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Cold Spray Process for Repair of Aluminum Parts

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Cold spraying is a technique that is prolonging the life of many high performance aluminum parts.  Durability is one of the advantages of aluminum but even the most resilient alloys are subject to wear and damage.

A relatively recent development, cold spraying allows manufacturers to protect and repair metal parts and products.  It is especially useful for critical components that would otherwise need to be replaced and offers many advantages over traditional techniques such as welding.

What is Cold Spraying?

Cold spraying, also known as gas dynamic cold spraying, is a material deposition process. It works by accelerating solid powders in a supersonic gas jet at velocities as high as 1200 m/s.  These powders can be made of several different materials, including metals, polymers, ceramics, and composite materials, and range in size from 1 to 50 micrometers in diameter.  When the powder collides with the surface at high speed it achieves plastic deformation, causing them to adhere to the surface.

At the molecular level, the kinetic energy that is present in the particles is generated by the gas expansion and is then converted into plastic deformation energy during the bonding process.  There is one crucial difference in the cold spraying technique that separates it from thermal spraying techniques, such as plasma spraying, arc spraying, flame spraying or high velocity oxygen fuel: the powder is not melted during the process.

This process was first developed by Russian material scientists in the 1990’s.  It was discovered somewhat accidentally while experimenting with particle erosion during the high-velocity flow of fine powder in a wind tunnel.  The scientists realized that there was an unexpected rapid formation of coatings occurring in certain circumstances.  They were able to commercialize the technique within a few years.

What are the basic principles of cold spraying?

The phenomenon of cold spraying is based upon adiabatic shear instability.  This occurs at the particle substrate interface when the critical velocity is reached by a spherical particle.  Upon impact, a strong pressure field is generated from the particle and substrate at the point of contact.  This pressure field forms into a shear load, which causes the material to accelerate laterally under localized shear straining.

The occurrence of adiabatic shear instability leads to viscous flow of material in an outward flowing direction.  Without any applied heating of the powder, the temperatures still come close to the melting temperature of the material.

There are other parameters recognized in cold spraying that manufacturers need to be aware of, especially when working with aluminum.  These factors can have a significant impact on the quality of and efficiency of cold-sprayed coatings.

The type of gas used can have a profound effect on how the coating is applied. Typical gas options include air, nitrogen, and helium; each one behaving differently.  Also important are the pressure and temperature of the gas.

Another important consideration is the size of the particles in the powder being used.  The density, strength and melting temperature of the feedstock material must also be considered.

The application of the surface treatment can also be controlled by the type of nozzle employed, as well as the traverse speed, scan velocity, the number of passes that are made, the distance between the nozzle and the substrate, and more.

All these parameters have a role to play in the performance of the final surface treatment. Finally, when selecting a cold spray material, it’s wise to assess the correlations between process parameters and final material properties.

What are the advantages of cold spraying?

Cold spraying has proven very effective compared with thermal spray techniques and has several recognized advantages. Because heating is not used, the initial physical and chemical properties of the particles are retained.  This means there is a cold-worked microstructure of coatings without melting or solidification.  Because of this, cold spraying can be used with thermally sensitive materials.  It’s also appropriate for significantly dissimilar material combinations because the adhesion is purely mechanical in nature.

The resulting coating exhibits high thermal and electrical conductivity.  It is also high in density and hardness, with excellent homogeneity and low shrinking.  The possible powder materials can be extremely small, with nanomaterials as an option.

Little to no special surface preparation is required, with low amounts of energy consumption during the process. Because of the high power feed rate, the process has high productivity, deposition rates, and efficiencies.

Finally, it’s possible to collect and reuse 100% of the particles, without any toxic waste or combustion.  With no high temperature gas jets or radiation, there is greater operational safety over comparable methods.

What are the disadvantages of cold spraying?

While the advantages are many, it’s important to recognize that cold spraying has some drawbacks.  Hard, brittle materials are vulnerable since mechanical adhesion through plastic deformation is not as effective as when using ductile particles.

Other problems include a near-zero ductility in the as-sprayed condition and the need for a ductile substrate.  Finally, there is the high cost of helium that must be taken into consideration.

 

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