heat flow in welding
TRANSCRIPT
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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
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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 ~ 106-108Wm-2
Heat intensity ~ 106-108Wm-2
Heat intensity ~ 104
-106
Wm-2
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Welding Arc
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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
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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
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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
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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
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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
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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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Suranaree University of Technology Sep-Dec 2007
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
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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
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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
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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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Heat source efficiency measurement
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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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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
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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
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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
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(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
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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
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Conical angle of
electrode tip
The arc becomes
more constricted
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Analysis of heat flow in welding
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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
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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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