Basic Principles of Spot Welding
Spot welding is a resistance process, relying on the electrical resistance of the components to generate heat when a current is passed through them.
The heat generated is proportional to:
- The square of the current (I2)
- The resistance of the components. (R)
- The time for which the current is passed. (T)
Weld current
Weld currents typically vary from 500A on very thin materials (<0·2mm) up to 30kA for heavy sheet of projection weld applications.
Weld time
The thicker the material, the longer it will take to heat it up to the required temperature. In general, therefore, as the thickness of the component increases, so the weld time needs to increase.
Weld Force
The purpose of the force applied to the welding electrodes is to contain the expansion which occurs when the metal is heated. If this expansion is not contained, metal will be expelled from the weld area, leaving reduced nugget size and the possibility of an incomplete nugget composition, this will adversely affect the weld strength.
As metal thickness increases, the expansion force increases, so the weld force must increase. The thicker the material, the higher the force required.
Metal type will also affect force required
Galvanised metal has a low initial electrical resistance requiring higher currents to generate the heat. Higher currents cause faster expansion, so higher forces are required.
Stainless steels are harder and require more force to create a join at the sheet interface during the weld, so require high forces, typically 50% above those for mild steel.
Spot diameter
The heat generated must be thought of as ‘heat per unit area’. Spot diameter increases with material thickness (too small a spot on sheet of a given thickness which requires a given current to heat it up and a given force will indent too far into the sheet, causing splashing (metal expulsion) and adversely affecting weld strength.