Ultrasonic soldering aluminum at low temperature

A majority of aluminum alloys are heat sensitive and preferably joined at low temperature to avoid deterioration of mechanical properties. Tin- (Sn-) based solders with low melting points are very attractive for the joining of heat-sensitive aluminum alloys. In this study, ultrasonic soldering of pure Al at 300°C was investigated with Sn-based solders. The tensile strength of the joints only reaches 63± 2.8 MPa if using pure Sn as solder, and the joints fail along the interface between Al/Sn during the tensile test. However, if using Sn-Zn alloys, the tensile strength could be raised up to more than 80 MPa, and the joints fail in the filler metal layer. For Sn-9Zn and Sn-20Zn solders, joints all fail at the interface of Zn-rich and Sn-Zn eutectic phases in the bond metal. For hypoeutectic solder, the joints fail across ß-Sn and Sn-Zn eutectic phase, and have maximum elongation. It indicates that Zn can strengthen the interface between the base metal and the solder through forming an Al-based solid solution layer containing Zn and Sn. The thickness of the solid solution increases with the content of Zn in the solder, but the strength of joints does not increase. Interface stronger than filler metal and best ductility could be realized with 4 wt-% Zn.