A spot welder joins overlapping metal sheets by squeezing them between copper electrodes and sending a large electric current through a small contact area. The electrical resistance of the metal turns electrical energy into heat, so the sheets melt locally and form a small fused region called a weld nugget. Spot welding matters because it is fast, repeatable, and widely used in cars, appliances, battery tabs, and sheet metal fabrication.
It is a practical example of electricity, heat transfer, force, and materials science working together in one machine.
The basic cycle is squeeze, current, hold, and release. First, the electrodes press the sheets together to reduce gaps and keep the joint aligned. Then a short pulse of high current creates heat mainly at the interface where the two sheets touch, and the hold time allows the molten nugget to solidify under pressure.
Good welds require the right balance of current, time, electrode force, sheet thickness, material conductivity, and clean contact surfaces.
Key Facts
- Joule heating in the joint is given by Q = I^2Rt, where Q is heat energy, I is current, R is electrical resistance, and t is time.
- Spot welding uses low voltage and very high current, often thousands of amperes, to heat a small region safely and quickly.
- Copper electrodes conduct current well and carry heat away, which helps focus heating inside the overlapped sheets.
- The welding cycle is squeeze time, weld time, hold time, and off time.
- Electrode force affects contact resistance: too little force can cause arcing, while too much force can reduce heating and make a weak weld.
- A strong spot weld forms a weld nugget large enough to hold the sheets without excessive surface burning or deep electrode marks.
Vocabulary
- Resistance spot welding
- A welding process that joins overlapping metal sheets by passing high current through them while they are clamped between electrodes.
- Weld nugget
- The small fused region of metal that forms between the sheets during a spot weld.
- Electrode
- A conductive part, usually made of copper alloy, that presses on the workpieces and delivers welding current.
- Contact resistance
- The electrical resistance at the surfaces where materials touch, which helps determine where heat is produced.
- Hold time
- The time after current stops when the electrodes continue pressing the joint so the molten metal solidifies properly.
Common Mistakes to Avoid
- Using dirty or painted metal surfaces, which is wrong because coatings and dirt change contact resistance, create fumes, and can prevent a reliable weld nugget from forming.
- Increasing current without adjusting time or force, which is wrong because excessive heat can burn through the sheet, cause spatter, or damage the electrode tips.
- Assuming higher electrode force always makes a stronger weld, which is wrong because too much force lowers contact resistance and can reduce the heat needed to fuse the joint.
- Touching electrodes or workpieces immediately after welding, which is wrong because the metal can remain hot enough to cause burns even after the glowing spot disappears.
Practice Questions
- 1 A spot welder sends 6000 A through a joint with a resistance of 0.00008 ohm for 0.20 s. Using Q = I^2Rt, how much heat energy is produced?
- 2 A welding transformer delivers 2.5 V at 8000 A during a weld pulse lasting 0.10 s. Using P = IV and E = Pt, find the power during the pulse and the energy delivered.
- 3 Two welds use the same metal sheets and current, but one has dirty, oxidized surfaces and the other has clean surfaces. Explain how the surface condition can change heating, weld quality, and safety.