Heat Treatment of Gas-Atomized Powders for Cold Spray Deposition
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gas dynamic spraying (cold spray) is an additive process used for producing metal coatings and deposits. This deposition is achieved by accelerating metal powder particles (5 to 50 lm in diameter) with a supersonic gas jet produced by expanding heated, high-pressure gas through a De Laval nozzle. When these metal particles collide with another piece of metal, the incoming particle will deform and form a metallurgical bond with the substrate.[1] Unlike traditional thermal spray techniques, metal powders used in cold spray never melt during the process. For this reason, cold spray is a low-heat input process that can be used to repair damaged components made from alloys highly sensitive to heat input, for example 2000, 6000, and 7000 series aluminum alloys.[2] One of the chief challenges for cold spray deposition is the quality of the feedstock powders. Inert gas atomization is one of the most commonly used methods for producing powders for cold spray, particularly for aluminum alloys.[3] Metal powders produced this way
WILLIAM A. STORY and LUKE N. BREWER are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Box 870202, Tuscaloosa, AL 35487-0200. Contact e-mail: [email protected] Manuscript submitted September 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
typically exhibit a cellular/dendritic microstructure, with solute atoms such as zinc or copper segregating along the cell boundaries.[4,5] For precipitation-strengthened aluminum alloys, this means that there will be less solute to form strengthening precipitates as most of the solute atoms are segregated on the cell boundaries, forming a brittle intermetallic network. Such a microstructure will result in lower strength and ductility. Typically to eliminate such a cast structure, a metal is heated to above the solvus temperature for the second phase but below the solidus temperature and held long enough for intermetallics to dissolve. Once the intermetallics are fully dissolved, the metal is quenched at a rate high enough to prevent re-precipitation of the second phase. After such a solutionizing heat treatment, aluminum alloys are then strengthened by either natural or artificial aging. As cold spray is often used to repair damage to high-value aircraft components, such a heat treatment to the material after deposition would be impractical in many applications. Additionally, by removing the intermetallic network through heat treatment, particle formability will be improved.[6] As cold spray is a process based on deformation of individual particles, we would expect to see increased deposition efficiency and higher quality deposits because of this enhanced formability.[7] In order to heat treat metal powders, several factors must be taken into consideration. Fine, spherical aluminum powder is highly flammable, so heat treatment must take place in an inert atmosphere. For heat treatment of precipitation-strengthened aluminum alloys, the solutionizing temperature is normally close to the solidus temperature, which can re
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