Semi-Empirical Analytical Determination of the Transient Thermal Evolution within a Substrate during Low-Pressure Cold Spraying

  • Author / Creator
    Mahdavi, Amirhossein
  • Hard-faced coatings are used to protect surfaces against wear, corrosion, and thermal degradation. Cold-gas dynamic spraying is a coating fabrication process in which a supersonic gas flow is produced to propel un-molten metal or alloy powder particles to deposit, with high impact forces, upon a substrate to form a highly adherent coating. Due to the high velocity of the under-expanding gas jet upon the substrate, and the high temperature of the impinging jet, a significant amount of thermal energy is expected to be transferred from the gas jet to the substrate. The quality of the final coating, as well as the deposition efficiency of the particles, can be significantly affected by the substrate surface or particle-substrate interface heating during the cold spraying process. With knowledge of the significant role that substrate temperature plays on the quality of cold-sprayed coatings, it becomes important to study the gas-substrate heat exchange. In this regard, the first phase of this PhD research project focused on developing a semi-empirical analytical model to determine the heat transfer coefficient of an impinging air jet generated by a cold spraying nozzle. A method involving Green’s functions was employed to solve a transient two-dimensional heat conduction problem to obtain an expression for the temperature distribution within the substrate. By coupling the analytical results of temperature distribution and experimental surface temperature data, the spatially-varying heat transfer coefficient of the impinging air jet upon the substrate, in the form of the non-dimensional Nusselt number, was estimated. The results showed that the maximum values of the heat transfer coefficient were close to the stagnation point of the air jet. It was found that the heat transfer coefficient was independent of the time that the cold spray nozzle remained stationary over the substrate surface. It was found further that by increasing the stand-off distance of the nozzle, the radial variations of the heat transfer coefficient became negligible, compared to those for small stand-off distances. In the second phase of the PhD research project, the effect of the air temperature and pressure, as process parameters, and surface roughness and thickness, as substrate parameters, on the heat transfer coefficient of the impinging air jet was investigated. It was found that increasing the total pressure would increase the Nusselt number of the impinging air jet, while the total temperature of the air jet had negligible effect on the Nusselt number. It was also found that increasing the roughness of the substrate enhanced the heat exchange between the impinging air jet and the substrate. As a result, higher surface temperatures on the rough substrate were measured. The study of the effect of the substrate thickness on the heat transfer coefficient showed that the Nusselt number that was predicted by the model was independent of the thickness of the substrate. The surface temperature profile, however, decreased with increasing radial distances from the stagnation point of the impinging jet as the thickness of the substrate increased. The final phase of this PhD research project focused on developing a mathematical model to predict the surface temperature profile of a substrate that was exposed to the impingement of a moving cold spray heat source. In this regard, a three-dimensional heat conduction model that was coupled with the travelling wave solution technique was developed. The analytically-predicted surface temperature profile was in good agreement with experimentally-measured data. Analytical modeling was further utilized to investigate the effect of the non-dimensional characteristic velocity of the travelling heat source on the surface temperature profile of the substrate. It was found that both the maximum surface temperature and the spatial variation of the surface temperature profile of the substrate decreased as the non-dimensional characteristic velocity increased.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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