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Acta Metall Sin  2017, Vol. 53 Issue (5): 631-640    DOI: 10.11900/0412.1961.2016.00263
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Numerical Simulation of Physical Characteristics of Variable Polarity Plasma Arc Welding
Shujun CHEN,Bin XU,Fan JIANG()
Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, Beijing University of Technology, Beijing 100124, China
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Abstract  

Variable polarity plasma arc (VPPA) is a kind of source to provide heat and force at welding process. It can remove the oxide layer with high melting point on the surface of base metal using the cleaning action of cathode spots (the special property of VPPA). So variable polarity plasma arc welding (VPPAW) is a very suitable method to join aluminum alloys which always have extremely tenacious surface oxides. It is great significant to understand clearly the physical characteristics of VPPA for predicting welding defects and making the welding process stable. Therefore, modeling and simulating VPPA are necessary and helpful to understand welding process theory and promote its application further. In this work, a three dimensional transient calculated model of VPPA was established. To describe the electrical characteristics of VPPA at different polarities, a sequential electric conducting model was proposed. With finite difference method, the temperature field, fluid flow and current density of VPPA were solved out. And the distribution of plasma arc pressure on the anode surface, as well as its evolution process as the time going on were analyzed. Arc pressure was measured experimentally to verify the calculated model. The results show that the arc temperature field of electrode negative (EN) is more compressed than that of electrode positive (EP). The range of high temperature at EN is a little larger. Arc pressure and current density of EN at central area are both higher than EP. Nonetheless, the magnitude of these values begins to reverse at a certain distance to center in radial direction. Moreover, the arc pressure rapidly responses to welding current. Pressure at EP is about 20% lower than that of EN. The pressure reduces to the lowest value when the current pass through 0. After that, while the current reaches to normal value, the pressure will immediately impact to a larger value, then quickly recover to an average value. Otherwise, to compare the experimental results with calculated results of arc images and arc pressure, they are in good agreement with each other.

Key words:  variable polarity plasma arc      three dimensional model      numerical simulation     
Received:  27 June 2016     
Fund: Supported by National Natural Science Foundation of China (No.51505008) and National Science and Technology Major Project of China (No.2014ZX04001-171)

Cite this article: 

Shujun CHEN,Bin XU,Fan JIANG. Numerical Simulation of Physical Characteristics of Variable Polarity Plasma Arc Welding. Acta Metall Sin, 2017, 53(5): 631-640.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00263     OR     https://www.ams.org.cn/EN/Y2017/V53/I5/631

Fig.1  Measure system of arc pressure and image of variable polarity plasma arc (VPPA)
Fig.2  Schematic of VPPA at different polarities (EN—electrode negative, EP—electrode positive)
Fig.3  Solution area and boundary conditions
Boundary v / (ms-1) T / K ? / V A / (Wbm-1)
ABC ?v?n=0 1000 ?=?VPPA ?A?n=0
APOTXUQLGFEDC, APNSWVRMHIJKB ?v?n=0 ?T?n=0 ???n=0 ?A?n=0
LGHM, XTONSW, UQLMRV, FGHI, DEJK - 1000 ???n=0 ?A?n=0
OPN - 3000 Eqs.(10)~(13) ?A?n=0
UXWV, EFIJ v=vconstant 1000 ???n=0 ?A?n=0
CBKD ?(ρv)?n=0 1000 ???n=0 0
Table 1  Boundary conditions
Fig.4  Temperature and flow field distributions of VPPA at EN (a) and EP (b)
Fig.5  Radial temperature distributions of VPPA (x=7 mm)
Fig.6  Radial pressure distributions of VPPA (x=7 mm)
Fig.7  Plasma velocity distributions of VPPA at the axis
Fig.8  Radial current density distributions of different polarities at work-piece surface
Fig.9  Surface morphology of aluminum alloy
Fig.10  Images of VPPA at EN (a) and EP (b)
Fig.11  Evolution of arc pressure at the center of VPPA
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