A Theoretical Study of Plasma Spraying in a Hybrid Torch
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A THEORETICAL STUDY OF PLASMA SPRAYING IN A HYBRID TORCH JOHN W. McKELLIGET* AND NAGY EL-KADDAH** * University of Lowell, Dept. of Mechanical Engineering, Lowell, MA 01854. "*The University of Alabama, Dept. of Materials and Metallurgical Engineering, Tuscaloosa,
AL 35486.
ABSTRACT A mathematical model for the analysis and design of RF, and hybrid DC/RF plasma torches for the thermal spraying of materials under conditions of high particle loading is presented. The model is based upon a numerical solution of Maxwell's equations for the electromagnetic field, the turbulent Navier/Stokes equations for the plasma velocity field, and the thermal energy balance equation for the temperature field. The interaction between the plasma and the injected particles is calculated using the Particle Source in Cell technique. The trajectories and thermal histories of nickel particles injected into a hybrid torch are compared to those of particles injected into a conventional RF torch. It is demonstrated that hybrid torches possess some superior characteristics over conventional plasma spray systems and may constitute a viable future technology for the spraying of advanced materials.
INTRODUCTION The use of thermal plasmas for particle treatment and spraying is a widely used and rapidly growing technology[l]. Traditional applications of plasma spraying involve the production of high quality ceramic and metallic coatings for wear resistance and thermal barrier applications. Recently, however, plasma spray techniques have begun to break out into many new areas including; the manufacture of near net shape products; the production of metal matrix composites; and the processing of superconducting materials. While commercially available DC plasma torches are capable of producing high spray velocities and high temperatures in the immediate vicinity of the torch exit, the plasma volumes (processing regions) are generally small, and are further reduced under conditions of high particle loading. The need for extended plasma volumes has led to increased interest in the use of the RF induction plasma for spraying. Despite the anticipated advantages of RF plasmas the complex nature of the flow fields in such systems has somewhat limited their usefulness. More specifically, the electromagnetically driven recirculating flow repels small particles from the plasma fireball and, as with the DC torch, the particles significantly reduce the plasma temperature with a consequent reduction in particle melting and heat utilization efficiency. In a recent publication by the authors[2] it was shown that it is possible to minimize some of the problems associated with the RF torch through modification of the velocity and temperature fields by the superposition of a low power DC torch (the hybrid plasma). In this paper a mathematical model of the electromagnetic, temperature, velocity, and particle fields is used to assess the feasibility of applying the hybrid plasma to plasma spraying.
THE HYBRID TORCH Figure 1 shows a sketch of the hybrid plasma torch an
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