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5403 |
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Pulsed current and pulsed powder operation of the
one-cathode-one-anode-plasma-generator (OCOAPG)
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Stephan Zimmermann* / University of the Federal Armed Forces Munich, Germany Alexander Atzberger/ University of Federal Armed Forces Munich IPM (LPT), Germany Jochen Schein/ University of Federal Armed Forces Munich IPM (LPT), Germany Michal Szulc/ Zierhut Messtechnik GmbH, Germany Jochen Zierhut/ Zierhut Messtechnik GmbH, Germany |
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In most plasma spraying SMEs a One-Cathode-One-Anode-Plasma-Generator (OCOAPG) is used due to its cost-effectiveness. To achieve high deposition rates, the highest possible fraction of the injected powder has to be melted and accelerated towards the substrate. Adequate to the amount and size of the particles, a sufficiently long and reproducible residence time in plasma is therefore needed. This can be achieved by a long plasma jet with little or no temporal variation in length and temperature. In OCOAPG an arc is operated between a cathode and a central tubular anode, which causes different instabilities in the effluent plasma jet. Due to the instable interaction between the plasma jet and the carrier gas jet continuously incorporating the powder, fluctuations occur resulting in reduced coating quality. Coating systems with a higher amount of electrodes (and hence using several arcs) show higher stability and therefore can provide higher coating quality. However, due to their complexity and cost intensity, the investment hurdle for SMEs usually turns out to be too high. Recently, research steps to improve the plasma spraying process with OCOAPG have been undertaken by using a controllable current source to create a uniform particle gas jet interaction. As the movement of the anodic arc attachment point can be actively controlled by current pulses, the plasma jet can be lengthened and shortened at periodic intervals. Furthermore, by pulsing the particle delivery synchronously to the power modulation an improved particle penetration and consistent residence times can be achieved. First diagnostic results, including coatings, are presented and discussed within the paper. |