HEAT TRANSFER AND HEAT FLOW IN ARC WELDING

Abhishek TaoriBT09MME066

Index

Introduction

Equipment

Methods of arc welding

Heat flow around weld metal

Temperature distribution around welds

Cooling of weld

Mathematical analysis of heat transfer during welding

Introduction

Arc welding is a type of welding that uses a welding power supply to create

an electric arc between an electrode and the base material to melt the metals at

the welding point. They can use either direct (DC) or alternating (AC) current,

and consumable or non-consumable electrodes. The welding region is

sometimes protected by some type of inert or semi-inert gas, known as

a shielding gas, and/or an evaporating filler material. The process of arc welding

is widely used because of its low capital and running costs. Getting the arc

started is called striking the arc. An arc may be struck by either lightly tapping the

electrode against the metal or scratching the electrode against the metal at high

speed.

fig 1 Arc of arc welding

Equipment

To supply the electrical energy necessary for arc welding processes, a number of

different power supplies can be used. The most common classification is

constant current power supplies and constant voltage power supplies. In arc

welding, the voltage is directly related to the length of the arc, and the current is

related to the amount of heat input. Typical currents are 50 to 500 amps,

depending on the size of weld required; 100 amps is typical for manual welders.[12] Voltage output is typically 20 to 50 volts during welding, [13] though some power

supplies also include a small high voltage source to aid in initially striking the arc.

Constant current power supplies are most often used for manual welding

processes such as gas tungsten arc welding and shielded metal arc welding,

because they maintain a relatively constant current even as the voltage varies.

This is important because in manual welding, it can be difficult to hold the

electrode perfectly steady, and as a result, the arc length and thus voltage tend

to fluctuate. Constant voltage power supplies hold the voltage constant and vary

the current, and as a result, are most often used for automated welding

processes such as gas metal arc welding, flux cored arc welding, and submerged

arc welding. In these processes, arc length is kept constant, since any fluctuation

in the distance between the wire and the base material is quickly rectified by a

large change in current. For example, if the wire and the base material get too

close, the current will rapidly increase, which in turn causes the heat to increase

and the tip of the wire to melt, returning it to its original separation distance.[14]

The direction of current used in arc welding also plays an important role in

welding. Consumable electrode processes such as shielded metal arc welding

and gas metal arc welding generally use direct current, but the electrode can be

charged either positively or negatively. In welding, the positively

charged anode will have a greater heat concentration and, as a result, changing

the polarity of the electrode has an impact on weld properties. If the electrode is

positively charged, it will melt more quickly, increasing weld penetration and

welding speed. Alternatively, a negatively charged electrode results in more

shallow welds.[15] Non-consumable electrode processes, such as gas tungsten

arc welding, can use either type of direct current (DC), as well as alternating

current (AC). With direct current however, because the electrode only creates the

arc and does not provide filler material, a positively charged electrode causes

shallow welds, while a negatively charged electrode makes deeper welds.[16] Alternating current rapidly moves between these two, resulting in medium-

penetration welds. One disadvantage of AC, the fact that the arc must be re-

ignited after every zero crossing, has been addressed with the invention of

special power units that produce a square wave pattern instead of the

normal sine wave, eliminating low-voltage time after the zero crossings and

minimizing the effects of the problem

Methods of arc welding

There are two methods of arc welding based upon the type of electrode used.

Namely consumable electrode and non consumable electrode. Consumable

electrode method is the most commonly used method for arc welding.

Heat flow around weld metal

Heat Flow in and Around Weld Metal - The sufficiently high temperature* of the

welding arc quickly heats up the base metal and the electrode or filler metal and

they melt to form the molten weld pool. For controlling metallurgical events in

welding, the thermal conditions in and near the weld metal must be established. 

Important factors are:

(i) The distribution of maximum temperatures in the weld heat affected zone, 

(ii) The length of time at temperature,

(iii) Cooling rates in the weld metal and in the heat affected zone, 

(iv) The solidification rate of the weld metal.

The heat flow theory indicates the minimum heat input rate to form a weld of any

given width, and the essential variables which govern the heating rate and

cooling rate in the heat affected zone and in the weld metal. 

The type of thermal cycle in the weld metal and heat affected zone has important

effects on the properties of certain alloys, particularly harden able alloy steels,

and for such materials control of the thermal cycle may be a prerequisite to

successful welding.

Temperature distribution around welds

The temperature distribution around a metallic arc butt weld Electrode (i.e., the

arc) is moving from right to left. The leading edge of the temperature pattern is

compressed, because the arc is continually moving toward cold metal and the

trailing edge becomes extended because the arc leaves preheated metal in its

wake.

Cooling of weld

The cooling rate of a particular piece increases with the increase in thickness of

the workpiece.

The thicker the workpiece more it acts as an heat sink. This heat sink effect leads

to better cooling down of the weld.

For example brass with higher melting point than that of aluminum is used as a

heat sink in aluminium welding to increase its cooling rate.

Mathematical analysis of heat flow in arc welding

The welding process involves phase change, hence the movement of the phase

front is also a determining factor for the quality of weld obtained. Apart from

conduction from the weld pool to the remaining part of the metal being joined,

radiation also plays an important role. Th solid-liquid interface movement is also

a major determining factor of the quality of the resulting weld.

Mathematical Model

Figure2 shows a schematic model of weld pool. The weld pool is usually treated

as a spherical coordinate problem approximating to a hemisphere. Using the

concept of conservation of energy relation within a domain Ω, and assuming

there is no mass transport with temperature dependent thermal properties then

the temperature distribution within the weld pool can be modeled using the

following relationships:

Fig2 Approximate representation of weld pool

The conservation of mass within volume of weld pool gives

(1)And the conservation of momentum gives

(2)Also, the conservation of heat yields

(3)

In equation (2) the Darcy and buoyancy source terms are included as

(4)

While the latent heat source terms are included in equation (3) as

(5)

Where ρ is density, t is time, v is velocity and p is the weld pool pressure, μ is the viscosity, α is thermal expansion coefficient, g is gravity, fl is the liquid metal fraction, L is the latent heat of fusion and To is the reference temperature.

The temperature distribution obtained is plotted on the graph at various points of the metal

References

Arena, M.; Acoff, V.L.; and El-Kaddah, N. 1999. Modeling Heat Transfer and

Fluid Flow in GTA Welding of Gamma Titanium Aluminum. Fluid Flow

Phenomena in Metals Processing the Metals, N. El-Kaddah; G.G.C. Robertson;

S.T. Johnson; and V.R. Voller (Eds.). Metals and Material Society, London,

England.

El-Kaddah, N.; Arena, M.; and Acoff, V.L. 1999. Heat Transfer and Fluid Flow in

Stationary GTA Welding of γ-TiAl based alloys: Effect of Thermo-capillary flow.

2nd Int. Conf. CFD in Minerals and Process Industries, CSIRO, Melbourne,

Australia.

Goldak, I.; Chakravarti, A.; and Bibby, M. 1984. A new finite element model for

welding heat sources. Mat. Trans. B 15B: 299-305

Glickstein, S.S.; and Friedman, E. 1983. Temperature resistance in gas tungsten

arc weldments Weld. Rev. 5: 72-76.

Glickstein, S.S.; and Friedman, E. 1984. Weld Modeling Applications. Weld J. 63:

38-42.

Kim, W.H.; Fan, H.G.; and Na, S.J. 1997. Effect of various driving forces on heat

and mass transfer in arc welding. Numer. Heat Transfer. Part A, 32: 633-52.

Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology.

Upper Saddle River, New Jersey: Pearson Education. Cary, Howard B. and Scott

C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New Jersey:

Pearson Education.

Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland:

Lincoln Electric

Weman, Klas (2003). Welding processes handbook. New York: CRC Press