<p>Brittle and hard Al₁₂Mg₁₇ intermetallic compound (IMC) are easily formed when magnesium is welded with aluminum alloys, leading to the degradation of joint properties. The interfacial microstructure and mechanical properties of welded joints between AZ31B magnesium alloy and 6061 aluminum alloy were investigated by ultrasonic welding, and the joint properties were optimized by adding Sn foil interlayer at the interface. The results showed that the thickness of the interlayer increased with the increase of welding energy, while the joint shear performance showed a trend of increasing and then decreasing. With the addition of a 30-μm-thick Sn foil, the maximum tensile shear was enhanced by 37.9% in comparison with the welded joints under the optimal parameters without the interlayer. Moreover, the Sn foil efficiently inhibited the formation of Mg-Al IMCs, which were substituted by a layer of composite Sn and Mg₂Sn-like structure.</p>