Only with hybrid construction it is possible to combine the advantages of dissimilar materials. Force-fitted or form-closed joints can be realized in an easy manner, but the strength of welded joints can be achieved rarely. However fusion welding of dissimilar materials is often impossible as a result of the different melting ranges. Moreover in many cases unrequested mixed phases occur in the weld area, whereby the attainable strength of the joints will be reduced. The aim of the submitted research project is to join Al-cast- and Mg-cast alloys by friction stirwelding (FSW), because both material groups are well-established in industrial applications, but there is no possibility to realize joints between them by fusion welding. FSW enables a joining by plastic deformation. To eliminate strength reducing effects by occurring brittle phases and to improve the stirring in the joining area additionally power ultrasound should be transferred into the welded area. An estimation of the quality of the produced welds will be carried out by monotonic and cyclic as well as non-destructive investigations. Furthermore detailed analysis and thermodynamical calculations will be carried out to understand the development of the microstructure in the bonding zone. To determine the complex problems of corrosion of the hybrid material joints detailed examination of the corrosion behavior especially of the welding area are planned.