Ultrasonic-assisted soldering of Mg alloy joints using Cu-foam/Sn composite solder foils

•Reliable Mg alloy joints reinforced with Cu-foam/Sn composite solder foils (CSF) were realized by ultrasonically soldering.•Unique structures in Mg/CSF/Mg joints can suppress crack propagation and block Mg2Sn growth.•MgSnCu grains with high hardness were generated on the surfaces of Cu skeletons after ultrasonically soldering. Mg alloys have been increasingly utilized in our daily life due to the low density and high specific strength and stiffness. However, the poor formability of these alloys restricted their adoption to some extent. The joining technique can effectively manufacture complex components, but some challenges are unique for Mg alloys. In this paper, reliable Mg alloy joints were realized by ultrasonically soldering at 250 °C for only 5 s. By using Cu-foam/Sn composite solder foils, the average shear strength of Mg alloy joints rose to 53.06 MPa, a 44.4% increase compared without using composite solder. The strengthening mechanism was attributed to the unique structure of 3D continuous Cu skeletons and the formation of high-hardness MgSnCu grains wrapped on skeletons after ultrasonically soldering, which could suppress crack propagation and block the excessive growth of Mg2Sn grains. The theoretical calculation confirmed that the formation energy of MgSnCu phase was lower than that of Mg2Sn or Cu6Sn5 phase despite evidence that it not existed in the Mg−Sn−Cu equilibrium phase diagram. Our study could contribute to the development of ultrasonic soldering technology with composite solder and open an opportunity for further expanding the usage of Mg alloys.