Joining light metals such as aluminum alloys or magnesium alloys is complicated by the presence of rapidly forming oxide layers as well as their inherent high electrical and thermal conductivity. Spot welding of both of these alloy types is typically difficult and requires preparation of the workpiece surface, high currents, high forces, and mechanically well-aligned and stable electrodes. Short electrode lives are common because of reaction between the workpiece alloy and copper electrode. The resultant weld can have high strength, but poor high cycle fatigue performance under some conditions, particularly for magnesium alloys.
An alternative to obtain the large surface area, strong, stiff joint that is formed with this invention is structural adhesive bonding. This joining technique requires the use of an expensive adhesive along with specially coated substrates to prevent environmental degradation of joints. Adhesives are designed to cure in ovens used for curing paint, however, a fixturing method such as riveting or spot welding must be used in conjunction with the adhesive to temporarily secure the structure prior to adhesive cure as well as provide added structural performance after cure. These techniques require additional joining equipment. For riveting, a large inventory of expensive rivets is needed to join the various gauge combinations encountered in a structure. For spot welding, the coating and adhesive must be compatible with the welding process.
There remains a need for a process for forming large-area welds for aluminum alloy workpieces or magnesium alloy workpieces. Such a welding process could be of use, for example, in joining lightweight body panels or body structure components in making automotive vehicles. Such a welding process might also have utility in joining a galvanized steel component or aluminum-coated steel component to an aluminum or magnesium component.