materials & manufacturing research areas
Cold Spray or High Velocity Particle Consolidation (HVPC)
The Metals and Ceramic Processing Department has been working in Cold Spray for more than a decade and has several active research programs. The process is also known as Cold Gas Dynamic Spraying, Kinetic Metallization, and Supersonic Particle Deposition.
Cold Spray is the process of applying metal and composite (metal/ceramic) powders onto a substrate by accelerating to velocities ranging from 400 to 1000 m/s. Upon impact the metal particles deform and bond with the substrate. Particles continue to impact the surface bonding with the already deposited particles building up very dense coatings. The deposition rate for the Cold Spray process can be as much as 3 kg/hr or higher. (Please review a PDF of iMAST Newsletter, Issue 2007 No. 1 , article by Timothy J. Eden, Ph.D. - on page 3 of the newsletter).
There are many different coating/substrate combinations that can be produced using the Cold Spray Process. Metal and metal/composite coatings have been applied to aluminum, alloys, steel alloys, titanium alloys, copper, copper alloys and ceramics. The coating must contain some ductile material to allow for plastic deformation during the deposition process. A partial list of the coating/substrate combinations is listed below.
|Partial List of Coating/Substrate Combinations|
|CP Aluminum(commercially pure)||Al 6061, Al 2024, Al 7050, steel Alloys, Magnesium Alloys||Corrosion Resistance|
|HP Aluminum (high purity)||Magnesium Alloys||Corrosion Resistance|
|Zinc, Zinc alloys||
|Nickel||Steel alloys, Titanium Alloys||Corrosion Resistance|
|Nickel – Chrome Carbide||Steel Alloys||Wear Resistance|
|Nickel Chrome-Chrome Carbide||Steel Alloys||Wear Resistance|
|Copper||Aluminum Alloys, Ceramics||Direct Write electrical circuits and antennas|
|Copper composite||Ceramics||Direct Write electrical circuits and antennas
|Aluminum Composite||Aluminum Alloys||Wear Resistance|
|Nickel –Aluminum-Bronze||Aluminum Alloys||Wear/Corrosion Resistance|
|7XXX Al||7XXX Al||Corrosion Resistance|
|5XXX Al||5XXX Al and Magnesium Alloys||Corrosion Resistance|
|Stainless Steel||Stainless Steel||Part Restoration|
|Nickel-Solid Lubricant||Ti64||Fretting Wear Resistance|
Product: AAV Panel
Product: Circuit Board
HVPC Coating Application
The Cold Spray Process operates at low process temperatures and high particle velocities. The difference between the Cold Spray Process and other thermal spray processes is illustrated below:
Thermal spray processes typically result in the development of voids and inclusions in the material, as shown in the illustration to the right.
In the cold spray process, advances in nozzle design, process optimization, and powder processing have lead to the ability to deposit very dense coatings. Aluminum, aluminum alloys, copper, stainless steel and nickel have all been deposited with densities greater that 99%. The process also produces excellent material interaction at the coating/substrate interface. An example of the unique microstructure and coating/substrate interface that can be produced by Cold Spray is shown in the figure below. The substrate is Al-6061 and the coating is copper powder. The coating is 99.9% dense. As a result of the process, the high velocity particles are subject to severe amounts of plastic deformation. This plastic deformation can also occur in the substrate as well. The result is a coating/substrate interface where there is mechanical mixing of the material that is free of voids and inclusions. The extent of the interfacial mixing is dependent on the coating/substrate material system.
Micrograph of copper powder deposited on Al-6061. The mixing of the materials at the interface is clearly visible.
Improvements in nozzle design have lead to higher deposition velocities and the ability to deposit larger particles. Gas dynamic models were used to design nozzles that can substantially increase particle velocities, which results in denser coatings and higher deposition efficiency. Increasing the length of the nozzle has been shown to have a significant effect on particle velocity. For example, by increasing the length of the nozzle from 83mm to 211mm, with nitrogen as the carrier gas, the calculated velocity of a 12µm copper particle can be increased from 553m/s to 742m/s. This is a 33% increase in particle velocity. The increased velocity leads to an increase in the deposition efficiency from less than 10% to close to 80%. However, there are fabrication and material constraints that limit the practical length of the nozzles. Other nozzle improvements include the use of new materials to improve powder flow through the nozzle and design optimization to minimize the gas flow through the nozzle.
A software package called Cost Analysis Software (CAS) was developed to accurately calculate the cost of applying a coating using the Cold Spray Process. Code inputs include the cost of the powder, gas, and electricity, nozzle dimensions, carrier gas, powder mass flowrate, substrate dimensions, deposition efficiency, desired coating thickness, start up and shut down times, and if desired, labor, burden and equipment amortization. The output includes cost by category (gas, powder, labor, burden, and amortization), cost per unit area, spray time, and the number of passes required to achieve the specified coating thickness. The CAS can also be used to determine deposition efficiency given the process parameters. The CAS was calibrated through experimentation and on the Navy Mantech Project C0934 AAV Enhanced Appliqué Armor Kit (EEAK) Product Improvement. It was demonstrated during this program that the Cold Spray process was cost competitive with wire-arc spray for the deposition of aluminum on steel. Future plans for the CAS include developing modules for complex geometries and to predict the cost of other thermal spray processes.