Sound dissimilar lap joints were achieved via ultrasonic spot welding(USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle Al12Mg17 intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure in both MgAl and Mg-high strength low alloy(HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the MgAl dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.
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