What do the aerospace, shipping, oil, gas, and mining industries all have in common? They have high-dollar components that cause even higher-dollar down-time when they require repair or replacement.
These industries have traditionally relied on thermal spray processes, like flame spraying and arc spraying. These modalities have been in development since the early 1900s, to repair and enhance original equipment.
Plasma spraying, another method of equipment repair, arrived in the industry about 50 years later in the ‘70s.
Cold spray processes are a much more recent development, gaining popularity in the 1990s. Portable, high-pressure, cold spray equipment, like that developed by VRC Metal Systems, is at the forefront of this emerging technology.
You can find more information on cold spray vs. thermal spray below. To determine how cold spray technology may apply to your next project, contact the experts at VRC Metal Systems.
What is it?
Cold spray is a coating deposition process in which powdered material is accelerated to supersonic speeds, as high as Mach 3. The accelerated powder is deposited onto a similar or dissimilar material surface. The powders reach high velocity using a high-pressure, electrically heated carrier gas, like nitrogen or helium.
Sending the gas and powders through a de Laval nozzle accelerates the powder to supersonic speed. The de Laval nozzle is a tube that contains an internal asymmetric hourglass shape and converts the heat energy of the flow into kinetic energy. Jet engines and steam turbines also use these types of nozzles.
Once the particles achieve these high speeds and impact the material surface, the particles undergo a plastic deformation causing them to bond to the target surface. The mechanical interlocking of the particles, as well as recrystallization at the surface interfaces, creates a strong bond between particle and surface.
There are two types of cold spray:
High-pressure cold spray (HPCS) – uses nitrogen, helium, or air at pressures ranging from 300-1000 psi, and is used for spraying high strength metals and alloys.
Low-pressure cold spray (LPCS) – uses the same compressed gases, at pressures below 300 psi, and is used for spraying soft metals and mixtures of metals and ceramic powders.
Using cold spray
Cold spray powders can be pure metals, alloys, or be a mixture of metallic and non-metallic particles. These powders allow for the application of material coatings, the repair of a surface with similar or improved materials, or the component features by spraying onto a surface and then machining the desired features into the cold spray.
Cold spray materials can create wear-resistant coatings, improving the life-cycle of a component subject to severe conditions. The cold spray process can also provide a corrosion protection coating, perfect for marine and industrial applications using bronze, stainless steel, zinc, aluminum, or even tantalum, just to name a few.
This process improves or repairs aluminum, nickel, or titanium components, among many others. Dimensionally modifying or repairing the original piece is most often a more economical solution to replacing the component completely.
Why is it used?
Cold spray is a “green” type of technology. There is no chemical heating and no melting in the process, so there are no toxic fumes. Furthermore, the waste materials are recyclable and repairing vs. replacing parts saves energy and reduces CO2 emissions.
Due to the focused particle spray path, minimal masking of the substrate area is required, and there is no heat-affected zone either. There is also no oxidation, alloy decomposition, or combustion product entrapment with a cold spray process.
Using cold spray is also an extremely cost-effective alternative to replacing the whole part. For example, a 40” x 40” panel on a B1 bomber can cost up to $200K to replace with an 18-month lead time. Depending upon the extent of the damage, you can repair this same panel using cold spray technology in days instead of months and for a tiny fraction of the replacement cost.
The portable cold spray equipment developed by VRC Metal Systems also makes cold spray the optimal solution for parts where removal for repair is not an option.
What is it?
Thermal spray is another coating deposition process. However, the material in this process is in the form of molten, or semi-molten, droplets that are sprayed onto a surface. The materials used can be heated by electrical or chemical means.
Typical materials include metals, ceramics, and polymers, but can include anything that melts or becomes plastic during the heating process. You can typically use thermal spraying to apply coatings of .1 to 1mm in thickness but sometimes thicker in limited cases.
A thermal spray bond is mechanical in nature, not metallurgical or fused. The condition of the substrate surface is critical and must be clean and roughened before spraying.
Below, we provide more information on the several types of thermal spraying.
Plasma spraying: uses an electrical means to heat the coating material. An electric arc forms a high-temp plasma jet. The coating material feeds into the plasma jet as well as an inert gas, which expands rapidly to create a high-velocity spray of particles.
Arc spraying: is another process that uses an electrical means to heat the coating material. An electrical arc initiates between two coating material sources, causing them to melt. Compressed air atomizes the coating and propels the droplet towards the desired surface.
Flame spraying: uses a chemical means of heating. A fuel gas such as propane or hydrogen, and oxygen, mix to heat the coating material either in wire or powder form. Inert gas propels the coating at the substrate.
HVOF (High-velocity oxy-fuel) spraying: is another chemically heat-produced process. The heat and pressure generate from combusting a liquid or gas fuel mixed with oxygen. The spray particles heat and expand in a chamber forcing the exhaust gases out at supersonic speeds towards the substrate.
How is it used?
Similar to cold spray, thermal spray produces coatings on substrate materials, enhancing the characteristics of the original materials, changing their appearance, or creating dimensional surface repairs.
Thermal spray coatings help significantly in the manufacturing and repair of oil field equipment, diesel engines, gas turbines, and coating medical implants.
Why is it used?
One of the main advantages of using thermal spray is that coatings can be applied at high deposition rates, but generally lose strength and toughness at thicknesses greater than 1mm. Thermal spray can be used as an alternative to nickel and chrome plating, nitride or heat treat processes, weld overlay, or anodizing.
Like cold spray, thermal spray can sometimes repair parts at a fraction of the cost to replace the same part, but it depends greatly on the types of materials involved and the sensitivity of the component. Thermal spray can also be used to apply wear-resistant coatings to extend the life of a component, but again with certain limitations. In many cases, the current limitations of these higher temperature processes can be overcome by cold spray.
Cost-effective restoration and repairs
VRC Metal Systems has revolutionized the process of applying cold spray technology. Our cold spray system is both high-pressure and portable and provides fast, cost-effective restoration and repairs. Contact us today to find out how we can support and improve your next project with our cold spray products and services.