Improving mechanical stability of Al/Cu ultrasonic bonded joint for battery tab by adopting electroplated Ni interlayer

The stability of the Al/Cu battery tab joint in the harsh environment has become a serious issue in the field of electric vehicles. The Al/Cu ultrasonic welding (USW) joint can easily degrade under thermal aging conditions. The growth of massive intermetallic compound (IMC) layers and the formation of Kirkendall void at the Al/Cu interface were the main causes of joint degradation after the thermal aging. To address these issues, a Ni interlayer was adopted to suppress the growth of IMCs in the Al/Cu USW joint. Comparative investigation revealed that, compared to the as-bonded lap-shear strength, the strength of joints with a Ni interlayer remained relatively stable after aging for 1000 h, whereas the strength of Al/Cu joints decreased over time. Without a Ni interlayer, the growth of IMCs occurred, leading to the formation of CuAl₂, Cu₉Al₄, and CuAl, ultimately resulting in Kirkendall voids and micro voids. The combination of stress concentration at the Cu/Cu₉Al₄ interface and void formation at the Al/Cu joint facilitated crack propagation along the IMC layer, causing brittle fracture and decreased joint strength. Conversely, the presence of a Ni interlayer prevented the formation of clear IMCs. By suppressing excessive IMC growth and void formation at the interface, the Ni interlayer significantly improves the mechanical stability of USW joints between Al and Cu battery tab interconnections.