Defining resistance spot welding:

• RESISTANCE SPOT WELDING (RSW) is a process in which faying surfaces are joined in one or more spots by the heat generated by resistance to the flow of electric current through work pieces that are held together under force by electrodes.

• The contacting surfaces in the region of current concentration are heated by a short-time pulse of low-voltage, high amperage current to form a fused nugget of weld metal.

• When the flow of current ceases, the electrode force is maintained while the weld metal rapidly cools and solidifies.

• The electrodes are retracted after each weld, which usually is completed in a fraction of a second.

Many assemblies of two or more sheet metal stampings that do not require gas-tight or liquid-tight joints can be more economically joined by high-speed RSW than by mechanical methods. Containers frequently are spot welded. The attachment of braces, brackets, pads, or clips to formed sheet-metal parts such as cases, covers, bases, or trays is another common application of RSW.

Major advantages of resistance spot welding:

• Major advantages of spot welding include high operating speeds and suitability for automation or robotization and inclusion in high-production assembly lines together with other fabricating operations.
• With automatic control of current, timing, and electrode force, sound spot welds can be produced consistently at high production rates and low unit labor costs using semiskilled operators.

Most metals can be resistance spot welded if the appropriate equipment is used coupled with suitable welding conditions.

This is particularly true for thin sheet or strip steel products, whether uncoated or coated.

Equipment

The equipment needed for RSW can be simple and inexpensive or complex and costly, depending on the degree of automation. Spot welding machines are composed of three principal elements:

• electrical circuit, which consists of a welding transformer, tap switch, and a secondary circuit
• control circuit, which initiates and times the duration of current flow and regulates the welding current
• mechanical system, which consists of the frame, fixtures, and other devices that hold and clamp the workpiece and apply the welding force

Specifications for resistance welding equipment have been standardized by the Resistance Welder Manufacturers Association (RWMA), and specifications for controls are issued by the National Electrical Manufacturers Association (NEMA).