Microstructure and properties of ultrasonic-assisted soldered 6061Al/Sn-xAg joints

Sn-based filler metals are attractive for soldering heat-sensitive aluminum alloys. However, practical application has been limited by the risk of severe galvanic corrosion between Al and Sn. In this study, 6061 Al joints were fabricated using ultrasonic-assisted soldering with Sn-xAg (x = 3.5, 6.5, 10 wt%) filler metals, and the corrosion behaviors were investigated by tailoring the morphology and distribution of interfacial Ag₂Al. The Ag₂Al can evolve from bulk particles to a continuous layer with increasing Ag content, and becomes gradually adherent to the Al substrate with increasing ultrasonic action time. A continuous and layered Ag₂Al fully adhered to Al substrate can be obtained when Ag content exceeds 6.5 wt% and the ultrasonic duration is not less than 10 s. The joints soldered with higher Ag content showed less than 25 % strength deterioration even after 72 h salt spray exposure, while those lower or free-Ag counterparts almost completely failed after 24 h. Numerical simulations combined with electrochemical analysis indicated that the Al/Ag₂Al/Sn interface structure exhibits enhanced corrosion resistance, compared to the Al/Sn interface structure. This is ascribed to the reduction in peak corrosion current density, from 0.62 A/m² (Al/Sn interface) to 7.98 × 10⁻³ A/m² (Al/Ag₂Al interface). The layered Ag₂Al restricted electron reception due to its lower exchange current density, which in turn suppressed the anodic corrosion.