A typical effect in the DC plasma spraying process using single-cathode single-anode plasma generators (F4/F6/MC60) is the permanent instability of the DC arc as the source of energy for plasma generation. Resulting fluctuations of the arc voltage are caused by the axial and radial movements of the arc. In consequence the power of the plasma torch as well as the plasma free jet geometry are fluctuating during the Plasma physicsprocess, which favors the inconsistency of the coating result. A qualitative and objective assessment of the complete arc movement and plasma generation would lead to an improved process characterization.
Therefore the subject of present work is the development of a preventive arc-based analytical method for DC plasma spraying. The evaluation is based on a representative data volume and realized by the automatic analysis of high-velocity recordings of the arc dynamics in the plasma generator and corresponding arc voltage values. In this case, the anode output (including DC arc) is displayed with a resolution of 365 dpi and a frame rate of 100 000 fps on the CMOS sensor of the high-speed camera via an optical path consisting of deflecting mirror, neutral density filter and lens. The developed algorithm (software MATLAB, The MathWorks, Inc.) automatically detects for all images of the video sequence (at least 3 200 images) the anode center axis, the arc orientation and the position of anode attachment point on the anode surface referring to the anode center, with simultaneous compensation of aberrations. This allows an objective assessment of the complete arc movement. The reproducibility of the measurement results could be verified within individual tests, after process interruption and for variable experimental setups (electrical power, gas distributor ring and anode type). The subsequent validation of the analysis software was carried out by means of static coating experiments (spray spots) with varying position of the cathode tip (decentered by 50 μm, 100 μm, 150 μm). Thus, the detection limit of the developed measuring system was gradually determined and the effects on the arc behavior and finally the coating process could be quantified. In this case, the cathode positioning with a deviation of already (50 ± 10) μm has been identified as one of the main reasons for the divergence of spray spot position and deposition efficiency, qualitatively characterizable by arc orientation respectively position of anode attachment point.
An interpretation of the test results is given by the numerical flow simulation of the cold gas flow (without arc generation), by software Ansys CFX (Ansys Inc.). In particular, the effects of anode geometry and gas distributor ring geometry as well as the effects of cathode position on the axial and radial direction vector of the vortex flow are confirmed, which among other things determine the dynamics of the arc (detectable by arc orientation and anode attachment position).
Thus, the developed automated analysis of the arc dynamics in the DC plasma generator corresponds to an arc-based process characterization in DC plasma spraying process and is similarly relevant for developers (data basis for simulations) and end users (process control).
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/3(E), Dissertation Abstracts International|
|Keywords:||Plasma physics, DC plasma spraying, DC plasma generator|
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