Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder

Ultrasound-assisted soldering of 7075 Al alloy was performed using Ni-foam reinforced Sn composite solder. The phase composition, interfacial microstructure and mechanical properties of Al/Ni-Sn/Al joints soldered for different times were investigated. Results showed that, the bonding ratio of joint was increased with increasing ultrasonic soldering time and was stabled at approximately 98% when the soldering time was longer than 20 s. The Ni-foam in joint was compressed into a strip type and a Ni3Sn4 intermetallic compound (IMC) layer was formed on the Ni skeleton surface, whilst an Al3Ni IMC layer was formed on the Al substrate surface. The Al3Ni phase was dot-distributed in joint soldered for 5 s then formed continuously in joint soldered for 15 s. However, further increasing the soldering time to 30 s resulted in the drastic growth of Al3Ni IMC layer, accompanied with the depletion of Ni3Sn4 phase. The formation and microstructure evolution mechanisms of the Al3Ni IMC layer was discussed. The measured shear strength of joint was first increased then decreased with increasing ultrasonic soldering time, and a highest shear strength of 58.0 MPa was obtained for joint soldered for 20 s.