Why Coated Steel Welds Differently: Galvanised, Primed and Painted Sheet

Most of the sheet metal that arrives at our bench is not bare steel. It is galvanised, or primed, or painted, and in each case there is a second material in the joint that was not there when the welding procedure was written. The coating is thin, but it decides how the joint starts, how the heat escapes and how the weld looks a year later.

This article sets out what the coating actually does in a weld, why resistance and laser welding cope with it differently, and what we check on the bench before quoting a programme for coated sheet.

The Coating Is the First Thing the Current Meets

Zinc melts at around 419 degrees Celsius and boils at around 906, both far below the melting point of steel. In a resistance weld the interface therefore behaves in stages: the coating melts first, then vaporises, and only afterwards does the steel reach the temperature where a nugget forms. The vapour has to go somewhere, and in a lap joint pressed together by an electrode it goes through the molten steel, which is how porosity and expulsion appear in a joint that looked properly set.

That sequence also changes the electrical path. A liquid zinc layer has a different resistance from the steel it sits on, and it is not evenly distributed, so the current density across the intended spot becomes less uniform than on bare sheet. The same programme produces a different nugget, and usually a smaller one.

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A galvanised sheet enclosure on a rotary fixture during a laser welding trial on our bench.

Why Resistance Welding Copes Better Than Expected

Given the vapour problem, galvanised sheet should be much harder to spot weld than it is. The reason it works is force and time: the electrode pressure is high enough to displace the softened coating sideways out of the contact area before the steel melts, and the weld time is short enough that the vapour does not have long to accumulate.

The practical levers follow from that. A short, hard weld with a slightly higher current and a well-dressed cap is the conventional answer. Adding time instead of current tends to make things worse, because the extra time is spent heating a coating that is already boiling rather than growing the nugget.

Laser Welding Coated Sheet: the Gap Question

A laser weld makes a much smaller molten pool, and it does not press the sheets together, so the vapour has fewer ways out. In a zero-gap lap joint the zinc vapour is trapped at the interface and pushes into the pool, producing blowholes and an unstable seam that looks acceptable on the surface and fails a leak test.

The countermeasure is counter-intuitive to anyone used to resistance welding: leave a gap. A controlled gap of a few hundredths of a millimetre at the interface gives the vapour an escape route along the seam. In production this is achieved with a fixture feature, a dimple or a slight pre-set in the part, and it has to be designed in rather than adjusted at the machine.

Where a gap cannot be held, the alternatives are to remove the coating locally, to weld with filler wire so the pool has more material to absorb the disturbance, or to change the joint to a flange or a fillet where the vapour can leave upward. Each of these is a design decision, and each has a cost that belongs in the quotation rather than in the commissioning week.

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The corner seam of a galvanised enclosure from a trial run. The pale band along the joint is where the coating has been burned back by the weld.

Primed and Painted Sheet Is a Third Case

Primer and paint are not coatings in the same sense as zinc. They are insulators until they break down, so the first part of the weld cycle is spent burning through them. The result is a joint with a wide, inconsistent contact area, a burnt ring around the spot and a nugget that depends on how thick the paint happened to be.

Weld-through primers exist for exactly this reason and they do work, but they behave more like a high-resistance coating than like bare metal: the current has to be raised, the electrodes soil faster, and the appearance of the joint is not the appearance of a clean weld. If a drawing specifies painted sheet and a structural weld, that conflict is better resolved before the parts are cut.

The practical test is a coupon of the same paint on the same substrate, welded and peeled. A trial on bare steel says nothing useful about a painted part, and that mismatch between the drawing and the sample is one of the most common reasons a commissioned machine under-performs.

Coating Weight Is a Number Worth Asking For

Galvanised sheet is not one material. The coating is specified by mass per unit area, and the difference between a light and a heavy coating is the difference between a joint that sets comfortably and one that expels. The mill certificate carries the figure; the drawing usually does not.

At the bench the effect is easy to see. A heavier coating means more zinc to melt and displace before the steel can form a nugget, so the same programme produces a smaller weld and a dirtier cap. On the laser side the same increase in coating means more vapour looking for a way out of the joint, which is where the gap requirement stops being optional.

We ask for the coating weight whenever a trial part is galvanised, and where it is not available we measure the part and record the assumption in the report. A programme quoted against an unknown coating is a programme that will be re-tuned on the customer floor.

Electrode Pickup and the Cost of Skipping Tip Dressing

On coated steel the cap face collects brass and zinc and gradually alloys with them, which raises the resistance and changes the current density. The visible sign is a cap that looks discoloured and slightly rough; the functional sign is a nugget that shrinks over a shift. Dressing restores the face diameter and the surface, and it is the cheapest control in the whole process.

It is also the first thing to be skipped when a line is busy, which is why we write the interval into the schedule rather than leaving it to the operator judgement.

Symptom, Cause, and What to Change

This is the sequence we run through when a coated joint misbehaves, and it is a reasonable checklist for a buyer reviewing a trial report.

Symptom	Most likely cause	What to change first
Porosity in a laser lap weld	Zinc vapour trapped at zero gap	Introduce a controlled gap or remove coating locally
Expulsion in a spot weld on galvanised	Coating boiling before the nugget forms	Shorten time, raise force, dress the cap
Nugget shrinks across a shift	Cap face growth and alloying	Set a tip dressing interval in spot counts
Burnt ring, weak joint on primed sheet	Insulating coating breaking down unevenly	Use weld-through primer or strip the weld zone
Good coupon, failing production part	Gap or fit-up differs from the coupon	Measure the gap on the real part, not the sample

Conclusion

Coated sheet is not a harder version of the same job. It is a different interface, with a second material that melts first, boils early and changes the electrical path. Resistance welding handles it with force and short time; laser welding handles it with a gap and a design that lets the vapour out. Both depend on the same unglamorous habit, which is checking the real part rather than the drawing.

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.

Why Coated Steel Welds Differently: Galvanised, Primed And Painted Sheet