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Acta Metall Sin  2016, Vol. 52 Issue (11): 1467-1476    DOI: 10.11900/0412.1961.2016.00008
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INFLUENCE OF Fe VAPOUR ON WELD POOL BEHAVIOR OF PLASMA ARC WELDING
Xiaoxia JIAN1,2,Chuansong WU1()
1) Key Laboratory for Liquid-Solid Structural Evolution and Materials Processing (Ministry of Education), Shandong University, Jinan 250061, China
2) School of Mechanic & Electrical Engineering, Henan University of Technology, Zhengzhou 450001, China;;
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Abstract  

Plasma arc welding (PAW) is an important joining technology for plates with medium thickness because of the heat source characteristics, however, most models of PAW neglect the vaporization of metal. An axisymmetrical unified PAW model was developed by taking into account the influence of Fe vapor behavior from the molten pool surface as an anode in this work. The simulation region includes tungsten cathode, plasma arc, weld pool, keyhole and their self-consistence coupling using one set conservation equations. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of iron vapor. The main physical properties of Ar plasma are set as function of temperature and mass fraction of Fe vapor and are updated every iterate step to reflect the influence of Fe vapor in real time. The process of keyhole formation in stationary plasma arc welding is simulated under welding currents of 150, 170 and 190 A. The transient production, diffusion and concentration in the plasma arc of Fe vapor were presented. The effects of Fe vapor on the plasma arc behavior and formation of weld pool and keyhole are studied. It was shown that the evaporation rate of Fe was greatly dependent on the temperature of the weld pool. Most Fe evaporates from the top part of the keyhole surface and little from the keyhole bottom. The diffusion of Fe vapor is accelerated in the radial direction and is prevented in the axial direction due to the effect of plasma jets flow and at last it tends to be confined to the fringe of the plasma arc closed to the anode. The mixing of Fe vapor in the plasma results in the increase of radiation losses and the decrease of current density of the arc plasma in the fringe, but it had insignificant influence on the arc center. The heat flux from the plasma arc to the anode is also affected by Fe vapor due to its influence on the plasma arc properties. It is found that the calculation result of the width of the molten pool becomes more accurate to consider the effect of Fe vapor.

Key words:  plasma      arc      welding,      Fe      vapor,      weld      pool,      keyhole,      plasma      arc,      numerical      simulation     
Received:  06 January 2016     
Fund: Supported by National Natural Science Foundation of China (No.50936003)

Cite this article: 

Xiaoxia JIAN,Chuansong WU. INFLUENCE OF Fe VAPOUR ON WELD POOL BEHAVIOR OF PLASMA ARC WELDING. Acta Metall Sin, 2016, 52(11): 1467-1476.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00008     OR     https://www.ams.org.cn/EN/Y2016/V52/I11/1467

Fig.1  Schematic of the plasma are welding (PAW) model simulation domain (r—the radial coordinate, z—the axial coordinate, d—the nozzle diameter)

工件采用6 mm厚的SUS 304不锈钢板. PAW焊枪喷嘴直径2.8 mm, 喷嘴端部与工件距离5 mm. W极简化为倒圆台形, 尖端直径1 mm, 角度60o, 内缩量2 mm. 采用纯Ar气体作为保护气和离子气, 离子气流量为2.8 L/min, 保护气流量为20 L/min.

Boundary Vz / (ms-1) Vr / (ms-1) T / K ?/ V Ar / (Tm) Az / (Tm)
ABC ?vz?n=0 ?vr?n=0 ?T?n=0 ???n=0 ?Ar?n=0 ?Az?n=0
CP - - k?T-εαT4 0 ?Ar?n=0 ?Az?n=0
PQ - - k?T-εαT4 ???n=0 ?Ar?n=0 ?Az?n=0
QD - - 1000 ???n=0 0 0
DF Constant 0 1000 ???n=0 ?Ar?n=0 ?Az?n=0
FEGH - - k?T ???n=0 ?Ar?n=0 ?Az?n=0
HI Constant 0 1000 ???n=0 ?Ar?n=0 ?Az?n=0
IK - - 1000 ???n=0 ?Ar?n=0 ?Az?n=0
KA - - 3000 j ?Ar?n=0 ?Az?n=0
Table 1  External boundary conditions of the PAW model
Nomenclature Value Unit
Freezing point 1670 K
Melting point 1727 K
Density 7200 kgm-3
Electric conductivity 7.7×105 Sm-1
Surface tension 1.2 Nm-1
coefficient
Surface tension 1×10-4 Nm-1K-1
temperature gradient
Work function 4.65 V
Table 2  Main physical properties of SUS 304 used in this model
Fig.2  Relation between maximum temperature of the weld pool surface and maximum concentration of Fe vapor above the weld pool under the welding current of 170 A
Fig.3  Variation of the maximum mass fraction of Fe vapor above the weld pool with welding time under three welding currents
Fig.4  Distributions of anode temperature, mass fraction of Fe vapor (a, c) and temperature and fluid flow of plasma arc (b, d) at welding times of 1.1 s (a, b) and 2.0 s (c, d) under the welding current of 150 A
Fig.5  Distributions of anode temperature, mass fraction of Fe vapor (a, c) and temperature and fluid flow of plasma arc (b, d) at welding times of 0.9 s (a, b) and 1.6 s (c, d) under the welding current of 170 A
Fig.6  Distributions of anode temperature, mass fraction of Fe vapor (a, c) and temperature and fluid flow of plasma arc (b, d) at welding times of 0.7 s (a, b) and 1.3 s (c, d) under the welding current of 190 A
Fig.7  Influence of Fe vapor on radiation loss at cross section 1 mm above the anode surface (z=0.005 m) at the time of keyhole formation under the welding current of 190 A
Fig.8  Influence of Fe vapor on current density of plasma under the welding current of 190 A
Fig.9  Surface temperatures on the weld pool surface under different welding currents at the time of keyhole formation(a) 150 A, 2.0 s (b) 170 A, 1.6 s (c) 190 A, 1.3 s
Welding Width of topside weld pool / mm Width of backside weld pool / mm
current / A Calculation Calculation Experiment Calculation
without vapor
Calculation
with vapor
Experiment
without vapor with vapor
150 13.1 11.2 9.6 2.1 2.3 3.0
170 14.1 11.9 9.9 2.6 2.5 3.1
190 14.3 11.8 10.3 2.9 2.8 3.2
Table 3  Comparison of predicted and measured weld pool widths
Fig.10  Electrical heat flux (a) and total heat flux (b) on the weld pool surface at the time of keyhole formation under the welding current of 190 A
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