The Weld You Cannot See: How To Read A Spot Weld

The Weld You Cannot See: How to Read a Spot Weld

Six marks on a galvanised panel, in two rows of three, all the same size and all the same depth. To anyone looking at the part they are six spot welds. Peel the panel apart and one of them leaves a hole with nothing behind it: the electrode came down, the metal got warm, the surface took a dent, and no nugget ever formed.

That is the central difficulty of resistance spot welding, and it is worth understanding before you approve a sample, a run-off or a shipment. The thing that decides whether the joint holds is inside the stack, and the only evidence on the outside is a dent.

A galvanised panel from a trial run with six spot welds in it. From the visible side they are indistinguishable, and at least one of them was not a weld.

What Is Actually Being Made

Two sheets are clamped between two electrodes and a current is pushed through them. The resistance is highest at the interface between the sheets, because that is where two surfaces meet through a film of oxide and contamination rather than through solid metal. That is where the heat is generated. A lens of molten metal forms, grows outward until it reaches the electrode faces, and solidifies under pressure when the current stops.

Nothing about that process is visible from outside. The surface records two things only: that an electrode pressed there at a certain pressure, and that the metal under it got hot enough to yield. Whether the two sheets actually fused is a third question, and the surface does not answer it.

Three Numbers Decide It

A spot weld is set by current, time and force, and the first of those is usually written down wrongly. What matters is not the current in the programme but the current density at the interface, which is current divided by contact area. The same 12 kA through a 16 mm electrode face and through a worn, mushroomed 20 mm face are two different welds, and the second one is weaker.

Force does two jobs that pull in opposite directions. More force flattens the contact and spreads the current, which lowers density; but more force also holds the expanding nugget in and prevents it from blowing out at the edge. There is a window, and it is narrower than most people assume. Time is the crudest of the three and the one most often used to compensate for the other two, which is how a weld that looks right ends up with a wide heat-affected zone and a small nugget.

Why the Surface Lies

A polished, shiny dent means the electrode face was clean and the pressure was even. It does not mean the nugget is there. A blue or straw-coloured ring means heat reached the surface, which is heat that did not stay at the interface. A deep dent with a dull centre is the signature of force without current: the electrode pushed, the circuit was poor, and the metal yielded mechanically instead of melting.

Expulsion is the most misread of all. A spark at the edge of the weld means the nugget grew past the electrode face and blew out. The weld behind it may still be sound, or it may be a hole with a thin rim. We have seen buyers accept a batch because the welds "burned nicely", which is a description of the failure mode rather than of the joint.

The Second Weld Steals From the First

Once a nugget exists, the path through it is far easier for current than the path through two sheets that are merely touching. Weld a second spot close to the first and part of the current goes through the finished weld instead of the new one. The second weld comes out weaker than the first, and the third weaker again if it is in the same run.

The practical consequence is spacing. As a working rule on our own fixtures, weld pitch is kept at four electrode-face diameters or more; below that we assume a shunt and set the schedule accordingly, or we sequence the welds so that no two neighbours are made consecutively. This is a decision that has to be made when the fixture is designed, not when the part is rejected.

Coated Steel Moves the Target

Zinc changes the electrical picture before the first weld is made. It melts at around 419 °C, well below the steel, and its contact resistance is several times that of bare cold-rolled sheet. A schedule that produces a good nugget in 1.0 mm cold-rolled steel will produce a smaller one, or none, in the same thickness galvanised, because the extra resistance at the interface is spent heating zinc rather than steel. Coating is a subject of its own, but it belongs in the same sentence as current density: it is one more variable standing between the programme and the nugget.

The Two Checks That Actually Work

There are only two non-laboratory tests, and both destroy the part. The peel test bends the top sheet back until the weld tears; what you are looking for is the button, the disc of metal pulled out of one sheet and left attached to the other. Its diameter is the number that matters. A button noticeably smaller than the electrode face, or a clean round hole with no button, means the nugget was smaller than the sheet thickness and the joint is marginal whatever it looked like.

The chisel test drives a chisel between the sheets at the weld and opens them. It is fast, it works on a production line, and it tells you whether a nugget exists. It does not tell you how big it is, because opening the joint also deforms it. Both tests consume the weld they examine, which is why the honest version of "we check every part" is "we check a coupon from every shift".

Measuring the nugget itself needs a section: cut through the weld, polish, etch, measure under a microscope. That is a laboratory job and nobody does it on a shop floor. What a shop floor can do is keep the buttons. A box of peeled buttons, one per shift, dated, is a better quality record than any number of photographs.

The same kind of joint, peeled apart. The torn hole in the bent piece is where the nugget was; the round button on the flat piece is the nugget itself, pulled out whole.

What We Got Wrong

We used to sign off a run-off with photographs. Two hundred brackets, no rejects, a picture of every twentieth weld, and a report that said the welds were consistent. They were consistent in appearance. The customer peeled ten of them three months later and found two with no nugget, and the reason was not the machine: they had changed coating supplier, the new coil was heavier coated, and the schedule had never been re-set for it.

The fix cost us a trip and a re-run, and it changed our paperwork. A run-off now closes with a destructive test sheet: the schedule used, the coating and thickness of the material actually on the machine, the button diameter measured on five coupons, and the coupons themselves, bagged and labelled. It takes an hour. It has also, twice since, caught a material substitution before the machine was packed.

What to Ask Before You Approve

Whether you are buying a machine or a million welded parts, these are the questions that separate a demonstration from evidence.

• What destructive test was done, on how many pieces, and what was the measured button diameter?
• Was the material tested the same coil, coating and thickness as the production material?
• How far apart are the welds, and was the shunt allowed for in the schedule or only in the drawing?
• How often are electrodes dressed or changed, and who measures the face when they are?
• If the coating or the supplier changes next year, what happens to the schedule?

None of this is exotic. It is the ordinary physics of a process that hides its result, and the reason a weld schedule is a document rather than a setting.

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

Related Products

• Spot Welding Machines
• Medium Frequency Inverter Seamless Spot Welding Machine

Finished spot welders waiting on the floor in Dongguan. Every one of them leaves with a weld schedule and, on a good day, with a box of peeled buttons.

Contact PDKJ

Send us your part drawing or a sample, and our engineers will run a welding feasibility trial on your own material and recommend the equipment configured for your application.