Resistance welding processes
Resistance welding is a fusion welding process that requires the application of both heat and pressure to achieve a sound joint. The simplest form of the process is spot welding where the pressure is provided by clamping two or more overlapping sheets between two electrodes (Fig. 9.1).A current is then passed between the electrodes, sufficient heat being generated at the interface by resistance to the flow of the current that melting occurs, a weld nugget is formed and an autogenous fusion weld is made between the plates. The heat generated depends upon the current, the time the current is passed and the resistance at the interface. The resistance is a function of the resistivity and surface condition of the parent material, the size, shape and material of the electrodes and the pressure applied by the electrodes.
There are a number of variants of the resistance welding process including spot, seam, projection and butt welding. It is an economical process ideally suited to producing large numbers of joints on a mass production basis. Spot welding in particular has been used extensively in the automotive industry, albeit mostly for the joining of steel and in the aerospace industry for airframe components in aluminium alloys. Seam welding is used in the production of thin sheet, leak-tight containers such as fuel tanks. Projection welding is generally used for welding items such as captive nuts onto plate. This variation is not normally used on aluminium and is not covered in this chapter. Flash welding, unlike spot and seam welding that require a lap joint, is capable of making butt welds. This is achieved by resistance heating the abutting faces and then forging them together.
There are a couple of characteristics of aluminium that make it more difficult to resistance weld than steel. The most significant is its high electrical conductivity, requiring high welding currents and large capacity equipment. Secondly, the electrodes are made from copper which alloys with aluminium, resulting in rapid wear and a short electrode life.
9.1 Principles of spot welding process.
As with conventional fusion welding, resistance welding suffers from similar problems of oxide entrapment and hot cracking, the latter not being helped by the lack of a more crack-resistant filler metal, and porosity.