lecture welding processes manufacturing technology

8/11/2019 Lecture Welding Processes Manufacturing Technology

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Welding Processes


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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/v

E = Arc Voltage (Volts)

I = Arc Current (Amps)

EI = Process power, converted to Heatv = Welding Travel Speed

Hnet = f1H = f1P/v = f1EI/v

f1 = Heat Transfer Efficiency

Not all the arc energy goes into the work


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Arc Length

long

short

f1 = 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 Fusion

Heat Requi red to melt

a Given Volume of Weld

=

ofFusionLatentHeatL

remtemperatuusuallyrooperatureInitialTemT

peratureMeltingTemT

CmassrgythermalenetyHeatCapaciC

volumemassDensity

LTTCQ

o

m

o

p

omp

,

/(

)/(

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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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

SourceArc 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

WireFeederCV

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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Welding Power Sources

Each type of power source has fundamentalelectrical differences that best suit particularprocesses

Welding machine

Must meet changing arc load and environmentalconditions instantly

Must deliver exact amount of electric currentprecisely at right time to welding arc

Available in wide variety of types and sizes


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Four Types of Power Source

Engine-driven generators

Powered by gas or diesel combustion engine

Can be found with a.c. or d.c. electric motor

No longer being manufactured and rarely found

Inverters

Increases frequency of incoming primary power

Constant current, constant voltage, or both

Produce a.c. or d.c. welding current


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Four Types of Power Source

A.C. transformers

Used to step down a.c. line power voltage to a.c.

welding voltage

Transformer-rectifiersUse basic electrical transformer to step down a.c.

line power voltage to a.c. welding voltage

Welding voltage then passed through rectifier to

convert a.c. output to d.c. welding currentMay be either d.c. or a.c.-d.c. machines


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Bridge Rectifier


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Output Slope

Two basic types

Constant current

Constant voltage


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Current Controls

Amperage

Quantity of current (flow)

Determines amount of heat produced at weld

VoltageMeasure of force of current (push)

Determines ability to strike an arc and maintain its

consistency


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Constant CurrentWelding Machines

Used for shielded metal arc welding and gas tungsten

arc welding

Available in both d.c. and a.c. welding current

Current remains fairly constant regardless of changes in arc

length

Total Wattage stays the same

Voltage drops as amps increase (dropping arc voltage(DAV) machine)

Enables welder to control welding current in specificrange by changing length of arc


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Open Circuit and Arc Voltage

Open circuit voltage runs between 50-100 volts (no

welding being done, volts high, no amps)

Drops to arc voltage when arc struck

Arc voltages (Voltage generated between electrode and work duringwelding, voltage lower, amps higher)

Range: 36 volts (long arc) to 18 volts (short arc)

Determined by arc length held by welder and type of

electrode used

Arc lengthened, arc voltage increases and currentdecreases


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Polarity

Electrode negative and electrode positive used in d.c.

welding

DCEN (d.c. electrode negative)

Electrode connected to negative terminal of power source

and work connected to positive terminal (current flows

from neg to pos) flow from electrode to work = more

electrode consumption.

DCEP (d.c. electrode positive)

Electrode connected to positive terminal of power sourceand work connected to negative terminal


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D.C. Transformer-Rectifier WeldingMachines

Copyrigh t The McGraw-Hill Companies, Inc. Permission required for reprodu ction or di splay.

The Lincoln Electric Co.

Miller Electric Mfg. Co.

Have many designs and purposes

Flexibility one reason for wide acceptance

Deliver either DCEN or DCEP

May be used for:Stick electrode welding

Gas tungsten arc welding

Submerged arc welding

Multi-operator systemsStud welding


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Transformer-Rectifier Machines

Have two basic parts

Transformer for producing and regulatingalternating current that enters machine

rectifier that converts a.c. to d.c.

Third important part is ventilating fan

Keeps rectifier from overheating

Design improves arc stability and makes iteasy to hold short arc which is soft and steady

No major rotating parts so consume littlepower


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A.C. Transformer WeldingMachines

Most popular a.c. welding machine

Function of transformer

Step down high voltage of input current to high

amperage, low voltage current required forwelding

Especially suited

for heavy work


The Lincoln Electric Co.Miller Electric Mfg. Co


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Advantages of a.c. PowerSources

Reduces tendency to arc blow

Can use larger electrodes

Resulting in faster speeds on heavy materials

Lower cost Decreased power consumption

High overall electrical efficiency

Noiseless operation

Reduced maintenance


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D.C. and A.C.-D.C. Inverter WeldingMachines

Portable, lightweight, and versatile

May be either constant current, constant

voltage or both

Can perform several different processes


Miller electric Mfg. Co.The Lincoln Electric Co.


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Duty Cycle

Percentage of any given 10-minute period that

machine can operate at rated current without

overheating or breaking down

Rating of 100% means machine can be used atrated amperage on continuous basis

Required by continuous, automatic machine welding

Rating of 60% means machine can be used at its

capacity 6 out of every 10 minutes without

damage

Satisfactory for heavy SMAW and GTAW


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Duty Cycle

Duty Cycle Amperes TimeAvailable

30% 250 3 Min/10 Min

40% 225 4 Min/10 Min

50% 200 5 Min/10 Min

60% 190 6 Min/10 Min70% 180 7 Min/10 Min

80% 170 8 Min/10 Min

90% 155 9 Min/10 Min

100% 140 10 Min/10 Min


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lecture welding processes manufacturing technology

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