Metal cold spray - also known as supersonic particle deposition, or cold spray additive manufacturing - is a cutting-edge technology that has gained popularity over the past decade as a high-speed metal additive manufacturing process and a method for applying metallic coatings.

The process involves accelerating fine metal powders to high velocities and spraying them onto a substrate. Upon impact, they deform and create a solid-state metal. And, as the layers build up, they form dense coatings or parts.

This process offers many benefits over traditional thermal spray methods, such as improved coating properties, better adhesion, lower porosity, lower heat input, and the ability to coat a broader range of materials.

The metal cold spray also has benefits over melt-based metal additive manufacturing techniques due to the size of parts enabled by the process and the speed at which the parts can be produced. And, as the metal is not melted, it enables the combination and fusion of metals that typically cannot be produced together.

The importance of metal cold spray lies in its ability to enhance the performance of various metallic parts and components in different industries. In the aerospace industry, cold spray can be used to repair and improve the wear resistance of turbine blades, additively manufacture new parts, and repair corrosion damage. In the automotive industry, it can be used to coat engine parts to improve their corrosion resistance and repair damaged components.

In this article, we'll explain the fundamentals of metal cold spray, including the principles of the process and the materials that can be used. We will also examine the various applications of cold spray in different industries. We'll also discuss the methods used to optimize the cold spray process and what we can expect for cold spray in the future.

Fundamentals of Metal Cold Spray

As mentioned, the cold spray works by accelerating very fine metal powders to supersonic speed, and then spraying them onto a substrate. As the particles collide, they do so with such force that they create a solid-state metal.

Accelerating the metal particles to supersonic speeds is done by injecting the metal powders into a fast-moving gas stream. The carrier gas can be air, nitrogen, or helium.

A 'de Laval nozzle' (also known as a converging-diverging nozzle) is used to accelerate the gas to supersonic speeds. It works by using converging and diverging shapes to create a supersonic gas flow. Imagine you have a tube that gradually narrows and then gets wider again.

As the gas enters this tube at subsonic speeds, it increases in speed to sonic speeds through the narrower section and starts to 'choke'. As this 'choked' gas exits the narrow section, it expands and accelerates to supersonic speeds. The metal particles that are suspended in the gas are also carried to supersonic speeds.

In summary, the 'de Laval nozzle' uses a particular shape to make gas flow faster and faster by gradually increasing the space for the gas to expand as it flows through the nozzle. This concept is used in high and low-pressure cold spray processes to accelerate the metal particles to high velocities.

Cold spray versus thermal spray

One of the critical differences between cold spray and traditional thermal spray methods is that cold spray operates at much lower temperatures. In contrast, a heat source is used in traditional thermal spray to melt the metal particles, which are then propelled onto the substrate.

On the other hand, cold spray uses kinetic energy to deform the metal particles, eliminating the need for a heat source. This results in improved coating properties, less effect on the substrate, better bonding, and a higher coating or part density.

Many materials can be used in cold spray, including metals, metal/ceramic blends, and polymers. The properties and suitability of a material for cold spray depend on its melting point, particle size, and shape. 

The 'de Laval nozzle' is the component that is responsible for accelerating the carrier gas, but a complete system for manufacturing parts or for coating metal consists of numerous components such as:

1. Powder feeder: The system where the metal powder is loaded into and then assists in delivering the powder to the gun.

2. Spray gun: This contains the 'de Laval nozzle' and is responsible for heating the gas which thereby again increases the speed of the gas.

3. Robot: Used to control the motion of the gun 

Applications of Metal Cold Spray

Cold spray has found use cases for repairing gearboxes, shafts, blades and numerous other components that suffer wear and corrosion.

In the automotive industry, cold spray can be used to coat engine parts to improve their corrosion resistance and repair damaged components. One of the most significant applications for cold spray in the automotive sector is for repairing engine blocks, gearboxes, and transmission housings, where repairing with welding leads to substrate damage and heat-affected zones.

The defence industry has identified use cases for cold spray on-site battle damage repair. It is also used to repair and improve the wear resistance of military equipment and vehicles. Additively manufactured cold spray structures are being explored for armour protection as well as novel hybrid weapons barrels offering weight reduction and improved longevity.

In the marine industry, cold spray can be used to repair and improve the wear resistance of ship components, such as propellers and rudders, repair corrosion where welding is not acceptable due to substrate and component sensitivity. Large lightweight additively manufactured structures for example doors and casting replacements are also in use.

The mining industry struggles with corrosion and wear resistance due to the challenging environment and aggressive materials in use, cold spray has the ability to apply novel material combinations to wear surfaces expanding the life of mission critical components.

In general industry, cold spray can be used for the repair of bearing housings, additive manufacturing of seamless titanium tanks, and many other components.

The advantages of cold spray in these applications include improved wear resistance, corrosion protection, and the ability to repair parts without the need for excessive heating. 

Cold spray also allows for the application of specialised coatings, such as ceramic or mixed metal coatings, which can improve the performance of parts in high-temperature and high-stress environments. 

This is just a small subset of the vast array of industries and use cases where cold spray adds value.

Suitable Metals for Cold Spray

Ductility: The more ductile a metal is, the easier it will be to cold spray. Ductility is a material's ability to deform plastically (permanently) without breaking. This feature of a metal allows it to deform on impact and thereby create a solid metal again. The key to cold spray is knowing how to optimize your process to get the best performance as the metals you use become less ductile and have higher performance.

Particle size: Smaller particle sizes are more suitable for cold spray because they can be accelerated to higher velocities and are more easily deformed upon impact, but there is a fine balance between too fine and too coarse, and the challenge is to find the optimum balance of particle sizes to get the best performance.

Shape: Metal particles that are spherical or near-spherical in shape are more commonly used in cold spray due to availability. However, almost any shape of the requisite particle size and metal type can be sprayed.

Flowability: Metal powders with good flowability are more suitable for cold spray because they can be easily fed into the spray gun.

Thermal properties: The metal should have thermal properties that match the intended application. For example, if the coating is intended to protect against high temperatures, the metal should have a high melting point and high thermal conductivity.

Cost: The metal should be cost-effective, as the process is already cost-effective, highlighting the importance of this process in manufacturing.

Overall, a suitable metal for cold spray can be easily deformed upon impact, has good flowability, and has the correct particle size in powder form.

Cold Spray Process Optimization

The performance of cold spray is affected by several factors, such as:

Particle velocity: The velocity of the metal particles plays a key role in the deformation and bonding of the particles upon impact. Higher particle velocities lead to greater deformation and better bonding but also increase the risk of particle fragmentation and nozzle erosion.

Gas Temperature: The temperature of the gas, as well as the geometry of the 'de Laval nozzle', affect the particle velocity.

Spray distance: The distance between the nozzle and the substrate can affect the impact velocity of the particles and the amount of deformation and bonding that occurs. A shorter spray distance can lead to higher impact velocities and better bonding but also increases the risk of substrate damage.

Methods to optimize the cold spray process include computational modelling and experimentation. Computational modelling involves using computer simulations to predict the behaviour of the metal particles during the cold spray process. This modelling can help to optimize the process parameters, such as particle velocity and temperature, to improve the performance of the coating. Experimentation involves using test samples to evaluate the performance of the coating under different conditions, such as different particle velocities or temperatures.

Despite the advantages of cold spray, optimizing the process is challenging and a challenge that researchers has spent years on solving. Some challenges include controlling the particle velocity and temperature, minimizing the fragmentation and erosion of the particles, and minimizing the risk of substrate damage.

Future research directions in cold spray include:

• Developing new nozzle designs to improve the performance of the process.

• Developing new methods for controlling the particle velocity and temperature.

• Developing new methods for measuring and controlling the process in real-time.

Additionally, developing new materials suitable for cold spray and exploring new application areas will also be important in the future.

Conclusion

As metal cold spray research continues, and companies embrace the benefits that cold spray provides, we can expect to see this process become as common as welding in the general engineering environment and become a well known, viable and well used additive manufacturing process. 

Glossary of Terms

Cold Spray: A solid-state coating and additive manufacturing process in which metal particles are accelerated to high velocities and then sprayed onto a substrate.

Substrate: The surface or material onto which the coating is applied.

Particle velocity: The speed at which particles are propelled during the cold spray process.

Feedstock: The material in powdered that is used as the source for the particles in the cold spray process.

Gas Atomisation: A method of producing fine metal powders for use as feedstock in cold spray.

Powder particle size: The size of the individual particles in the feedstock powder.

Carrier gas: The gas, such as nitrogen, air or helium, that is used to propel the particles during the cold spray process.

Spray gun: The device used to accelerate and direct the particles during the cold spray process.

de Laval nozzle: A convergent, divergent nozzle used to accelerate the carrier gas

Impact energy: The energy imparted to the particles as they collide with the substrate during the cold spray process.

Adhesion (Bond Strength): The strength of the bond between the coating and the substrate.

Porosity: The presence of small voids or holes within the coating/part.

Density: The lack of porosity with the coating/part.

Density: The mass of a substance per unit volume.

Deposition rate: The rate at which material is deposited onto the substrate during the cold spray process.

Heat-affected zone: The area of the substrate that is affected by the heat generated during the cold spray process.

Microstructure: The structure of a material at the microscopic level.

Hardness: The resistance of a material to indentation or scratching.

Toughness: The ability of a material to absorb energy and deform plastically before breaking.

Tensile strength: The maximum amount of tensile stress a material can withstand before breaking.

Young's modulus: A measure of the stiffness of a material.

Compression strength: The maximum amount of compression a material can withstand before breaking.

Fatigue strength: The ability of a material to withstand cyclic loading without failure.

Creep: The deformation of a material under a constant load over time.

Ductility: A material's ability to deform without breaking.