chapter  10
15 Pages

Electric resistance welding

Resistance welding is a joining process belonging to the pressure welding sector. With its locally applied heat and pressure it has an obvious relationship with the forge welding technique practised by blacksmiths when joining metal. The resistance welding process was invented in 1877 by Professor E. Thomson of Philadelphia, USA, when an accidental short circuit gave him the idea for what was originally termed short-circuit welding. From the beginning of the twentieth century it was used on a small scale in industry, but it was only after the Second World War that resistance spot welding had its real beginning in the automobile industry. It has since grown to be the most important method of welding used in the construction and mass production of vehicle bodies. Resistance welding is extensively used for the

mass production assembly of the all-steel body and its component sheet metal parts. Its wide adoption has been brought about by its technical advantages and the reductions in cost. Most mass produced car bodies are assembled entirely by welding steel pressings together to produce an integral rigid chassis and body structure. Low-carbon steel thicknesses used in this unitary construction range from 0.8-1 mm for skin or floor panels to 3 mm for major structural pressings such as suspension brackets. Intermediate gauges such as 1.2 mm are used for hinge reinforcements, 1.6 mm for chassis structural members, and 1.8-2.5 mm for suspension and steering members. With the introduction of high-strength steels (HSS steels), car manufacturers are producing body panels as thin as 0.55 mm, and structural members with gauges of between 1.2 and

2 mm. This reduction in thickness can be made without loss of strength. There are a number of resistance welding pro-

cesses. Resistance spot welding is the most widely used welding process in car body construction; there are approximately 4500-6000 spot welds per body, which accounts for approximately 80 per cent of the welding used. A further 10 per cent is composed of other resistance welding processes: seam, projection, flash and butt welding. The remaining 10 per cent is divided between MIG/MAG welding and gas welding. Of themanywelding techniques used in the mass production of car bodies, resistance welding dominates the field. The fundamental principle upon which all resis-

tance welding is based lies in the fact that the weld is produced by the heat obtained from the resistance to flow of electric current through two or more pieces of metal held together under pressure by electrodes which are made from copper or copper alloys. The engineering definition of heat (heat being the essence of all welding) is energy time. This indicates a balance between energy input and weld time; therefore the faster the welding, the greater the clamping force. However, the engineering definition of resistance is such that the higher the clamping force, the greater the current needed to produce a constant heat. Heat is generated by the resistance of the parts to be joined to the passage of a heavy electrical current. This heat at the junction of the two parts changes the metal to a plastic state. When the correct amount of pressure is then applied fusion takes place. There is a close similarity in the construction of all

resistance welding machines, irrespective of design and cost. The main difference is in the type of jaws or electrodes which hold the object to be welded.