L. T. An, Y. Gao, Dalian Maritime University, Dalian, China
In order to improve the voltage of the plasma arc used for thermal spray, a new plasma torch was designed and constructed. The torch, called bi-anode torch, has two anodes that were inter-insulated and have different distances from the cathode tip. The position of the arc root can be controlled to attach to either anode surfaces during operation. When the anode arc root attached to the first anode, which is near to the cathode, the arc behaves just like a normal one and the arc root position can be determined by Steenbeck’s minimum principle. When it is attached to the second anode, the arc voltage is remarkably enhanced because the arc length is increased. Experimental work has been carried out to compare the electric characteristics of the arcs whose anode roots were attached to different anode surfaces. The results show that the arc has different electric characteristics when changing the anode arc root from first anode to second one. Their mechanisms were discussed in this investigation.
Summary: 1.Introduction:
Over the past decades, plasma spraying, as a well-established technology in modern surface engineering, has been applied widely to industry practice in depositing coatings with various functions [1][2]. Plasma jet, as the heat source of the plasma spraying, can reach a temperature up to 13,000K, which is much higher than the melting point of any known material. However, due to the millisecond range of the residence time for in-flight particles receiving heat flux in the plasma jet, it is necessary to increase the arc power when spraying hard-melted materials such as ceramics and hard alloy. The common practice to increase the arc power is achieved by increasing the arc current. In practical spraying, the arc power ranges from 40-200kW, which requires an arc current up to 1000A. Large arc current not only decreases the thermal efficiency of the torch, but also accelerates the erosion and corruption of the electrode, especially for the anode nozzle. Alternative way is to increase the arc voltage, but it is difficult to achieve by using conventional torch design.
Generally, the arc voltage is not an independent spraying parameter in conventional plasma spraying. It is determined by processing parameters such as arc current, gas flow rate, gas composition, and nozzle diameter [3]. Conventional non-transferred plasma torch, shown as fig.1, consists of a stick-typed cathode and a nozzle-shaped anode. The arc stretches between the cathode tip and the water-cooled anode wall. The working gas is introduced axially from the circumference of the cathode. The arc diameter is small at the cathode tip because the cooling effect on the cathode surface makes the arc column intensively shrink. With the arc developing to the downstream direction, the gas from the cold boundary layer is continuously entangled into the arc column, which makes the arc diameter increase gradually and the thickness of the cold boundary layer reduce. The arc strikes the anode surface to form the anode arc root when the cold boundary layer is thin enough that the arc voltage can break down it. Theoretically, The anode arc root position can be determined by SteenbeckĄ&hibar;s minimum principle, which postulates that the anode arc root will be in such position that makes the total arc voltage be minimum for a given current, working gas flow and torch configuration. This principle has been proved useful by the work of Seungho Paik and Li-Heping etc. [4][5]. Based on this principle and according to the industrial practice, the traditional-designed plasma torch canĄ&hibar;t get a high voltage. Besides, the anode arc root is unsteady under the general spray conditions. Two forces act on the connecting column that crosses the cold boundary layer and connects the arc column and the anode arc root. They are drag force from the flow and Lorenz forces due to the self-magnetic field. Generally, because the former is much larger than the latter, the anode arc root is pushed to the downstream direction and the arc voltage rises up until a short-circuit occurs with a new re-arcing in the upstream direction, which results in the fluctuation of arc voltage [6] and is disadvantageous for the spraying.
Some works have been carried out to study the influences of the anode arc root attachment on the plasma characteristics. Sooceok Choi etc. [7] used a stepped nozzle to enhance the arc voltage and stabilize the plasma arc. Katashi Osaki etc. [8] applied two parallel stick-typed anodes to form a U-shape arc. In this research, a bi-anode plasma torch was designed to study the influence of the anode arc root position on the characteristics of plasma arc. The torch has two inter-insulated anodes connected in serial and with different distance from the cathode tip. The anode arc root can be controlled to attach to either anode surfaces to get a normal arc or a lengthened arc. The thermal characteristics of the bi-anode torch have been studied in the previous work [9]. This paper will concentrate on the electric characteristics of the bi-anode torch.
4. Conclusions:
A special bi-anode plasma torch was designed and constructed to improve the performance of plasma torch. The electric characteristics of the new torch were compared with conventional plasma torch. The conclusions are stated as below:
1. Changing the anode arc root from anode I to anode II not only increases the arc length and leads to a higher arc voltage, but also increases the stability of the anode arc root and reduces the fluctuation of the arc voltage.
2. In anode I mode, the arc always shows rising voltage-current characteristic, which is consistent with a traditional one. In anode II mode, the voltage-current characteristic of the arc relates to the thermal conductivity of the plasma gas, pure argon arc of low thermal conductivity shows a rising voltage-current characteristic while Ar-H2 arc of high thermal conductivity shows a dropping voltage-current characteristic.
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