Influence of tool pin profile on friction stir processed pure aluminium based composites: A zonal analysis

The use of different tool pin profiles for friction stir processing is a common practise. Generally, the pin is responsible for material breaking, initial heating, mixing and transportation. Attainment of different zonal features by different pin profiles is therefore highly expected. The present work utilized H13 tool steel tools with square (SQ), cylindrical threaded (CT) and taper threaded (TT) pin profiles of sides 7×7 mm, diameter 7 mm, and 7 mm major diameter and 5 mm minor diameter respectively, all being 5 mm in length, with a common shoulder diameter of 21 mm. The study sought to map the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and the heat affected zone (HAZ) geometrical, macrostructural and microstructural features. The SZ widths attained by the pins were 9.433 mm for the CT pin, 10.185 mm for the TT pin and 10.34 mm for the SQ pin. The attained TMAZ thickness for the CT and TT pins were 0.568 mm and 0.214 mm, while the SQ pin had no visible TMAZ. The CT, TT, and SQ pins had particle sizes of 17-88 µm, 23-135 µm, and 9-73 µm respectively. The macrostructural and microstructural results revealed fingerprint or onion ring features for the TT and CT pins, with pronounced zonal features, while the SQ pin had no visible onion rings and zonal features. The obtained results show the capability of the CT and TT pins to severely stir and mix the matrix with the reinforcements better than the SQ pin, which on the other hand attained the best grain refinement.