What is Cold Spray?

The Process

Cold spray, also 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 re-crystallization at highly strained particle interfaces.

Cold Spray can create mixtures of metallic and nonmetallic particulates to form a coating or free­standing structure by means of ballistic impinge­ment upon a substrate. The Cold Spray process is applicable to corrosion-resistant coatings (zinc and aluminum), dimensional restoration and repair (nickel, stainless steel, titanium, and aluminum), wear-resistant coatings (chromium carbide – nickel chromium, tungsten carbide – cobalt, and tungsten copper), electromagnetic interference (EMI) shielding of components and structures, high strength dissimilar material coatings for unique manufacturing solutions, and field repair of components and systems.

Benefits of Cold Spray

Cold spray is an efficient method for the application of Metals, Metal Alloys, and Metal blends for numerous applications:

  • Very low heat input with no “heat-affected zone”
  • Structural properties can be achieved
  • No real limit on deposition thickness
  • High deposit efficiency typically > 80%
  • Bond strengths > 10 ksi
  • Coating Strengths > 40 ksi
  • Porosity commonly below 1%
  • Powder microstructure and properties are preserved
  • No oxide formation, alloy decomposition, combustion product entrapment
  • Compressive residual stresses in coating, rather than tensile

How Cold Spray Works

Physics and Metallurgy

Using very high particle velocities you can create metal coatings that are anywhere from 2 to 10 times stronger than conventional thermal spray coatings, depending on the material deposited. Physics and MetallurgyThe step change in performance is the result of a shift from predominantly mechanical interlocking to primarily metallurgical bonding resulting from a high degree of re-crystallization at highly strained particle interfaces.  The greater the extent of the re-crystallization across the particle boundaries, the closer the properties of the coating come to meeting book values for wrought materials.

Particle interface with recrystallized nano-grains in Al 6061 cold spray.

EBSD map of Al 7075 cold spray.

Microstructural Observations

Cold spray presents some fascinating microstructural characteristics:

  1. Because the process occurs rapidly (<<1 sec), there is virtually no time for particle oxidation or phase transformation, even if using Air as the carrier gas, or exposing the particle to high temperatures.
  2. Coatings show signs of true dynamic recrystallization and the formation of nano-grains at particle-particle interfaces.
  3. Coatings generally have high dislocation density and consequently similar or higher hardness than the base alloy.
  4. Therefore, coatings also generally have much lower ductility than the base metal, HOWEVER, high pressure cold spray can have ductility of 3-5% or more.
  5. Cold spray heat input to a substrate can be minimized, and has been shown to create no measurable heat affected zone (HAZ) in substrates of aluminum alloys like 7075.

Cold Spray High Pressure Gas Carrier

Helium vs Nitrogen

In a supersonic nozzle, the speed of the gas is related to the speed of sound of the gas and its Mach number. For Helium, the speed of sound at standard atmospheric conditions is 1007 m/s, but for Nitrogen it is only 349 m/s. This translates to higher particle velocities when Helium is used versus Nitrogen, thus, if cost and availability were not in question, then all cold spray deposits would probably use helium as the preferred gas.

Gas Mixing

A cost saving approach to pure helium is gas mixing of helium and nitrogen. Using the VRC Gen III cold spray system, any percentage of helium from 0-100% can be selected to optimize both cost and quality in your cold spray depositions. In this way, higher particle velocities can be achieved than nitrogen alone, but at a lower cost than pure helium.

What About Air?

Our heaters are designed to work with Air also, and Air can work with cold spray as well as Nitrogen for many applications. It is made up of approximately 78% Nitrogen, 21% Oxygen, and 1% other gases, and with a speed of sound of 343 m/s it is nearly equivalent to Nitrogen. However, the additional Oxygen can have a detrimental effect for some materials, nevertheless it can be a enabler for a large number of applications due to the significant cost savings.

Benefits of High Pressure

  • Better properties
  • Higher Mach numbers at the nozzle
  • Higher strengths with Nitrogen or Air

Higher gas pressures can be used to achieve extremely high strength coatings using cold spray. This is accomplished in two ways. First, higher pressures increases the density of the gas which can improve its ability to accelerate particles in the gas. Second, a higher expansion ratio nozzle can be used to increase the Mach number of the gas. Higher expansion ratios require higher stagnation pressures at the nozzle inlet.

By designing the VRC Gen III cold spray system with a working pressure of 1000 psi, we have increased the maximum possible Mach number available at the nozzle. At higher pressures, greater nozzle expansion ratios can be used which increase the Mach number, which can increase the gas velocity and subsequently particle velocities. Higher pressures also open the door for better quality deposits with nitrogen gas and compressed air. In this way, particle velocities that are closer to Helium at lower pressures can be realized.

Advanced computational fluid dynamic (CFD) modeling, can be a helpful way to understand the benefits of high pressure cold spray, as well as provide a method for predicting processing parameters for cold spray deposits needed to achieve a given particle velocity and temperature.

Cold Spray vs. Thermal Spray

Cold Spray is in the family of thermal spray processes; however, it has the lowest overall temperatures and highest velocities of the thermal spray family. As a result, cold spray coatings are deposited in the solid state and possess the highest strengths of any thermal spray process. Cold spray coatings cause almost no microstructural changes in the powder materials deposited except for extreme plastic deformation, and it does not increase the oxide content in the coating over the base oxygen level present in the starting powder. Because it is a solid-state process the coatings are in a generally compressive rather than tensile residual stress state. This can have a positive impact on fatigue and mechanical strength of the coating.

Benefits of Cold Spray vs. Thermal Spray

  • No heat affected zone
  • No oxidation of cold spray materials
  • Higher strength coatings for most metal alloys
  • No intermetallic formation for dissimilar metal coatings
  • No limit on deposition thickness
  • Minimal masking requirement due to focused particle spray path
  • No toxic fumes
  • Precise gas temperature control
  • Hand operable

Cold Spray Applications

Recent advancements in cold spray technology are dramatically expanding what you can do with thermal spray processes.  Cold spray can be used to deposit metals on sensitive or difficult-to-weld surfaces for never before possible combinations of unique materials.  Cold spray can also be used to repair damaged areas of a finished part, without causing additional problems like warping, cracking, or softening of the part.  Furthermore, properties approaching, or in some cases even exceeding the base material properties can now be achieved.  This opens the door for cold spray to be used on more than just cosmetic surface restoration, but loaded areas can be repaired including rebuilding entire features on a part.

Cold spray has been applied to numerous high strength aluminum parts, but it has also been used to deposit stainless steel and other high strength steels, bronze alloys, nickel alloys, titanium, and even exotic elements like tantalum and niobium. Cold spray doesn’t fix everything, but it has the ability to solve challenges that other technologies simply can’t touch. And just as importantly, once a cold spray process has been developed, it can be applied reliably and repeatedly for a given application.

Industrial Applications

  • Mining
  • Oil & Gas
  • Transportation
  • Heavy Industry
  • Automotive
  • Metal Art & Statuary
  • Shipping
  • Rail
  • Power Plants
  • Structural Corrosion Repair
  • Corrosion Resistant Coatings
  • Electronics
  • Heat Exchangers
  • And many more…

 

Unfortunately, it isn’t possible to show most of the industrial applications that have been developed in order to protect the often-proprietary nature of the work. There are however, some examples that can be shown of repair applications that are included to the right (Figure 1):

Figure 1: Cast aluminum engine transmission case with two (2) large holes (0.5-in diameter) in the top side of the case. The repaired holes are shown in the bottom right, and the inside of the case showing the repair zones (bottom left).

Department of Defense Components

The technology has also been under development for the military as a method to dramatically reduce maintenance costs across the Department of Defense by repairing previously un-repairable assets, such as the KC-135 Ruddervator part shown in Figure 3.  Another example is a very large aluminum 6061 Navy valve actuator with corrosion damage and internal bore sealing surface wear, which was cold sprayed, re-machined, and returned to service with a savings of over $40,000 per part compared to a new replacement.  A collage of the part and the repair process are shown in Figure 4.  The as-sprayed 6061 coating had a tensile strength of 38 ksi [262 MPa], 3% ductility, with adhesion strengths well in excess of 10 ksi [69 MPa].

Figure 2.  KC-135 Ruddervator Aluminum 7075-T6 Fitting Repair with Al 7075 sprayed on to rebuild a damaged bearing mating surface, with 50% of the re-machining complete.

Figure 3.  Cold spray repair of the 3.5 inch internal bore portion and external corroded surfaces of a large Al 6061 valve actuator for an application in the Navy.1

Figure 4.  Ti-3Al-2.5V alloy tube cold sprayed with commercially pure titanium for wear protection to prevent chaffing at contact points with the tube.

Another application developed is a chaffing prevention and possible repair coating for titanium hydraulic lines, as shown in Figure 5.

1 C. A. Widener, M. J. Carter, O. C. Ozdemir, R. H. Hrabe, B. Hoiland, T. E. Stamey, V. K. Champagne & T. J. Eden (2016). Application of High-Pressure Cold Spray for an Internal Bore Repair of a Navy Valve Actuator, International Journal of Thermal Spray, 25(1-2), 193-201.

Cold Spray Materials

Cold spray has been successfully demonstrated on a very broad range of metallic, ceramic, and thermosetting polymeric materials. While not exhaustive, the materials below represent some of the deposits that we have specifically evaluated to date using our system.

Aluminum Alloys

Al 6061

The most common high strength Al alloy, cold spray strengths >40 ksi while maintaining >3% ductility are repeatable

Al 7075

Used in aerospace, the highest strength cold spray deposits have been achieved with this alloy (>60 ksi).

Al 2024

Also commonly used in the aerospace industry, strengths over 50 ksi with 4-6% elongation as-sprayed have been achieved.

Steel Alloys

SS 316

Adhesion Strength > 10 ksi

4340

Adhesion Strength > 10 ksi

M300

Adhesion Strength > 10 ksi

Copper Alloys

CP Cu

CP Copper on 1100 Cu
Adhesion Strength > 10 ksi
Hardness: 180 HV

DT 31

Adhesion > 10 ksi
Tensile strength > 56 ksi

500A Cu

Copper on Al 7075
Adhesion Strength > 10 ksi
Hardness: 190 HV

Other Materials and Alloys

CP Ni

CP Nickel on Aluminum
Adhesion Strength > 10 ksi
3-lug Shear Strength > 18 ksi
Hardness: 210 HV

CP Ti

CP Titanium on Ti 6Al-4V
Adhesion Strength > 10 ksi

Tantalum

The most common high strength Al alloy, cold spray strengths >40 ksi while maintaining >3% ductility are repeatable.

316 SS + CrC

Adhesion strength > 10 ksi
High strengths > 100ksi
Hardness:

Ti + BAM

BAM is a unique and patented high hardness low-friction material developed by AMES and can be licensed from New Tech Ceramics.