Physics of Welding

Physics of Welding

Lesson ObjectivesWhen 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 Activities1. Read Handbook

pp 32-622. Look up

Keywords3. View Slides; 4. Read Notes, 5. Listen to lecture6. Do on-line

workbook7. 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 input

Heat Loss

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 deliveryUsed 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

Hnet = f1H = f1P/v = f1EI/v f1 = Heat Transfer Efficiency

Not all the arc energy goes into the work

Arc Lengthlong

short

f1 = Heat Transfer Efficiency

Reinforcement

HeatAffectedZone

Melted Base Metal

Aw = Cross Section of Weld = Am + Ar

For Autogenous Weld (no filler metal)Aw = Am

Q =Heat Required to elevate solid to MP

+ Latent Heat of Fusion

Heat Required to melta Given Volume of Weld

=

Enthalpy of Melting

Q =Heat Required to elevate solid to MP

+ Latent Heat of Fusion

Heat Required to melta Given Volume of Weld

=

ofFusionLatentHeatL

remtemperatuusuallyrooperatureInitialTemT

peratureMeltingTemT

CmassrgythermalenetyHeatCapaciC

volumemassDensity

LTTCQ

o

m

op

omp

,

/(

)/(

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/vResistance: H = I2RtElectroslag: H = EIt

Laser:

EB:

H = Heat generated, joulesE = Voltage, voltsv = Travel Speed, mm/secI = Current, ampsR = Resistance, ohmst = Time, secPD = Power DensityP1 = Input power 2

14PD

f

P

lens oflenght Focalf

divergence beam of angle

wavelength constant laser A

EIPD

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.

Convert.exe

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

I I

DCEP DCEN

Anode

Cathode

Cathode

Anode

Welding Electrode or "Electrode"

Work Electrode or "Work"

StraightSPEN

ReverseRPEP

Gas is hot enough so that high energy collisionsproduce free electrons

-eA A

Plasma may only be a few % electrons

Plasma StatePlasma State

Conduction of Current in the Arc

Plasma

Electron

Ion

Neutral Gas Atom

Ionization Free

Recombination T>10,000K

Thermal

Cathode

Anode

Electrons Emitted

Electrons Absorbed

ArgonArc

Thermionic Work Function

V

I I/e electrons/second

Energy into

Cathode

Anode

emitted electrons = I x WF

Energy deposited by impinging electrons = I x WF

I/e electrons/second

(from arc)

(into anode)

Energy Required for electron to escape a solid surface

Work Function of pure Tungsten = 4.4 eVWork Function of Thoriated W = 4.1 eV

Ionization

Free Electron with

Collision

Ionization

FreeIon

FreeElectron

"Neutral" Atom

Energy > Ionization Potential

Ionization Potentials: He 24.6 eV Ar 15.8N 15.6Fe 7.9 Na 5.1

I

} }}

Will total voltage change if we change the amount of current (say from 200 amps to 300 amps)?

Arc V-I Characteristic

Welding Power Source

A

V

Welding Arc

I

V

V

I 0

20

30

40

10

50 100 150 200 250 300 0

h1

h2

h3

h=0

h

Unstable

We see that current and arc length have an effect, what happens if we change from Ar to some other ionizing gas?

Ionization PotentialHe 24.6 evAr 15.8N 15.6Fe 7.9Na 5.1P 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?