Resistance Welding: Definition, Process Types and Welding Parameters
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What Resistance Welding Is
Resistance welding joins two overlapping metal parts by passing a high current through them while a mechanical force holds them together. The electrical resistance at the interface between the faying surfaces generates the heat; no filler metal, flux or shielding gas is required. Because the heat is generated inside the joint rather than supplied from outside, the process is fast, easily automated and economical on high-volume lines.
The transformer delivers a low voltage at a high current, typically from a few kiloamperes on thin sheet up to more than 100 kA on heavy gauges. Water is circulated through the electrode holders and caps to keep the copper alloy electrodes from annealing and losing shape.
How the Weld Nugget Forms
Current passes from the electrode through the upper workpiece, across the faying interface and out through the lower workpiece. The highest resistance usually sits at the interface, so melting starts there and spreads until a lens-shaped nugget forms. The electrode force holds the plastic metal in place while the nugget grows, and it is maintained through the hold time so that the nugget solidifies under pressure. Coating the surface with a thin layer of zinc, or heavy oxide scale, changes the contact resistance and therefore the current and force settings required.
For low carbon steel spot welds the minimum acceptable nugget diameter is commonly taken as four times the square root of the sheet thickness, and this rule of thumb appears in the procedure standards used for the automotive and appliance industries.
Main Process Variants
| Variant | How the current is applied | Typical use |
|---|---|---|
| Spot welding | Electrode tips pressed on both sides at a single point | Sheet and strip assemblies, brackets, panels |
| Seam welding | Roller electrodes with overlapping pulses | Leak-tight joints in tanks and drums |
| Projection welding | Domes or embosses concentrate the current | Nuts, studs and fasteners welded to sheet |
| Flash welding | Arc flashes across a butt joint, then forging | Wires, tube ends, rings, tool joints |
| Upset welding | Continuous current plus axial upsetting force | Wire and rod butt joints |
Parameters and Procedure Standards
Four parameters decide a resistance weld: welding current, electrode force, weld time and hold time. Weld time is usually expressed in cycles, and on a 50 Hz supply one cycle equals 20 milliseconds. Power supplies are single-phase alternating current, medium-frequency direct current in the 1 kHz range, or inverter types; the medium-frequency and inverter units give a more stable current at the same setting and are preferred for coated and dissimilar thickness stacks.
Recommended practices and quality requirements for the process are set out in AWS C1.1M/C1.1 for resistance welding practice, ISO 14554-1 and ISO 14554-2 for quality requirements, EN ISO 14373 for spot welding of uncoated and coated low carbon steels, ISO 15614-12 for procedure qualification covering spot, seam and projection welding, and ISO 5821 for the dimensions of spot welding electrode caps. Test methods used to evaluate the welding behaviour of coated and high-strength sheet are given in AWS D8.9M.
Resistance Welding of Stainless Steel
Austenitic stainless steel 304 has an electrical resistivity of about 0.72 micro-ohm metre at 20 C, roughly three times that of low carbon steel, and a thermal conductivity of about 16 W/(m K) at 100 C, roughly one third of low carbon steel. The combination means heat is generated closer to the interface and is not conducted away as quickly, so stainless sheet is welded with a lower current and a shorter weld time than the equivalent carbon steel gauge, but with a higher electrode force and more frequent electrode dressing to control surface marking and pickup.
Weld current shunting through previously made spots, poor fit-up and mill scale are the common causes of undersized nuggets on stainless work. Regular peel, chisel or tensile tests on production coupons remain the practical way to confirm that the settings are still valid after a change of coil or electrode.
Frequently Asked Questions
Q: Does resistance welding need filler metal or shielding gas?
A: No. The joint is produced by the resistance heating of the workpieces themselves under pressure, so no filler wire, flux or shielding gas is used.
Q: What is the difference between spot welding and seam welding?
A: Spot welding makes separate nuggets with shaped electrode tips; seam welding uses rotating roller electrodes and overlapping pulses to produce a continuous leak-tight seam.
Q: Why are resistance welding electrodes made of copper alloys?
A: The electrode must carry very high current with low losses while resisting deformation, so precipitation-hardened copper alloys with high electrical and thermal conductivity are used and are cooled with internal water channels.
Q: Which standard covers welding procedure qualification for spot and seam welding?
A: ISO 15614-12 covers procedure qualification for spot, seam and projection welding, while AWS C1.1M/C1.1 provides the recommended practices for the process.
Q: Is the minimum nugget size fixed by a standard?
A: Procedure standards for low carbon steel commonly use a minimum nugget diameter of four times the square root of the sheet thickness; for other materials the acceptance size is agreed between the parties and verified by coupon testing.
Q: Can stainless steel be spot welded to carbon steel?
A: It can, but the different resistivity and thermal conductivity of the two materials produce an asymmetric nugget that sits mostly in the stainless side, so the settings and the acceptance criteria must be established by trial welds.
