What is Resistance Welding?

Resistance welding is one of the oldest of the electric welding processes in use by industry today. It is the most widely used welding method in pressure welding. Although its joint form is subject to certain restrictions, the applicable structure and parts materials are very wide, such as: carbon steel, low alloy steel, stainless steel, aluminum, etc. It can mainly weld aircraft fuselage, car body, bicycle steel ring, safe box, etc.

Process of Resistance welding

The Resistance weld is done by passing an electric current through two pieces of metal pressed together. There are four main methods of resistance welding, spot welding, seam welding, projection welding and butt welding.The pieces coalesce at the surfaces of contact because more resistance and heat are concentrated there. The heat is localized where needed, the action is rapid, no filler metal is needed, the operation requires little skill and can easily be automated, and these advantages make the process suitable for large-quantity production.

Figure  1    Main methods of Resistance Welding

To prevent arcing on the contact surface and to forge the weld metal, pressure should always be applied during welding. When performing resistance welding, the surface of the workpiece to be welded is important to obtain a stable welding quality. Therefore, the contact surface between the electrode and the workpiece must be cleaned before welding.

All the common metals and dissimilar metals can be welded by resistance welding, although special precautions are necessary for some. The parent metal is normally not harmed, and none is lost. Many difficult shapes and sections can be processed.

Figure  2    Spot welding machine

Principle of Resistance Welding

Resistance welding is usually done with AC from the line stepped down through a transformer and applied for a length of  time controlled by a timer. A typical circuit is depicted in Figure 3. The heat generated in a circuit is H=I²RTK, where / is the current in amperes, R the resistance in ohms, T the time of duration of the current flow in seconds and K a conversion factor from KW to the unit of heat desired.

The electrodes that carry the electricity to the work also press the pieces together. The current meets resistance in the metal but more so at the surfaces of contact.’ The most heat is generated where the resistance is highest. Thus the aim is to put the most resistance and heat at the faying surfaces between the workpieces to make a sure weld without harming the electrodes. Under pressure, melting of the metal is not always necessary for coalescence. The heat distribution that occurs in a satisfactory operation is indicated by the diagram of Figure 3.

Figure   3    Principle of Resistance Welding

Advantage of Resistance Welding

(1) When the nugget is formed, it is always surrounded by a plastic ring, the molten metal is isolated from the air, and the metallurgical process is simple.

(2) The heating time is short and the heat is concentrated, so the heat affected zone is small, and the deformation and stress are also small. Usually, it is not necessary to arrange calibration and heat treatment processes after welding.

(3) No filler metals such as welding wires and electrodes, and no welding materials such as oxygen, acetylene, hydrogen, etc. Low welding cost.

Disadvantages of Resistance Welding

(1) Equipment cost is high. Ample work must be ready to justify the investment. Some jobs call for special equipment, such as fixtures, which add appreciably to the investment.  A high order of skill is required to set up and maintain the apparatus.

(2) At present, there is still a lack of reliable non-destructive testing methods, and the welding quality can only be checked by the destructive testing of process samples and workpieces, and by various monitoring techniques.

(3)  The lap joints of spot and seam welding not only increase the weight of the component, but also form an angle around the welding nugget of the two plates, resulting in a lower tensile strength and fatigue strength of the joint.