Abstract: A method of electrically welding a part such as a bolt around a bore of a composite sheet comprising an intermediate layer of a substantially insulating material disposed between two metal layers. Said sheet is pre-heated in the vicinity of said bore in order to remove said substantially insulating material therefrom, after which said part is welded onto said sheet by a conventional resistance spot welding method. Preferably, said pre-heating operation is conducted by passing an electric current between a counter-electrode applied to the sheet from the same side as said part to be welded, in the vicinity of said bore, and an electrode applied on said part to be welded.

Abstract: This welding electrode is one wherein the welding cap 3 of the electrode body 1 carries a contact tip 4 which is securely fastened to it by brazing. Thus, the replacement of this contact tip can be carried out very easily by stopping the circulation of the cooling water in the electrodes, by applying them to a resistive metal sheet and by passing through them a current of sufficient intensity to cause the brazing ring 5 which securely fastens the contact tip 4 to the welding cap 3 to melt. The removal of a worn tip 4 and the replacement of a new tip 4 are carried out in the same manner. The welding electrode also has a contact tip 4 provided with a brazing ring 5, as well as to an automatically operating machine for replacing the tips 4 away from the line for welding the metal sheets T.