was investigated. To achieve it, the powders were initially cold compacted at three different compaction pressures (i.e. 50, 200 and 380 MPa) and then subjected to wear testing and FSP. The wear testing results showed that the co-efficient of friction (COF) of the green compacts increased with increasing compaction pressure. The FSP was carried out at a constant tool rotation and traverse speed i.e. 1525 rpm and 1 mm/min respectively. The green densities of the samples compacted at 50, 200 and 380 MPa were 2.1, 2.3 and 2.5 g/cm3, respectively. The density increased to 2.7 g/cm3 after FSP, suggesting 100% densification. Green density found to have significant 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 same. Based on the size of the nugget zone, the strain rate during the FSP was calculated and was found to be increase with compaction pressure. Peak temperature and coefficient of friction (COF) were found to increase with compaction pressure. It was argued that with increase in compaction pressure bounded powder particles tightly thereby increasing COF values. Increase in friction played important role in increasing peak temperature, though FSP parameters were same. Due to higher strain rate and temperature, the sample compacted at highest compaction pressure (380 MPa) showed fine and recrystallized grains whereas, the sample compacted at lowest compaction pressure (50 MPa) showed elongated grains with higher dislocation density. 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.