Lesson 11.11CoatChapter 11 · Coatings and surface finishing

Cold spray

Solid metal grains weld on by sheer speed

Cold spray fires solid metal powder at a part so fast that the grains weld on when they hit, without ever melting. Think of it as thermal spraying (Thermal spraying) without the flame. It repairs worn aircraft and ship parts, and can even build whole metal parts layer by layer.

Photo: Александр Шоринов, CC BY-SA 4.0, via Wikimedia Commons

Cold spray · heater, powder feed, supersonic nozzle, impact close-upThe top shows the gun from heater to nozzle; the close-up shows what happens at impact. Circled numbers match the steps below.

How it works

Heat the gas. Nitrogen or helium at high pressure (often tens of times normal air pressure) is warmed, typically to a few hundred degrees and at most about 1,000 °C. That stays well below the melting point of the powder.
Add the powder. A feeder meters fine metal grains, typically about 10–50 µm across, into the gas stream.
Accelerate. A converging-diverging nozzle (one that narrows, then widens, called a de Laval nozzle) makes the gas supersonic. It drags the grains up to roughly 500–1,000 m/s.
Impact and bond. Each solid grain squashes flat on impact. The crash breaks its thin oxide skin, and clean metal presses against clean metal and welds in the solid state (without melting).
Build up and machine. A robot sweeps the nozzle across the part, and grains pile up into a dense deposit from a fraction of a millimeter to several centimeters. Because it is rough, the deposit is then machined to shape.

Key facts

Materials: Ductile metals: aluminum, copper, titanium, nickel, tantalum, steels; some metal-ceramic mixes. Hard, brittle powders spray poorly
Typical parts: Repaired housings and shafts, corrosion coatings, copper layers, rocket nozzles, rings
Part size: Small repairs up to deposits several centimeters thick on large parts; limited by robot reach
Precision: Coarse as sprayed, around ±1 mm; final shape comes from machining
Surface finish: Rough and matte as sprayed; machined surfaces look like solid metal
Batch size it suits: Repairs, one-offs and small series; production coatings
Tooling and machine cost: No molds; systems often hundreds of thousands of dollars, plus gas (helium is expensive)
Speed: Fast deposition, typically kilograms per hour for aluminum or copper

Strengths and limits

Strengths

No melting: no oxidation, little heat damage to the part
Dense deposits, from coatings to thick builds
Can join metals that are hard to weld together
Fast and good for repairs on site

Limits

Works only with ductile (bendable) metals
Rough, near-net shape that must be machined
Line of sight, like any spray
Helium for top quality is costly, and the jet is very loud

Where it's used

Aerospace and defenseRestoring corroded magnesium and aluminum gearbox housings
SpaceCopper and nickel layers for rocket engine nozzles
Electronics and energyCopper on aluminum for heat sinks and electrical contacts
Oil, gas and marineCorrosion-resistant layers, shaft and valve repair
ManufacturingAdditive builds of rings, flanges and near-net-shape parts

How to recognize it

A thick, dense metal build-up on an old part, often of a different metal than the part itself.
No heat tint, scale or burn marks around the repaired area, unlike welding.
As sprayed, a rough, matte surface like coarse sandpaper; after machining, plain metal.
Under a microscope, flattened grains packed tightly with almost no pores.