Avoiding degradation and increasing joint strength of Al alloys by rapid ultrasonic soldering at low temperature

Abstract

To avoid substrate softness, AA7075 were rapidly soldered using a Sn-9Zn solder at 200 °C under ultrasonication. The effect of acoustic intensity on the microstructure and mechanical properties of the soldered joint was studied. The acoustic intensity inside the solder was improved by decreasing the lap width without increasing the input ultrasonic power. Results showed that the softness of the substrate was almost avoided with minimal hardness decrease. The acoustic intensity increased by four times when the width of the lap clearance was reduced from 0.8 mm to 0.2 mm. Pronounced erosion of the substrate occurred under high acoustic intensity. Thus, the content of Al in the joint seam increased from 3.35% to 10.84% after reducing the lap width from 0.8 mm to 0.2 mm. The joint seam was characterized by interlaced fine/coarse Sn-Zn eutectic. The Sn and Zn grains had average sizes of 2.31 and 0.31 μm in the fine eutectic and 3.03 and 0.41 μm in the coarse eutectic. Increasing the acoustic intensity during soldering can effectively increase joint strength. The maximum shear strength of 65.4 MPa and hardness of 29.5 HV were obtained in the joint soldered at 0.2 mm. The cavitation bubble with a nucleation diameter of 10 μm had a large liquid velocity of 7653 m/s, a temperature over 1800 K, and a pressure of 4 × 1012 Pa during its collapse.