Welding and joining

Resistance spot welding: what goes wrong and where to inspect

Sheet assemblies for vehicles, appliances and cabinets are joined by passing high current through electrodes pressed onto the sheets, forming a weld nugget in milliseconds.

IATF16949 PPAPAutomotive SPCAutomotive

01How the process works

Weld current, time and electrode force create a molten nugget at the sheet interface. Projection welding concentrates current on embossed points, typically to attach nuts and studs. Electrode tips wear and mushroom, reducing current density and nugget size unless they are dressed regularly.

Spot weld quality is invisible from outside. It is verified destructively by peel and chisel tests or teardown, and non-destructively by ultrasonic spot weld testers, with sampling plans linked to electrode dressing intervals.

Schedule set-up. Current, time, force for the stack-up.
Electrode dressing. Tip geometry restored.
Welding. Spots placed by robot or press.
Monitoring. Current and resistance signals monitored.
Testing. Chisel, peel or ultrasonic checks.

02What typically goes wrong

Defect	Typical cause	How it is found
Undersized nugget	Electrode wear, low current	Peel test, ultrasonic
Stick weld (no fusion)	Coating, contact	Chisel test, ultrasonic
Expulsion	Excess current	Visual
Cracks and porosity in nugget	Material, schedule	Teardown, metallography
Missing welds	Program error	Visual count
Indentation	Excess force	Depth measurement

03Where an inspector should look

Before production

Weld schedules per stack-up
Test plan and sampling
Electrode dressing interval

During production

Destructive tests at start of shift
Process monitoring alarms

After production

Spot count and position
Ultrasonic sampling
Assembly dimensions

04Standards and references

IATF16949 PPAPAutomotive SPCAutomotive

ISO 14329 and ISO 10447 (testing of resistance welds), ISO 18278 (weldability), customer body in white standards.

Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.

05How we support buyers of spot welding parts

Manufacturing and Special Process AuditsProcess audits of casting, machining, welding, heat treatment, plating and moulding, with VDA 6.3 style evaluation. Independent Quality ControlIncoming, in-process and final quality control performed independently at the supplier, with measured results and clear acceptance decisions.