Bonding and strengthening mechanism of ultrasonically soldered 7075 Al joint using Ni-foam/Sn composite solder foil

Ni-foam reinforced Sn-based composite solder was employed to join ultrafine grain 7075 Al alloy with the assistance of ultrasound. The solder seam was mainly composed of Ni skeletons, Sn-based solder, α-Al phase and very fine (hundreds of nanometers to several microns diameter) Ni3Sn4 particles. Discontinuous networks surrounded by the fine Ni3Sn4 and α-Al particles were in-situ formed in the solder seam, the diameter of which was decreased with increasing soldering time. Smooth transition of the lattice from Al substrate to Sn-based solder was achieved by the formation of an amorphous Al2O3 interlayer at the interface. The microstructure evolution mechanism and the amorphous Al2O3 interlayer formation mechanism are discussed in detail. The shear strength of Al/Ni-Sn/Al joints increased with prolonging soldering time. The Al/Ni-Sn/Al joint ultrasonically soldered for 60 s exhibits a shear strength of 71.4 MPa, which was approximately 37% higher than that soldered with pure Sn solder.