Resistance spot welding

Fusion: a pulse of current melts a nugget between clamped sheets

Resistance spot welding joins two overlapping metal sheets at small points by squeezing them between copper electrodes and passing a huge electric current. Each spot takes well under a second and needs no filler metal or gas. It is how car bodies are built: look inside any car door opening and you will see rows of small round dimples.

Photo: BMW Werk Leipzig, CC BY-SA 2.0 de, via Wikimedia Commons

Resistance spot welding · one spot in four moments: squeeze, weld, hold, releaseRead the panels left to right: they show one spot weld at four moments of its cycle, and their numbers match the steps below.

How it works

Squeeze. A robot or press closes two water-cooled copper-alloy electrodes on the overlapping sheets. They press with a force of a few kilonewtons, roughly the weight of a few hundred kilograms.
Weld. A current of about 5,000–20,000 amps flows for a tenth to a few tenths of a second, at only a few volts. Resistance (a material's opposition to current) turns it into heat, as in a toaster wire. The contact between the two sheets resists most, so it heats first. There a lens-shaped nugget (a small pocket of melted metal) forms, typically 4–8 mm across.
Hold. The current stops, but the electrodes keep pressing while the nugget cools and freezes. Water flowing inside the electrodes pulls the heat out, so the copper stays cool and does not weld to the sheet.
Release. The electrodes open, leaving a shallow dimple on each face. The robot moves to the next spot, usually a few centimeters along the flange (the flat overlapping edge).

Why only the middle melts. Copper conducts electricity and heat several times better than steel, so the electrodes barely warm up. The sheet-to-sheet contact is the hottest point, so the nugget grows from the inside out, hidden from view.

Key facts

Materials: Low-carbon and zinc-coated steel sheet, stainless steel; aluminum with special high-current machines; nickel strips for batteries
Typical parts: Car bodies, doors and hoods, appliance cabinets, steel shelving, battery packs
Part size: Sheets typically 0.5–3 mm thick each; assemblies as large as a whole car body
Precision: Spot placed within about ±1 mm by a robot; overall part accuracy comes from the clamping fixture
Surface finish: A small round dimple on each face; usually hidden in a flange or painted over
Batch size it suits: Mass production; small bench machines also serve repair shops and battery builders
Tooling and machine cost: A few hundred dollars for a small bench unit; around $100,000 or more for a robot with its weld gun
Speed: Well under a second per spot; a robot makes roughly one spot every 1–3 seconds, including moves

Strengths and limits

Strengths

Very fast and easy to automate
No filler, gas or flux: very low cost per weld
Heat stays local, so parts barely distort
Repeatable once current, force and time are tuned

Limits

Only for overlapping (lap) joints in thin sheet
Joins at points, not a continuous seam, so it does not seal against leaks
The nugget is hidden inside, so quality is hard to check without destructive tests
Electrode tips wear and must be redressed (reshaped); aluminum needs much higher current

Where it's used

AutomotiveBody-in-white (the bare welded body shell), doors, hoods, seat frames
AppliancesWashing machine, oven and refrigerator cabinets
BatteriesNickel tabs joining cells in e-bike and power-tool packs
Metal furniture and enclosuresSteel shelving, lockers, electrical cabinets
RailStainless-steel railcar body panels

How to recognize it

Rows of small round dimples, about 5–8 mm across, evenly spaced along a flange.
The same dimple appears on both faces of the joint.
On bare or stainless metal, a faint ring of heat color around each spot.
Open a car door or hood: the edges of the body shell are lined with them.

Compare with

Arc welding7.1makes continuous seams and handles thick plates, but adds filler and far more heat.Laser welding7.4can replace rows of spots with continuous stitches for stiffer car bodies, at a much higher equipment cost.Ultrasonic welding7.7also joins overlapping thin parts at spots, but with vibration and no melting; it suits foils, wires and plastics.

Watch it

Welding Fabrication World · The process, its strengths and its uses, for beginners

The Weld Nugget · How current, force and time form the nugget

FANUC America Corporation · Robots welding a car body side, with a spark at each spot

ABB Robotics · A real car-body line at Volvo

See real photos

Wikipedia: Spot welding — photos of spot-welding robots and machines, plus a process diagram
Wikimedia Commons photo search

Remember

Current, squeezing force and time are the three settings of every spot weld.
Heat appears where the sheets touch, because that contact resists the current most.
A car body is held together by several thousand of these spots, nearly all made by robots.