CFD ANALYSIS OF GAS METAL ARC WELDING1

CFD ANALYSIS OF GAS METAL ARC WELDING

PRESENTED BY UNDER GUIDANCE OFPRATIK S. JOSHI Dr. N. YAGNESH SHARMAREG.NO.110926007 PROFESSOR,M.TECH (MET) DEPT. OF MECH. AND MFG.ENGG.

MIT,MANIPAL

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CONTENT• Introduction• Literature review• Problem definition • Objectives• Methodology • Theory • Modeling• Result and Analysis• Conclusion• Future scope of work• References

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INTRODUCTION

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GAS METAL ARC WELDING(GMAW)

• Gas metal arc welding (GMAW) is defined as(10) “an arc welding process that produces coalescence of metals by heating them with an arc between a continuous filler metal electrode and the work piece. Shielding is obtained entirely from an externally supplied gas.”

• The GMAW process is multi-energy process involving plasma physics, heat flow, fluid flow, and metal transfer. 4

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Fig.1 Energy involved in GMAW(6)

GMAW PROCESS

• GMAW process uses solid electrode that continuously feed into the weld pool. The wire electrode is consumed which becomes the filler metal.

• GMAW is done using DCEP( direct current electrode positive). AC is never used for GMAW process.

• GMAW is constant voltage process.

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FIG.2 GMAW machine(12)

WELDING ARC

• A welding arc can be defined as “A controlled electrical discharge between the electrode and the workpiece that is formed and sustained by the establishment of a gaseous conductive medium, called an arc plasma.”

• The amount of heat that an arc produces mainly depends on arc current and arc length.

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Fig.4 Arc-conversion device

METAL TRANSFER MODES

• There are basically two metal transfer modes in GMAW process1. Short circuit transfer2. Globular transfer• Metal transfer modes is mainly depends on the current

and voltage value set on the machine.

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SHORT CIRCUIT TRANSFER

• In the short-circuiting mode, metal transfer occurs when the electrode is in contact with the weld pool.

• In this mode of metal transfer, the relationship between the electrode melt rate and its feed rate into the weld zone determines the intermittent establishment of an arc and the short circuiting of the electrode to the workpiece.

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Fig.5 short circuit transfer(12)

GLOBULAR TRANSFER

• The filler material transfers in the form of globules propelled by arc forces.

• The metal transfers across the gap in the form of large ,irregularly shaped droplets. The drops are usually higher than that of electrode wire diameter.

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Fig.6 Globular transfer(12)

LITERATURE REVIEW

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• Anthony B. Murphy (1) ,studied the transport properties of arc plasma. Calculated values of viscosity, thermal conductivity, and electrical conductivity of argon and helium at high temperatures were presented.

• T.W. Eagar and Y.S. Kim(2), studied droplet size produced in the GMAW electrode both theoretically and experimentally. The transition of metal transfer mode was investigated experimentally using high speed videography. The causes for the deviation of predicted droplet size from measured size are discussed with suggestion for modification in theory in order to model more accurately metal transfer in GMAW process.

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• P. G. Jonsson and J. Szekely[3] studied the arc parameters and the metal transfer in GMAW process using mild steel and helium and argon gases as shielding gases. The governing equations for the computational domain are developed. The solution of the governing equations, boundary conditions, and source terms was obtained . The arcs behaved very differently for the argon and helium atmospheres and have pronounced effect on the system performance.

• J. Hu, H.L. Tsai(4) prepared a unified comprehensive model to simulate transient phenomenon occurring during the GMAW process. Based on the unified model, a thorough investigation of the plasma arc characteristics during the gas metal arc welding process was conducted. It was found that the droplet transfer and the deformed weld pool surface have significant effects on the transient distributions of current density, arc temperature and arc pressure, which were normally assumed to be constant.

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• M. Schnink, M.Dreher(5)studied experimental methods for visualization and quantification of gas flows in GMAW process. Advanced Particle Image Velocimetry(PIV) and Schilerin technique were used for characterization of flow field in the direct vicinity of the arc.

• Takehiko TOH, Jun TANAKA et.al [9] studied the behavior of DC arc plasma under a magnetic field imposed perpendicular to the plasma current. The behavior is studied both theoretically and experimentally by changing various parameters such as plasma electric current, nozzle diameter, argon flow rate and magnetic flux density. DC plasma was mathematically modeled by use of three dimensional magneto hydrodynamics (MHD) theory and numerical simulation performed using finite volume approach. By experimental and theoretical analysis controlling parameters of DC plasma are stated.

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PROBLEM DEFINITION

• The problem being taken up for the computational analysis pertains to GMAW process. This domain is two phase domain consisting of mixture of molten metal and shielding gas. The need for determining effect of nozzle geometry on shielding gas flow and consequently on welding arc characteristics is felt much actual in GMAW process and problem will be solved covering both.

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OBJECTIVES

1. Developing a numerical computational model to represent complex two phase GMAW process.

2. To study welding arc characteristics such as electric potential, current density, Joule heat.

3. To study effect of nozzle geometry on fluid dynamics of shielding gas flow.

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METHODOLOGY• Using computational fluid dynamics (CFD)as a tool to

understand complex physics involved in the GMAW process.• Preparing numerical domain which is two dimensional

axisymmetric model to reduce computational analysis time.• Suitable boundary conditions to be imposed corresponding to

shielding gas enveloped molten metal so as to satisfy physics of the problem.

• The computation will be carried out as a transient fluid flow coupled with heat transfer by stating relevant initial conditions.

• A grid independent solution will be obtained after several trials with different grid geometry and size and the output results are obtained in the form of contours for distribution of temperature and velocity and welding arc characteristics. 16

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THEORY

• Theoretical study is based on static force imbalance theory(2).

• Static force imbalance theory• The static force imbalance theory postulates that the drop

detaches from the electrode when the static detaching force exceeds the static retaining force.

• Four different forces are considered-gravitational force, electromagnetic force, plasma drag force are detaching forces while the surface tension force is retaining force.

• Gravitational force is due to mass of the droplet ……(1)

where R= droplet radius in m =density of drop in kg/m3

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• The electromagnetic force is given by Lorentz law ……(2)where J=current density in a/m2

B= magnetic flux in Tesla By assuming current density on the drop uniform, the total force can be obtained by integrating equation (2) over the droplet surface as

f ..….(3) where =permeability of the free surface I =welding current in a f=

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• The plasma drag force is given by ……(4) where CD=drag coefficient

Af = projected area in m2

= density of fluid in kg/m3

= velocity of fluid in m/s• Surface tension force is given by

…….(5) where r=radius of the electrode in m

= surface tension of liquid metal in N/m • According to theory droplet size is calculated as

………(6)19

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GOVERNING EQUATION

• Governing equation is derived based on following assumptions(3):

1. The arc is axially symmetric.2. The arc is in Local Thermodynamic Equilibrium (LTE) that is

the electron and heavy particle temperatures are nearly same.

3. The plasma is optically thin so that radiation may be accounted.

4. The consumable electrode is cylindrical and tip of the electrode and workpiece are flat.

5. The consumable electrode is in quasi state.20

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• Conservation of mass ….(7)

• Conservation of radial momentum

.…(8)• Conservation of axial momentum

.…(9)Where = Mass density in kg/m3 r=Radial distance in m

z= Axial distance in m u= radial velocity in m/sw= axial velocity in m/s Jr=Radial current density in a/m2

Jz= axial current density in P= pressure in N/m2

a/m2

= Viscosity in N-s/m2 =Azimuthal magnetic field in Tesla

• Conservation of energy

…(10)Where h= Enthalpy in joule k=Thermal conductivity in w/m-k Cp=Specific heat at =Electrical conductivity in1/Ω-m

constant pressure SR=Radiation heat loss Kb=Boltzmann constant

term 8.617 332× 10−5 eV/ke=elementary charge 1.602176× 10−19 C

• Conservation of electric charge

…(11) where =Electric potential in V

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MODELLING

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CFD MODELLING OVERVIEW

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GAMBIT

ANSYS FLUENT

Fig.5 CFD modeling overview (16)

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Fig.7 Elements of nozzle(12) Fig.8 Modeling parameters (10)

MODELLING PARAMETERS FOR GEOMETRY

STANDARDS USED FOR MODEL

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Table of AWS standards(11)

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THERMOPHYSICAL PROPERTIES OF ARGON

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Fig.9 Graph of temperature versus properties of argon(1)

MATERIAL PROPERTIES

Density(Kg/m3)

Specific heat(Cp) (j/kg-k)

Thermal conductivity(w/m-k)

Electrical conductivity(1/ohm-m)

Magnetic permeability (h/m)

Viscosity (kg/m-s )

Steel 8030 502.48 16.27 8.33e6 1.257e-06 NA

Argon 1.6228 520 1.58 1000000 1.257e-06 2.125e-05

Oxygen 1.299 919.31 2.46 1000000 1.257e-06 1.919e-05

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Property

GEOMETRIC MODEL

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Fig.10 Geometric model with boundary condition

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Fig.11 meshing of domainElement type: Quad-map(solid),Quad –pave(fluid) No. of elements:89553

No. of nodes:90104

INLET VELOCITY

• Calculations for inlet velocity

where Q= flow rate(m3/min)=14 e-3m3/min An= exit area of nozzle(m2) V= velocity of gas flow(m/s) )=6.09e-4 m2

• Inlet velocity(V)=74.8 m/s

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SOLVER INITIAL SETTING

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Initial boundary conditions

• Initial velocity=74.8m/s• Electric potential(electrode)=25 V• Electric potential(workpiece)=0V• Electric current=275 A• Temperature= 300K

Models used

• Multiphase-Volume of fluid• Energy-On• Viscous-Standard k-ε• MHD model-Electric potential

Solution methods

• Scheme-SIMPLE• Discretization-Second order upwind• Transient Formulation-First order implicit

Residuals & time step

• Residuals-10e-5

• Time step-10e-4

SCALED RESIDUALS

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RESULT AND ANALYSIS

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CONTOUR OF VELOCITY IN (m/s)

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Fig. 12(a) Contour of velocity at t=100 ms

CONTOUR OF VELOCITY IN (m/s)

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Fig. 12(b) Contour of velocity at t=400 ms

CONTOURS OF VELOCITY(m/s)

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Fig. 12(c) Contour of velocity at t=800 ms

CONTOURS OF TURBULENT ENERGY(m2/s2)

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Fig.13(a) Contour of turbulent energy at t=100ms


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Fig.13(b) Contour of turbulent energy at t=400ms


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Fig.13(c) Contour of turbulent energy at t=800ms

CONTOUR OF ELECTRIC POTENTIAL(V)

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Fig.14(a) Contour of electric potential at t=100ms


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Fig.14(b) Contour of electric potential at t=400ms


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Fig.14(c) Contour of electric potential at t=800ms

CONTOUR OF CURRENT DENSITY(a/m2)

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Fig. 15(a) Contour of current density at t=100ms


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t=300ms

Fig. 15(b) Contour of current density at t=400ms


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Fig. 15(c) Contour of current density at t=800ms

GRAPH OF JOULE HEAT VS DISTANCE

•The formula for joule heat is given as, .…(12)

• The current value is 275 A and voltage value is 25 V for 1 second of time.

W • Heat generated per unit volume is given as,

....(13) ....(14)

• Diameter of electrode is 1.6mm and length is 5mm, thus

W/m3

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Fig 16(a) graph of joule heat vs distance at t=100 ms


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Fig 16(b) graph of joule heat vs distance at t=400 ms

Fig 16(c) graph of joule heat vs distance at t=800 ms

CONTOUR OF TEMPERATURE (K)

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Fig. 17 (a) Contour of temperature at t=100 ms


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Fig. 17 (b) Contour of temperature at t=400 ms


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Fig. 157(c) Contour of temperature at t=800 ms

CONCLUSION• A unified model has been developed to simulate the transport

phenomenon occurring during GMAW process.• The heat transfer and fluid flow in the arc column were studied based

on the transient distributions of velocity, current, temperature in the arc plasma region.

• From the study it is found that as the arc struck the shielding gas is accelerated towards axis. When the plasma reaches towards workpiece axial momentum of gases is changed to radial momentum and flows away from workpiece. The shielding gas also carries current from electrode to workpiece which helps in reducing spatter of arc and concentrated arc is obtained.

• There are two distinct regions of electric potential contour observed. One is around electrode with upside contour showing current diverges from centre and another is at cathode with downside contour showing current converges to centre. The electric current density is concentrated at the tip of electrode causing large amount of heat generation.

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SCOPE OF FUTURE WORK• Transient simulation of molten metal droplet and influence of

shielding gas flow on weld bead characteristics.• Experimental validation of the theoretical results.

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REFERENCE1. Anthony B. Murphy, “Transport coefficients of helium and argon plasmas”,

IEEE transaction on plasma science,Vol.25,No.5 Oct. 19972. T.W. Eagar and Y.S. Kim, “Analysis of metal transfer in gas metal arc

welding”, Welding research supplement, June 1993.3. P. G. Jonsson and J. Szekely, “Heat and mass transfer in gas metal arc

welding using argon and helium”, Metallurgical and Materials transaction B, Volume 26B, April1995, 383-395.

4. J. Hu, H.L. Tsai, “Heat and mass transfer in gas metal arc welding”, International journal of heat and mass transfer, Vol.50,Oct. 2006,833-846.

5. M. Schnink, M.Dreher, “Visualization and optimization of shielding gas flows in arc welding”, Welding in the world, Vol.56, No.01, 2012.

6. H.G. Fan and R. Kovacevic, “A unified model of transport phenomenon in gas metal arc welding including electrode, arc plasma”, Journal of physics D: Applied Physics, Vol. 37, 2004, 2531-2544.

7. G.Wang, P.G. Huang, Y.M. Zang, “Numerical analysis of metal transfer in GMAW under modified pulsed current condition”, Metallurgical and material transaction B, July2003.

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REFERENCE8. U. Fussel, M.Dregher, M. schinck, “Numerical optimization of GMAW

torches using ANSYS CFX”, 63rd Annual assembly and International conference of international institute of welding, Istanbul, Turkey, 11-17 July 2010.

9. Takehiko Toh, Jun Tanata, Yasho Maruki, “Magneto hydrodynamic simulation of D.C. arc plasma” ISIJ International, Vol.45,No.7,2005, 947-953.

10. Larry Jeffus, “Welding principles and applications”, Delmar publications, 4th edition, ISBN 0-8273-8240-5.

11. W. Hoffman, "Modern welding", The goodheart-willcox co. ltd., 3rd edition.

12. Desineni naidu, Selahattin Ozcelik, “Modelling sensing and controlling of GMAW”, Elsevier publications, 1st edition, ISBN 0-0804-4066-5.

13. Praxair shielding gas selection manual.14. Suhas V. Patankar, “Numerical heat transfer and fluid flow”, McGraw-Hill

book Company, New york, ISBN 0-07-048740-5. 15. Ansys, Inc.: Ansys-GAMBIT 2.4 user Guide. Canonsburg / U.S.A.16. Ansys, Inc.: Ansys-FLUENT 14 Solver Theory Guide. Canonsburg / U.S.A.

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THANK YOU

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CFD ANALYSIS OF GAS METAL ARC WELDING1

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