Lesson 8.8ShapeChapter 8 · Additive manufacturing

Wire arc additive manufacturing (WAAM)

A welding robot stacks arc-melted wire into beads (directed energy deposition)

Wire arc additive manufacturing (WAAM) is 3D printing with a welding robot. An electric arc melts ordinary welding wire, and the robot stacks bead after bead into a part that can be meters long. It is the cheapest and fastest way to print big metal parts, from ship propellers to a pedestrian bridge.

WAAM robot cell · close-up of the arc and a cut through the wall, 5 stepsTop left: the robot cell. Top right: a close-up of the torch tip. Bottom: a cut through the wall before and after machining. Circled numbers match the steps below.

How it works

Feed the wire. A feeder pulls welding wire, typically about 1–1.6 mm thick, off a spool and pushes it through the torch.
Strike the arc. An electric arc (a continuous spark, thousands of degrees hot) jumps from the wire tip to the part. It melts the wire, and droplets fall into a molten pool on top of the wall.
Shield the pool. Gas flowing out of the nozzle, usually argon or an argon mix, keeps air away. Without it, the hot metal would oxidize (react with oxygen) and turn weak and porous.
Stack the beads. The robot drags the torch along the wall, and the pool freezes into a bead behind it. After each layer, the torch rises about 1–2 mm and lays the next one.
Cool, then machine. Each layer must cool a little before the next, or the wall sags. At the end, a milling machine cuts the wavy sides flat to the final size.

Key facts

Materials: Almost any weldable wire: carbon and stainless steels, aluminum, titanium, nickel alloys and bronze
Typical parts: Large near-net-shape parts (close to final shape, still needing machining): propellers, pipe fittings, flanges, aerospace frames, architectural structures
Part size: From about 0.2 m to several meters, limited by the robot's reach, its rails or a gantry
Precision: About ±0.5–1 mm as built; machining brings it to about ±0.05 mm
Surface finish: Rough and wavy as built, with ripples you can feel; machined faces are smooth
Batch size it suits: One-offs and small batches of large parts
Tooling and machine cost: No tooling. Built from standard welding robots and power sources: typically a few hundred thousand dollars, much less than laser systems. Wire costs far less than metal powder
Speed: Typically 1–4 kg of metal per hour, about ten times faster than powder-bed fusion

Strengths and limits

Strengths

Very high build rate for big parts
Low-cost equipment and cheap wire
Almost no size limit: the robot can ride on rails
Little wasted metal compared with machining from a solid block

Limits

Rough, wavy surfaces: machining is always needed
Coarse detail: no small holes, thin walls or fine features
Heat builds stress and distortion, so parts can warp
Waiting for each layer to cool slows the build

Where it's used

MaritimeShip propellers and propeller blades in bronze
AerospaceLarge titanium and aluminum parts printed as blanks, then machined
Oil, gas and energyPipe fittings, flanges and valve bodies, often as fast spare parts
Construction and architectureSteel nodes and structures, such as a printed pedestrian bridge in Amsterdam
SpaceRocket thrusters and large structural parts

How to recognize it

Walls made of visible stacked weld beads, like a coil of rope.
Often a rainbow heat tint (straw, blue and purple colors) on steel and titanium surfaces.
A thick base plate still attached, or saw marks where it was cut off.
Machined flat faces right next to rough, rippled ones.

Compare with

Directed energy deposition8.7the same weld-it-in-place idea with a laser and powder: finer and more precise, but slower and costlier.Arc welding7.1WAAM uses the same torches and wire; it stacks beads into a shape instead of joining two parts.Sand casting1.1also makes large metal parts, but needs a pattern and a mold; WAAM needs neither, so it suits one-offs.

Watch it

theworacle · A lab demo of titanium WAAM, including rolling the wall between layers

MX3D · A robot printing a rocket thruster bead by bead

MX3D · A large printed impeller seen up close

MX3D · The printed stainless-steel bridge lifted into place over a canal

See real photos

Wikipedia: Directed energy deposition — the process family WAAM belongs to; text with few photos
Wikimedia Commons photo search
Wikimedia Commons search: MX3D bridge — photos of the printed steel bridge

Remember

WAAM is robotic arc welding used to stack beads into a part.
It is the fastest, cheapest route to large metal parts, but they come out rough.
Expect to machine every functional surface after printing.