Abstract: A spot welding electrode is made of a copper base material such as copper, copper alloy, or alumina dispersion strengthened copper, and has a Sn coating layer formed at the part making contact with the material being welded. The formed Sn coating layer reduces electrode wear and prolongs electrode life.

Abstract: A process of fixing a double-walled tubing structure to a titanium tube sheet, the tubing structure comprising a non-titanium outer tube and a titanium inner lining tube, comprising the steps of: pressure-welding a titanium collar to an end face of the outer tube to form an integral titanium end portion at one end of the outer tube; inserting the titanium lining tube in the outer tube and fixing the titanium lining tube to the outer tube in pressed contact with the inner surface of the outer tube to form the titanium-lined double-walled tubing structure; and inserting an end section of the tubing structure in a hole in the tube sheet, and welding the titanium end portion of the outer tube and an end portion of the lining tube to the tube sheet. Instead of the titanium collar being directly welded to the outer tube, an auxiliary collar made of a metal material for sound welding to the outer tube may be pressure-welded at its one end to the titanium collar, and fusion-welded at the other end to the outer tube.