The electrification of the traffic sector increases the importance of the joining of aluminum with copper. During the fusion welding of this joint, brittle intermetallic phases which exert a great influence on the load-bearing capability are developing. Until today, systematic investigations of the different, developing phases neither have been carried out nor has a direct correlation with the mech. properties been made. Within the scope of this paper, electron beam welding on Al-Cu plates is examined The focus is put on the identification of the different phases and on their influence on the tensile strength. To this end, the mixture ratio and the energy-per-unit length are varied within the framework of test series. As a result, the microstructure of the welded joint can be characterized via electron backscatter diffraction (EBSD) tests. Further, a direct correlation of these tests with the failure of the welded joint will be made. The greatest weld joint strength of 104 MPa was achieved when the microstructure of the joint featured a lamellar structure, whereas formation of the Al2Cu phase resulted in brittle fracture of the welded joints along the Al2Cu/lamellar matrix interfaces.