welding in a third step in which the welding itself takes place for this purpose a pressure force is applied axially to the closure element 7 by means of the sonotrode 28 and the sonotrode 28 is simultaneously excited at high frequency by means of a torsional welding plant which is not illustrated in any more detail in this way the material at the lower edge region of the closure element 7 connects to the material on the abutment shoulder 3 of the pump housing 10 the material which fuses on or melts on in the process is indicated in fig7 and fig9 by reference numeral 30 at the same time the negative form of the sonotrode 28 is impressed on the top side of the closure element 7 because said sonotrode is in particular of corrugated form on its contact surface facing the closure element 7 fig1 shows a variant of the connection of the closure element 7 to the pump housing 10 and the cylindrical bore 12 thereof the figure shows the chamfer 2 at the top edge of the cylindrical bore 12 and a step 32 which is formed on the closure element 7 and which projects into the cylinder bore 12 and in so doing covers the chamfer 2 in the axial direction of the bore 12 with said step 32 on the closure element 7 it is achieved that chips formed at the.