Mathematical Modeling and Experimental Validation of the Warm Spray (Two-Stage HVOF) Process
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H. Katanoda, T. Kiriaki, T. Tachibanaki, J. Kawakita, S. Kuroda, and M. Fukuhara (Submitted November 10, 2008; in revised form December 24, 2008) The warm spray (WS) gun was developed to make an oxidation-free coating of temperature-sensitive material, such as titanium and copper, on a substrate. The gun has a combustion chamber followed by a mixing chamber, in which the combustion gas is mixed with the nitrogen gas at room temperature. The temperature of the mixed gas can be controlled in the range of about 1000-2500 K by adjusting the mass flow rate of nitrogen gas. The gas in the mixing chamber is accelerated to supersonic speed through a converging-diverging nozzle followed by a straight barrel. This paper shows how to construct the mathematical model of the gas flow and particle velocity/temperature of the WS process. The model consists of four parts: (a) thermodynamic and gas-dynamic calculations of combustion and mixing chambers, (b) quasi-one-dimensional calculation of the internal gas flow of the gun, (c) semiempirical calculation of the jet flow from the gun exit, and (d) calculation of particle velocity and temperature traveling in the gas flow. The validity of the mathematical model is confirmed by the experimental results of the aluminum particle sprayed by the WS gun.
Keywords
gas dynamics, mathematical model, thermodynamics, warm spray
1. Introduction In the history of the recently developed thermal spray processes from the 1980s, the larger kinetic energy of the process gas, that is the supersonic thermal spray process, rather than the larger thermal energy, has been used to accelerate the spray particle. This is because the higher gas velocity is advantageous in obtaining the higher particle velocity, resulting in higher bond strength and denser coating. In order to obtain the supersonic gas flow, the stagnation pressure of twice the atmospheric pressure or above is necessary. The stagnation temperature, on the other hand, does not work to accelerate the stagnant gas to the supersonic speed. Therefore, a wide range of gas temperature from room temperature to over several thousand K can be employed in the supersonic thermal spray processes. Figure 1 summarizes the comparison of the gas temperature and the particle velocity in supersonic thermal spray processes operated in the atmosphere. The vertical axis of the figure shows the temperature of the gas in which the spray particle travels in the spray gun, and the H. Katanoda, T. Kiriaki, T. Tachibanaki, and M. Fukuhara, Department of Mechanical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan; and J. Kawakita and S. Kuroda, National Institute for Materials Science, Ibaraki, Japan. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
horizontal axis shows the particle velocity in the thermal spray gun. The high-velocity oxyfuel (HVOF) thermal spray gun (Ref 1) was developed in the 1980s to provide a wear-resistant coating of WC powder. In the HVOF process, the liquid or gaseous fuel is c
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