Structural Welding (Squeeze-Type Resistance Spot Welding)
How the weld forms
Current passes through copper electrodes and through the panels; resistance at the sheet-to-sheet interface generates the heat, while electrode force holds the panels together as the nugget forms. Quality depends on current, weld time, electrode force, tip condition and alignment, and the material stack.
Rules
- Dress tips and keep them square to the flange. Mushroomed or worn tips reduce current density and produce small or cold welds.
- Coatings and adhesives change resistance. Zinc coating raises the current requirement; welding through non-weld-through adhesive is not allowed.
- Nugget size: a widely used rule of thumb is a minimum nugget diameter of 4√t, where t is the thinnest sheet in millimeters. Always check the OEM specification.
- Weld-through primer must be the correct type and applied thin. Heavy primer increases resistance and causes weak welds and expulsion.
- Replicate OEM weld count and spacing. Extra welds in the wrong place can change how the part collapses.
- Verify with a destructive test on a coupon of the same stack, looking for a button pulled from the parent metal rather than an interfacial separation.
Worked example
Welding 0.8 mm coated steel to 1.2 mm steel on a replacement rail produces a coupon that separates at the interface. The reasoning: the thinnest sheet governs nugget size, and the coating plus a slightly undersized tip reduced current density. Cleaning the tips, correcting alignment, and re-running the coupon produces a proper button before any production welds are made.
Traps that cost marks
Welding through the wrong adhesive, misaligned electrodes, skipping the coupon test, and accepting an interfacial failure as acceptable.