Laser welding

Fusion: a focused laser drills a keyhole for narrow, deep welds

Laser welding melts the joint with a tightly focused beam of light, making narrow, deep welds at high speed. Because the heat is so concentrated, parts barely warp. It joins car bodies, battery cells and medical implants, usually with robots or fast-moving mirrors.

Photo: Krorc, CC BY-SA 3.0, via Wikimedia Commons

Laser welding · keyhole side view, with end views of laser and arc seamsCircled numbers match the steps below. The box on the right compares the finished seam with an arc weld of the same plates.

How it works

Make and deliver the beam. A laser source, today usually a fiber laser of about 1–10 kW, makes an intense infrared beam. An optical fiber carries it to the welding head.
Focus it. A lens squeezes the beam into a spot a few tenths of a millimeter wide. That puts more than a million watts on each square centimeter.
Drill a keyhole. The metal under the spot melts and partly boils. The vapor pressure holds open a thin channel, the keyhole, which lets the beam reach deep into the joint.
Let the melt flow around. As the head moves, molten metal flows around the keyhole and closes up behind it.
Shield the pool. A gas such as argon, helium or nitrogen protects the hot metal from air and pushes away the plume (the cloud of metal vapor above the spot).
Freeze fast. The small pool solidifies almost instantly into a narrow, deep seam. The heat-affected zone (metal changed by heat but not melted) stays thin.

Two modes. At lower power density the laser only melts the surface, giving a shallow, smooth seam (conduction mode) for thin sheet and visible joints. Above a threshold it opens a keyhole, which can make welds up to about ten times deeper than they are wide.

Key facts

Materials: Carbon and stainless steels, aluminum, titanium, nickel alloys; copper with green or blue lasers or special settings; some plastics
Typical parts: Car roofs and doors, tailored blanks (sheets of different thickness joined before stamping), battery tabs, gears, pacemaker cases
Part size: From foils under 0.1 mm to plates of roughly 20 mm in one pass with high power; parts as large as a car body with robots
Precision: Beam placed within about ±0.1 mm; very low distortion, but parts must fit with gaps of roughly 0.1 mm or less
Surface finish: Narrow, smooth seam, often nearly flush; little spatter
Batch size it suits: Large series with automation; handheld units now also serve job shops
Tooling and machine cost: Roughly $10,000 and up for a handheld fiber laser welder; several hundred thousand dollars or more for a robotic cell
Speed: Typically 1–10 m of seam per minute on thin sheet; scanner heads jump between welds in milliseconds

Strengths and limits

Strengths

Narrow, deep welds with little heat and distortion
Very fast and easy to automate, even from a distance with scanning mirrors
Welds from one side, without touching the part
Precise enough for tiny medical and electronic parts

Limits

Expensive equipment and strict laser safety: the beam and its reflections can blind
Needs tight fit-up: gaps and misalignment cause defects
Shiny metals like copper and aluminum reflect much of the beam
Very fast cooling can make hard, brittle welds in high-carbon steels

Where it's used

AutomotiveRoofs, doors, tailored blanks, gears and transmission parts
Batteries and e-mobilityCell tabs, busbars (thick connecting strips), battery housings
MedicalPacemaker and implant housings, surgical instruments
ElectronicsSensor housings, connectors, sealed packages
Kitchen equipmentStainless sinks, appliance panels and tubes

How to recognize it

A very thin, even seam, often only 1–2 mm wide, with fine regular ripples.
Little or no heat tint or warping beside the seam.
Rows of tiny overlapping dots on small parts, made by a pulsed laser (for example on medical devices or repaired jewelry).

Compare with

Arc welding7.1cheaper and more forgiving of gaps, but wider, shallower and hotter, with more distortion.Electron beam welding7.5an even deeper and cleaner weld from a beam of electrons, but it needs a vacuum chamber.Resistance spot welding7.2joins sheets only at points; a laser can make continuous stitches instead.

Watch it

Mech Vidya · The keyhole principle, animated

Ricky Explains Lasers · The laser source, fiber and welding head, opened up

TRUMPF Inc. · A scanner head jumping between welds at high speed

TRUMPFtube · A laser welding cell in a real factory

See real photos

Wikipedia: Laser beam welding — principle, keyhole and conduction modes, with links to photos
Wikimedia Commons photo search

Remember

A focused laser concentrates so much power that it opens a vapor keyhole and welds deep in one pass.
Narrow seams mean little heat, little distortion and high speed.
The price is costly equipment, strict safety rules and parts that must fit together closely.