Multi-pass friction stirred clad welding of dissimilar joined AA6061 aluminium alloy and brass

In the past years, joining of copper and aluminium has attracted much attention to allow the utmost use of the special properties of both the metals. Friction stir welding (FSW) technology is feasible to join dissimilar materials compared to conventional fusion welding methods because of its solid-state nature. The present article provides an extensive insight into dissimilar copper to aluminium materials joined by FSW technology. The effect of overlapping between consecutive passes of FSW on the microstructure and mechanical properties of AA6061-brass was examined. FSW was carried out at three shoulder-overlapping percentages, i.e. 0, 50, and 100, and at a constant rotational speed of 1055 rpm and traverse speeds of 5.5 mm/min of the FSW tool. Microstructural analysis was determined by optical microscopy, SEM and XRD while mechanical properties were evaluated by micro-hardness and peel tests. The increase of micro-hardness in all SZ compared to base metal is caused by plastic deformation and dynamic recrystallisation at the same time during the welding process. The micro-hardness and grain size of SZ for all samples remained approximately constant without variation between passes. Hardness across TMAZ/HAZ of Al is almost constant and lower than that of the Al-base metal, due to artificially aged precipitation hardened aluminium alloy. The peel results indicated that the 50% shoulder had the highest peel strength due to shoulder influenced region that increased tool penetration under the constant axial load, producing a better weld quality. In contrast, 0% overlapping sample has a too low overlapped region that produced low interface strength, resulted in inappropriate metallurgical bonding between the Al-Cu. Moreover, the sample with too overlapping of 100% overlapping rises the peak temperature of the overlapped zone due to the pin-preheating effected by the previous pass, which caused grain coarsening, liquation and rearrangement of IMCs, thus produced surface grooves at the Al/Brass interface that lower the peel strength. The XRD results revealed that the multi-spectrum in the SZ is made up of CuAl2, Cu4Al9, CuAl and CuZn intermetallic compounds, which affected the strength of the resulting welds.