Measurement and Application of Arc Separability Introduction In 1802 Petrov discovered the phenomenon of continuous electrical discharge, and in 1911 Mathers described the plasma as a heat source for the first time (Refs. 1, 2). Subsequently, scientists proposed many arc welding methods including submerged arc welding (SAW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and plasma arc welding (PAW) (Ref. 3). Scientists also proposed many hardfacing methods, including plasma spray, plasma transferred arc weld surfacing (PTAWS), and plasma transfer wire arc (PTWA) thermal spraying (Refs. 4–8). As a special arc welding process, PAW belongs to the category of highenergy density welding. Its unique mechanical, heat, and magnetic compression in its small orifice results in its distinguished directionality and high restraint (Refs. 9–11) that are useful for welding applications such as space shuttles, airplanes, and rockets (Refs. 12–14). Similar to PAW, the heat source of PTAWS is also plasma arc (PA).The PTAWS process has been used in surface modification, repairing techniques, and additive manufacturing (Refs. 4, 15–18). Better controllability for the distribution of pressure and heat flow undoubtfully will further expand its use. Generally, scientists consider that WELDING RESEARCH the pressure and the heat flow of PA are coupled together. If the heat flow increases, the pressure also increases. It is also thought that the electron flow as determined by the welding current is carried by the arc plasma. The arc plasma, consisting of equal numbers of ions and electrons, and the electron flow, which ionizes the shielding gas to form the arc plasma, are hard to separate. In our previous study, the authors found (Refs. 19, 20) that the plasma arc can be separated from the electron flow: as the arc plasma has been formed through ionization, the electron flow can be separated from the arc plasma partly or completely. For PA, because of the directionality and high restraint, separating the electron flow has little effect on the form of arc plasma. However, the pressure and heat are changed, providing an opportunity to provide better controllability of the plasma arc to meet different needs. This paper first presents results on distributions of pressure and heat of the separated, partially or completely, arc plasma and electron flow for the effects from major parameters including the plasma gas flow rate, distance from the cathode to workpiece, and welding current. Then a novel separated plasma transferred arc weld surfacing (SPTAWS) process is proposed to realize the controllability made possible by separating the arc, and also to experimentally test for the results due to this improved controllability. The results from this study provided fundamentals to better under- in Plasma Arc The distribution of pressure and heat from a separated arc plasma was analyzed for a novel welding system based on a split anode BY S. J. CHEN, R. Y. ZHANG, F. JIANG, Z. Y. YAN, AND Y. M. ZHANG KEYWORDS • Plasma Arc • Arc Separability • Arc Pressure • Arc Heat • SPTAWS S. J. CHEN, R. Y. ZHANG, F. JIANG (jiangfan@bjut.edu.cn), and Z. Y. YAN are with the Ministry of Education Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Beijing University of Technology, China. Y. M. Zhang is with the Institute for Sustainable Manufacturing and Department of Electrical and Computer Engineering, University of Kentucky, Lexington, Ky. JUNE 2016 / WELDING JOURNAL 219-s ABSTRACT The authors recently demonstrated that the arc plasma and electron flow in an arc could be separated. This separability provided the foundation to develop an ability to adjust the heat and arc pressure distribution without changing the current. To better understand this property, a novel system was developed based on the split anode. The distribution of the pressure and heat from the separated arc plasma was analyzed quantitatively. It was verified that the arc pressure mainly concentrated in the arc plasma. Also, the heat input from the arc plasma exceeded that from the eletron flow. To explore possible applications of this separability, a novel process, namely, separated plasma transferred arc weld surfacing (SPTAWS) is proposed. Grade D steel workpieces were hardfaced using this process. As the separated electron flow increased, the heataffected zone (HAZ) of the deposited bead was reduced. The microstructure with SPTAWS was found to be better than with conventional plasma transferred arc weld surfacing (PTAWS), especially in the finegrain zone. The grain is more refined and uniform.
Welding Journal | June 2016
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