Fraunhofer Institute for Mechanics of Materials IWM
Due to its excellent electrical conductivity and formability, copper is the standard material for electrical wiring; however, it is more expensive and heavier than aluminum. In the field of electromobility, aluminum and copper are therefore used in parallel and must be joined at specific points in a way that ensures both materials compatibility and electrical conductivity. The oxide layers that form spontaneously on aluminum and the intermetallic phases at the joint can cause significant difficulties.
The materials engineering challenges in joining aluminum and copper lie in avoiding undesirable intermetallic phases, which are brittle and promote cracking, as well as in the different coefficients of thermal expansion of the materials, which complicate thermal joining processes. Additionally, the spontaneous oxide layer on aluminum must be overcome. The differences in hardness between aluminum (~40 HV1) and copper (~100 HV1) and the intermetallic phases (>600 HV, up to 1000 HV) lead to complex local stress conditions in the joint zone, which are critical for joint reliability.
Magnetic pulse welding (MPW) is used to address these challenges. It is based on a short, intense electrical pulse that accelerates a flyer material (in this case, aluminum) to a very high velocity (~400 m/s) via a coil and causes it to impact the target material (copper). This high-speed impact generates very high local pressures, enabling a materials-bonded joint without the traditional thermal issues. MPW is classified as a cold joining process and can also join combinations of materials that are normally considered unweldable.
The Fraunhofer institutes IWM and IWU are collaborating on the application-specific configuration of the process. The joining process is adjusted so that the intermetallic phase boundaries remain as narrow and locally confined as possible to avoid brittle fractures and ensure high strength. Joint reliability is evaluated through fractographic analyses.
The microstructure and bonding in the joint zone are examined using high-resolution analyses and model-based simulations to understand local bonding regions and mechanical properties.