Interface structure characterization of Fe36Ni alloy with ultrasonic soldering

The ultrasonic soldering of Fe36Ni alloy using Zn-based filler metal (with and without Si) has been investigated at 420 °C. For the solder without Si, apparent double reaction layers formed at the Fe36Ni and ZnAl interfaces, including the C-Fe₄Zn₉ facing Fe36Ni alloy and Γ₂-Fe₆Ni₅Zn₈₉ upon which numerous cracks had grown into the solder. When 0.4 wt% Si was added to the solder, the bond microstructure was the Zn-Al eutectic phase and g-Zn phase without cracks; only a thin intermetallic compound with constant thickness was formed on the ZnAlSi/Fe36Ni interface even though the process parameters had changed. Joints using ZnAlSi solder reached the highest compressive shear strength of approximately 102-115 MPa with their fracture paths propagating primarily through the intermetallic compound layers at the ZnAlSi/Fe36Ni interface. The mechanism for the effect of Si is explored through a reaction model of the ZnAl(Si)/Fe36Ni system with ultrasonic soldering. This study provides a joining method characterized by low temperature and low stress for low-expansion alloys.