MIL-STD-3021 w/Change 2
1. This standard is approved for use by all Departments and Agencies of the Department of Defense.
2. The purpose of this standard is to develop the manufacturing process controls for a cold spray operation utilizing a high-velocity jet of solid-phase particles. The jet temperatures are below the melting thresholds of many engineering materials. This allows the process to be used to apply deposits on a wide variety of substrates, such as, alloys, ceramics, and plastics, as well as to produce bulk materials and near-net shaped parts. The uniqueness of cold spray lies in its ability to produce a coating, provide dimensional restoration or even produce a near-net shaped part at temperatures well below the melting point of the powders being applied, thereby avoiding or minimizing many deleterious high temperature reactions which are characteristics of typical thermal spray processes. This ensures that the input powder materials do not experience any grain growth, oxidation or phase changes when deposited. It is this characteristic of cold spray that makes it attractive as a method for coatings or for dimensional restoration, and/or the production of bulk materials or near-net shaped, while retaining their own unique material properties.
3. This standard is required because the cold spray deposition process has significant differences from thermal spray coating technologies such as high velocity oxy-fuel (HVOF), detonation gun, plasma spray, flame spray, and arc spray. These process differences result in application criteria, process equipment, and operating parameters that are considerably different than previously documented processes. The objective is to enable the application of Cold Spray Deposition with the success of the aforementioned processes when properly applied.
4. Comments, suggestions, or questions on this document should be addressed to: Director, U.S. Army Research Laboratory, Material & Manufacturing Science Directorate, Materials Manufacturing Technologies Branch, Specifications and Standards Office, Attn: RDRL-WMM-D, Aberdeen Proving Ground, MD 21005-5069 or emailed to firstname.lastname@example.org. Since contact information can change, you may want to verify the currency of this address information using the ASSIST Online database at https://assist.dla.mil.
MIL-STD-3021 w/Change 2
The procedures covered by this standard are intended to ensure that cold spray operations, either manual or automated, meet prescribed requirements. This standard ensures that cold spray operations are repeatable and consistent. However, additional qualification requirements may be required based on each specific application. Therefore, for structural applications and other applications that are considered critical - (safety critical or FOD critical), a qualification plan shall be provided by the Cognizant Engineering Authority (CEA) (see 3.13) and shall follow the recommendations specified in Appendix A and shall be specified in the contract or purchase order (see 6.2).
Cold spray is a process whereby powder particles are utilized to form a coating, provide dimensional restoration and/or the production of bulk, or near net shaped, materials by means of supersonic impingement upon a substrate. The powders can be metal, ceramic, and polymeric or combinations thereof, typically range in particle size from 1.0 to 100 micrometers (µm) and are accelerated by injection into a high-velocity stream of gas. The powder particles remain solid throughout the deposition process, which is the primary difference between cold spray and traditional thermal spray processes. The high velocity gas stream is generated through the expansion of a pressurized, preheated, gas through a nozzle. The nozzle may be a contracting expanding supersonic type or a contracting sonic type. The pressurized gas is expanded in order to achieve high velocity, with an accompanying decrease in pressure and temperature. The powder particles, initially carried by a separate gas stream, are injected into the nozzle either at the nozzle entrance (High Pressure Powder Injection) or at a lower pressure point downstream of the entrance (Low Pressure Powder Injection). The particles are then accelerated by the main nozzle gas flow and are impacted onto a substrate after exiting the nozzle. The solid particles that impact the substrate above a threshold (critical) velocity for the powder and substrate combination will deform and bond in a dense layer. As the process continues, particles continue to impact and form bonds with the previously deposited material resulting in a uniform deposition with very little porosity and high bond strength.
Applications for cold spray technology often occur in situations where conventional thermal metal spray technology cannot be successfully used and where cold spray will result in an improved deposit. These situations often occur when high temperatures cannot be tolerated by the substrate. Cold spray is often used for dimensional restoration and repair of mating surfaces, but is not limited to those applications. Good corrosion protection is achieved by dense cold sprayed deposits. Wear resistant, hard surfaces, such as MCrAlYs, can be deposited by cold spray when operated at its high-temperature end. The cold spray process has been used to produce dense, pure, thick and well bonded deposits of many metals and alloys, such as aluminum (Al), copper (Cu), nickel (Ni), tantalum (Ta), titanium (Ti), silver (Ag), and zinc (Zn), as well as stainless steel, nickel-base alloys (Inconels, Hastalloys), and bond coats, such as MCrAlYs. Cold spray can produce composites, such as metal-metal like copper-tungsten (Cu-W) or copper chromium, metal-carbides like aluminum-silicon carbide (Al-SiC), and metal-oxides like aluminum-alumina. Polymeric materials have also been deposited by the cold spray process. Cold spray has been used to produce protective coatings and performance enhancing layers, ultra thick deposits for dimensional restoration, freeform and near net shape parts. Typical protective coatings produced by cold spray include MCrAlY coatings for high temperature protection and bond coats for thermal barriers, copper-chrome layers for oxidation protection, and corrosion resistant aluminum and zinc coatings for oil and auto industries and others."
To view the General Cold Spray Manufacturing Process Standard document in its entirety, please visit http://everyspec.com/MIL-STD/MIL-STD-3000-9999/download.php?spec=MIL-STD-3049.047765.pdf or download the MIL-STD-3049 pdf file for the latest release on 04-MAR-2015.