An Investigation on Powder Injection in the High-Pressure Cold Spray Process
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JTTEE5 18:320–330 DOI: 10.1007/s11666-009-9329-y 1059-9630/$19.00 ASM International
An Investigation on Powder Injection in the High-Pressure Cold Spray Process T. Han, B.A. Gillispie, and Z.B. Zhao (Submitted March 30, 2007; in revised form April 9, 2009) High-pressure cold spray process is a relatively new coating process that uses high-velocity powder particles to form coatings. One of the requirements for this process is to inject the particles to be sprayed into a prenozzle chamber where both the particles and the powder feed gas are entrained into the primary gas stream. In this study, we investigated the effects of powder injection on coating formation through both experimental studies and computational simulations. Several issues related to powder injection will be examined, including the size of powder injector, the differential pressure, powder gas flow, and injector clogging. It is shown that an improved powder injector design not only enables the use of reduced amount of powder carrier gas flow but also maintains steady, clogging-free spraying conditions. Combining with properly selected injection conditions, it can also lead to enhanced coating deposition by kinetic spray process.
Keywords
coating deposition, coating formation, high-pressure cold spray process, kinetic spray, powder injector, spray nozzle
1. Introduction The high-pressure cold spray is a process that utilizes solid-state particles of various material types to form coatings (Ref 1-7). In this coating process, powder particles are accelerated by a supersonic gas stream generated by a de Laval-type spray nozzle. When the particles exit the nozzle at high velocities and impinge on a substrate, the powder particles undergo significant plastic deformations as a result of collisions and bond to the substrate and one another to form a coating. A schematic of the highpressure cold spray nozzle is shown in Fig. 1(a). The powder particles to be sprayed are injected through a powder injector (made of a stainless steel tube), as shown in Fig. 1(b), into the prenozzle chamber where the powder particles and the powder carrier gas intermix with the primary gas. While the primary gas is typically heated to elevated temperatures, the gas flow carrying the particles from the powder feeder through the injector is at room temperature. Thus, the mixture of the primary gas and powder gas serves as the propellant gas that accelerates powder particles. Coating formation by the high-pressure cold spray process is primarily controlled by particle velocities. Generally, there is a critical particle velocity for a given material being sprayed; particles above this velocity would T. Han, GM R&D Center, Warren, MI 48090; B.A. Gillispie, Delphi Powertrain, Flint, MI 48556; and Z.B. Zhao, First Solar, Toledo, OH 43697. Contact e-mails: [email protected] and [email protected].
320—Volume 18(3) September 2009
stick to the substrate and particles below this velocity would bounce off or erode the substrate (Ref 2, 3). The key design variables include
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