A Study on Arc Instability Phenomena of an Axial Injection Cathode Plasma Torch
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P. Mohanty, Jovan Stanisic, Jelena Stanisic, A. George, and Y. Wang (Submitted April 29, 2009; in revised form September 24, 2009) Axial injection in plasma gun through the cathode has clear benefit of longer particle residence time and optimum particle trajectory in the plume; however, accelerated wear of the cathode seem to be the major issue in this approach. This study investigates the arc instability phenomena in an axially injecting single cathode plasma torch design. Gun voltage measurements were used to evaluate the arc behavior. For comparison purpose, arc fluctuations with a standard solid cathode torch design under identical operating parameters have also been studied. A comparison of different internal hardware configurations is also done to understand and establish the important factors in the design of the axial injection and solid cathode systems. Further, this study presents the influence of plume elongation and accelerated gas velocities on the arc behavior in different configurations under low pressure environment.
Keywords
arc instability, axial injection cathode, low pressure, TaC
1. Introduction Most conventional plasma spray systems employ radial injection of powders into the plasma stream. Even though temperatures in a plasma plume can exceed 15000 C, radially injected feedstock does not follow an optimum trajectory within the plasma jet, which reduces the melting fraction of the powder. Further, there are some materials that cannot simply be sprayed because of the short interaction time between the precursor powders and the plume. Therefore, achieving an extended residence time and a uniform trajectory in the plume has been the aim of many design modifications and developments. There have been two main techniques employed to extend this residence time, namely, by means of a low pressure environment to extend the plume or by changing the injection point of the powder. The benefits of low pressure plasma spray (LPPS) have been realized by the industry for some time now. This article is an invited paper selected from presentations at the 2009 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Expanding Thermal Spray Performance to New Markets and Applications: Proceedings of the 2009 International Thermal Spray Conference, Las Vegas, Nevada, USA, May 4-7, 2009, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2009. P. Mohanty, Jovan Stanisic, Jelena Stanisic, A. George, and Y. Wang, Additive Manufacturing Process Laboratory, University of Michigan, Dearborn, MI. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
Many investigations have been carried out on the effects of a low pressure environment on plasma plume elongation. Reports have shown that the plume can be expanded to 10 times (~104 Pa) its original length in some instances (Ref 1). Although limited, there have been reports on the useful
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