<P> The spot welding process tends to harden the material, causing it to warp . This reduces the material's fatigue strength, and may stretch the material as well as anneal it . The physical effects of spot welding include internal cracking, surface cracks and a bad appearance . The chemical properties affected include the metal's internal resistance and its corrosive properties . </P> <P> The basic spot welder consists of a power supply, an energy storage unit (e.g., a capacitor bank), a switch, a welding transformer, and the welding electrodes . The energy storage element allows the welder to deliver high instantaneous power levels . If the power demands are not high, then the energy storage element isn't needed . The switch causes the stored energy to be dumped into the welding transformer . The welding transformer steps down the voltage and steps up the current . An important feature of the transformer is it reduces the current level that the switch must handle . The welding electrodes are part of the transformer's secondary circuit . There is also a control box that manages the switch and may monitor the welding electrode voltage or current . </P> <P> The resistance presented to the welder is complicated . There is the resistance of secondary winding, the cables, and the welding electrodes . There is also the contact resistance between the welding electrodes and the workpiece . There is the resistance of the workpieces, and the contact resistance between the workpieces . </P> <P> At the beginning of the weld, the contact resistances are usually high, so most of the initial energy will be dissipated there . That heat and the clamping force will soften and smooth out the material at the electrode - material interface and make better contact (that is, lower the contact resistance). Consequently, more electrical energy will go into the workpiece and the junction resistance of the two workpieces . As electrical energy is delivered to the weld and causes the temperature to rise, the electrodes and the workpiece are conducting that heat away . The goal is to apply enough energy so that a portion of material within the spot melts without having the entire spot melt . The perimeter of the spot will conduct away a lot of heat and keep the perimeter at a lower temperature . The interior of the spot has less heat conducted away, so it melts first . If the welding current is applied too long, the entire spot melts, the material runs out or otherwise fails, and the "weld" becomes a hole . </P>

Two sheets of different material but same thickness can be spot welded by