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 2.

3.

4.

5.

6.

32-62 Look up Keywords View Slides; Read Notes, Listen to lecture Do on-line 7.

workbook 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 H net = f 1 H = f 1 P/v = f 1 EI/v f 1 = Heat Transfer Efficiency

f 1 = Heat Transfer Efficiency short Arc Length long

Melted Base Metal Reinforcement Heat Affected Zone A w = Cross Section of Weld = A m + A r For Autogenous Weld (no filler metal) A w = A m 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



T m



T o

 

L



Density

(

mass

/

volume

)

C p



HeatCapaci ty

(

thermalene rgy

/

mass o C T m



MeltingTem perature T o L

 

InitialTem perature LatentHeat ofFusion

,

usuallyroo mtemperatu re

Not all the net heat transferred goes into melting

Melting Efficiency

f 2 = Melting efficiency, the fraction of the process heat energy per unit length delivered to the metal which is required to melt the metal f 2 = QA w /H net From previous slide: H net = f 1 H = f 1 P/v = f 1 EI/v f 2 = QA w v/f 1 EI 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 = I 2 Rt Electroslag: H = EIt Laser:

PD   4

f P

1   2

EB:

PD



EI 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 P 1 = Input power



f

    Focal  angle lenght of of lens beam divergence

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 I DCEP 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

V I I/e electrons/second Cathode Energy into emitted electrons = I x WF (from arc) I/e electrons/second Energy deposited by impinging electrons = I x WF (into anode) Anode

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

I "Neutral" Atom

Ionization

Free Electron 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

} } }

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 V A V Welding Arc h 40

Unstable

30 20 h3 h2 h1 h=0 10 0 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?