Influence of brass interlayer and water cooling on microstructure, mechanical and corrosion behaviour of friction stir welded AA6061-T6 alloy

This study investigated the effect of brass interlayer and water cooling on the mechanical properties and corrosion behaviour of friction stir welded AA6061 alloy. Although grain refinement was observed in the stir zone (SZ) of FSW without interlayer, strength loss occurred due to thermal softening. The insertion of a brass interlayer resulted in more grain refinement which could be attributed to brass particles acting as a nucleation site for further recrystallization. The weld strength with interlayer improved due to the formation of strengthening intermetallic compounds (IMCs) such as Al2Cu and Al4Cu9. However, the strength recovery is not attainable in a heat-affected zone (HAZ). Moreover, hard and brittle IMCs formed in SZ of weld with interlayer resulted in premature failure. Therefore, water cooling was used to enhance the hardness in HAZ. Very fine grains in SZ with a high fraction of high angle grain boundaries were obtained with water cooling FSW with interlayer, while HAZ softening was also reduced. Moreover, the water-cooling FSW with interlayer inhibited the formation of brittle intermetallics and decreased volume fraction of IMCs attributed to faster cooling rate. Thereby the strength of the overall weld joint was enhanced. The corrosion test was performed in 3.5% NaCl solution. The corrosion findings stated that the decrease in the number of intermetallics formed low galvanic coupling with aluminium matrix resulted in better corrosion resistance and minimum corrosion rate for water-cooled FSW than natural cooling.