The use of metal-plastic composites (MPCs) plays an important role for lightweight products. MPCs provide an innovative substitute for solid metal sheets due to their low weight, excellent stiffness, and due to their thermal and acoustic insulation. A challenge that links all the different applications is the need to efficiently process the MPCs at high quality and join them with other materials, using suitable joining techniques. Due to their special features (layer structure, material mix, etc.), conventional manufacturing processes are therefore only of limited or of no use at all. In particular, the polymer core layer is a barrier for the application 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 energy of ultrasonic waves, and displacement of the molten plastic material by pressure on the cover sheets. Two different strategies are investigated for this purpose. In the first approach, the joining tool is directly superimposed by ultrasonic waves and the polymer is displaced before and during joining. In the second approach, the polymer layer is displaced in the joining area during a prior sheet forming process step. This paper describes the simulation of the ultrasonic-assisted displacement of the polymeric layer. These simulations are validated using experimental results. An exact analysis of the process parameters and infrared images are used for the validation of the simulation results. The results lead to an optimal design of special ultrasonic tools, which enable clinching and resistance spot welding in one step or allow pressing in a forming tool.