Welding for engineers chapter 1

Welding Technology for Engineers

1 Welding for Engineers

Content:

Chapter 1: Welding Processes and Equipment

Chapter 2: Materials and their Behaviours in Welding

Chapter 3: Design and Construction

Chapter 4: Fabrication and Application Engineering


Welding for Engineers 3

Chapter 1:

Welding Processes and Equipment


Welding Introduction

Requirements for Joining Materials


Surface Roughness


To make up atomic interaction of materials

(1) Deform them – Pressure Welding

– Solid State Bonding, Hot Pressing , …etc

(2) Introduce molten metal between them – Brazing, Soldering, ..etc

– Hot Pressing with metal insert, ..etc

(3) Melt them – Fusion Welding

– Arc Welding, Resistance Welding , …etc



Typical Weld Joints

(a) Fusion Welding (b) Pressure Welding (c) Brazing


Fusion Welding – A welding process where metal

workpieces are joined through melting (fusing) and

solidifying. Molten metal id formed by heating, and is made

up from base metal, or from mixture of base metal and filler

metal.

Pressure Welding – A welding process that forms a weld

joint by pressure of mechanical force after heating up the

joint by friction or other heat effects.

Brazing and Soldering – joining processes that form a joint

by filling gap with molten brazing filler metal after heating

the joint. Capillary force induces the filling. Brazing filler

metal has a lower melting point than that of the base metal

so that the base metal does not melt.

Classification of Metal Joining Methods

Brazing and Soldering



Classification of Joining Methods of Metals


Fusion Welding Advantages: (1) Joint efficiency is high

(2) Air and water tightness is excellent

(3) Structure of joint can be simplified

(4) Thickness of joint ranges is wide

(5) Reduction of material usage and saving of workforce

Limitations:

(1) Newly formed weld joint is heterogeneous to the base metal

(2) Quality of the base metal locally deteriorates by the welding heat

(3) Weld strain and deformation occur by local heating and cooling

(4) Residual stress develops and deteriorates the joint strength

(5) It is difficult to confirm quality of the weld joint


Classification of Gas Shielded Arc Welding


Overview of Common Welding Methods

Welding Positions

Sketch of a Weld Joint


Welding Positions


Power Sources for Welding AC Arc Welding Power Source

• Shielded (Manual ) Metal Arc Welding

• Electro-Slag Welding

• TIG for Aluminum Alloys (cleaning action)

• Submerged Arc Welding

DC Arc Welding Power Source

• MIG/MAG Welding

• Electro-Gas Arc Welding

• CO2 Gas Arc Welding with Flux Cored Wire

• Self Shielded Arc Welding

• TIG for Steel

• Plasma Welding and Cutting

• Stud Welding

• Submerged Arc Welding with small diameter wire


Characteristics of Arc


Characteristic of Arc • Voltage-Current relationship

• Voltage distribution

(a) Distribution of Arc Voltage (b) Arc Characteristics

Heat (Energy) Sources • Electric energy

Arc Welding, Electro-Slag Welding, Resistance Welding,

Electron Beam Welding, etc…

• Mechanical energy Friction Welding, Friction Stir Welding, Ultrasonic

Welding, etc…

• Chemical energy Gas Welding, Thermit Welding, etc…

• Photon energy Laser Welding, etc…



Temperature Profile of TIG Arc


Structure of Arc

• Arc Voltage is a sum of cathode drop voltage, arc column voltage

and anode drop voltage.

• Arc Column Voltage increases as Arc Length increases.


Relationship between Welding Current and Arc Voltage


Electromagnetic Pinch Effect

Electromagnetic attractive force causes the cross section of the

arc to shrink – Electromagnetic Pinch Effect.

Arc also shrinks to reduce its surface area to suppress heat loss

when the arc is cooled from ambient – Thermal Pinch Effect


Plasma Gas Flow

• Magnetic field is made up around the arc by welding current.

• The induced flow of gas directs from the electrode towards the

workpiece, and its speed is high. This induced gas flow is Plasma Gas

Flow.

• The plasma gas flow strongly influences the transfer of molten metal

droplets and penetration shape of weld.


Arc Blow

(a) Effect of Work Piece Lead Connection (b) Effect of Work Piece Shape

• Arc deflects from its intended direction by asymmetric magnetic field

and welding current circuit (residual magnetic field) – Arc Blow.

• Arc Blow tends to occur at DC welding of easily magnetized

material, e.g. ferritic steel.

• Elimination: Managing workpiece connection, leads (cables) &

demagatizing workpieces.


Waveform Traces of Welding Voltage and Current of AC Arc

P = Reignition Voltage

Q = Transitional Voltage

R = Usual Arc Voltage

• In AC Welding (e.g. MMAW), the polarity alternates every half cycle.

• Welding current becomes null at the crossover. The arc once extinguishes at the

crossover and reignites in the following half cycle. This arc voltage is called

reignition voltage, P.

• The reignition voltage, P is higher than both a transitional arc voltage, Q and

the usual arc voltage, R.

• In an open circuit voltage of a power source, Po must be higher than the

reignition voltage, P for AC arc to be sustained.


Influence of Shielding Gas Type over Metal Transfer

Globular Transfer Spray Transfer


Mode of Droplet Transfer in Consumable Electrode Welding


Welding current – Low

With Active Gas (CO2) – Unstable

With Active Gas (CO2) – Stable

With Inert Gas (Argon)

Classification of Molten Metal Transfer Mode


Welding Condition and Droplet Transfer Mode

27%


Effective Factors on Weld Penetration


Effect of Welding Condition on Bead Formation

Low Current

High Speed

High Current

High Speed

High Current

Low Speed


Characteristics of Power Source

(a) Mechanism of Arc Stability in a

Welding Power Source with drooping

Characteristics

(b) Mechanism of Arc Stability in a

Constant voltage characteristics

welding power source

Drooping – Manual Welding

Constant – Automatic or Semi-Automatic (high current – self regulation


Important Actions of Arc Plasma (1) Magnetic pinch effect

• Droplet transfer

(2) Magnetic arc blow • Magnetized base metal

• DC currents (arc stiffness)

(3) Plasma gas flow • Electro-Magnetic interaction

(4) Thermal pinch effect • Stability as plasma phase

(5) Cleaning action ( on cathode ) • Reduction of oxides

(6) Heat input ( on anode ) • Anode > Cathode, due to work function of the material

(7) Digging action • By the pressure of Arc Plasma

Exercise 1:

Which are the gas shielded metal arc welding ? • Shielded metal arc welding

• MAG, MIG welding

• TIG welding

• Electro-gas arc welding

• Submerged arc welding

• Self-shielded arc welding

• Plasma arc welding

• Stud arc welding


Exercise 2.

Arrange following welding processes in the below table.

a. Arc welding b. Brazing c. Cold pressure welding

d. Electron beam welding e. Explosion welding f. Flash welding

g. Friction welding h. Gas welding i. Laser welding

j. Resistance welding k. Riveting l. Soldering

m. Thermit welding

Joining Energy Electrical Energy Chemical Energy Mechanical Energy Light Energy

Joining Mechanism

Mechanical Joining

Weld

ing

Pro

cesse

s

Fusing Welding

Pressure Welding

Brazing/Soldering



Arc Welding Equipment


External Characteristics of Welding Power Source

& Operation Point

e.g. SMAW, SAW e.g. TIG, PAW e.g. GMAW (MAG & MIG)


Self-Regulation of Arc Length by Constant Voltage

Characteristic Welding Power Source

WF: Electrode Fee rate

MR: Electrode Melting Rate

WF = Constant, I1 < Io < I2


Movable-Shunt-Core AC Welding Power Source


Voltage-Ampere

Characteristic of

Arc

High Current

Low Current

Working Principle of Movable-Shunt-Core AC Welding Power Source


Thyristor Controlled Welding Power Source


Inverter Controlled Welding Power Source


Advantages of Inverter Controlled Power Source


Schematic Diagram of Inverter Controlled AC Welding Power Source

This is especially suitable for TIG welding of aluminium and its alloy


Handling of Welding Power Source

Allowable Cycle (%) = Rated Welding Current (A)

Max Welding Current of Usage(A)

2

x Rated Duty Cycle (%)

A welding power source must ne be used continuously for a

long time without care!

For Example:

When a power source of a rated output 350A and a rated duty cycle 60% is

used at 300A, the allowable duty cycle is given as below.

Allowable Cycle (%) = 350(A)

300 (A)

2 x 60(%) = 82%


Allowable Duty Cycle of Welding Power Source


For example:

The max welding current for continuous welding (Im) is a

welding current with which continuous welding can apply

without burn out of a welding power source

In the case of rated output of 350A and rated duty cycle of

60%. . Im can also be calculated as below.

100(%) = 350(A)

Im (A)

2 x 60(%)

Im = 350 (A) x √ 60%

100% = 271 (A)

Thus, Consequently, the power source does not get burnt out at continuous

welding as far as the power is used at an output current below 270A.


Effect of Welding Lead Length on Arc Stability


Exercise 3:

Fill in all the technical terms – Welding Processes

(a) Sketch of a Weld Joint (b) Welding Positions

Arc Welding

– Shielded Metal Arc Welding (SMAW or MMAW)

– GMAW (MAG & MIG) Welding

– TIG Welding

– Electrogas Arc Welding

– Submerged Arc Welding

– Self-Shielded Arc Welding

– Plasma Arc Welding

– Stud Arc Welding


Shielded Metal Arc Welding – SMAW

Manual Metal Arc Welding – MMAW • Several types of covered electrodes

• Coated flux dissolved

– Generate gasses Stable arc

– Make slag De-oxidation and shield weld metal



Set-up of Manual Metal Arc Welding (MMAW) Equipment


Characteristics of MMAW or SMAW Diameter of electrode – 3.2mm to 6.4mm

Welding current – 100A to 2,000A

Welding power source – A moveable-shunt-core type

When arc length becomes higher, the electrode feed speed is increased to

shorten the arc length.

The arc length is autogenously controlled constant with self-regulating of

arc by a constant voltage power source.

Limitations: (1) Limited welding position – flat &

horizontal

(2) Limited weld line of linear, of semi-

linear and of large radius curve

(3) No applicability to weld complex line

(4) Requirements of strict groove

preparation

(5) Heat affected zone (HAZ) softened

or embrittled by large heat input

(6) Relatively expensive machine

Advantages: (1) Highly efficient welding with

high welding current.

(2) Deep penetration of weld

(3) Unnecessary of an arc

protector for optical radiation

(4) Rare spatter and fume

(5) Little disturbance from wind

Arc Welding



– TIG Welding








Set-up of Gas-Shielded Metal Arc Welding Equipment

Schematic View of MAG Welding MAG Welding Equipment

Gas Metal Arc Welding (MAG & MIG)

MAG: Metal Active Gas (CO2 or CO2+Ar)

MIG: Metal Invert Gas (Ar)


Periodic Table



Balance of Wire Feed Rate and Wire Melting Rate


Control of Welding Current Waveform in MAG Welding

(a) Increasing rate control of short circuiting current

(b) Suppression of short circuiting current

(c) Decreasing rate control of arc current

(d) Promotion of short circuiting

(e) Retarding control of increasing timing for short

circuiting current

(f) Breaking current control of the short circuiting

(g) Suppression of arc reignition current


Pulsed Gas-Shielded Metal Arc (Pulsed-MAG & Pulsed-MIG) Welding

(a) A peak current and a base current repeat at a given pulse frequency.

(b) The peak current level is chosen to be higher than a transition current for

spray transfer.

(c) A droplet is transferred by strong electromagnetic pinch force at a given

time.

(d) Sputter rarely occurs in a spray transfer mode as there is no short circuiting

happened.


Buried Arc

Droplet Transfer Diagram of MIG Welding

Cross Section Shape of Bead


Effect of Pulsed Current on the transfer

Pulsed Current Waveform


Power sources for welding

AC arc welding power sources

• Movable iron core / Movable coil type

• Thyristor type

• Inverter type

DC arc welding power sources

• Engine or motor driven generator type

• Thyristor type

• Inverter type


Comparison of DC and AC Welding Power Sources

DC Welding Power Source AC Welding Power Source

Thyristor

Controlled

Inverter

Controlled

Single Phase

Transformer

Inverter

Controlled

Open Circuit

Voltage Low Low High Low

Stability of Arc Good Excellent Poor Good

Magnetic Arc

Blow Often Occurs Often Occur Hardly Occurs Hardly Occurs

Power Factor High Very High Low Very High

Arc Welding



– TIG Welding (GTAW)








Set-up of TIG Welding Equipment


Characteristics of TIG Welding A filler metal (a rod or a wire) must be added when deposited metal is

necessary.

Separate addition of a filler material means that welding heat input and

amount of deposited metal can be controlled separately.

Advantages (1) All positional welding is possible.

(2) Easiness of bead formation at a

root pass.

(3) Highly clean weld metal of

excellent toughness, elongation

and anti-corrosion.

(4) Availability of clean bead surface –

no oxidation

(5) No necessity of removal of slag

(6) Applicable to all metals

Limitations (1) Slow welding speed

(2) Low efficiency

(3) Expensive shielding gas of argon

and helium

TIG Welding (Tungsten Invert Gas Welding)



Ignition Methods of TIG Arc and Their Characteristics

Welding for Engineers

Ip: Peak Current

Tp: Peak Time

T: Pulse Time (= Tp + Tb)

f = Pulse frequency (=1/T = 1/Tp + Tb)

Ib: Base Current

Tb: Base Current Time

Pulsed TIG Welding


Effect of Electrode Polarity in TIG Welding


Effect of EP Time Ratio Control

Arc Welding



– TIG Welding

– Electrogas Arc Welding (EGW)






Electrogas Arc Welding – EGW



Characteristics of EGW

• EGW fundamentally applies in single pass welding.

• Thickness of plates – 10 to 35mm; for heavy thickness, oscillating torch

or multi-pass welding can be used.

• Applications – for butt joints in vertical up position in a ship hull, a

storage tank, a pressure vessel, a bridge, ..etc

Advantages (1) High work efficiency because of

high welding current.

(2) Little angular distortion because

of a small number of passes.

(3) Large tolerance in groove

preparation and in groove set up.

Limitations (1) Deterioration of mechanical properties

of joints because of large heat input.

(2) Long starting time after the

interruption of welding

(3) Applicability only to the vertical up

position

Arc Welding



– TIG Welding







Submerged Arc Welding – SAW



Side Beam with Submerged Arc Welding Equipment

Arc Welding



– TIG Welding



– Self-Shielded Arc Welding (FCAW-S)





Self-Shielded Arc Welding (FCAW-S)

Self-Shield Arc Welding



Characteristics of FCAW-S

• Arc length to keep as short as possible to secure the shielding.

• Longer stick out aiming to preheat flux in the electrode.

• Retract start of arc to eliminate defects.

• Applications: welding of steel structures, steel pipe piles, ..etc

Advantages (1) No necessity of preparation of

shielding gas.

(2) Easy handling of welding torch by

its light weight.

(3) Less disturbance from wing

Limitations (1) Large volume of fume with some wire.

(2) Deterioration of mechanical properties

and occurrence of blowholes caused by

insufficient control of the arc length.

(3) Shallow penetration.


Exercise 4.

Which type of power sources are used for following

processes ? Fill in either AC or DC in the ( ).

a. Shielded (Manual ) Metal Arc Welding ( )

b. MIG/MAG Welding ( )

c. CO2 Gas Arc Welding with Flux Cored Wire ( )

d. TIG for aluminum alloys ( )

Arc Welding



– TIG Welding



– Self-Shielded Arc Welding (FCAW-S)

– Plasma Arc Welding (PAW)



Plasma Arc Welding



Comparison of TIG and Plasma Arcs


Electron Beam Welding

(EBW)


Set-up of Electron Beam Welding Equipment


Characteristics of EBW • Electrons, emitted from a heated cathode, are accelerated in high voltage and are

converged to a high energy density electron beam with a magnetic coil.

• The electron beam is projected onto a workpiece in vacuum.

• A deflection coil is used to irradiate the beam onto a welding position of the

workpiece.

• Energy density of the electron beam reaches to more than thousands times of that

TIG arc.

• High quality welding with high efficiency.

Advantages

(1) Deep penetration with small heat

input.

(2) Narrow heat affected zone and less

deterioration of base metal.

(3) Small weld strain and deformation

Limitations

(1) Necessity of vacuum.

(2) Precise preparation of a groove face.

(3) Expensive equipment


Laser Beam Welding

(LBW)


Characteristics of Laser Beam Welding (LBW) • A welding method uses a laser light beam as heat source.

• Laser light is photons of the same wavelength in a synchronized phase.

• Laser is focused with mirrors or lenses onto a workpiece.

• Energy density of laser reaches to more than thousands times of that of

arc, like an electron beam as depicted below.

Advantages (1) Possibility of welding in an atmosphere.

(2) No influence from magnetic field.

(3) Possibility of welding non-metallic

materials.

Limitations (1) Dependence of light absorption upon

surface conditions of a workpiece.

(2) Safety protection from laser light.

(3) Low energy efficiency esp at a laser

generator.

(4) Expensive instruments.


Set-up of Laser Beam Welding Equipment (A)

CO2 Gas Laser – Use a continuous wave mode and wavelength is 10.6µm.

– An optical fibre cannot pass through the 10.6µm wave.

– Mirrors are used to convey the light.

– Laser gas: a mixture of helium, nitrogen and CO2, circulated for reuse and also

deteriorated during services.


Set-up of Laser Beam Welding Equipment (B)

YAG Laser – Can generate both a pulse wave and a continuous wave.

– The light is oscillated in a YAG rod excited by Kr arc lamps, Xe arc lamps or

lights of diode laser (LD).

– The wavelength is 1.03µm or 1.06µm; the light can pass through an optical fibre.

– An optical fibre is used for transmission.


Duty cycle Electric Energy/sec =VI = RI 2

Power = Energy / sec (J/s, VA, W)

( When IActual is different from IRated , duty cycle must be changed. )

Actual power for welding < Rated power for welding

Allowable Duty Cycle (%) =

Rated Secondary Welding Current

Actual Welding Current

2

X Rated Duty Cycle (%)

rActual Duty Cycle < r Rated Duty Cycle I Rated

IActual

2

rActual Duty Cycle < r Rated Duty Cycle 2

R(I Rated) 2

R(I Actual)


Exercise 5:

Rated duty cycle: 40%

Rated secondary welding current: 400A.

When the welding current is 300A, how much duty cycle is

allowable ?

Welding for engineers chapter 1

Documents

welding introduction

fusion welding arc welding

arc voltage welding

welding processes

ac welding

resistance welding

thermit welding

ultrasonic welding