Resistance spot welding (RSW) is a process that joins two or more metal sheets together by passing a high electrical current through them at a single point while clamping them under pressure. The heat generated at that point melts a small area of metal, forming a solid joint called a weld nugget. It’s the dominant method for joining sheet metal in manufacturing, and a typical car body contains 4,000 to 6,000 individual spot welds holding it together.
How the Process Generates Heat
The physics behind spot welding comes down to a simple principle: when electrical current flows through a material that resists it, that resistance generates heat. The governing equation is Q = I²Rt, where Q is the heat produced, I is the welding current, R is the electrical resistance at the joint, and t is the duration of current flow. Because the current is squared in that formula, even small increases in current produce significantly more heat.
What makes this useful is that the highest resistance in the setup sits right where the two metal sheets meet. That interface heats up faster than the surrounding material, concentrating a large amount of energy into a tiny area in a very short time, typically 10 to 100 milliseconds. The result is a localized melt zone that fuses the sheets together without overheating the rest of the workpiece.
The Four Stages of a Weld Cycle
Every spot weld follows a timed sequence of four phases, each measured in cycles (one cycle equals 1/60 of a second on a standard 60 Hz power supply).
- Squeeze time: The electrodes close on the metal sheets and build up clamping force before any current flows. This ensures consistent contact and prevents arcing. Squeeze time typically ranges from 25 to 90 cycles depending on the equipment.
- Weld time: Current passes through the clamped sheets, generating heat and forming the molten nugget. For steel, weld time is roughly 1/5 of a second.
- Hold time: Current stops, but the electrodes stay clamped. This lets the molten metal solidify under pressure. Because the copper electrodes are water-cooled, holding too long can pull heat out of the nugget too quickly, acting like a quench and potentially making the joint brittle.
- Off time: The electrodes retract and the workpiece advances to the next weld location.
Getting the balance between these four stages right is the core challenge of setting up a spot welding operation. The three primary variables an operator controls are welding current (measured in kiloamperes), electrode force (measured in pounds or kilonewtons), and time for each phase.
Welding Current and Electrode Force
Spot welding uses far more current than arc welding processes. Typical values fall in the range of 8 to 15 kiloamperes, thousands of times more than a household outlet delivers. This enormous current is what allows the weld to form in a fraction of a second.
Electrode force serves two purposes. It ensures tight contact between the sheets so current flows predictably, and it forges the molten nugget as it solidifies, compressing out porosity and improving joint strength. Too little force allows molten metal to splash out from between the sheets. Too much force spreads the contact area, reducing current density and producing a weak, undersized nugget.
Which Metals Work Best
Low-carbon and mild steel are the easiest metals to spot weld. They have relatively high electrical resistance (which helps generate heat at the joint) and low thermal conductivity (which keeps that heat concentrated where it’s needed). Stainless steel, low-alloy steel, nickel, and nickel alloys like Monel and Inconel are also regularly spot welded, though each requires its own parameter adjustments.
Aluminum and other highly conductive metals are more difficult. Because aluminum conducts electricity and heat so efficiently, it takes much more current to build enough heat at the joint. Machines rated well above 20 KVA are needed to produce sound welds in aluminum. The same challenge applies to copper, magnesium, and brass.
Joining two dissimilar metals, like copper and steel, introduces an additional problem: one side heats up much faster than the other. Welders compensate by using a smaller electrode tip on the more conductive side, which concentrates the current and evens out the heat distribution across the joint.
Thickness matters too. Most spot welding covers relatively thin sheet metal. For uncoated mild steel, combined thickness of the two sheets can reach up to about 1/4 inch on a standard 20 KVA machine. Stainless steel tops out around 0.300 inches for the thinnest outside piece, while magnesium alloys are limited to about 0.128 inches.
Electrode Materials and Wear
The electrodes that deliver current and clamping force are made from copper alloys, not pure copper. The Resistance Welding Manufacturers Association (RWMA) classifies these into groups and classes. The two most common are Class 1 and Class 2 from Group A.
Class 1 electrodes are nearly pure copper (99% copper, 1% cadmium) with the highest electrical conductivity at 92% of the International Annealed Copper Standard. They’re the go-to choice for welding aluminum, magnesium, brass, and bronze, where minimizing heat buildup in the electrode is critical. Class 2 electrodes (99.2% copper, 0.8% chromium) sacrifice a small amount of conductivity for greater strength and durability, making them the general-purpose option for welding steel and most other materials.
A useful rule of thumb: as the RWMA class number goes up, electrode strength increases but conductivity decreases. Lower conductivity means the electrode itself heats up more during welding, which accelerates tip wear. Selecting the right class is a balance between the strength needed to withstand the clamping force and the conductivity needed to avoid premature degradation.
What a Good Weld Looks Like
The quality of a spot weld is judged primarily by the diameter of the nugget it produces. In the automotive industry, the standard target is a nugget diameter of 4.25 times the square root of the sheet thickness. A nugget that meets this minimum ensures reliable joint strength under the loads and impacts a car body experiences. Welds are tested and qualified against the AWS D8.9 standard, which specifies the mechanical testing methods for evaluating spot weld performance in automotive sheet steel.
Common Defects and Their Causes
The most frequent problem in spot welding is expulsion, where molten metal squirts out from between the sheets during the weld. This happens when the welding schedule is set too aggressively, particularly when current is pushed close to the maximum to produce larger nuggets and faster production cycles. Expulsion causes thinning of the joint and voids inside the nugget, both of which significantly weaken the weld and reduce crash performance in automotive applications.
On high-volume production lines, conditions constantly shift. Electrode tips gradually mushroom from wear, changing the contact area. Parts don’t always fit together perfectly, creating gaps. Surface contamination from oils, coatings, or oxide layers alters resistance at the joint. All of these variables push the process closer to its limits. A welding schedule that produces perfect results with fresh electrodes and clean parts can start producing expulsion or cold welds (insufficient fusion) as conditions drift. Deeper surface indentation on the finished part is a visible indicator that expulsion occurred, with more severe expulsion creating deeper marks.
AC vs. MFDC Power Supplies
Traditional spot welding machines use single-phase alternating current (AC). Newer systems use medium-frequency direct current (MFDC) inverter technology, and the difference in performance is measurable. MFDC systems generally produce larger weld nuggets than AC systems when both use the same effective welding current. They also consume less energy per weld, and this efficiency gap widens as current increases. For high-volume production where thousands of welds are made per shift, the energy savings and improved consistency of MFDC have made it the preferred technology in modern automotive plants.
Where Spot Welding Is Used
Automotive manufacturing is by far the largest application. Those 4,000 to 6,000 spot welds per car body are placed by robotic welding cells that can complete each weld in under a second and reposition for the next one almost instantly. Beyond automotive, spot welding is used in appliance manufacturing, aerospace sheet metal work, electronics enclosures, and HVAC ductwork. Anywhere two overlapping sheets of metal need a fast, strong, repeatable joint without filler material or shielding gas, spot welding is a natural fit.