Where the Heat Goes in Resistance Spot Welding

Two identical machines, the same programme and the same sheet thickness can produce one joint that peels cleanly and one that leaves a mark with almost no nugget. The difference is rarely the machine. It is where the heat actually went, and how much of the current reached the interface instead of travelling around it.

Resistance spot welding is decided by four things: current density at the interface, the force holding the sheets together, the time the current flows, and the parallel paths that carry current somewhere other than the spot you care about. This article walks through them in the order they cause trouble on our own test bench.

Heat Is I Squared R, and Force Sets the R

The heat generated at a spot is the product of current squared, resistance and time. Current is the lever everyone reaches for first because it has the largest effect, and it is also the one that punishes a small error hardest. What gets overlooked is resistance, because it is not a setting: it is a property of the surfaces in contact.

Interface resistance is set by the material, the coating, the oil film and the roughness of the sheet, and it is then modified by the force applied. Higher force flattens the contact points, lowers the resistance and spreads the current over a wider area. That is why force is not a clamping detail: it is the parameter that turns the same kiloamps into a different current density.

In practice this means the electrode force curve matters as much as the current value. A machine that applies force slowly gets a soft start and a wider heat-affected zone; one that snaps to full force before the current starts produces a tighter nugget. Neither is wrong, but they are not interchangeable on a coated material.

Nugget Growth Has a Ceiling

Once the interface melts, the nugget grows outward while the electrodes press the sheets together. It grows quickly at first, then the growth rate falls as the contact area increases and the current density drops. Push the current or the time past that point and the melt no longer has anywhere to expand into: it finds the path of least resistance, which is out of the sheet as expulsion.

This is the practical reason a stronger weld is not a matter of turning the dial further. There is a window of current and time within which the nugget grows without expelling, and the window moves when the material, the coating or the electrode condition changes. Finding it is a bench exercise, not a calculation, and it is why we ask for the actual material rather than a thickness.

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A multi-head spot welding machine with twelve guns on one gantry. Every gun is an independent current path, which is why neighbouring guns interact.

The Shunt Path: Where the Current Goes Instead

Current takes every path available, not only the intended one. Weld a second spot close to the first and part of the current flows through the nugget already made, because steel is a conductor and the finished nugget is a better one than the unwelded sheet. Weld on a conductive fixture, and the fixture takes a share. Fire two guns on the same part at the same moment, and the two circuits are now in parallel through the workpiece.

On a multi-gun gantry this is not a corner case, it is the normal condition, which is why gun timing is part of the weld schedule rather than a machine setting. The symptom of unmanaged shunt current is consistent under-welding that no amount of extra current fixes cleanly, because raising the current also raises what is being shunted.

The measurement is straightforward and rarely done: a current meter on each gun, fired in the real sequence, on the real part. It takes an afternoon and it explains most of the "the machine is not consistent" complaints that arrive months later.

Coating Changes the Resistance, Not the Setting

A zinc coating alters the interface before any current flows. Zinc melts at a much lower temperature than steel, so the first thing that happens at the interface is a thin liquid layer, followed by vapour as the temperature climbs. That sequence changes both the resistance and the path the current takes, which is why a programme that is perfect on cold-rolled sheet can be marginal on the same thickness galvanised.

Paint behaves differently again: it is an insulator until it is broken down, so the first cycles act on the coating rather than the steel. Weld-through primer is the exception that proves the rule, and even then the resistance is higher and less predictable than bare metal.

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An operator running an automated welding line from its control console, where the weld schedule is set per station.

Electrode Wear Is a Slow Drift, Not a Failure

A copper cap does not usually fail; it flattens. The face grows from its original diameter as it mushrooms, the contact area increases, the current density falls and the nugget shrinks. Nothing on the machine reports this, because the machine is still delivering the programmed current. The weld is simply getting smaller.

This is why tip dressing is a production parameter and not housekeeping. On high-duty lines the interval is set in spot counts and checked against a peel test at the start of each shift, not judged by eye. A cap that looks acceptable can be well outside the diameter that the schedule assumed.

Thickness Is Not the Specification

Questions about spot welding arrive as a thickness: one millimetre, or two, or a range. Thickness is the least informative part of the answer, because the same total thickness can be made up of different stacks. Two sheets of one millimetre and a stack of 1.2 plus 0.8 both measure two millimetres, and they weld differently: the current has to pass through two interfaces in one case and a single, unequal one in the other, and the thinner sheet in a mismatched stack heats faster and marks first.

What we ask for instead is the stack-up, the material and the coating on each side, the number of spots and their spacing, and the surface condition as it arrives. Those five answers usually decide the schedule on paper, before anything is clamped.

The same logic explains why a supplier who quotes from a drawing without asking about the stack is guessing, and why the first samples either confirm the guess or cost a week.

The Four Levers, Side by Side

The table below is the one we work through when a joint will not behave. Each row is a lever, what it actually changes, and the symptom at either end of its range.

Lever	What it really changes	Too little	Too much
Current	Heat input at the interface	Small or no nugget, cold weld	Expulsion, surface marking, electrode wear
Force	Contact resistance and current density	High resistance, expulsion, sheet marking	Wide shallow nugget, low density, indentation
Time	How long the nugget grows	Nugget too small, brittle interface	Growth stops, heat spreads, distortion
Shunt control	How much current reaches the spot	Under-welding that resists tuning	Not applicable - the fix is sequence, not power

Conclusion

Spot welding is often described as a simple process with a difficult machine. It is closer to the reverse: the machine is straightforward and the process is subtle, because the four levers interact and one of them, the shunt path, is invisible unless it is measured. Getting a stable joint is mostly a matter of controlling the interface and the current path, and then holding both steady as the electrodes wear.

Requirements vary by destination market. Always confirm against the current edition of the applicable standard and with your certification body.

PDKJ continues to provide customized laser welding and resistance welding solutions based on different materials, workpiece structures, welding requirements, and production applications.

Where The Heat Goes In Resistance Spot Welding