Welding Processes System Exploration Workshop

8/17/2019 Welding Processes System Exploration Workshop

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Welding Processes


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History of joining techniques

• During the Bronze age, metals were

used for domestic tools, weapons etc• The first development of forge welding

was by the Syrians (1400 BC).

• Welding was then increasingly utilised in

response to military requirements forexample,riveting of suits of armour.

• A commercial scale for bolting, riveting,

soldering and finally

welding was introduced.

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• A commercial scale for bolting, riveting,

soldering and finally welding was

introduced.

• Metal joining is required

Whenever the desired component cannot

be made by means of simple fabrication

processes such as casting, forging,

rolling, extrusion, etc.

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• Metal joining is indispensable to manufacturing industrybecause it is necessary in large constructions,

components required for modern living.

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Heat input

Heat Loss


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Heat Input

H = energy input, energy/unit length, joules /

H = Power/Travel Speed, = P/v

P = total input power, Watts

v = travel speed of heat source, mm/se

Describes energy per unit length delivered,

not rate of delivery

Used in codes & specifications

This energy does not all go entirely to the wo


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Heat Input for Arcs

H = P/v = EI/vE = Arc Voltage (Volts)

I = Arc Current (Amps)

EI = Process power, converted to Heat

v = Welding Travel Speed

Hnet = f 1H = f 1P/v = f 1EI/v

f 1 = Heat Transfer Efficiency

Not all the arc energy goes into the work


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Arc Lengthlong

short

f 1 = Heat Transfer Efficiency


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Reinforcement

Heat

Affected

Zone

Melted Base Metal

Aw = Cross Section of Weld = Am + Ar

For Autogenous Weld (no fil ler metal)

Aw = Am

Q =Heat Required to

elevate sol id to MP+ Latent Heat

of FusionHeat Required to melt

a Given Volume of Weld=


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Enthalpy of Melting

Q =Heat Required to

elevate sol id to MP+ Latent Heat

of FusionHeat Requi red to melt

a Given Volume of Weld

=

ofFusion LatentHeat L

remtemperatuusuallyroo perature InitialTemT

perature MeltingTemT

C massrgythermalenety HeatCapaciC

volumemass Density

LT T C Q

o

m

o

p

om p

,

/(

)/(

Not all the net heat transferred goes into melting


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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 = QAw/Hnet

f 2 = QAwv/f 1EI

From previous slide:

Hnet = f 1H = f 1P/v = f 1EI/v

Melting Efficiency Depends On:

• Higher Thermal Conductivity - Lower Efficiency

• High Energy Density Heat Source - Higher Efficiency


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Polarity and Current Flow

I I

DCEP DCEN

Anode

Cathode

Cathode

Anode

Welding Electrode or "Electrode"

Work Electrode or "Work"

Straight

SPEN

Reverse

RPEP


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Conduction of Current in the Arc

Plasma

Electron

Ion

NeutralGas Atom

Ionization Free

RecombinationT>10,000K

Thermal

Cathode

Anode

Electrons Emitted

Electrons Absorbed


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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 eV

Work Function of Thoriated W = 4.1 eV


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} }}


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Arc V-I Characteristic

Welding

PowerSource

A

V

Welding

Arc

I

V

V

I0

20

30

40

10

50 100 150 200 250 3000

h1

h2

h3

h=0

h

Unstable


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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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Open-Circuit Voltage

V

I

CC

CV

Voc

VocHot Start

Arc


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Electrical Stability of the Arc

V

I

Rs

Unstable

Stable Arc

Source Arc willoperate here


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Manual Arc Control

F

B

F

B

B>FF>B

hh

F

B

B=F

h

F = Feed RateB = Burn-Off Rate


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Manual Arc Control Inputs

V

I

Arc

Current Varies

h

h

Power

I

Source

Vary

V

I

Arc

Power

I

Vary Current

Source

Vary


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Self-Regulation of the Arc

VmF

B

h

ContactTube or Tip

Feed Rolls

Wire Spoolor Reel

WireFeeder CV

Power

SourceV

I

V

I

CV

I=F/k1

arc too

arc tooshort

long

currentdrops

currentrises

F=constant, I varies

BF


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Ave I


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Pulsed Arc

Tandom

Arc


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Resistance Welding Processes


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Principal Types of Resistance Welds

Spot Weld Seam Weld Projection Weld

Upset Weld Flash Weld

After Welding After Welding

Introduction to Resistance Spot


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Introduction to Resistance Spot

Welding

Resistance

Distance

Typical Equipment of Resistance


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Typical Equipment of Resistance

Spot Welding

(a) (b)

[Reference: Welding Process Slides, The Welding Institute]

Process Operation of Resistance Spot


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Process Operation of Resistance Spot

Welding

[Reference: Welding Process Slides, The Welding Institute]

Advantages of Resistance Spot


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Advantages of Resistance Spot

Welding

l Adaptability for Automation in High-Rate Production of

Sheet Metal Assemblies

l High Speed

l Economical

l Dimensional Accuracy


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Basic Single Impulse Welding Cycle

Electrode Force

Welding Current

Welding Cycle

Squeeze Time Weld Time Hold

Time

Off

Time

[Reference: Welding Handbook, Volume 2, AWS, p.538]

Enhanced Welding Cycle


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Enhanced Welding Cycle

P

r e h e a t

T i m e

U

p s l o p e

T i m e

C o o l T i m e

WeldTime

C o o l T i m e

Preweld

Interval

Welding Cycle

Weld Interval Postweld Interval

D o w n s l o p e

T i m e

Q

u e n c h

T i m e

T e m p e r

T i m e

H o l d

T i m e

Pulse

ImpulseTempering

Current

Welding CurrentElectrode

Force

Forge Delay Time

Forge Force

[Reference: Welding Handbook, Volume 2,

AWS, p.539]

Squeeze time


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Contact-Resistance Measurement

Contact

Area

Electrode Force

Electrode Force

Small Current

Rec

Rec

Rsc

Rv

Rv

Rec

Rec

Rtotal

Rec = contact resistance

between electrode

and sheet surface

Rsc = contact resistance

at the faying surface

Rv = volume resistance of

the sheets

Resistivity as A Function of


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Resistivity as A Function of

Temperature

100 200 300 400 500 600 700 800

10

203040506070

8090

100110120

130

HSLA

Low Carbon

Temperature, °C

R e s i s t i v i t y , m W

- c m

[Reference: Welding in the Automotive Industry, D.W. Dickinson, p.125]


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Temperature Gradient of A Spot Weld

End of

“Weld Time”20% of

“Weld Time”

WaterTemperature

Water

Water

Electrode

Electrode

Work

Welding Temperature

[Reference: Resistance Welding Manual, RWMA, p.1-4]


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Expulsion at Interfacial Surface


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Zhang et al, “Expulsion Modeling in RSW of Steel and Al Alloys”,

AWS Sheet Metal Conf VIII, 1998


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Pulsing

Cool Time

Pulse 1 Pulse 2 Pulse 3

Pulse

Time


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Without Pulsing With Pulsing

Welding Processes System Exploration Workshop

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