Physics of Welding

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Physics of Welding
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Physics of Welding

• Learning Activities
• Read Handbook pp 32-62
• Look up Keywords
• View Slides
• Read Notes,
• Listen to lecture
• Do on-line workbook
• Do homework

• Lesson Objectives
• When you finish this lesson you will understand
• Heat input and heat transfer from arc to weld
• Metal melting and regions of weld.
• Arc physics and plasma properties of arc

Keywords Heat Input, Heat Transfer Efficiency,
Heat Affected Zone, Enthalpy of Melting, Latent
Heat, Melting efficiency, Plasma, Polarity,
Thermionic Work Function, Ionization, Cathode
Spot, Anode Spot, Arc I-V Characteristics
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Physics of Welding

• Heat Input Concepts
• Energy Sources
• Arc Characteristics
• Wire Melting

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Heat Input
Heat Loss
Heat input
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Heat Input
H energy input, energy/unit length, joules
/mm H Power/Travel Speed, P/v P
total input power, Watts v travel
speed of heat source, mm/sec Describes energy
per unit length delivered, not rate of
delivery Used in codes specifications This
energy does not all go entirely to the work
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Heat Input for Arcs
H P/v EI/v E Arc Voltage (Volts)
I Arc Current (Amps) EI Process
power, converted to Heat v Welding
Travel Speed
Not all the arc energy goes into the work
Hnet f1H f1P/v f1EI/v f1 Heat
Transfer Efficiency
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f1 Heat Transfer Efficiency
short
Arc Length
long
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Reinforcement
Melted Base Metal
Heat Affected Zone
Aw Cross Section of Weld Am Ar For
Autogenous Weld (no filler metal) Aw Am
Heat Required to elevate solid to MP

Latent Heat of Fusion
Heat Required to melt a Given Volume of Weld

Q
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Enthalpy of Melting
Heat Required to elevate solid to MP

Latent Heat of Fusion
Heat Required to melt a Given Volume of Weld

Q
Not all the net heat transferred goes into melting
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Melting Efficiency
f2 Melting efficiency, the fraction of the
process heat energy per unit length
delivered to the metal which is required
to melt the metal f2 QAw/Hnet f2 QAwv/f1EI
From previous slide Hnet f1H f1P/v f1EI/v

• Melting Efficiency Depends On
• Higher Thermal Conductivity - Lower Efficiency
• High Energy Density Heat Source - Higher
  Efficiency

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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• We can select a range of processes for arc
  welding from a tiny GTAW run at 15 volts and 100
  amps and 30 ipm to twin arc submerged arc welds
  run at 25 volts and total curretn of over 1000
  amps run at 8 ipm. What is the heat input in
  each of these welds? What do you think might
  happen to the cooling rate in the part being
  welded when the weld is stopped in each of these
  weld?

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Other Energy Sources
Arc H EI/v Resistance H I2Rt Electroslag H
EIt Laser EB
H Heat generated, joules E Voltage, volts v
Travel Speed, mm/sec I Current, amps R
Resistance, ohms t Time, sec PD Power
Density P1 Input power
A Area of focused beam
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OXYFUEL GAS WELDING
THERMIT WELDING
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Homework
Do Homework Assignment 4 Physics of Welding
From the Assignment page of the WE300 Website.
Often engineering calculations require conversion
of units. In the Slide Show Mode, clicking on
this icon will open a free program to help you
with conversions. You might want to bookmark
this program for later use as well. On your
first use, please click on help and register this
free program.
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Arc Characteristics
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Polarity and Current Flow
Welding Electrode or "Electrode"
Anode
Cathode
I
I
DCEP
DCEN
Cathode
Anode
Work Electrode or "Work"
Straight SPEN
Reverse RPEP
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Plasma State
Gas is hot enough so that high energy
collisions produce free electrons
Plasma may only be a few electrons
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Conduction of Current in the Arc
Cathode
Electrons Emitted
Thermal
Free
Ionization
Electron
Ion
Plasma
Tgt10,000K
Recombination
Neutral
Anode
Gas Atom
Electrons Absorbed
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Argon Arc
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Thermionic Work Function
Energy Required for electron to escape a solid
surface
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Work Function of pure Tungsten 4.4 eV Work
Function of Thoriated W 4.1 eV
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Ionization
Free
"Neutral"
I
Electron
Atom
Ionization
Collision
Free
Ion
Free Electron with
Energy gt Ionization Potential
Ionization Potentials
He 24.6 eV
Ar 15.8
N 15.6
Fe 7.9
Na 5.1
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Will total voltage change if we change the amount
of current (say from 200 amps to 300 amps)?
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Arc V-I Characteristic
Unstable
We see that current and arc length have an
effect, what happens if we change from Ar to some
other ionizing gas?
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Ionization Potential He 24.6 ev Ar 15.8 N
15.6 Fe 7.9 Na 5.1 P 4.3
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Questions?

• Turn to the person sitting next to you and
  discuss (1 min.)
• The arc characteristics that we looked at were
  for a Gas Tungsten Arc where the electrode is not
  melted so the metal ions in the arc do not come
  from molten electrode. What happens in GMAW
  where the wire (electrode) melts? Would you
  expect anything different to happen?