Due to the growing environmental concern and the increasing demand for more ecological methods of transportation, composite materials that combine low weight and high resistance have raised the interest of several industrial sectors. Among these materials, the metal-plastic composites (MPCs) can be highlighted, who combine a low weight and a relatively high mechanical resistance. Such materials are used in advertising signs, building panels, motor shielding, etc. Mechanical joining of MPC materials is to be avoided when protrusions are not allowed or because of aesthetical reasons. Welding is therefore considered as an interesting alternative. However, qualitative joining using welding of the MPC materials to other MPC materials or to metallic plates is a real challenge. Due to their special features (layer structure, material mix, etc.), conventional manufacturing processes are therefore only of limited or no use at all. In particular, the polymer core layer is a barrier for the use of conventional joining methods. This contribution presents a novel joining approach for MPCs. The basic approach is the local melting of the polymer layer by ultrasonic waves and displacement of the molten plastic material by pressure on the cover sheets. This work proposes the investigation of the use of non-conventional solid-state welding processes, such as ultrasonic welding (USW) and refill friction stir spot welding (RFSSW), for joining MPCs with aluminum sheets. Prior to the application of the joining processes, the intermediate plastic core of the MPC materials is displaced using ultrasonic vibrations, so that the materials can be joined as monolithical materials. These joining concepts are validated experimentally. The obtained weld quality is assessed based on destructive and non-destructive testing methods.