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An initial study was made to evaluate the feasibility of joining magnesium alloy AZ31 sheet to galvanized steel sheet in a lap configuration using friction stir welding (FSW). Two different automotive sheet steels were used for comparative evaluation of the dissimilar joining potential: a 0.8 mm thick, electrogalvanized (EG) mild steel, and a 1.5 mm thick hot-dipped galvanized (HDG) high-strength, low-alloy (HSLA) steel. These steels were joined to 2.33 mm thick AZ31B magnesium sheet. A single FSW tool design was used for both dissimilar welds, and the process parameters were kept the same. The average peak load for the AZ31-1.5 mm steel weld joint in lap shear mode was found to be 6.3 ± 1.0 kN. For the AZ31-0.8 mm steel weld, joint strength was 5.1 ± 1.5 kN. Microstructural investigation indicates melting of the Zn coating present on the steel sheets, and subsequent alloying with the Mg sheet resulted in the formation of a solidified Zn-Mg alloy layer.

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Thinning in friction stir lap joints and its relation with the process variables was investigated. Friction stir welds were made on 1 mm thick AA6111 aluminium alloy sheets in order to study the effects of rotation rate, traverse speed, plunge depth, tilt angle and pin height on faying surface defects. Sheet thinning on the advancing and retreating sides was quantified and the lap shear strength of the joints was evaluated. A decrease in the pitch of the runs at constant rotation rate increased the sheet thinning and reduced the joint strength in a linear manner. Process pitch and pin height were found to be the most critical factors in determining the faying surface lift up. A pin of the same height as the sheet thickness resulted in maximum lap shear strength. Tool tilt did not show a significant effect on the sheet thinning.