Advanced Diagnostics and Modeling of Spray Processes
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Diagnostics and Modeling of Spray Processes
James R. Fincke and Richard A. Neiser Introduction The microstructure and properties of thermally sprayed deposits depend critically on the thermal- and kinetic-energy histories of the particles entrained in the hot-gas jet. At impact, the particle temperature, molten fraction, size, velocity, and chemistry, along with substrate temperature and surface characteristics, control the morphology of individual particle splats. These factors control the adhesion, strength, microstructure, and porosity of a coating and influence the residual-stress state. In order to produce higher-quality coatings and expand the use of this versatile family of technologies, the ability to model and measure particle behavior is essential. Accurate modeling of particle heating and acceleration begins with an accurate model of the gas jet itself. These jets typically exhibit high temperatures and velocities; steep gradients of temperature, velocity, and composition; and a variety of temporal fluctuations. The short exposure times of particles to these flow fields and the rapid deceleration and solidification of the particles upon impact give rise to a number of complicating nonequilibrium conditions. Major advances in model sophistication and computational power over the past decade have enabled researchers to begin to handle these complex issues. A number of thermal-spray codes, particularly in the areas of plasma and highvelocity oxy-fuel (HVOF) spraying, have been developed that have reasonably good predictive capabilities. Of course, our ability to adequately capture the physics of this dynamic environment is ultimately limited by the availability of appropriate thermophysical data. Experimental measurements of in-flight particle size, velocity, and temperature
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are another essential component in improving our understanding and control of thermal-spray processes. These diagnostic tools are noninvasive optical techniques that sense either scattered laser light or thermal emission from the particles. These measurements are frequently used to validate and refine computational models. They are also used to establish quantitative relationships between processing conditions and thermal- and kinetic-energy distributions of particles in the gas jet. Some of the issues associated with making reliable measurements of in-flight particle characteristics are related to those that cause difficulty in modeling. The steep gradients in the jet raise questions about sampling schemes. Ensemble techniques tend to provide time-resolved, spatially averaged data, while single-particle techniques yield spatially resolved, timeaveraged information. The presence of low-temperature particles in the jet that may or may not be detected is another complicating element. Assumptions about particle shape and emissivity must be made when measuring particle temperatures and size. There are, for example, few emissivity data available for partially oxidized, complex alloys at elevated temperatures. Sensors for characterizing
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