Characterization of ultrasonic spot welded joints of Mg-to-galvanized and ungalvanized steel with a tin interlayer

• USWed Mg-to-bare steel, galvanized steel and bare steel with Sn were studied.
• USWed Mg-to-bare steel with Sn interlayer significantly improved the joint.
• Composite-like Sn and Mg2Sn eutectic structure was found at the weld joint.
• Partial nugget pull-out failure occurred in Mg-to-bare steel with Sn interlayer.

Ultrasonic spot welded (USWed) Mg-to-bare steel, Mg-to-galvanized steel and Mg-to-bare steel with Sn interlayer (placed in-between Mg and bare steel) were studied. Weak joining occurred in the USWed Mg-to-bare steel, since Mg and Fe do not react with each other. The intermetallic compounds (IMCs) of Mg7Zn3 and Mg2Zn11, which led to the failure of the joint, were largely present in the USWed Mg-to-galvanized steel joint. The introduction of a Sn interlayer in the USWed Mg-to-bare steel actively worked as an intermediate medium to join Mg to Fe, and led to the presence of a distinctive composite-like Sn and Mg2Sn eutectic structure. The lap shear strength of Mg-to-bare steel with Sn interlayer joint was significantly higher than that of the Mg-to-bare steel and Mg-to-galvanized steel joints. Failure during the tensile lap shear tests occurred mainly in the partial nugget pull-out mode in the dissimilar joints of Mg-to-bare steel with Sn interlayer. All the joints of Mg-to-galvanized steel failed from the interface (cohesive failure). The addition of Sn interlayer resulted in energy saving since the welding energy required to achieve the maximum strength decreased from 1750 to 1500 J in the Mg-to-steel joints.