Dependence of the microstructure and properties of joints on acoustic intensity during the ultrasonic soldering of 7075 Al alloys

In this work, 7075 Al alloys were ultrasonically soldered by pure Zn solder in air under different acoustic intensities. The dependence of the microstructure and mechanical properties of joints on acoustic intensity was revealed. Acoustic intensity is innovatively controlled by applying different lap widths without increasing the input ultrasonic power. The acoustic pressure inside the solder was simulated by finite element simulation, and results showed that the acoustic intensity increased by more than six times when the lap width decreased by 50 %. High acoustic intensity led to intense cavitation, thereby causing a pronounced erosion of substrate. The Al content inside the joint increased from 17.3%–24.5% when the clearance width decreased from 0.8 mm to 0.4 mm. The Clausius–Clapeyron equation helped explain that the melting point of Zn increased by approximately 73.8 °C under the high acoustic pressure caused by cavitation bubble collapse, which could cause the direct solidification of Zn and served as the main reason of grain refinement. Zn grains were refined from 6.9 μm to 4.2 μm when soldered at 0.8 mm and further to 3.5 μm when soldered at 0.4 mm because of the strong cavitation caused by high acoustic intensity. Acoustic intensity had a remarkable effect on the mechanical properties of joints. Joint shear strength and hardness increased from 117.3 MPa and 93.3 HV to 145.1 MPa and 109.4 HV, respectively, when the lap clearance width decreased from 0.8 to 0.4 mm. Particularly, the joint soldered at 0.4 mm failed through the substrate because of the high strength of the joint seam and the weakness of the substrate, which were explained through the Hall–Patch relationship, the Schmid factor, the dissolution of secondary phases, and the increase in high angle boundaries. This work provides a novel method by producing strong joints with fine grains without increasing input power.