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Numerical simulation of temperature and fluid in GTAW-arc under changing process conditions
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3752–3765
journal homepage: www.elsevier.com/locate/jmatprotec
Numerical simulation of temperature and fluid in GTAW-arc under changing process conditions Hua-yun Du a,∗ , Ying-hui Wei a,∗∗ , Wen-xian Wang a , Wan-ming Lin a , Ding Fan b a b
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this work, the heat and fluid flows in a free burning GTAW-arc under changing process
Received 22 April 2007
conditions have been developed by a steady two-dimensional (2D) axisymmetric model.
Received in revised form
The temperature profiles, velocity profiles, distribution of potential and current density of
1 August 2008
the arc have been studied. A series of arc models are established by changing the welding
Accepted 30 August 2008
process conditions, including welding current, flow rate of shielding gas, arc length and the kind of shielding gas. Governing equations, including heat and mass transfer equations, electromagnetic transport equations, are solved by finite volume method. © 2008 Elsevier B.V. All rights reserved.
Keywords: GTAW-arc Numerical simulation Changing process conditions Temperature profiles Fluid profiles
1.
Introduction
Numerical simulations of complex physical phenomena and behavior have only recently become feasible due to the development of computer technologies. The method of numerical simulations is now being widely used in many fields, which has proved very effective in gas tungsten arc welding (GTAW) research. Owing to the interferent of blazing arc light, real time measurement and inspect of welding are very difficult. Consequently, numerical approach has been adopted in many studies. Hsu et al. (1983), Etemadi (1982) and Masao and Fukuhisa (1982) simulated the arc plasma in early times. Much improvement has achieved in recent researches, and more factors are considered. A non-equilibrium model of transferred arc has been presented (Haidar et al., 1999), which considered the separate energy balance of the electrons and the heavy particles. Tanaka et al. (2002), Chuansong and Jinqiang (2002)
∗
and Ding and Jianhong (1998) built different static models of gas tungsten arc. The arc pressure (Fan and Shi, 1996; Ding et al., 1996) and arc plasma radiation (Masao et al., 1993) have been calculated. The effects of ambipolar diffusion on a burning arc model have been presented Sansonnens et al. (2000). A 3D fluid dynamic model has been presented by Freton et al. (2000). The anode boundary layer in free burning argon arcs with one-dimensional approach has been analyzed (Manabu et al., 1999). The heat and fluid flows in arc and welding pool has been coupling-calculated (Kim and Basu, 1998; Yongping and Yaowu, 2002). Two numerical models, i.e. the “potential” and the “magnetic” approaches have been established and compared (Marco et al., 2004). Nevertheless, understanding of the arc behavior still remains incomplete. In this paper, on the basis of previous studies (Ding et al., 2004; Huayun and Ding, 2006), the effect of process condition changes (different levels of welding current
Corresponding author. Tel.: +86 3516014426; fax: +86 3516014426. Corresponding author. E-mail addresses: [email protected] (H.-y. Du), [email protected] (Y.-h. Wei). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.08.038 ∗∗
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Table 1 – Source terms in the equations. Fr Source term
Fz
Fr = (jB) r
Q
Fz = (jB) z
Jz2 +Jr2
+
5 kB 2 e
∂T Jz ∂T ∂z + Jr ∂r
− SR
Table 2 – Boundary conditions. AB ϕ U V T
∂ϕ ∂r ∂u ∂r
=0 =0 V=0 ∂T ∂r = 0
BC ϕ=0 U=0 V=0 T = 1000 K
CD ∂ϕ ∂r ∂u ∂r ∂v ∂r
=0 =0 =0 T = 500 K
DE ∂ϕ ∂r ∂u ∂r ∂v ∂r
=0 =0 =0 T = 500 K
Fig. 1 – Domain of free burning arc in 2D, Torch and Armor Plat.
Fig. 2 – Temperature profiles of different currents. (A, 150 A; B, 200 A; C, 250 A; D, 300 A).
EA Eq. (9) U=0 V=0 T = 3000 K
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Fig. 3 – Temperature changes along axis (different currents). Fig. 4 – Potential changes along axis (different currents). , arc length, flow rate of shielding gas and the kind of shielding gas) is analyzed through numerical simulation, which has proved a useful intermediate step in the direction of developing a comprehensive representation of the correlations among these process conditions.
2.
Mathematical model
2.1.
Basal hypothesis
The model is based on the following main assumptions: the arc is symmetrical; gravity is neglected; the plasma is assumed to be thermal plasma, which satisfies conditions for local
thermodynamic equilibrium (LTE) in steady state; the flow is laminar; the arc anode interactions are not taken into account. The model uses cylindrical symmetry.
2.2.
Governing equations
With the above assumptions, the Navier–Stokes equations for the arc may be written as follows: Conservation of mass ∂ 1 ∂ (u) + (r) = 0 ∂z r ∂r
Fig. 5 – Distribution of velocity and streamtraces (A, 150 A; B, 300 A).
(1)
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Fig. 8 – Comparison of axial current density along axis (different currents).
Fig. 6 – Axial velocity changes along the axis (different currents).
Conservation of energy:
∂T
cP u
∂z
+v
∂T ∂r
=
∂ ∂z
∂T k
∂z
+
1 ∂ r ∂r
∂T kr
∂r
+Q
(4)
In these flow and temperature field equations, u and v represent the plasma velocity in the axial and radial directions, respectively. P is the plasma pressure and T is temperature. The material properties required in the flow and temperature field calculations are the density , dynamic viscosity , specific heat cp , thermal conductivity k and electrical
Fig. 7 – Radial velocity changes on the anode (different currents). Conservation of radial momentum:
∂u
∂u +v u ∂r ∂z
∂P + = Fr − ∂r
u ∂2 u 1 ∂u ∂2 u − 2 + 2 + 2 r ∂r ∂r r ∂z
(2)
Conservation of axial momentum:
∂v
u
∂r
+v
∂v ∂z
= Fz −
∂P + ∂z
1 ∂v ∂2 v ∂2 v + + 2 2 r ∂r ∂r ∂z
(3)
Fig. 9 – Comparison of radial current density along axis (different currents).
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conductivity . All of these properties of Ar, He and CO2 are obtained from reference of Lancaster (1984). As functions of temperature, , , cp and k are obtained from statistical thermodynamic analysis and calculations in a 1 atm environment (Fenggui et al., 2006). Fr and Fz present volume forces, i.e. electromagnetism force in the axial and radial directions, respectively. Q is source term of heat; SR is the radiant heat. kB is the Boltzmann constant. To solve Eqs. (1)–(4) the current density and circumferential magnetic fields need to be known. A series relevant electromagnetic transport equations need to be solved. Conservation of electrical charge: ∂ ∂z
∂ϕ
∂z
+
1 ∂ r ∂r
∂ϕ r
∂r
=0
(5)
ϕ presents potential. The current densities in the axial and radial directions can be obtained by Ohm law: Jr =
∂ϕ ∂r
Jz =
∂ϕ ∂z
(6)
Since the current distribution is axisymmetrical, the selfinduced azimuthal magnetic field is derived from ampere circumfluence law:
B0 =
0 r
r
Jz r dr
(7)
0
Maxwell equation: F = (jB)
2.3.
(8)
Boundary conditions
The domain used in this model is region of ABCDEA, as shown in Fig. 1. The electrode angle is 60◦ . The source terms in the equations are shown in Table 1. The source terms of energy consists of joule heating, transport of heat due to electron drift and radiation losses.
Fig. 10 – Temperature profiles of different flow rates (A, 0 L/S; B, 1 L/S; C, 5 L/S; D, 10 L/S).
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Fig. 11 – Temperature distribution along axis (different flow rates).
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Fig. 13 – Axial velocity along the axis (different flow rates).
anode is assumed to be a perfect conductor relative to the plasma. Boundary DE is the shielding gas inlet. At the cathode surface EA, the current density J is given by Jz = Jmax exp(−br)
Fig. 12 – Temperature distribution along the anode (different flow rates).
As shown in Fig. 1, all quantities use symmetry condition, and only half of the flow domain is considered in the calculation. The boundary conditions (Freton et al., 2000) are given in Table 2. Along the centerline AB, symmetry conditions are used. Zero velocities are specified along the solid boundaries BC and EA. Along far-field boundary CD, constant radial gradients for all variables are specified. A constant electrical potential is specified along the anode surface BC because the
Fig. 14 – Radial velocity along the anode (different flow rates).
(9)
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Fig. 15 – Distribution of velocity and streamtraces at different flow rates (A, 0 L/S; B, 1 L/S; C, 5 L/S; D, 10 L/S).
The maximum current density can be given by Jmax =
I 2rh2
(10)
Constant b is determined by
I = 2
rc
rJz dr
(11)
0
rh presents cathode spot radium and rc presents the cutoff radius of cathode. In fact the current density, the rh and rc depend critically on the cathode surface temperature. At the cathode surface EA, the temperature is assumed to be 3000 K. Thus the rc is assumed to be 3 mm, the rh is assumed to be 0.5 mm (Freton et al., 2000; Ding et al., 1996). Radiation loss is a function of plasma temperature. The radiative flux is calculated by reference (Ding et al., 1996), which is used as a parameter of the energy source term.
3.
Numerical analysis procedures
3.1.
Transformation of governing equations
This model is calculated by PHOENICS (Version 3.4) code, a general thermofluid-mechanics computer program (which is based on the SAMPLE algorithm and developed by CHEM, to solve coupled sets of partial differential equations governing heat, mass and momentum transfer) and body-fitted co-ordinates is used. In order to enhance the accuracy of calculation and to reduce the cost of analysis, the mathematical model employed a 60 × 60, fixed rectangular grid system for the calculation of the temperature and velocity fields. For a numerical solution of Eqs. (1)–(5), the problem domain is covered by a set of rectangular control volumes. Values of the variables within a control volume are presented in terms of the values at the associated node point. Based on the rectangular grid, a fully implicit control volume integration of the govern-
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Fig. 16 – Temperature profiles at different arc lengths (A, 3 mm; B, 5 mm; C, 8 mm; D, 10 mm).
ing Eqs. (1)–(5) results in the finite-difference scheme below:
3.2.
aP P = aN N + aS S + aE E + aW W + aL L + aH H + b
In order to analyze the effect of different arc parameters, a series of arc models with only one of the process conditions changed while other conditions remain unchanged are established. Under conditions of changing current values (150, 200, 250 and 300 A), the arc length remains 10 mm and the shielding gas Ar flow rate remains 0 L/S. Under conditions of changing Ar flow rates (0, 1, 5 and 10 L/S), the arc length remains 10 mm and the current remains 200 A. Under conditions of changing arc lengths (3, 5, 8 and 10 mm), current remains 200 A and the Ar flow remains 0 L/S. Besides, a set of models using different shielding gases such as He, Ar and CO2 are also constructed and studied, in which the arc length of 10 mm, the current of 200 A and the gas flow rate of 5 L/S all remain unchanged.
(12)
where presents any of axial velocity, radial velocity, potential and temperature. The subscripts indicate the appropriate nodal value of the dependent variable. The value of a represents the coefficients that results from the equations, whilst b is the source term from the equations. The SAMPLE algorithm was used to solve the governing equations with the associated source terms. The selected relaxation parameter is 1.7 and the number of sweeps is 150. Initial convergence difficulties are overcome by using a simultaneous solver for the pressure correction equation and false-time step relaxation on temperature and velocities. Convergence is accomplished when the spot values of the relevant dependent variables at the critical grid location remains fixed, but the residuals of all governing equations keep decreasing. Generally, the residual must decrease by at least three orders of magnitude with respect to the previous sweep before the run is terminated, so it is set to 0.001 in this case.
4.
Parameter changes in the models
Results and discussions
The calculated results on the conditions of 200 A current, 10 mm arc length and shield gas Ar flow 0 L/S are excellent
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Fig. 17 – Temperature changes along axis (different arc lengths).
agreement with many other works (Ding et al., 2004; Huayun and Ding, 2006) on the same conditions, Which can verify the model in a certain extent.
4.1.
Effects of current changes
Fig. 2 shows the temperature profiles with different welding currents. It can be seen that the profiles of temperature change little, but the values are obviously different. The highest temperatures are all in the centre of the arc near the cathode (between coordinates 3 and 4 mm). The arc plasma shows a contract state owing to electromagnetism force. And temperature declines along both of the axial and radial direction from the centre of the arc. The temperature changes along the axis are indicated in Fig. 3. The highest temperatures are 1.45E + 04, 2.11E + 04, 2.83E + 04 and 3.18E + 04 K when the current changes from 150 to 300 A. As can be seen, with the increase of the current, the arc temperature rises. Fig. 4 shows the potential changes along axis. The arc voltages are 10.66, 13.6, 16.11 and 19.4 V, respectively when the current increases from 150 to 300 A. It can be seen that with the increase of the current, the potential of cathode declines, i.e. the voltage of the arc increases. It can be noticed that all the curves become steeper near the cathode (3 mm), which indicates a sharp cathode potential drop. Fig. 5 presents the distribution of velocity and streamtraces when the current is 150 and 300 A, respectively. The streamtraces indicate the direction of the fluid flow and the shade of colors indicates the intensity of the velocity. The configuration of two swirls in both figures can be clearly observed, one clockwise and the other anticlockwise. The clockwise, i.e. counter rotating vortices may be driven by the self-induced azimuthal magnetic field, which shows the same streamtrace configura-
Fig. 18 – Temperature changes along the anode (different arc lengths).
tion in both profiles. And the anticlockwise swirl also shows the same result. Axial velocities along the axis are shown in Fig. 6. The highest axial velocities along the axis are 50.9, 69, 85.2 and 93.5 m/s, respectively. The highest absolute velocity of counter rotating vortices nearby the cathode are 30.1, 42, 52.1 and 60 m/s, respectively. It can be conclude that the change
Fig. 19 – Distribution of axial velocity along the axis (different arc lengths).
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Fig. 20 – Radial velocity on the anode (different arc lengths).
of current has no effect on direction of streamtraces, but the values of velocity change more significantly. With the increase of current, the velocity of fluid flow rises too. Fig. 7 presents the radial velocity on the anode. With the current increases from 150 to 300 A, the highest radial velocity on the anode are in turn 17.8, 23.8, 29.7 and 32.5 m/s. The higher the current, the larger the velocity, indicating same change trend with axial velocity. Fig. 8 shows the axial current density along axis. The absolute values are 4.88E + 07, 6.51E + 07, 8.13E + 07 and 8.88E + 07 A/m2 , respectively. It can be seen that the radial current density along axis and the current are in direct ratio, i.e. with the increase of the current, the axial current density along axis also increases. The highest values are at the end of the cathode. Fig. 9 presents the radial current density along axis. The highest values also at the end of the cathode are 4.74E + 07, 6.32E + 07, 7.90E + 07 and 8.62E + 07 A/m2 , respectively. With the same trend of axial current density, the radial current density increases with the increase of the current. With the increase of the welding current, the temperature, voltage of the arc, axial velocity, radial velocity on the anode and both the axial and radial current density also increase. The reason is that the increase of current brings more energy to the arc, and all properties of which are improved. This conclusion is in good agreement with the classical theories.
4.2.
Effects of protective flow rate
Fig. 10 shows the temperature profiles of different flow rates. It can be observed that with the increase of flow rate, the outside isotherms change their shapes, and the high temperature area beside the cathode shrinks. The increased flow rate compresses the arc and causes the energy emission much closer to
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the axis center, and the highest temperature is also increased. With the flow rate further increases, the energy emission area shrinks more, but the highest temperature declines. The reason may be that with the increase of flow rate, the energy convection speed also increases, and the energy loss increases as a result. Fig. 11 shows the temperature distribution along axis with different flow rates. The highest temperatures are 2.11E + 04, 2.18E + 04, 2.28E + 04 and 1.97E + 04 K, respectively corresponding to the flow rates of 0, 1, 5 and 10 L/S. Fig. 12 shows the temperature distribution along the anode. With the same trend of temperature distribution along axis, the center temperatures are evidently higher when the flow rates are 1 and 5 L/S as compared with the values at 0 and 10 L/S. It can be concluded from the two figures that with the increase of flow rate, the highest temperature of the arc both along the axis and the anode firstly rises and then drop. Under the conditions of this case, the arc with flow rate of 5 L/S shows the highest temperature. Fig. 13 shows a comparison of axial velocity along the axis with different flow rates. The highest axial velocity along the axis are 60.8, 62.2, 65.5 and 68.9 m/s, respectively, corresponding to the flow rates of 0, 1, 5 and 10 L/S. As can be seen, the highest axial velocity increases with the increase of flow rate. The highest axial velocity with flow rate of 0, 1 and 5 L/S is at 6–8 mm in the axial direction. As the flow rate increases to 10 L/S, the highest axial velocity appears at 4–5 mm, nearer to the cathode. Fig. 14 shows the radial velocity along the anode. It is obvious that the radial velocity along the anode near the arc centre decreases with the increase of the flow rate. However, in the region far from the arc center the radial velocity of 10 L/S is higher than that of 5 L/S. The distribution of velocity and streamtraces at different flow rates are shown in Fig. 15. From the distribution of velocity, it can be seen that with the increase of flow rate, the fluid field and streamtraces changed obviously. Especially with the flow rate of 10 L/S, the area of higher velocity in the arc pole has left the original position and moved close to the cathode and depart from the axis. The configuration changing of two swirls can be clearly observed. The clockwise swirl streamtraces is compressed with the increase of flow rate, and eventually vanishes when the flow rate increases to 10 L/S. The anticlockwise swirl streamtraces become larger as the flow rate increase and govern the arc region, thus the quality of flow protection is improved. The changing flow rate has no effect to the electromagnetism field, potential field and current density of both axial and radial direction.
4.3.
Effects of arc length
Fig. 16 shows the temperature profiles at different arc lengths. The calculation results indicate that the change of arc length shall significantly affect the arc temperature. The arc is compressed to a smaller region when the arc length is decreased, and the configuration of arc with longer arc length shall become more extended. The highest temperature in the arc center increases with the increase of the arc length. The curves in Fig. 17 show the temperature changes along axis. With the arc length increases from 3, 5, 8 to 10 mm, the
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Fig. 21 – Distribution of velocity and streamtraces of different arc lengths (A, 3 mm; B, 5 mm; C, 8 mm; D, 10 mm).
highest temperature along the axis are 1.63E + 04, 1.86E + 04, 1.99E + 04 and 2.11E + 04 K. The arc with length of 10 mm shows the highest temperature, and the arc with length of 3 mm shows the lowest temperature. Hence the temperature and arc length is in direct proportion. Fig. 18 shows the temperature changes along the anode. All curves show that the closer to the center axis, the higher the temperature. The arc with length of 8 mm has the highest temperature. It can thus be concluded that the temperature on the anode rises at first and then drops with the arc length increases from 3 to 10 mm. In another word, there has to be an optimal arc length that can assure the optimal heat input in the welding procedure. Fig. 19 shows the distribution of axial velocity along the axis. The highest axial velocities along the axis are in turn 42.3, 60.4, 65.8 and 64.7 m/s with the arc length increase from 3 to 10 mm. It can be seen that the velocity increases with the
arc length increase from 3 to 8 mm and then deceases a little with the arc length increase from 8 to 10 mm. Fig. 20 presents the radial velocity on the anode. Radial velocities on the anode are higher when the arc length is 5 and 8 mm, as compared with those when it is 3 and 10 mm. It can thus be concluded that the arc with moderate length can be better protected. Fig. 21 shows the distribution of velocity and streamtraces of different arc lengths. The configuration of two swirls at different arc lengths can be clearly observed. With the arc length of 3 mm, the clockwise swirl (counter rotating vortices) is comparatively larger, while the anticlockwise swirl is comparatively smaller. With the increase of the arc length, the clockwise swirl gradually shrinks and the anticlockwise swirl gradually dominates the arc area. It is obvious that the longer the arc length, the larger the area of arc pole and the higher the velocity in the arc pole.
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Fig. 22 – Comparison of potential along axis (different arc lengths).
Fig. 22 shows the potentials along axis. The arc voltages are in turn 8.78, 10.9, 11.8 and 13.6 V corresponding to the arc length of 3, 5, 8 and 10 mm. It can be clearly noticed that the longer the arc length, the larger the arc voltage. Fig. 23 shows the axial current density along axis. Corresponding to the arc length of 3, 5, 8 and 10 mm, the highest
Fig. 24 – Radial current density along axis (different arc lengths).
axial current density along axis are 6.39E + 07, 6.42E + 07, 6.21E + 07 and 6.51E + 07 A/m2 , respectively. The change of these values is little. The reason is that the arc voltage and arc length are in direct proportion, and the voltage grads (∂ϕ/∂z) changes little, thus the axial current density changes little. Fig. 24 shows the radial current density along axis. Corresponding to the arc length of 3, 5, 8 and 10 mm, the highest radial current density along axis are 3.27E + 07, 5.64E + 07, 5.63E + 07 and 6.32E + 07 A/m2 near the cathode tip. It can be concluded that with the increase of arc length, the axial current density varies little, but the radial current density along axis increases much more significantly. Values of the variables are significantly affected by the arc length. The temperature of arc center, the arc voltage and the velocity of fluid flows are in direct proportion with the arc length. However, there has to be an optimal and moderate arc length that can assure the arc with optimal heat input and better protection. As for the effect on the current density, the radial current density increase with the increase of arc length, but the axial density changes little.
4.4.
Fig. 23 – Axial current density along axis (different arc lengths).
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Effect of the composition of shielding gas
Fig. 25 shows the temperature field of different shielding gases with a comparison of temperatures along axis. A is arc with shielding gas Ar, B is with CO2 and C is with He. The arc temperature with shielding gas He is the lowest, while the one with CO2 is the highest. Here the difference is owing to their different thermal conductivity. The higher the thermal conductivity, the more the heat loss. Thermal conductivity of He is four times of that of Ar’s, and ten times of CO2 ’s. Thus the
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Fig. 25 – Temperature field of different shielding gas and their comparison of temperature along axis (A, Ar; B, CO2 ; C, He; D, comparison of temperature along axis).
highest temperature along the He arc axis is the lowest, only 13,000 K, while the highest temperature along the CO2 arc axis is 31,000 K. The He arc is comparatively relaxed and in contrast the CO2 arc is comparatively contracted.
5.
Conclusion
• With the increase of the welding current, the increase of current brings more energy to the arc, and all properties including the temperature, voltage of the arc, axial velocity, radial velocity on the anode and both the axial and radial current density are improved. Except that the streamtraces with all currents are still showed in the same configuration. • As for the changing of flow rate, there exists an optimum flow rate, with which the arc can acquire the highest temperature. With the increase of flow rate, the area of higher velocity in the arc pole has left the original position and the streamtraces change their configurations, the anticlockwise
swirl streamtraces become larger and govern the arc region, the protection effects are improved. The electro-magnetism field, potential field and current density of both axial and radial directions are not influenced. • With the increase of arc length, the temperature of arc, the potential and the velocity of the fluid flows are increased. The moderate arc length may be an optimal one that can assure the arc with optimal heat input and better protection. The radial current density increases with the increase of arc length, however the axial density changes little. • The higher the thermal conductivity of the shielding gas, the more the heat loss and the lower the arc temperature.
Acknowledgements The authors gratefully acknowledge gratitude to the Young Subject-Leader Foundation, Shanxi Province Natural Science Foundation and the Supporting Plan for Excellent Talents New
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3752–3765
Century and the Ministry of Education of P.R. China for financial support to this work.
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Chuansong Wu, Jinqiang Gao, 2002. Numerical simulation of the TIG welding arc behavior. J. Mater. Sci. Technol. 8, 43–48. Ding Fan, Huayun Du, Ruihua Zhang, 2004. Numerical simulation of the profiles of temperature and fluid in GTAW-arc. J. Electr. Weld. Mach. 34 (8), 7–9. Ding Fan, Jianhong Chen, 1998. Numerical analysis of the heat and mass transfer process for TIG arc. J. Chinese J. Mech. Eng. 34 (2), 39–45. Ding Fan, Masao Ushio, Fukuhisa Matsuda, 1996. Numerical computation of arc pressure distribution. J. Trans. JWRI 15 (1), 1–5. Etemadi K., 1982. PhD Thesis. University of Minnesota. Fan, H.G., Shi, Y.W., 1996. Numerical simulation of the arc pressure in gas tungsten arc welding,. J. Mater. Process. Technol. 61, 302–308. Fenggui Lu, Xinhua Tang, Hailiang Yu, Shun Yao, 2006. Numerical simulation on interaction between TIG welding arc and weld pool. J. Comput. Mater. Sci. 35, 458–465. Freton, P., Gonzalez, J.J., Gleizes, A., 2000. Comparison between a two- and a three-dimensional arc plasma configuration. J. Phys. D: Appl. Phys. 33, 2442–2452. Haidar, J., 1999. Non-equilibrium modeling of transferred arcs. J. Phys. D: Appl. Phys. 32, 263–272. Hsu, K.C., Etemadi, K., Pfender, E., 1983. Study of the free-burning high-intensity argon arc. J. Appl. Phys. 54, 1293–1301.
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Huayun Du, Ding Fan, 2006. Numerical simulation of the profiles of potential, current density and electromagnetic force in GTA-arc. J. Electr. Weld. Mach. 36 (9), 50–53. Kim, I.S., Basu, A., 1998. A mathematical model of heat transfer and fluid flow in the gas metal arc welding process. J. Mater. Process. Technol. 7, 17–24. Lancaster, J.F., 1984. The Physics of Welding. Oxford Pergamon Press, A Wheaton & Co. Ltd. Exeter, Great Britain, pp. 15–28. Tanaka Manabu, Ushio Masao, Chuansong Wu, 1999. One-dimensional analysis of the anode boundary layer in free-burning argon arcs. J. Phys. D: Appl. Phys. 32, 605– 611. Marco A. Ramirez, Gerardo Trapaga, John McKelliget, 2004. A comparison between different numerical formulations for welding arc representations. J. Mater. Process. Technol. 155–156, 1634–1640. Masao Ushio, Ding Fan, Manabu Tanaka, 1993. Contribution of arc plasma radiation energy to electrodes. Trans. JWRI 22 (2), 201–207. Masao Ushio, Fukuhisa Matsuda, 1982. Mathematical modeling of heat transfer of welding arc. Trans. JWRI 11 (1), 7–15. Sansonnens, L., Haida, J., Lowke, J.J., 2000. Prediction of properties of free burning arcs including effects of ambipolar diffusion. J. Phys. D: Appl. Phys. 33, 148–157. Tanaka Manabu, Terasaki Hidenori, Ushio Masao, Lowke, J.J., 2002. Steady state calculations of stationary gas tungsten arc welding from a unified arc-electrodes model. Trans. JWRI 31 (1), 19–24. Yongping Lei, Yaowu Shi, 2002. Numerical analysis of the two-way interaction between a mutually coupled weld-pool and weld-arc for GTA Welding process. J. Acta Metall. Sin. 37 (5), 537–542.
journal homepage: www.elsevier.com/locate/jmatprotec
Numerical simulation of temperature and fluid in GTAW-arc under changing process conditions Hua-yun Du a,∗ , Ying-hui Wei a,∗∗ , Wen-xian Wang a , Wan-ming Lin a , Ding Fan b a b
College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China College of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
In this work, the heat and fluid flows in a free burning GTAW-arc under changing process
Received 22 April 2007
conditions have been developed by a steady two-dimensional (2D) axisymmetric model.
Received in revised form
The temperature profiles, velocity profiles, distribution of potential and current density of
1 August 2008
the arc have been studied. A series of arc models are established by changing the welding
Accepted 30 August 2008
process conditions, including welding current, flow rate of shielding gas, arc length and the kind of shielding gas. Governing equations, including heat and mass transfer equations, electromagnetic transport equations, are solved by finite volume method. © 2008 Elsevier B.V. All rights reserved.
Keywords: GTAW-arc Numerical simulation Changing process conditions Temperature profiles Fluid profiles
1.
Introduction
Numerical simulations of complex physical phenomena and behavior have only recently become feasible due to the development of computer technologies. The method of numerical simulations is now being widely used in many fields, which has proved very effective in gas tungsten arc welding (GTAW) research. Owing to the interferent of blazing arc light, real time measurement and inspect of welding are very difficult. Consequently, numerical approach has been adopted in many studies. Hsu et al. (1983), Etemadi (1982) and Masao and Fukuhisa (1982) simulated the arc plasma in early times. Much improvement has achieved in recent researches, and more factors are considered. A non-equilibrium model of transferred arc has been presented (Haidar et al., 1999), which considered the separate energy balance of the electrons and the heavy particles. Tanaka et al. (2002), Chuansong and Jinqiang (2002)
∗
and Ding and Jianhong (1998) built different static models of gas tungsten arc. The arc pressure (Fan and Shi, 1996; Ding et al., 1996) and arc plasma radiation (Masao et al., 1993) have been calculated. The effects of ambipolar diffusion on a burning arc model have been presented Sansonnens et al. (2000). A 3D fluid dynamic model has been presented by Freton et al. (2000). The anode boundary layer in free burning argon arcs with one-dimensional approach has been analyzed (Manabu et al., 1999). The heat and fluid flows in arc and welding pool has been coupling-calculated (Kim and Basu, 1998; Yongping and Yaowu, 2002). Two numerical models, i.e. the “potential” and the “magnetic” approaches have been established and compared (Marco et al., 2004). Nevertheless, understanding of the arc behavior still remains incomplete. In this paper, on the basis of previous studies (Ding et al., 2004; Huayun and Ding, 2006), the effect of process condition changes (different levels of welding current
Corresponding author. Tel.: +86 3516014426; fax: +86 3516014426. Corresponding author. E-mail addresses: [email protected] (H.-y. Du), [email protected] (Y.-h. Wei). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.08.038 ∗∗
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Table 1 – Source terms in the equations. Fr Source term
Fz
Fr = (jB) r
Q
Fz = (jB) z
Jz2 +Jr2
+
5 kB 2 e
∂T Jz ∂T ∂z + Jr ∂r
− SR
Table 2 – Boundary conditions. AB ϕ U V T
∂ϕ ∂r ∂u ∂r
=0 =0 V=0 ∂T ∂r = 0
BC ϕ=0 U=0 V=0 T = 1000 K
CD ∂ϕ ∂r ∂u ∂r ∂v ∂r
=0 =0 =0 T = 500 K
DE ∂ϕ ∂r ∂u ∂r ∂v ∂r
=0 =0 =0 T = 500 K
Fig. 1 – Domain of free burning arc in 2D, Torch and Armor Plat.
Fig. 2 – Temperature profiles of different currents. (A, 150 A; B, 200 A; C, 250 A; D, 300 A).
EA Eq. (9) U=0 V=0 T = 3000 K
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Fig. 3 – Temperature changes along axis (different currents). Fig. 4 – Potential changes along axis (different currents). , arc length, flow rate of shielding gas and the kind of shielding gas) is analyzed through numerical simulation, which has proved a useful intermediate step in the direction of developing a comprehensive representation of the correlations among these process conditions.
2.
Mathematical model
2.1.
Basal hypothesis
The model is based on the following main assumptions: the arc is symmetrical; gravity is neglected; the plasma is assumed to be thermal plasma, which satisfies conditions for local
thermodynamic equilibrium (LTE) in steady state; the flow is laminar; the arc anode interactions are not taken into account. The model uses cylindrical symmetry.
2.2.
Governing equations
With the above assumptions, the Navier–Stokes equations for the arc may be written as follows: Conservation of mass ∂ 1 ∂ (u) + (r) = 0 ∂z r ∂r
Fig. 5 – Distribution of velocity and streamtraces (A, 150 A; B, 300 A).
(1)
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Fig. 8 – Comparison of axial current density along axis (different currents).
Fig. 6 – Axial velocity changes along the axis (different currents).
Conservation of energy:
∂T
cP u
∂z
+v
∂T ∂r
=
∂ ∂z
∂T k
∂z
+
1 ∂ r ∂r
∂T kr
∂r
+Q
(4)
In these flow and temperature field equations, u and v represent the plasma velocity in the axial and radial directions, respectively. P is the plasma pressure and T is temperature. The material properties required in the flow and temperature field calculations are the density , dynamic viscosity , specific heat cp , thermal conductivity k and electrical
Fig. 7 – Radial velocity changes on the anode (different currents). Conservation of radial momentum:
∂u
∂u +v u ∂r ∂z
∂P + = Fr − ∂r
u ∂2 u 1 ∂u ∂2 u − 2 + 2 + 2 r ∂r ∂r r ∂z
(2)
Conservation of axial momentum:
∂v
u
∂r
+v
∂v ∂z
= Fz −
∂P + ∂z
1 ∂v ∂2 v ∂2 v + + 2 2 r ∂r ∂r ∂z
(3)
Fig. 9 – Comparison of radial current density along axis (different currents).
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conductivity . All of these properties of Ar, He and CO2 are obtained from reference of Lancaster (1984). As functions of temperature, , , cp and k are obtained from statistical thermodynamic analysis and calculations in a 1 atm environment (Fenggui et al., 2006). Fr and Fz present volume forces, i.e. electromagnetism force in the axial and radial directions, respectively. Q is source term of heat; SR is the radiant heat. kB is the Boltzmann constant. To solve Eqs. (1)–(4) the current density and circumferential magnetic fields need to be known. A series relevant electromagnetic transport equations need to be solved. Conservation of electrical charge: ∂ ∂z
∂ϕ
∂z
+
1 ∂ r ∂r
∂ϕ r
∂r
=0
(5)
ϕ presents potential. The current densities in the axial and radial directions can be obtained by Ohm law: Jr =
∂ϕ ∂r
Jz =
∂ϕ ∂z
(6)
Since the current distribution is axisymmetrical, the selfinduced azimuthal magnetic field is derived from ampere circumfluence law:
B0 =
0 r
r
Jz r dr
(7)
0
Maxwell equation: F = (jB)
2.3.
(8)
Boundary conditions
The domain used in this model is region of ABCDEA, as shown in Fig. 1. The electrode angle is 60◦ . The source terms in the equations are shown in Table 1. The source terms of energy consists of joule heating, transport of heat due to electron drift and radiation losses.
Fig. 10 – Temperature profiles of different flow rates (A, 0 L/S; B, 1 L/S; C, 5 L/S; D, 10 L/S).
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Fig. 11 – Temperature distribution along axis (different flow rates).
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Fig. 13 – Axial velocity along the axis (different flow rates).
anode is assumed to be a perfect conductor relative to the plasma. Boundary DE is the shielding gas inlet. At the cathode surface EA, the current density J is given by Jz = Jmax exp(−br)
Fig. 12 – Temperature distribution along the anode (different flow rates).
As shown in Fig. 1, all quantities use symmetry condition, and only half of the flow domain is considered in the calculation. The boundary conditions (Freton et al., 2000) are given in Table 2. Along the centerline AB, symmetry conditions are used. Zero velocities are specified along the solid boundaries BC and EA. Along far-field boundary CD, constant radial gradients for all variables are specified. A constant electrical potential is specified along the anode surface BC because the
Fig. 14 – Radial velocity along the anode (different flow rates).
(9)
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Fig. 15 – Distribution of velocity and streamtraces at different flow rates (A, 0 L/S; B, 1 L/S; C, 5 L/S; D, 10 L/S).
The maximum current density can be given by Jmax =
I 2rh2
(10)
Constant b is determined by
I = 2
rc
rJz dr
(11)
0
rh presents cathode spot radium and rc presents the cutoff radius of cathode. In fact the current density, the rh and rc depend critically on the cathode surface temperature. At the cathode surface EA, the temperature is assumed to be 3000 K. Thus the rc is assumed to be 3 mm, the rh is assumed to be 0.5 mm (Freton et al., 2000; Ding et al., 1996). Radiation loss is a function of plasma temperature. The radiative flux is calculated by reference (Ding et al., 1996), which is used as a parameter of the energy source term.
3.
Numerical analysis procedures
3.1.
Transformation of governing equations
This model is calculated by PHOENICS (Version 3.4) code, a general thermofluid-mechanics computer program (which is based on the SAMPLE algorithm and developed by CHEM, to solve coupled sets of partial differential equations governing heat, mass and momentum transfer) and body-fitted co-ordinates is used. In order to enhance the accuracy of calculation and to reduce the cost of analysis, the mathematical model employed a 60 × 60, fixed rectangular grid system for the calculation of the temperature and velocity fields. For a numerical solution of Eqs. (1)–(5), the problem domain is covered by a set of rectangular control volumes. Values of the variables within a control volume are presented in terms of the values at the associated node point. Based on the rectangular grid, a fully implicit control volume integration of the govern-
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Fig. 16 – Temperature profiles at different arc lengths (A, 3 mm; B, 5 mm; C, 8 mm; D, 10 mm).
ing Eqs. (1)–(5) results in the finite-difference scheme below:
3.2.
aP P = aN N + aS S + aE E + aW W + aL L + aH H + b
In order to analyze the effect of different arc parameters, a series of arc models with only one of the process conditions changed while other conditions remain unchanged are established. Under conditions of changing current values (150, 200, 250 and 300 A), the arc length remains 10 mm and the shielding gas Ar flow rate remains 0 L/S. Under conditions of changing Ar flow rates (0, 1, 5 and 10 L/S), the arc length remains 10 mm and the current remains 200 A. Under conditions of changing arc lengths (3, 5, 8 and 10 mm), current remains 200 A and the Ar flow remains 0 L/S. Besides, a set of models using different shielding gases such as He, Ar and CO2 are also constructed and studied, in which the arc length of 10 mm, the current of 200 A and the gas flow rate of 5 L/S all remain unchanged.
(12)
where presents any of axial velocity, radial velocity, potential and temperature. The subscripts indicate the appropriate nodal value of the dependent variable. The value of a represents the coefficients that results from the equations, whilst b is the source term from the equations. The SAMPLE algorithm was used to solve the governing equations with the associated source terms. The selected relaxation parameter is 1.7 and the number of sweeps is 150. Initial convergence difficulties are overcome by using a simultaneous solver for the pressure correction equation and false-time step relaxation on temperature and velocities. Convergence is accomplished when the spot values of the relevant dependent variables at the critical grid location remains fixed, but the residuals of all governing equations keep decreasing. Generally, the residual must decrease by at least three orders of magnitude with respect to the previous sweep before the run is terminated, so it is set to 0.001 in this case.
4.
Parameter changes in the models
Results and discussions
The calculated results on the conditions of 200 A current, 10 mm arc length and shield gas Ar flow 0 L/S are excellent
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Fig. 17 – Temperature changes along axis (different arc lengths).
agreement with many other works (Ding et al., 2004; Huayun and Ding, 2006) on the same conditions, Which can verify the model in a certain extent.
4.1.
Effects of current changes
Fig. 2 shows the temperature profiles with different welding currents. It can be seen that the profiles of temperature change little, but the values are obviously different. The highest temperatures are all in the centre of the arc near the cathode (between coordinates 3 and 4 mm). The arc plasma shows a contract state owing to electromagnetism force. And temperature declines along both of the axial and radial direction from the centre of the arc. The temperature changes along the axis are indicated in Fig. 3. The highest temperatures are 1.45E + 04, 2.11E + 04, 2.83E + 04 and 3.18E + 04 K when the current changes from 150 to 300 A. As can be seen, with the increase of the current, the arc temperature rises. Fig. 4 shows the potential changes along axis. The arc voltages are 10.66, 13.6, 16.11 and 19.4 V, respectively when the current increases from 150 to 300 A. It can be seen that with the increase of the current, the potential of cathode declines, i.e. the voltage of the arc increases. It can be noticed that all the curves become steeper near the cathode (3 mm), which indicates a sharp cathode potential drop. Fig. 5 presents the distribution of velocity and streamtraces when the current is 150 and 300 A, respectively. The streamtraces indicate the direction of the fluid flow and the shade of colors indicates the intensity of the velocity. The configuration of two swirls in both figures can be clearly observed, one clockwise and the other anticlockwise. The clockwise, i.e. counter rotating vortices may be driven by the self-induced azimuthal magnetic field, which shows the same streamtrace configura-
Fig. 18 – Temperature changes along the anode (different arc lengths).
tion in both profiles. And the anticlockwise swirl also shows the same result. Axial velocities along the axis are shown in Fig. 6. The highest axial velocities along the axis are 50.9, 69, 85.2 and 93.5 m/s, respectively. The highest absolute velocity of counter rotating vortices nearby the cathode are 30.1, 42, 52.1 and 60 m/s, respectively. It can be conclude that the change
Fig. 19 – Distribution of axial velocity along the axis (different arc lengths).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3752–3765
Fig. 20 – Radial velocity on the anode (different arc lengths).
of current has no effect on direction of streamtraces, but the values of velocity change more significantly. With the increase of current, the velocity of fluid flow rises too. Fig. 7 presents the radial velocity on the anode. With the current increases from 150 to 300 A, the highest radial velocity on the anode are in turn 17.8, 23.8, 29.7 and 32.5 m/s. The higher the current, the larger the velocity, indicating same change trend with axial velocity. Fig. 8 shows the axial current density along axis. The absolute values are 4.88E + 07, 6.51E + 07, 8.13E + 07 and 8.88E + 07 A/m2 , respectively. It can be seen that the radial current density along axis and the current are in direct ratio, i.e. with the increase of the current, the axial current density along axis also increases. The highest values are at the end of the cathode. Fig. 9 presents the radial current density along axis. The highest values also at the end of the cathode are 4.74E + 07, 6.32E + 07, 7.90E + 07 and 8.62E + 07 A/m2 , respectively. With the same trend of axial current density, the radial current density increases with the increase of the current. With the increase of the welding current, the temperature, voltage of the arc, axial velocity, radial velocity on the anode and both the axial and radial current density also increase. The reason is that the increase of current brings more energy to the arc, and all properties of which are improved. This conclusion is in good agreement with the classical theories.
4.2.
Effects of protective flow rate
Fig. 10 shows the temperature profiles of different flow rates. It can be observed that with the increase of flow rate, the outside isotherms change their shapes, and the high temperature area beside the cathode shrinks. The increased flow rate compresses the arc and causes the energy emission much closer to
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the axis center, and the highest temperature is also increased. With the flow rate further increases, the energy emission area shrinks more, but the highest temperature declines. The reason may be that with the increase of flow rate, the energy convection speed also increases, and the energy loss increases as a result. Fig. 11 shows the temperature distribution along axis with different flow rates. The highest temperatures are 2.11E + 04, 2.18E + 04, 2.28E + 04 and 1.97E + 04 K, respectively corresponding to the flow rates of 0, 1, 5 and 10 L/S. Fig. 12 shows the temperature distribution along the anode. With the same trend of temperature distribution along axis, the center temperatures are evidently higher when the flow rates are 1 and 5 L/S as compared with the values at 0 and 10 L/S. It can be concluded from the two figures that with the increase of flow rate, the highest temperature of the arc both along the axis and the anode firstly rises and then drop. Under the conditions of this case, the arc with flow rate of 5 L/S shows the highest temperature. Fig. 13 shows a comparison of axial velocity along the axis with different flow rates. The highest axial velocity along the axis are 60.8, 62.2, 65.5 and 68.9 m/s, respectively, corresponding to the flow rates of 0, 1, 5 and 10 L/S. As can be seen, the highest axial velocity increases with the increase of flow rate. The highest axial velocity with flow rate of 0, 1 and 5 L/S is at 6–8 mm in the axial direction. As the flow rate increases to 10 L/S, the highest axial velocity appears at 4–5 mm, nearer to the cathode. Fig. 14 shows the radial velocity along the anode. It is obvious that the radial velocity along the anode near the arc centre decreases with the increase of the flow rate. However, in the region far from the arc center the radial velocity of 10 L/S is higher than that of 5 L/S. The distribution of velocity and streamtraces at different flow rates are shown in Fig. 15. From the distribution of velocity, it can be seen that with the increase of flow rate, the fluid field and streamtraces changed obviously. Especially with the flow rate of 10 L/S, the area of higher velocity in the arc pole has left the original position and moved close to the cathode and depart from the axis. The configuration changing of two swirls can be clearly observed. The clockwise swirl streamtraces is compressed with the increase of flow rate, and eventually vanishes when the flow rate increases to 10 L/S. The anticlockwise swirl streamtraces become larger as the flow rate increase and govern the arc region, thus the quality of flow protection is improved. The changing flow rate has no effect to the electromagnetism field, potential field and current density of both axial and radial direction.
4.3.
Effects of arc length
Fig. 16 shows the temperature profiles at different arc lengths. The calculation results indicate that the change of arc length shall significantly affect the arc temperature. The arc is compressed to a smaller region when the arc length is decreased, and the configuration of arc with longer arc length shall become more extended. The highest temperature in the arc center increases with the increase of the arc length. The curves in Fig. 17 show the temperature changes along axis. With the arc length increases from 3, 5, 8 to 10 mm, the
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Fig. 21 – Distribution of velocity and streamtraces of different arc lengths (A, 3 mm; B, 5 mm; C, 8 mm; D, 10 mm).
highest temperature along the axis are 1.63E + 04, 1.86E + 04, 1.99E + 04 and 2.11E + 04 K. The arc with length of 10 mm shows the highest temperature, and the arc with length of 3 mm shows the lowest temperature. Hence the temperature and arc length is in direct proportion. Fig. 18 shows the temperature changes along the anode. All curves show that the closer to the center axis, the higher the temperature. The arc with length of 8 mm has the highest temperature. It can thus be concluded that the temperature on the anode rises at first and then drops with the arc length increases from 3 to 10 mm. In another word, there has to be an optimal arc length that can assure the optimal heat input in the welding procedure. Fig. 19 shows the distribution of axial velocity along the axis. The highest axial velocities along the axis are in turn 42.3, 60.4, 65.8 and 64.7 m/s with the arc length increase from 3 to 10 mm. It can be seen that the velocity increases with the
arc length increase from 3 to 8 mm and then deceases a little with the arc length increase from 8 to 10 mm. Fig. 20 presents the radial velocity on the anode. Radial velocities on the anode are higher when the arc length is 5 and 8 mm, as compared with those when it is 3 and 10 mm. It can thus be concluded that the arc with moderate length can be better protected. Fig. 21 shows the distribution of velocity and streamtraces of different arc lengths. The configuration of two swirls at different arc lengths can be clearly observed. With the arc length of 3 mm, the clockwise swirl (counter rotating vortices) is comparatively larger, while the anticlockwise swirl is comparatively smaller. With the increase of the arc length, the clockwise swirl gradually shrinks and the anticlockwise swirl gradually dominates the arc area. It is obvious that the longer the arc length, the larger the area of arc pole and the higher the velocity in the arc pole.
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Fig. 22 – Comparison of potential along axis (different arc lengths).
Fig. 22 shows the potentials along axis. The arc voltages are in turn 8.78, 10.9, 11.8 and 13.6 V corresponding to the arc length of 3, 5, 8 and 10 mm. It can be clearly noticed that the longer the arc length, the larger the arc voltage. Fig. 23 shows the axial current density along axis. Corresponding to the arc length of 3, 5, 8 and 10 mm, the highest
Fig. 24 – Radial current density along axis (different arc lengths).
axial current density along axis are 6.39E + 07, 6.42E + 07, 6.21E + 07 and 6.51E + 07 A/m2 , respectively. The change of these values is little. The reason is that the arc voltage and arc length are in direct proportion, and the voltage grads (∂ϕ/∂z) changes little, thus the axial current density changes little. Fig. 24 shows the radial current density along axis. Corresponding to the arc length of 3, 5, 8 and 10 mm, the highest radial current density along axis are 3.27E + 07, 5.64E + 07, 5.63E + 07 and 6.32E + 07 A/m2 near the cathode tip. It can be concluded that with the increase of arc length, the axial current density varies little, but the radial current density along axis increases much more significantly. Values of the variables are significantly affected by the arc length. The temperature of arc center, the arc voltage and the velocity of fluid flows are in direct proportion with the arc length. However, there has to be an optimal and moderate arc length that can assure the arc with optimal heat input and better protection. As for the effect on the current density, the radial current density increase with the increase of arc length, but the axial density changes little.
4.4.
Fig. 23 – Axial current density along axis (different arc lengths).
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Effect of the composition of shielding gas
Fig. 25 shows the temperature field of different shielding gases with a comparison of temperatures along axis. A is arc with shielding gas Ar, B is with CO2 and C is with He. The arc temperature with shielding gas He is the lowest, while the one with CO2 is the highest. Here the difference is owing to their different thermal conductivity. The higher the thermal conductivity, the more the heat loss. Thermal conductivity of He is four times of that of Ar’s, and ten times of CO2 ’s. Thus the
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Fig. 25 – Temperature field of different shielding gas and their comparison of temperature along axis (A, Ar; B, CO2 ; C, He; D, comparison of temperature along axis).
highest temperature along the He arc axis is the lowest, only 13,000 K, while the highest temperature along the CO2 arc axis is 31,000 K. The He arc is comparatively relaxed and in contrast the CO2 arc is comparatively contracted.
5.
Conclusion
• With the increase of the welding current, the increase of current brings more energy to the arc, and all properties including the temperature, voltage of the arc, axial velocity, radial velocity on the anode and both the axial and radial current density are improved. Except that the streamtraces with all currents are still showed in the same configuration. • As for the changing of flow rate, there exists an optimum flow rate, with which the arc can acquire the highest temperature. With the increase of flow rate, the area of higher velocity in the arc pole has left the original position and the streamtraces change their configurations, the anticlockwise
swirl streamtraces become larger and govern the arc region, the protection effects are improved. The electro-magnetism field, potential field and current density of both axial and radial directions are not influenced. • With the increase of arc length, the temperature of arc, the potential and the velocity of the fluid flows are increased. The moderate arc length may be an optimal one that can assure the arc with optimal heat input and better protection. The radial current density increases with the increase of arc length, however the axial density changes little. • The higher the thermal conductivity of the shielding gas, the more the heat loss and the lower the arc temperature.
Acknowledgements The authors gratefully acknowledge gratitude to the Young Subject-Leader Foundation, Shanxi Province Natural Science Foundation and the Supporting Plan for Excellent Talents New
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3752–3765
Century and the Ministry of Education of P.R. China for financial support to this work.
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