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Acta Metall Sin  2015, Vol. 51 Issue (11): 1365-1376    DOI: 10.11900/0412.1961.2015.00020
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NUMERICAL SIMULATION AND ORTHOGONAL ANALYSIS ON COUPLED ARC WITH MOLTEN POOL FOR KEYHOLING PLASMA ARC WELDING
Xuannan WU1,Yanhui FENG1,2(),Yan LI1,Yafei LI1,Xinxin ZHANG1,2,Chuansong WU3
1 School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083
2 Beijing Key Laboratory of Energy Saving and Emission Reduction for Metallurgical Industry, University of Science and Technology Beijing, Beijing 100083
3 School of Materials Science and Engineering, Shangdong University, Jinan 250061
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Xuannan WU,Yanhui FENG,Yan LI,Yafei LI,Xinxin ZHANG,Chuansong WU. NUMERICAL SIMULATION AND ORTHOGONAL ANALYSIS ON COUPLED ARC WITH MOLTEN POOL FOR KEYHOLING PLASMA ARC WELDING. Acta Metall Sin, 2015, 51(11): 1365-1376.

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Abstract  

A 2D axial symmetrical mathematical model was developed for stationary keyholing plasma arc welding (PAW), to describe the transport process in coupled high-temperature flow arc and molten pool in the workpiece. The evolutions of electric, magnetic, velocity and temperature fields were simulated. The simulated fusion line of the weld bead is in quite good agreement with the experimental results, validating the mathematical model. It turns out that, both the current density and the temperature reach the maximum values near the tip of the tungsten cathode. The arc displays a typical bell-shape above the workpiece, but becomes slim cone-shape near the central axis as the arc enters the keyhole. The argon plasma slows down sharply when it strikes the inner wall of the keyhole, so high pressure appears in the keyhole and some argon plasma flows back. The combination of fluid flow and heat transfer contributes to the reversed bugle shaped fusion line. The simulation of orthogonal test was further conducted to study the effects of operational and structural parameters of the weld torch. The range analysis shows that the structural parameters of weld torch are more influential than the operational parameters. That is, more attention should be paid to control the gap between two electrodes, the electrode shrinkage and the nozzle diameter to guarantee the welding quality.
Key words:  plasma arc      welding      molten pool      coupling      orthogonal test     
Fund: Supported by National Natural Science Foundation of China (No.50936003)
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Xuannan WU
Yanhui FENG
Yan LI
Yafei LI
Xinxin ZHANG
Chuansong WU

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https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00020     OR     https://www.ams.org.cn/EN/Y2015/V51/I11/1365

Fig.1  Schematic of keyholing plasma arc welding (PAW) process
Fig.2  Geometric model of keyholing PAW (unit: mm, r—radial coordinate, z—axial coordinate)
Zone Source item Expression
Arc S r - ∂ p ∂ r - J z B θ
S z - ∂ p ∂ z + ρ g + J r B θ
S e J r 2 + J z 2 σ + 5 k B 2 e ( J r ∂ T ∂ r + J z ∂ T ∂ z ) - R
Molten pool S r - ∂ p ∂ r - μ l K v r
S z - ∂ p ∂ z + ρ g - μ l K v z
S e - ∂ ( ρ f l L a ) ∂ t - 1 r ∂ ∂ r ( r ρ v r f l L a ) - ∂ ∂ z ( ρ v z f l L a )
Table 1  Source items in momentum and energy equations
Fig.3  Temperature-dependent thermophyical properties of Ar plasma[34]

(a) density and dynamic viscosity

(b) specific heat

(c) thermal conductivity and electrical conductivity

Condition vr vz T φ A
Initial 0 0 300 K 0 0
Boundary
AB 0 0 3000 K -24 V ∂ A / ∂ n = 0
BC 0 0 3000 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
CD 0 0.758 m/s 300 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
DE 0 0 1000 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
EF 0 0 1000 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
FG 0 0 1000 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
GH 0 0.764 m/s 300 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
HI ∂ v r / ∂ n = 0 0 300 K ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
IJ 0 0 Eq.(17) 0 0
JK 0 0 Eq.(17) 0 0
KL 0 ∂ v z / ∂ n = 0 300 K ∂ φ / ∂ n = 0 0
LA ∂ v r / ∂ n = 0 ∂ v z / ∂ n = 0 ∂ T / ∂ n = 0 ∂ φ / ∂ n = 0 ∂ A / ∂ n = 0
Table 2  Initial and boundary conditions
Fig.4  Distributions of electrical potential and current density in arc column and workpiece at 2.0 s
Fig.5  Distribution of electromagnetic force in arc column at 2.0 s
Nomenclature Symbol Value
Solidus temperature Ts 1663 K
Liquidus temperature Tl 1723 K
Latent heat of fusion La 2.6×105 Jkg-1
Specific heat cp,metal 630 Jkg-1K-1
Density rmetal 7200 kgm-3
Elementary charge e 1.602×10-19 C
Boltzmann constant kB 1.381×10-23 JK-1
Magnetic permeability m0 4π×10-7 Hm-1
Radiation emissivity e 0.4
Welding current i 169 A
Arc voltage u 24 V
Working gas flow rate qw 3 Lmin-1
Shielding gas flow rate qs 30 Lmin-1
Gap between two electrodes ga 5 mm
Electrode shrinkage le 2 mm
Nozzle diameter dn 2.8 mm
Table 3  Thermophyical properties of stainless steel 304 and welding operational parameters[8]
Fig.6  Distributions of velocity and temperature in arc column and workpiece at 2.0 s (Tarc—temperature of arc, Twp—temperature of workpiece)

(a) whole calculation domain

(b) local region close to cathode

(c) distribution of velocity upon the top surface of workpiece

(d) distribution of velocity in the molten pool

Fig.7  Distributions of velocity streamline and pressure in arc column at 2.0 s (p—pressure)
Fig.8  Dynamic evolution of velocity and temperature fields in the arc column and workpiece (t—time)

(a) 0.2 s (b) 0.4 s (c) 0.6 s (d) 0.8 s (e) 1.0 s (f) 1.2 s (g) 1.4 s (h) 1.6 s (i) 1.8 s (j) 2.0 s

Fig.9  Comparison of weld bead cross-section between experimental image (left) and calculated result (right) (The workpiece is stainless steel 304 and its thickness is 6 mm; the welding current is 169 A; the arc voltage is 24 V; the working gas flow rate is 3 L/min; the shielding gas flow rate is 30 L/min; the gap between two electrodes is 5 mm; the electrode shrinkage is 2 mm; the nozzle diameter is 2.8 mm)
Level i / A qw / (Lmin-1) qs / (Lmin-1) ga / mm le / mm dn / mm rk / mm
1 169 3 20 5 2 2.8 2
2 159 1 10 3 0 1.8 1
3 179 5 30 7 4 3.8 3
Table 4  Table of factors and levels for orthogonal test simulation
Fig.10  Range analysis of top and bottom weld bead widths under different factors
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