---
type: process
name: "Resistance Welding"
category: "Joining"
subcategory: "Thermal"
materials: ["Metal"]
tolerances: "±1/32 in (±0.8 mm) on fixtured sheet assemblies; nugget diameter held to 4√t–5√t (t in mm); electrode indentation ≤ 20–25% of sheet thickness"
volumes: "5,000 to 10,000,000+ per year — the capital only pays back at production volume"
lead_time: "0.1–0.5 s of weld time and 1–2 s per spot on a robotic gun; 2–8 weeks for weld fixtures, guns and electrode tooling"
url: https://manufacturingprocesses.org/processes/joining/resistance-welding
---

# Resistance Welding

Resistance welding passes a heavy current through the joint so its own electrical resistance melts a nugget between the sheets.

- **Category**: [Joining](https://manufacturingprocesses.org/processes/joining.md)
- **Family**: Thermal
- **Materials**: Metal
- **Typical tolerances**: ±1/32 in (±0.8 mm) on fixtured sheet assemblies; nugget diameter held to 4√t–5√t (t in mm); electrode indentation ≤ 20–25% of sheet thickness
- **Typical volumes**: 5,000 to 10,000,000+ per year — the capital only pays back at production volume
- **Lead time**: 0.1–0.5 s of weld time and 1–2 s per spot on a robotic gun; 2–8 weeks for weld fixtures, guns and electrode tooling

## Overview

Resistance welding passes a high current — 5,000 to 20,000 A at only 1–10 V — through overlapping sheet metal held between two copper alloy electrodes. The joint's own electrical resistance is the heat source (Q = I²Rt), so a molten nugget forms at the sheet interface and nowhere else, and the whole cycle is over in 0.1–0.5 s.

It is the dominant joining process for sheet steel assemblies: a typical car body contains 3,000–5,000 spot welds. Three variants cover most work — spot welding for discrete nuggets, seam welding for pressure-tight continuous joints, and projection welding for embossed features and weld nuts made several at a time in one press stroke.

Sheet thickness runs from about 0.020 to 0.125 in (0.5–3 mm) per sheet in normal practice. There is no filler, no shielding gas and no consumable except electrode tips, so per-joint cost is essentially electricity and cycle time — which is why the process only makes sense at volume.

## How it works

1. **Squeeze.** The electrodes close on the stack-up and build force before any current flows — typically 200–1,500 lbf (0.9–6.7 kN) for sheet steel. Force sets contact resistance and holds the sheets together; too little causes expulsion, too much shunts heat into the electrodes.
2. **Weld.** Current flows for 5–30 cycles of 60 Hz line frequency (0.08–0.5 s), or an equivalent millisecond schedule on a medium-frequency DC inverter. Heating follows Q = I²Rt, and the highest resistance in the stack is the faying surface between the sheets, so that is where melting starts.
3. **Nugget growth.** A lens of molten metal grows outward from the interface, contained by surrounding solid metal and electrode force — the nugget is forged as well as melted. Target diameter follows d = 4√t to 5√t with t in millimeters, so 1 mm sheet wants a 4–5 mm (0.16–0.20 in) nugget.
4. **Hold.** Current stops but the electrodes stay closed for a few cycles while the nugget solidifies under pressure. Releasing early gives shrinkage porosity and center cracks.
5. **Off and index.** Water-cooled electrodes recover and the part indexes. Robotic guns average 1–2 s per spot including travel.

### How do the three variants differ?

**Spot welding** uses two opposed copper alloy electrodes (RWMA Class 2 CuCrZr for steel) to make one discrete nugget at a time — the default for sheet assemblies and the process behind every C-frame and X-gun on a body line.

**Seam welding** replaces the tips with two rotating copper wheels that weld continuously as the part passes between them. Overlapping nuggets by roughly a quarter of their diameter produces a pressure-tight seam — fuel tanks, radiators, drums, can bodies.

**Projection welding** embosses a dimple, ring or rib into one part so current is forced through that small area. Because geometry rather than the electrode locates the heat, flat platens replace pointed tips and several welds are made in one press stroke. Standard weld nuts and studs carry three projections and are attached this way in a single hit.

Electrode life is the practical process limit: several thousand welds on bare steel between tip dressings, often under a thousand on hot-dip galvanized, because the zinc alloys with the copper face.

## Design guidelines

### Nugget size and sheet thickness

Size the nugget with d = 4√t to 5√t (t in mm), and let the thinner sheet govern. In a stack-up, the thickness ratio between outer sheets should stay under about 3:1 — beyond that the thin sheet burns through before the thick one melts. Three-sheet stacks are routine; four is specialist work.

### Flange width, edge distance and spacing

Edge distance from nugget center to sheet edge should be at least 2x the nugget diameter, or molten metal expels sideways. Spot spacing needs to be at least 10x sheet thickness for steel; closer than that and a share of the current shunts through the previous weld, starving the new joint. In practice that means a flange 0.5–0.75 in (13–19 mm) wide. Gauge conversions are on the [sheet metal gauge chart](/charts/sheet-metal-gauge-chart).

### Access and appearance

Both sides of the joint must be reachable, and the flange has to be presentable to a gun throat — design flanges that run in one plane and avoid deep re-entrant pockets. The electrodes also leave a visible depression; standard automotive acceptance practice limits indentation to 20–25% of sheet thickness, and deeper than that means the weld has thinned the sheet. On a Class A visible surface, put the spot weld on a hidden flange or switch to adhesive bonding or laser welding.

### Which material pairs actually weld?

| Pair | Resistance weldable? | Notes |
| --- | --- | --- |
| Low-carbon steel to low-carbon steel | Excellent | The reference case — high enough resistivity, moderate conductivity |
| HSLA and advanced high-strength steel | Yes | Narrower current window; often needs post-weld tempering pulses to avoid a brittle martensitic nugget |
| Galvanized / galvannealed steel | Yes | Needs roughly 20–50% more current and destroys electrode tips far faster; specify a tip dress interval |
| 300-series stainless to itself | Yes | Higher resistivity means lower current and shorter time than carbon steel |
| Aluminum to aluminum | Difficult | Roughly 3x the current of steel at a fraction of the time; oxide film must be broken; electrode life is poor |
| Copper or brass to itself | Poor | Conductivity is too high — heat goes into the electrodes, not the joint |
| Aluminum to steel | No | Brittle intermetallics; use self-piercing rivets, adhesive bonding or friction stir |
| Painted or oiled surfaces | No | Insulating layers block current entirely — weld before paint |

Conductivity and resistivity figures behind those rows are on the [material properties chart](/charts/material-properties).

### How are resistance welds inspected?

Nugget quality cannot be seen from outside, so inspection combines destructive sampling with in-process monitoring. Chisel and peel tests on production samples must produce a **button pull-out** — base metal tearing in a ring around the nugget — rather than interfacial fracture through the weld. Macro-sections confirm nugget diameter and penetration into each sheet. Phased-array ultrasonic spot inspection is the non-destructive option, and adaptive controllers watching dynamic resistance and electrode displacement are the primary line-side control.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Nugget diameter | 5√t (t in mm) | 4√t minimum | Below this the joint fails interfacially instead of pulling a button |
| Edge distance | 2.5x nugget diameter | 2x nugget diameter | Molten metal expels through the free edge |
| Spot spacing | 15x sheet thickness | 10x sheet thickness | Current shunts through the adjacent weld |
| Flange width | 0.75 in (19 mm) | 0.5 in (13 mm) | Edge distance plus electrode access |
| Thickness ratio in a stack | 2:1 | 3:1 | Heat balance shifts toward the thick sheet |
| Electrode indentation | 10% of sheet thickness | 20–25% | Deeper indentation means the sheet has been thinned |

## Variants

- Projection Welding
- Spot Welding
- Seam Welding

## Cost drivers

Per-joint consumable cost is nearly zero — no filler, no gas, only electricity and copper tips — so resistance welding's economics are dominated by equipment and cycle time. A pedestal spot welder is inexpensive; a robotic weld cell with servo guns, transformers, water cooling and adaptive controls is a capital project. That capital only amortizes across large quantities, which is why the process owns automotive body-in-white and almost nothing at prototype volumes.

The second driver is spot count. Each spot is 1–2 s of robot time on a production line, which is why body engineers fight over weld counts. Electrode maintenance is the hidden operating cost: tip dressing intervals collapse on galvanized steel, and unplanned dressing stops the line.

1. **Justify every spot.** Load-path analysis usually shows spacing can be opened up away from high-stress regions; going from 10x to 20x sheet thickness halves the spot count.
2. **Consider weld bonding.** A bead of structural adhesive plus a reduced spot count gives higher stiffness and fatigue life than spot welds alone.
3. **Design one-sided access out of the part.** Deep-throat and offset guns cost force accuracy and cycle time; a flange in an accessible plane is free.
4. **Use projection welding for hardware.** Attaching four weld nuts in one press stroke beats four separate spot welding operations.
5. **Budget tip dressing on coated steel.** If the part must be galvanized, plan the dress interval into cycle time rather than discovering it in production.

## FAQ

### What size should a spot weld nugget be?

The standard rule is d = 4√t to 5√t with t the thinner sheet thickness in millimeters — so 1 mm sheet needs a 4–5 mm (0.16–0.20 in) nugget and 2 mm sheet needs 5.7–7 mm (0.22–0.28 in). Below 4√t the joint tends to fail interfacially through the weld instead of pulling a button out of the base metal.

### How far apart should spot welds be spaced?

At least 10x the sheet thickness for steel, and 15x is a better default. Welds placed closer shunt part of the welding current through the previously made nugget, which starves the new joint and produces an undersized weld even though the machine settings never changed.

### Can you spot weld aluminum?

Yes, but it is much harder than steel. Aluminum's high electrical and thermal conductivity means roughly three times the current for a fraction of the time, the tenacious oxide film has to be broken down, and electrode life is poor because aluminum alloys with copper. Most volume aluminum body structures use self-piercing rivets or friction stir welding instead.

### How do you inspect a spot weld?

Destructively, by chisel or peel testing production samples — an acceptable weld pulls a button of base metal out of one sheet rather than fracturing through the nugget. Non-destructively, by phased-array ultrasonic inspection. On production lines the real control is adaptive monitoring of dynamic resistance and electrode displacement during each weld.

### Why does galvanized steel wear out spot welding electrodes so fast?

Zinc melts and alloys with the copper electrode face, so tip geometry and contact area change weld by weld. Bare steel runs several thousand welds between dressings; hot-dip galvanized often needs dressing under a thousand, and also requires roughly 20–50% more current for the same nugget.

### What is projection welding used for?

Attaching hardware and making multiple welds in one stroke. An embossed dimple or ring on one part concentrates current where you want the weld, so flat platens replace pointed electrodes. Standard weld nuts and weld studs carry three projections and are attached to sheet steel in a single press hit.

## Alternative processes

- [Arc Welding](https://manufacturingprocesses.org/processes/joining/arc-welding.md): Arc welding melts and fuses metal with an electric arc between an electrode and the work, shielded from the atmosphere by gas or flux.
- [Riveting](https://manufacturingprocesses.org/processes/joining/riveting.md): Riveting sets a rivet through aligned holes and upsets its tail so the joint clamps permanently, with no heat and no access to both sides for blind types.
- [Adhesive Bonding](https://manufacturingprocesses.org/processes/joining/adhesive-bonding.md): Adhesive bonding joins parts with a cured polymer layer that spreads load over the whole bond area instead of concentrating it at fastener points.
- [Mechanical Fastening](https://manufacturingprocesses.org/processes/joining/mechanical-fastening.md): Mechanical fastening joins parts with screws, inserts, clinch hardware or snap fits so the joint can be assembled and taken apart again.

## Related processes

- [Sheet Metal Fabrication](https://manufacturingprocesses.org/processes/forming/sheet-metal-fabrication.md): Sheet metal fabrication cuts, bends and joins flat stock into a finished assembly, typically laser cutting followed by press braking, hardware and welding.
- [Metal Stamping](https://manufacturingprocesses.org/processes/forming/metal-stamping.md): Metal stamping shears and forms sheet metal between matched dies in a press, producing flat and shallow-formed parts at very high rates.
- [Power Beam Welding](https://manufacturingprocesses.org/processes/joining/power-beam-welding.md): Power beam welding fuses metal with a laser or electron beam focused to a very small spot, producing deep, narrow welds with little distortion.
- [Punching and Blanking](https://manufacturingprocesses.org/processes/cutting/punching-and-blanking.md): Punching and blanking shear sheet metal between a punch and a die, producing holes and flat outlines at very high speed.

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/joining/resistance-welding)*

*Last updated: August 11, 2026*