Friction Stir Lap Welding of AZ31B and AA6061 Alloys Using Tin as an Inter-Layer

The formation of Mg17Al12 and Al3Mg2 is inevitable even in solid-state joining of Al and Mg alloys by friction stir welding (FSW). In the present study, the effect of a thin inter-layer of Sn (tin) and FSW parameters (pin length and tool rotation speed) on the microstructure and strength of friction stir lap welds of 3 mm thick AA6061 Al alloy and AZ31B Mg alloy sheets was investigated. A similar to 20 mu m thick Sn layer was deposited on AZ31B alloy by displacement plating. The heat generated during the welding melted the Sn layer and the rotation action of the tool pushed the molten Sn to the hooks on the advancing side and retreating side. The extent of Mg17Al12 and Al3Mg2 intermetallic compound formation was reduced with Sn inter-layer with limited formation of Mg2Sn. The Sn inter-layer helped improve the lap shear strength at lower tool rotational speed (600 RPM) and pin lengths of 3.25 and 3.75 mm compared to joints without Sn inter-layer. Lap welds prepared with a 3.25 mm pin at 600 RPM had a lap shear strength of 245 N/mm which is the highest value reported so far and is 116% higher compared to joints without Sn inter-layer. This was due to elimination of hook at the advancing side and retreating side and disintegration of intermetallic compounds.