ABSTRACT
Lightweight structural materials like magnesium (Mg) and aluminum (Al) alloys can replace steel components in automobile industries. Dissimilar combination of Mg to Al and Mg to Mg alloys offers significant weight reduction. However, the dissimilar joining of Mg/Al alloys and Mg/Mg alloys by fusion welding is challenging. In the case of dissimilar joining of Mg/Al alloys, the formation of the intermetallic compounds like Al3Mg2 and Al12Mg17 are inevitable. Further, solidification cracking is observed due to formation of low temperature eutectics. These intermetallics are brittle, can act as a crack initiator, and are detrimental for the joint strength. While there are no intermetallic compound related problems in dissimilar joining of different Mg alloys, the high temperatures gradients involved in the fusion welding reduces the strain hardening and precipitate strengthening effects, which are very sensitive to the temperature, and eventually affect the joint strength. Friction Stir Welding (FSW) is one of the solid-state welding processes and has been considered for joining the dissimilar materials due to the low peak temperature during welding. The formation and growth of intermetallic compounds are controlled by diffusion. Hence, it is difficult to avoid the complete formation of intermetallic compounds, even with FSW. However, it is possible to modify the formation of intermetallic compounds, which are less detrimental to the joint strength, by adjusting the welding parameters. In the present work, AZ31B-H24 Mg alloy and AA6061-T6 Al alloy were joined using FSW in the lap configuration. The results showed the lap shear strength of the Al and Mg dissimilar joint increased by improving the mechanical interlock and dispersion of the intermetallic compounds in the stir zone. A Sn interlayer placed at the joint interface during welding eliminated the hook of the joint and reduced the extent of intermetallic compounds in the stir zone, thereby increasing the lap shear strength of the joint. Dissimilar joining AZ31B-H24 and WE43-T5 Mg alloys were also carried out using FSW in butt configuration. In dissimilar joining, the quality of the joint mainly depends on the weld nugget properties. Due to the difference in both base material physical and mechanical properties, the weld nugget quality depends on the base materials' contribution in the nugget zone. Hence, base materials contribution in the weld nugget was varied by changing the base materials position and tool offset to the joint interface. The mechanical properties of the joint and material mixing in weld nugget with respect to base material and tool offset was studied. It is shown that optimization of the FSW parameters such as pin length, rotation speed and offset can lead to improved strength of dissimilar joints.