Physics of Welding
Physics of Welding
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
Learning Activities
1. Read Handbook pp
32-62
2. Look up Keywords
3. View Slides;
4. Read Notes,
5. Listen to lecture
6. Do on-line
workbook
7. Do homework
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
Physics of Welding
• Heat Input Concepts
• Energy Sources
• Arc Characteristics
• Wire Melting
Heat Loss
Heat input
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
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
f1 = Heat Transfer Efficiency
short
Arc Length
long
Melted Base Metal
Reinforcement
Heat
Affected
Zone
Aw = Cross Section of Weld = Am + Ar
For Autogenous Weld (no filler metal)
Aw = Am
Q=
Heat Required to melt
a Given Volume of Weld
=
Heat Required to
elevate solid to MP
+
Latent Heat
of Fusion
Enthalpy of Melting
Q=
Heat Required to melt
a Given Volume of Weld
=
Heat Required to
elevate solid to MP
+
Latent Heat
of Fusion
Q  C p Tm  To   L
  Density (mass / volume)
C p  HeatCapacity(thermalenergy / mass oC
Tm  MeltingTem perature
To  InitialTem perature, usuallyroo mtemperature
L  LatentHeatofFusion
Not all the net heat transferred goes into melting
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
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?
Other Energy Sources
Arc H = EI/v
Resistance: H = I2Rt
Electroslag: H = EIt
4 P1
Laser: PD 
2
  f 
EI
EB: PD 
A
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
f  Focal lenght of lens

   angle of beam divergence

wavelength
  laser
constant
A = Area of focused beam
OXYFUEL GAS WELDING
THERMIT WELDING
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.
Polarity and Current Flow
Welding Electrode or "Electrode"
Anode
Cathode
I
DCEP
I
DCEN
Cathode
Anode
Work Electrode or "Work"
Reverse
RPEP
Straight
SPEN
Plasma State
Gas is hot enough so that high energy collisions
produce free electrons

A A e
-
Plasma may only be a few % electrons
Conduction of Current in the Arc
Cathode
Thermal
Ionization
Electrons Emitted
Free
Electron
Ion
Plasma
T>10,000K
Anode
Recombination
Neutral
Gas Atom
Electrons Absorbed
Argon
Arc
Thermionic Work Function
Energy Required for electron to escape a solid surface
I
I/e electrons/second
Cathode
V
Energy into
emitted electrons = I x WF
(from arc)
I/e electrons/second
Anode
Energy deposited by
impinging electrons = I x WF
(into anode)
Work Function of pure Tungsten = 4.4 eV
Work Function of Thoriated W = 4.1 eV
Ionization
I
Free
Electron
"Neutral"
Atom
Ionization
Collision
Free
Ion
Free Electron with
Energy > 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)?
Arc V-I Characteristic
V
I
Welding
Power
Source
A
V
Welding
Arc
V
h
40
30
20
10
0
0
Unstable
h3
h2
h1
h=0
50 100 150 200 250 300
I
We see that current and arc length have an effect, what happens if we change from Ar
to some other ionizing gas?
Ionization Potential
He 24.6 ev
Ar 15.8
N 15.6
Fe 7.9
Na 5.1
P 4.3
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?