Resistance Spot Welding

Why spot welding matters

Spot welding is the invisible skeleton of almost everything made from thin steel. It is a fast, resistance-based fusion process: rather than dragging a heat source along a seam, it dumps an enormous burst of electrical energy into a single point and forms a self-contained molten puddle in a fraction of a second. That speed, and the fact that it needs no consumable filler, gas, or flux, is what made it the default joining method for high-volume sheet-metal assembly.

• Automotive body-in-white. The dominant use. Robots place thousands of welds per car at cycle times measured in tenths of a second.
• Appliances and enclosures. Washing-machine drums, oven cavities, electrical cabinets, HVAC ductwork.
• Battery and electronics packaging. Tab-to-terminal welds on cells, though many of these have moved to ultrasonic or laser welding for very thin/dissimilar foils.
• Rail cars, containers, and furniture. Anywhere flat panels overlap and a continuous seam is not required.
• Wire and mesh. Cross-wire welding of reinforcing mesh, fencing, and grilles is the same physics applied to round stock.

The economics are stark: a spot weld can cost a fraction of a cent in consumables (there are none — only electricity and electrode wear), versus arc or laser processes that need wire, gas, or expensive optics. The trade-off is that RSW only joins overlapping sheets, only up to a few millimetres thick per sheet, and only at discrete points — not a hermetic continuous seam.

How it works, step by step

A spot weld is a tightly choreographed sequence, all of it completed in well under a second. The controller commands it as a weld schedule: a timeline of force and current.

1. Squeeze. The electrodes close on the stacked sheets and build the set force (typically 2–5 kN). This must complete before current flows, so the parts are in intimate contact and the contact resistance is stable.
2. Weld (current on). A low-voltage, high-current pulse flows through the stack. Current is timed in cycles of the AC line (1 cycle = 1/60 s ≈ 16.7 ms in North America). The interface heats fastest and a molten nugget nucleates and grows.
3. Hold (forge). Current stops but force is maintained while the nugget solidifies. The pressure feeds solidification, suppresses porosity, and refines the grain — effectively forging the joint.
4. Off / release. Electrodes retract; the gun steps to the next location. On coated or thick steel, a pulsation or tempering post-heat pulse may follow to control the metallurgy.

The reason it works at all is a race between two paths for heat. Joule heating deposits energy fastest at the point of highest resistance — the loosely mated faying interface between the two sheets. Meanwhile the copper electrodes are excellent thermal conductors and are internally water-cooled, so they continuously pull heat out of the outer surfaces. The net effect is a temperature peak buried at the interface, exactly where fusion is wanted, while the surfaces stay solid.

The governing equation: Joule heating

All resistance welding rests on Joule's first law. The heat generated in a resistive element is:

Q = I² · R · t

where every symbol has a physical meaning and unit:

• Q — heat energy generated, in joules (J).
• I — welding current, in amperes (A). Because it enters as I², it is by far the most sensitive knob — doubling current quadruples the heat.
• R — total electrical resistance of the current path, in ohms (Ω). Milliohm-scale in RSW.
• t — weld time, in seconds (s), usually specified as an integer number of 60 Hz cycles.

The current path is a series stack of resistances: the two electrode-to-sheet contact resistances, the bulk resistance of each sheet, and — crucially — the faying (sheet-to-sheet) contact resistance in the middle. Contact resistance dominates early in the weld because real surfaces touch only at scattered asperities and are covered by oxide films. As the interface heats, asperities soften, the true contact area grows, and the faying resistance collapses; the weld then transitions to being governed by the sheets' bulk resistivity, which rises with temperature. This dynamic R(t) is why real weld growth is nonlinear and why current alone does not tell the whole story.

For steel, the interface must exceed roughly 1,500 °C (the melting point of low-carbon steel) to nucleate a nugget. A useful mental model: energy in ≈ (energy to raise the small nugget volume to melting) + (energy conducted away into the electrodes and surrounding metal). Because the conduction loss is large and fast, the process needs a high power density — hence thousands of amperes squeezed into a spot only a few millimetres across.

Worked example: sizing a weld for 1 mm steel

Suppose we spot-weld two sheets of 1.0 mm mild steel. A common rule of thumb sizes the target nugget diameter as d ≈ 5·√t, where t is the sheet thickness in millimetres. Here that gives d ≈ 5·√1.0 = 5 mm, a widely used minimum for automotive-grade welds.

Estimate the energy. Take the current path resistance at roughly R ≈ 100 µΩ (1.0×10⁻⁴ Ω, a representative dynamic average for this stack), a current of I ≈ 9 kA, and a weld time of 10 cycles = 10/60 s ≈ 0.167 s:

Q = I²Rt = (9,000)² × 1.0×10⁻⁴ × 0.167 ≈ 1,350 J

Roughly a kilojoule delivered in a sixth of a second — an instantaneous power of about 8 kW concentrated in a spot the size of a pencil eraser. Only a fraction of that Q ends up in the nugget; much is lost to the electrodes and the surrounding sheet, which is exactly why the numbers look so extreme for such a tiny joint.

The schedule that produces this sits inside the weldability lobe — the region on a current-versus-time plot bounded below by the minimum needed to reach the target nugget size, and above by the onset of expulsion. Widening that lobe (more forgiving process) is the goal of every process engineer.

Representative spot-weld schedules (uncoated mild steel, per common practice)

Sheet thickness	Electrode force	Weld current	Weld time	Min. nugget dia.
0.5 mm	~1.3 kN	~6 kA	~6 cycles	~3.5 mm
1.0 mm	~2.3 kN	~9 kA	~10 cycles	~5.0 mm
1.5 mm	~3.2 kN	~11 kA	~14 cycles	~6.1 mm
2.0 mm	~4.2 kN	~13 kA	~18 cycles	~7.1 mm

Values are illustrative ranges; real schedules depend on alloy, coating, machine type (AC vs. mid-frequency DC), and electrode geometry, and are validated by destructive peel or chisel tests and metallographic nugget measurement.

Materials, coatings, and where it gets hard

How material choice changes the spot-welding problem

Material	Why RSW likes or dislikes it
Low-carbon (mild) steel	Ideal: moderate resistivity, high melting point, wide lobe. The classic body-panel material.
Galvanized / zinc-coated steel	Zinc (melts ~420 °C) shunts current and alloys with copper tips, shortening electrode life and needing higher current — but very common in cars for corrosion resistance.
Advanced high-strength steel (AHSS)	Higher resistivity welds easily but the hard martensitic nugget can be brittle; risk of interfacial fracture. Post-weld tempering pulses help.
Aluminium	Difficult: very high conductivity (low R) and tenacious oxide demand ~2–3× the steel current, huge force, and fast electrode wear — a key reason aluminium bodies lean on self-piercing rivets and adhesives instead.
Copper / brass	Nearly impossible with standard RSW — the workpiece conducts as well as the electrode, so heat won't localize.

The electrodes themselves are a consumable in disguise. They are usually chromium-zirconium copper (RWMA Class 2), chosen for high conductivity plus enough hot hardness to resist mushrooming under 2–5 kN. On coated steels, zinc pickup alloys the tip and grows its face, which drops current density and shrinks the nugget over time. Tips are therefore dressed (re-cut to shape) at intervals and replaced after thousands of welds — an important cost and quality-control variable on a production line.

Common misconceptions and failure modes

• "More current is always better." Wrong — past the top of the lobe you get expulsion: molten metal blows out as spatter, leaving a smaller, porous, weaker nugget with a deep surface dent.
• "The heat comes from the electrodes." No. The electrodes are water-cooled heat sinks. The heat is generated inside the sheets by resistance, peaking at the faying interface.
• "A bigger surface mark means a stronger weld." Surface indentation indicates heat and force at the skin, not nugget size. Undersized or "stuck" (cold, unfused) welds can look fine on the surface yet peel apart — hence destructive and ultrasonic testing.
• "Force doesn't matter much." Too little force raises contact resistance and invites expulsion and gaps; too much force lowers resistance and can starve the weld of heat. Force is a first-class schedule parameter.
• "Shunting is negligible." If welds are placed too close together, current leaks (shunts) through the adjacent existing weld and bypasses the new joint, undersizing it. Minimum weld spacing exists for this reason.
• "It's a fusion weld like arc welding, so it needs filler." It fuses, but only base metal — no filler, no gas. That's its defining efficiency.

The chief defects to know: expulsion (over-heating blowout), cold/stuck welds (insufficient fusion), undersized nuggets (below the strength minimum), porosity and shrinkage voids (poor solidification), interfacial fracture (brittle nugget in AHSS pulling apart across the weld plane instead of tearing a button out of the parent metal), and excessive indentation from worn tips or over-force.