The Influence of Powder Porosity on the Bonding Mechanism at the Impact of Thermally Sprayed Solid Particles
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RODUCTION
HIGH-VELOCITY oxy-fuel (HVOF) thermal spraying produces coatings with higher density, superior bond strengths, and less decarburization or oxidation than other high-temperature thermal spray processes, because of its unique advantage of high velocity and low temperature output for sprayed particles. A computational investigation of HVOF sprayed WC-Co particles shows that most particles are in solid state prior to impact.[1,2] The impingement of liquid droplets including spreading, breakup, air entrapment, and solidification has been studied and reported in References 3 and 4, whereas the solid particle impact and its subsequent bonding mechanism are less well understood. Recently, the authors have developed a finite element model to examine the dynamic process of solid particle impingement. A parametric study has been performed to clarify the influence of impact velocity, temperature, particle size, and more importantly, the shape of the particles.[5] Such numerical models provide insights of the high-speed physical process inaccessible by conventional experimental techniques. Experimental results[6] revealed that powder structure, particularly porosity, influences its deformability and, consequently, the deposition behavior of cermet powder such as WC-Co. However, the detailed behavior, including densification and deformation, is not understood clearly for HVOFsprayed porous powders, because adequate knowledge is SPYROS KAMNIS, Researcher, and MICHALIS VARDAVOULIAS, Managing Director, are with PyroGenesis S.A., Technological Park of Lavrio, 19500 Lavrio, Greece. Contact e-mail: [email protected] SAI GU, Senior Lecturer, is with the School of Engineering Science, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted April 21, 2010. Article published online September 30, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
lacking about the transition of particles from porous structure to dense deposition layer during impingement. In this article, the previous particle impact model[7] is developed more broadly to investigate the influence of particle porosity in deposition efficiency. The WC-Co powder is used for consistency with the previous study, whereas the particle parameters prior to impact are taken from the computational fluid dynamics (CFD) in-flight particle models reported in Reference 2. The description of the CFD model is not repeated here because it is out of the scope of the current study. An extensive discussion examines the combined effects of strain hardening and thermal softening for different temperatures and velocities. The temperature-dependent critical velocity for successful bonding is derived to take into account particle porosity.
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MODEL DEVELOPMENT
The solid impact dynamics are analyzed by using the finite element commercial solver ABAQUS/Explicit (Dassault Systemes, Suresnes, France). The model accounts for strain hardening, thermal softening, and heating resulting from frictional and plastic dissipation. Because of the
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