Heat Flow in Welding

8/7/2019 Heat Flow in Welding

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Heat flow in weldingHeat flow in welding

Subjects of Interest

Suranaree University of Technology Sep-Dec 2007

Heat sources

Heat source and melting efficiency

Analysis of heat flow in welding

Effects of welding parameter

Weld thermal simulator


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ObjectivesObjectives

This chapter provides information of heat flow during

welding, which can strongly affect phase transformation,

microstructure, and properties of the welds.

Students are required to indicate heat source and powerdensity used in different welding methods, which affect the

melting efficiency.



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Welding heat sources


Electrical sources

Chemical sources

High energy sources

Mechanical sources

Other sources

Arc welding

Resistance welding

Electroslag

Oxyfuel gas welding

Thermit welding

Laser beam welding

Electron beam welding

Friction (stir) welding

Ultrasonic welding (15-75 KHz)

Explosion welding (EXW)

Diffusion welding

Heat intensity ~ 1010-1012Wm-2



Heat intensity ~ 104

-106

Wm-2


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Welding Arc


A welding arc consists of a sustained electrical discharge

through a high temperature, conducting plasma, producing

sufficient thermal energy as to be useful for the joining of metal by

fusion. Gaseous conductor changes electrical energy into heat.

Arc produces sources ofheat + radiation (careful required

proper protection)

Welding arc Gas metal arc welding

http://en.wikipedia.org

Characteristics

(ionic gas or plasma

with electric current

passing through)

bell shaped arc


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Emission of electron at cathode


Emission of electrons at cathode occurs when an amount

of energy required to remove the electron from a material

(liquid or solid). This amount of energy per electron is

called work function. (analogous to ionization potential)


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Plasma formation


States of matter

Solid

Liquid

Gas

Plasma

Melting

Vaporization

Ionization

(neutral

atoms/molecules)

(negative charges

and positive ions)

Plasma consists of ionized state of a

gas composed of nearly equal

numbers ofelectrons and ions, whichcan react to electric or magnetic fields.

Electrons, which support most of the

current conduction, flow from cathode

terminal(-) to anode terminal(+).

Neutral plasma can be established

by thermal means by collision

process, which requires the attainment

of equilibrium temperature according to

ionization potential of the materials.

www.fronius.com


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Ionization potential


Ionization potential, Vi, required to strip an

electron from an outer shell of and atom or M+.

3.9Cs

4.3K

5.1Na

7.6Ni

7.9Fe

8.2Si

11.3C

14.1CO

13.8CO2

12.1O2

15.6N2

15.4H2

15.8Ar

24.6He

Ionization Potential (Volts or eV)Element/Compound

Plasma temperature = Ionization potential x 1000 K

Energy


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Temperature in the arc and heat loss


Plasma temperature contour in the arc

The arc temperature ~ 5000-30,000 K

depending on the nature of plasma and

current.

The arc temperature is determined bymeasuring the spectral radiation

emitted.

www.geocities.com

Heat losses in the arc

Energy losses by heat conduction

and convection, radiation and

diffusion.

InArgas, radiation loss ~ 20%

while in other welding gas, radiation

loss


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Polarity


There are three different types of current used in arc welding

1) Direct-Current Electrode Negative (DCEN)

2) Direct-Current Electrode Positive (DCEP)

3) Alternating current (AC)


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Direct-Current Electrode Negative (DCEN)

Also called straight polarity.

Electrons are emitted from the negative

tungsten electrode and accelerated whiletravelling through the arc.

Most commonly used in GTAW.

Relatively narrow and deep weld poolis

produced due to high energy.

DCEN in GMAWmakes the arc unstable

and causes excessive spatter, large droplet

size of metal and the arcs forces the droplets

away from the workpiece. This is due to a

low rate of electron emission from the negativeelectrode.


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Direct-Current Electrode Positive (DCEP)

Also called reverse polarity.

The electrode is connected to the positive

terminal of the power source, therefore the

heating affectis now at the tungsten electrode

rather than the workpiece. shallow weld for

welding thin sheets.

At low current inAr, the size of the droplet ~ the

size of the electrode Globular transfer.

The droplet size is inversely proportional to the

currentand the droplets are released at the rate

of a few per second.

At above the critical current the droplets are

released at the rate of hundreds per second(spray mode).

Positive irons clean offthe oxide surface.


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Surface cleaning action


DCEPcan be employed to clean the surface of the workpiece by knocking

off oxide films by the positive ions of the shielding gas.

Ex: cleaning ofAl2O3 oxide film

(Tm ~2054oC) on aluminium tomake melting of the metal

underneath the oxide film easier.

Surface cleaning action in GTAW with

DC electrode positive.


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Alternating Current (AC)

Reasonably good penetration and

oxide cleaning action can be both

obtained.

Often used for welding aluminium

alloys.


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Heat source efficiency


In the case ofarc welding, having a constant voltage Eand a

constant current I, the arc efficiency can be expressed as;

EI

Q

EIt

Qt

tQ

Qt

weld

weld

weldalno

weld ===min

Eq.2

In cases of electron beam and laser beam welding, Qnominal is the power

heat source of the electron beam and laser beam respectively.

The term, heat input per unit length of weld often refers to

VEIor

VQ alno ,min Eq.3

Where Qnominalor EI is the heat input

V is the welding speed

Qnominal/ V is heat input per unit length of weld

Where Q is the rate of heat transfer

Qnominal is the heat input

tweld is the welding time


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Heat source efficiency measurement


Heat source efficiency can be measured using

a calorimeter(by measuring the heat transfer

from the heat source to the workpiece and then to

the calorimeter). The temperature rise in the cooling water

(Tout-Tin) can be measured using thermocouples

or thermistors. Heat transfer from the workpiece

to the calorimeter is given by

dtTTWCdtTTWCQt inoutinoutweld =

0 0)()(

Eq.4

Where W is the mass flow rate of waterC is the specific heat of water

Tout is the outlet water temperature

Tin is the inlet water temperature

t is time

Note: This integral corresponds

to the shaded area, and can be

used to calculated the arcefficiency.


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The arc efficiencycan also be measured

using Seebeck envelope calorimeter. This

technique utilises thermocouple junctions for

sensing temperature difference.

The heat transfer from the workpiece to

the calorimetercan be determined by

measuring the temperature different Tand

hence gradient across a gradient layer of

material of known thermal conductivity kand thickness L.

=

0dt

TkAQtweld Eq.5

Where A is the area for heat flow

T/L is temperature gradient

Note: this type of calorimeter is used to determine the arc

efficiencies in PAW, GMAW, and SAW.

Layer of temperature gradient for heat

source efficiency measurement.


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In GMAW the arc, metal droplets, and the

cathode heating contribute to the efficiency

of the heat source.

Lu and Kou used a combination ofthreecalorimeters to estimate the amounts of

heat transfer from the arc, filler metal

droplets and the cathode heating to the

workpiece in GMAW of aluminium.

(a) Heat transfer from metal droplets

(c) Heat inputs from arc and metal droplets.

(b) Total heat inputs

(a) Measured results, (b) breakdown of power inputs.


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Heat source efficiency in various

welding processes


LBWHeat source efficiency is low

because of the high

reflectivity.

PAWHeat source efficiency is

much higher than LBW (no

reflectivity).

EBWHeat source efficiency is high

due to the keyhole acting like

a black body trapping the

energy from electron beam.

SAWHeat source efficiency is

higher than GTAW or SMAW

since the arc is covered with

thermally insulating blanket of

molten slag and granular flux.

Heat source efficiencies in several

welding processes.


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Melting efficiency


The melting efficiency of the arc m can be defined as follows

weld

fillerweldfillerbaseweldbase

mEIt

HVtAHVtA

)()( +=

Where

V is the welding speedHbase is the energy required to raise a unit volume of

base metal to the melting point and melt it.

Hfiller is the energy required to raise a unit volume of

filler metal to the melting point and melt it.

tweld is the welding time.

Eq.7

Note: the quantity inside the parentheses represents the volume of material

melted while the denominator represents the heat transfer from the heat

source to the workpiece.

mV

tweld

Aweld= Afiller+Abase

Melting efficiency is the ability of the heat source to

melt the base metal (as well as the filler metal).

Cross section of weld


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Melting efficiency


(a) shallow welds of

lower melting

efficiency,

(b) (b) deeper weld ofhigher melting

efficiency.

Aweld= Afiller+Abase

Low heat inputLow welding speed

High heat inputHigh welding speed


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Power density distribution of heat source


Power density distribution is influenced by

1) Electrode tip angle

2) Electrode tip geometry

Effect of electrode tip angle on shape and power

density distribution of gas-tungsten arc.

Blunter electrode

Arc diameter

Power density distribution

Sharp electrode

Arc diameter

Power density distribution


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Effect of electrode tip angle on shape ofgas tungsten arc and power density


Conical angle of

electrode tip

The arc becomes

more constricted


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Analysis of heat flow in welding


Heat or temperature distribution occurring during welding greatly affect

microstructure of the weld, hence, the weld properties

Temperature distribution round a typical weld

The temperature-distance profile

shows that the heat source travels

along the weld in the directionA-Aat

a constant speed.

As the heat source moves on, the

cooling rates around the weld are very

high.

A more intense heat source will give

a steeper profile and the HAZ, whichwill be confined to a narrower region.


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Effect of temperature gradient onweld microstructure

Suranaree University of Technology Sep-Dec 2007Microstructures occurring in a weld and its HAZ.

The temperature gradients in the liquid weld material are substantially higher

than in most casting processes. This leads to high solidification rates which

produce a finer dendritic structure than that observed in most castings.


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Effect of welding parameters


Effect of heat input Q and weldingspeed V on the weld pool.

Effect of heat input on cooling rate.

Effect of the power density

distribution of the heat source on the

weld shape.

Heat sink effect of workpiece.


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Effect of heat input and welding

speed on the weld pool

The shape and size of the weld pool is

significantly affected by heat input Qand

the welding speed V.

Heat input

Welding speed

The weld pool

becomes more

elongated.

Note: the cross indicates the

position of the electrode.


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Effect of heat input on cooling rate

Heat input per

unit length EI/V

Cooling rate

The cooling rate in ESW(high Q/V)

is much smaller than that in arc

welding.


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Effect of power density distribution

on weld shape

Power density

Weld penetration


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Heat sink effect of the workpiece

The cooling rate increases with the

thickness of the workpiece due to

the heat sinkeffect.

Thicker workpiece acts as a better

heat sinkto cool the weld down.

Brass with a higher melting point than

that of aluminium is used as a heat sink

to increase the cooling rate in

aluminium welding.

Blass heat sink is clamped behind

aluminium to be welded.


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ReferencesReferences

Kou, S., Welding metallurgy, 2nd edition, 2003, John Willey and

Sons, Inc., USA, ISBN 0-471-43491-4.

Gourd, L.M., Principles of welding technology, 3rd edition, 1995,

Edward Arnold, ISBN 0 340 61399 8.


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Heat Flow in Welding

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