Theoretical and Experimental Particle Velocity in Cold Spray
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Victor K. Champagne, Dennis J. Helfritch, Surya P.G. Dinavahi, and Phillip F. Leyman (Submitted March 11, 2010; in revised form June 22, 2010) In an effort to corroborate theoretical and experimental techniques used for cold spray particle velocity analysis, two theoretical and one experimental methods were used to analyze the operation of a nozzle accelerating aluminum particles in nitrogen gas. Two-dimensional (2D) axi-symmetric computations of the flow through the nozzle were performed using the Reynolds averaged Navier-Stokes code in a computational fluid dynamics platform. 1D, isentropic, gas-dynamic equations were solved for the same nozzle geometry and initial conditions. Finally, the velocities of particles exiting a nozzle of the same geometry and operated at the same initial conditions were measured by a dual-slit velocimeter. Exit plume particle velocities as determined by the three methods compared reasonably well, and differences could be attributed to frictional and particle distribution effects.
Keywords
aluminum, CFD, modeling, nozzle, velocity measurement
1. Introduction The U.S. Army utilizes metal coatings in many of its weapons systems for the strengthening or protection of vulnerable substrates. The quality of these coatings is characterized by the non-porous nature of the metal coating and its ability to adhere to the substrate. Extremely non-porous and adherent metal coatings can be applied to surfaces by impacting metal particles onto the surface at supersonic velocities. This cold spray process is carried out at the U.S. Army Research Laboratory Center for Cold Spray (ARLCCS) in Aberdeen, MD. The cold spray system accelerates micron-sized particles to high velocities by entraining the particles in the flow of a supersonic nozzle as shown in Fig. 1. High velocity is necessary for optimal particle deposition and coating density, and several parameters, including gas conditions, particle characteristics, and nozzle geometry, affect the particle velocity. It has been well established that impacting particles must exceed a ‘‘critical velocity’’ to deposit instead of bouncing off. The magnitude of the critical velocity can be estimated through the use of empirical relationships, which generally depend on particle material characteristics, such as density, ultimate strength and melting point as well as the particle temperature Victor K. Champagne, US Army Research Laboratory, Aberdeen, MD; Dennis J. Helfritch, Dynamic Science, Aberdeen, MD; Surya P.G. Dinavahi, Lockheed Martin Company, Army Research Lab, Aberdeen Proving Ground, MD; and Phillip F. Leyman, Data Matrix Solutions, Sterling, VA. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
immediately before impact (Ref 1). Typically, the velocities and temperatures of particles prior to impact are calculated as functions of particle diameter. Those particles with velocities higher than critical velocity will deposit. The known particle volume distribution as a function of diameter then allows the calculation of d
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