Joining aluminium with other metals is an important task for lightweight construction. Joints of aluminium and copper are interesting for lightweight design of electric conducting applications. As the solidus temperature during the friction stir welding (FSW) process is not exceeded, the amount of formed intermetallic compounds is far smaller compared to fusion welding processes. However, a small intermetallic layer of a few hundred nanometres at the Al-Cu-interface is the key joining mechanism. The joint properties correlate with this layer thickness. The layer thickness follows the laws of inter-diffusion processes with the thermal activation provided by the process heat. This knowledge in combination with dynamic control of the process parameters, e.g. temperature control, leads to reproducible joints of high quality. The generated knowledge seems also be applicable for joints of e.g. aluminium and titanium.