Investigation of accelerated long-term tests of thermal aging of ultrasonically welded copper and aluminum compounds for electric drives

The substitution of copper conductors by aluminum coils in the wheel hub motor provides an innovative approach in terms of light-weighting solutions and a high slot filling ratio. The contacting of aluminum-to-copper is being developed by a solid-state joining process of ultrasonic spot welding (USW). The aim is to prevent the formation of hard and brittle intermetallic phases considerably. In this study, the long-term behavior of USWed Al-Cu electrical contacts is investigated via subjecting to accelerated aging conditions. The development of the interface structure at various temperatures and times is analyzed under the influence of the atmospheric environment with and without electric current application. The characteristics of the constituent phases, i.e., Al4Cu9 (g2), Al2Cu3 (d), Al3Cu4 (z2), AlCu(h2), and Al2Cu (q), present within the interdiffusion layer at the contact interface are studied by scanning electron microscopy with spectrum/point analysis. In order to predict the equivalent field time of Al-Cu contacts in-service conditions, the activation energy of the intermetallic phases is calculated to employ in the Arrhenius model. In addition, the effect of thermal aging on the electrical and mechanical properties is examined for the aging conditions in a furnace and by direct current.