A Beginner’s Guide to Cold Spray Terminology

Table of Contents

Automotive, Aerospace, and Mining Industries, among many others, currently use cold spray technology for the cost-effective repair of metal componentry. These repairs include the reformation of cosmetic features, enhancing metallurgical qualities, and completely rebuilding physical attributes.

According to the experts at VRC Metal Systems:

“Cold spray, referred to as supersonic particle deposition, is a high-energy solid-state coating and powder consolidation process. Cold spray uses an electrically heated high-pressure carrier gas, like nitrogen or helium, to accelerate metal powders through a supersonic de Laval nozzle above a critical velocity for particle adhesion. The bonding mechanism is a combination of mechanical interlocking and metallurgical bonding from recrystallization at highly strained particle interfaces.”

Well, that was certainly a mouthful. Let’s look a little more closely at each of those terms, some additional industry vocabulary, and see if we can all speak the same language. Then, contact the experts fluent in cold spray technology at VRC Metal Systems to partner together on your next cold spray technology solution.

Five industrial helium canisters of varying sizes

The supplies

Let’s start with the physical items needed to apply cold spray technology.

Alloy: a blend of metals (or other elements) in a solid solution. Steel is an example of an alloy of iron and carbon, often containing other alloying elements as well.

Metal powder: a powder consisting of a single, pure metal. Size range for cold spray ranges from 5-100 micrometers in diameter.

Alloy powder: powder created from a single molten alloy

Composite powder: a powder combining particles with two or more individual materials

Process gas: provides the driving force for accelerating the metal powders. The Process Gas is heated prior to being blended with the carrier gas and injected into the De Laval nozzle.

Carrier gas: the gas conveying the powder from the powder feeder through to the surfacing gun

Helium: a low-density inert gas used for filling balloons and airships. Often used in the cold spray process due to its high speed of sound.

Nitrogen: another inert gas often used as a carrier gas in cold spray technology. Nitrogen makes up over 78% of the earth’s atmosphere, yet it will not support life or combustion. Nitrogen gas is cheap and widely available, however, it has a lower speed of sound and cannot achieve the particle speeds that helium can.

De Laval nozzle: a nozzle that converts heat energy in the gas to kinetic energy (speed!). A slow and high-temperature gas goes in, and a fast and lower temperature gas comes out.

Substrate: the base material to which the coating is applied

Although far from an exhaustive list, the above items cover the basics. Now let’s move on to the cold spray process itself.

Flow regulator gages

The cold spray process

The cold spray process is a supersonic particle deposition where the powder is accelerated to a critical velocity by an inert gas. The resulting bonding is a combination of mechanical interlocking and metallurgical bonds from the recrystallization of highly strained particle interfaces.

Here’s a look at some of the terms related to that description.

Feed rate: The rate at which the consumable metal, alloy, or composite powder is fed through the cold spray device.

Gas flow rate: the rate at which the gas, usually measured in liters per minute, flows through the spray nozzle.

Deposition rate: the weight of material deposited onto the substrate over a given time, usually measured in kilograms/hr (kg/h) or pounds/hr (lb/h).

Critical velocity: the speed at which a spray of particles transitions from eroding the target surface to coating the target surface.

Mechanical interlocking: a mechanical mixing or blending of the impacting particles and the substrate or previously deposited layers of particles that effectively interlock the layers together. This interlock can transfer load and resist motion even if the particles are not metallurgically bonded to the substrate or other particles.

Metallurgical bond: is the type of chemical bond between atoms in a metallic element. Metallic bonding is what yields property characteristics in metals such as ductility, conductivity, and strength.

Recrystallization: new grain formation within the metal structure, often occurring to replace highly deformed (strained) grains. Dynamic recrystallization in the cold spray process can result in nanograin formation, which resists dislocation movement (high strength).

Plastic deformation: when a particle changes shape due to applied stress, pressure, heat, or force. The forces break atomic bonds, and the deformation is permanent.

Elastic deformation: deformation in the material below the yield strength that is fully recovered when the load is removed. This is similar to stretching and releasing a spring.

Metal part being cold sprayed

The results

Once again, from the experts at VRC Metal Systems:

“The cold spray process is applicable to corrosion-resistant coatings…, dimensional restoration and repair …, wear-resistant coatings …, electromagnetic interference (EMI) shielding of components and structures, high strength dissimilar material coatings for unique manufacturing solutions, and field repair of components and systems.”

And that would mean…

Corrosion: the chemical or electrochemical reaction of the substrate material with its environment leading to the deterioration of the base component. Rust is a typical example of electrochemical corrosion.

Wear: loss of material from a component surface due to some mechanical action such as erosion, abrasion, or fatigue.

Electromagnetic interference (EMI) shielding: using conductive or magnetic materials to form a barrier against electromagnetic fields. Electromagnetic shielding that blocks radio frequencies is known as RF shielding.

Field repair: Portable cold spray systems allow you to take the solution to the component, which is an extremely cost-effective way to handle the extensive component repair.

The benefits

In addition, some of the benefits of applying cold spray technology include:

Bond strength: the strength of the adhesion between the coating and substrate. For cold spray, this is typically > 10 kilopounds per square inch (KSI).

Coating strength: also referred to as spray deposit strength. Typical values achieved exceed 40 ksi.

Porosity: the amount of void space in a material, usually below 1% for cold spray coatings.

Oxidation: chemical reaction on the metal surface that occurs at high temperatures. Cold spray temperatures are low enough that no oxidation of metal powders occurs in the deposition process, leading to little to no oxide contamination in the coating.

Heat affected zone: the area of the substrate that increases in temperature due to coating application. Cold spray has inappreciable heat input, thereby minimizing the heat-affected zone.

Overspray: the portion of feedstock not deposited on the substrate. Cold spray technology achieves tightly focused particle spray paths, which minimizes the overspray.

The terminology detailed above is a rudimentary list.

The engineers at VRC Metal Systems have been working to commercialize this technology alongside engineering experts from the South Dakota School of Mines and Technology (SMSM&T), the Department of Defense, and the Army Research Laboratory for over ten years.

These solutions have military, as well as commercial applications.

VRC Metal Systems has the only high-pressure, portable cold spray system on the market. This combination of technologies has transformed the process applications.

Today, there is little need to disassemble equipment and bring large bulky plates to a repair facility. VRC Metal Systems sprayers go to the site, providing for cost-effective repairs and restorations.

Now that you’re familiar with some of the basic terminology used in the cold spray industry, contact the experts at VRC Metal Systems who truly speak the language. We’ll start working towards your next project solution.

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