Resistance Spot Welding Gun Product

Overview

Resistance spot welding (RSW) joins two overlapping sheet metal parts (0.5–4 mm typical) in under 1 second by passing high current through [[spot-welding-gun-electrode-caps|copper electrodes]] pressed together. The electrical resistance between the overlapping surfaces generates intense Joule heat, melting a small circular nugget of metal. The [[spot-welding-gun-actuator|pneumatic squeeze force]] holds the parts during solidification, creating a fusion bond.

A typical automotive body assembly uses 200–500 spot welds per car, applied by handheld guns or robotic arms. No filler metal, no skill, and nearly 100% success rate with proper setup make spot welding the standard for high-volume sheet fabrication.

The [[spot-welding-gun|handheld gun]] shown here is operator-paced; single-phase transformer power source (not detailed) provides secondary current. Industrial systems often use three-phase welders with primary taps for current range selection.

How it works

Electrode Positioning: The operator squeezes the gun trigger, energizing the [[spot-welding-gun-solenoid-valve|solenoid air valve]]. Compressed air flows into the [[spot-welding-gun-pneumatic-cylinder|actuating cylinder]], pushing the upper electrode arm downward. A [[spot-welding-gun-flow-control-valve|speed valve]] meters the descent rate to ~50 mm/s, ensuring gentle electrode seating on the workpiece.

The lower electrode arm rests on the work. Both electrodes mate, clamping the overlapping sheet zone between them.

Squeeze Phase (0–500 ms): During the initial squeeze phase (adjustable 100–300 ms), the [[spot-welding-gun-actuator|pneumatic force]] (typically 2–4 kN) seats the electrodes on the sheet, establishing metal-to-metal contact. No welding current flows yet. This phase allows any surface irregularities to collapse and ensures consistent contact pressure.

Weld Phase (10–1000 ms): At the end of squeeze time, the [[spot-welding-gun-timer-control|weld timer]] energizes the [[spot-welding-gun-contactor|three-phase contactor]], applying primary voltage to the transformer. Secondary current (5–50 kA) flows between the electrodes through the sheet stack.

Electrical resistance causes rapid heating:

$$P = I^2 R = (20,000 ext{ A})^2 imes (0.0001 , Omega) = 40 ext{ kW}$$

The hottest zone is at the sheet-to-sheet interface (faying surface), where resistance is highest. Temperature reaches 1500 °C within 200–500 ms, melting the metal. The upper and lower electrode tips (maintained at ~200 °C by [[spot-welding-gun-cooling-jacket|water cooling]]) remain solid, ensuring localized melting.

Hold Phase (0–1000 ms): When the weld timer expires, the contactor de-energizes, stopping current. But the [[spot-welding-gun-actuator|electrode force]] remains applied for an additional hold period (adjustable 200–500 ms). During hold, the molten nugget solidifies under compressive force, consolidating the weld and preventing shrinkage voids.

Release: After hold time, the solenoid valve de-energizes and air exhausts from the cylinder. The upper electrode retracts, and the operator removes the gun. The workpiece is now permanently bonded at that spot.

Nugget Formation and Mechanics

The weld nugget is a metallic button 3–8 mm diameter, 0.5–2 mm thick, completely fused to both sheets. Strength comes from:

1. Solid-state fusion: Sheets melt and re-solidify together, creating grain continuity.
2. Forging action: Compressive force during hold squeezes out oxides and voids.
3. Rapid cooling: The massive copper electrodes act as heat sinks, quenching the nugget and producing fine-grained, ductile metal.

With proper parameters, the nugget is as strong as the base metal. Failure under tensile load occurs in the base metal outside the nugget, not in the weld itself.

Electrode Wear and Maintenance

Copper electrodes are consumables. As current passes, the electrode tips erode slightly:

1. Alloying: The electrode surface reacts with the base metal, forming Cu–Fe compounds. These increase hardness and reduce current conduction.
2. Mushrooming: Thermal cycling causes the electrode face to bulge outward. Over-sized electrodes produce larger, weaker nuggets.
3. Pitting and cracking: Repeated heating and cooling stress the electrode, eventually causing surface cracks.

Expected electrode life: 1000–5000 welds, depending on current and cooling water quality. When electrode mushroom exceeds 1 mm diameter growth, they must be replaced or "dressed" (machined back to original mushroom shape).

Stainless and aluminum are harder on electrodes than mild steel. Aluminum welds require platinum-faced electrodes or special copper alloys to reduce alloying.

Squeeze, Weld, and Hold Timing

Squeeze (0–500 ms): Longer squeeze times ensure electrode seating on wavy or corroded sheet. Too short and the weld is weak (poor contact). Typical 100–300 ms.

Weld (10–1000 ms): Larger or thicker material requires longer weld times to build sufficient nugget depth. A rule of thumb: 1 second per 0.5 mm sheet thickness. Typical 300–600 ms for automotive (0.8 mm galvanized). Over-welding (excessive time) wastes energy and electrode life.

Hold (0–1000 ms): Cooling time prevents weld cracking, especially in high-carbon steels. Hold times match or exceed weld times. Typical 300–500 ms.

Total cycle time: Sum of squeeze + weld + hold. A single spot might take 0.8–2 seconds. A robot can apply 20–40 spots/minute on a complex assembly.

Current and Voltage Selection

Secondary current is selected based on material type and thickness:

Material	Thickness	Current	Voltage
Mild steel	0.8 mm	8–12 kA	3–4 V
Mild steel	1.5 mm	12–18 kA	4–6 V
Galvanized	0.8 mm	10–14 kA	3–5 V
Stainless 304	1.0 mm	14–20 kA	4–7 V
Aluminum	1.0 mm	12–20 kA	2–3 V

Aluminum weals at much lower voltage due to its high conductivity. Stainless and galvanized demand higher current because the oxide layer (Cr₂O₃, ZnO) raises contact resistance.

Transformers provide a fixed voltage at a given tap. Manual systems use tap selection; modern systems use a [[spot-welding-gun-contactor|solid-state controller]] to regulate current feedback.

Water Cooling Necessity

Electrodes dissipate ~10 kW during a 500 ms weld. Without cooling, the electrode face would soften and deform in 5–10 welds. A [[spot-welding-gun-cooling-jacket|continuous cooling loop]] at 15–30 L/min keeps electrodes at 100–150 °C, maintaining hardness.

Water must be clean (filtered to 20 micron) and treated (biocide and corrosion inhibitor) to prevent:

• Scale buildup (blocks cooling passages)
• Rust (electrode cap corrosion from internal scale flaking)
• Algae (clogs filters)

Coolant change every 3–6 months ensures electrode longevity. Neglect leads to electrode failure and weld quality collapse.

Pneumatic vs. Servo Force Control

Pneumatic: Simple, reliable, adjustable via pressure regulator. A 100 mm bore cylinder at 6 bar produces ~3 kN force. Drawback: pressure varies with ambient temperature and supply fluctuation.

Servo: Proportional or stepper-driven actuator providing precise, repeatable force. More expensive but enables dynamic force programming (start soft, ramp hard mid-weld) for better nugget formation on varying material.

Most job shops use pneumatic; automotive OEMs use servo for statistical process control and Six Sigma quality.

Troubleshooting and Quality

Poor weld pull-off strength (nugget breaks easy): Insufficient weld time (increase timer), low current (check transformer tap and primary voltage), or electrode mushroom too large (dress or replace electrodes).

Electrode sticking (workpiece adheres to electrode): Excessive weld time or hold force, dirty sheets (scale), or low cooling (check water flow). Stuck workpieces are safety hazards—cool under air before manually separating.

Indentation marks around nugget (expulsion): Current too high for material thickness (reduce current or increase weld time slightly), or squeeze force too low (increase pneumatic pressure). Expulsion creates a sharp lip that can cut hands.

No nugget formed (cold weld): No secondary current (check contactor closing, transformer fuses, cable continuity), or loose electrode-to-arm connection (tighten or replace contact).

Electrode mushroom (diameter growth): Normal wear. Replace or dress electrodes when growth exceeds 1 mm. Over-mushroomed electrodes produce large, shallow nuggets that fail in shear.

Advantages and Limitations

Advantages: Fast (0.5–1 s/spot), low skill (push button), repeatable, minimal heat distortion, no consumables (except electrodes), clean (no smoke).

Limitations: Limited to thin sheet (0.5–4 mm practical range), requires high current infrastructure (substantial primary power), joint must be overlapped (not butt-jointed), and nugget size is small (not suitable for high-stress structures needing large fillet welds).

For large or dissimilar-metal welds, or where flexibility and higher heat input are needed, MIG or TIG welding is preferred.