Resistance welding is one of the oldest of the electric welding processes in use by industry today. The weld is made by a combination of heat, pressure and time. As the name resistance welding implies, it is the resistance of the material to welded, to current flow that causes a localized heating in the part.
The pressure exerted by the tongs and electrode tips, through which the current flows, holds the parts to be welded in intimate contact before, during and after the welding current time cycle. The required amount of time current flows in the joint is determined by material thickness and type, amount of time current flowing and the cross-sectional are of the welding tip contact surfaces.
Fundamentals of Resistance Welding
Resistance spot welding is accomplished when current is caused to flow through the electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electric current flow causes localized heating in the joint and the weld is made.
The resistance spot weld nugget is formed internally with relation to the surface of the base metal. The below illustration shows a resistance spot weld nugget compared to a gas tungsten – arc spot weld.
The gas tungsten – arc spot is made from one side only. The resistance spot weld is normally made with the electrodes on each side of the workpiece. Resistance spot welds may be made with the workpiece in any position.
The resistence spot weld nugget is formed when the interface of the weld joint is heated due to the resistance of the joint surfaces to electric current flow. In all cases, of course, the current flow MUST flow or the weld can not be made. The pressure of the electrode tips on the work piece holds the part in close and intimate contact during the making of the weld. Remember however, that resistance spot welding machines are NOT designed as force clamps to pull the workpieces together for welding.
1 – Heat Generation
A modification of Ohm’s Law may be made when watts and heat are considered synonymous. When current is passed through a conductor the electrical resistance of the conductor to current flow will cause heat to be generated. The basic formula for heat generation may be stated:
H = I R where:
H= HEAT, I2 = WELDING CURRENT SQUARED, and R= RESISTANCE
The secondary portion of a resistance spot welding circuit, including parts to be welded, is actually a series of resistances. The total additive value of this electrical resistance affects the current output of the resistance spot welding machine and the heat generation of the circuit.
The key fact is, although current value is in the same in all parts of the electrical circuit, the resistance values may vary considerably at different points in the circuit.
The resistance spot welding machines manufactured by Miller Electric Mfg. Co. are constructed so minimum resistance will be apparent in the transformer, flexible cables, tongs and electrode tips. Miller resistance spot welding machines are designed to bring the welding current to the weldment in the most efficient manner. It is at the weldment that the greatest relative resistance is required. The term “relative” means with relation to the rest of the actual welding circuit.
There are six major points of resistance in the work area. They are:
- The contact point between the electrode and top workpiece.
- The top workpiece.
- The interface of the top and bottom workpieces.
- The bottom workpiec.e
- The contact between the bottom workpiece and the electrode.
- Resistance of electrode tips.
2 – Time
Resistance spot welding depends on the resistance of the base metal and the amount of current flowing to produce the heat necessary to make the spot weld. Another important facto is time. In most cases several thousands of amperes are used in making the spot weld. Such amperage values flowing through a relatively high resistance will create a lot of heat in a short time. To make good resistance spot welds it is necessary t have close control of the time the current is flowing.
3 – Pressure
The effect of pressure on the resistance spot weld should be carefully considered. The primary purpose of pressure is to hold the parts to be welded in intimate contact at the joint interface. This action assures consistent electrical resistance and conductivity at the point of the weld. The tong and electrode tips should NOT be used to pull the workpiece together. The resistance spot welding machine is not designed as an electrical “C” clamp. The parts to be welded should be intimate BEFORE pressure is applied.
4 – Electrode Tips
Copper is the base material normally use for resistance spot welding tips and tongs. The purpose of the electrode tips are to conduct the welding current to the workpiece, to be the focal point of the pressure applied to the weld joint, to conduct heat from the work surface and to maintain their integrity of shape and characteristics of thermal and electrical conductivity under working conditions.
Information courtesy of Miller Electric