Faculty of Engineering

Mechanical Dept.

ME 410: Casting and Welding Engineering

Welding processes

Importance of joining

Wide use in manufacture

Occurs late in manufacturing process

Large number of practitioners

Cost is high proportion of manufactured item

Risk and cost of defective welds is high

Science is complex

Overview of joining methods

Mechanical methods

Screwed fasteners, rivets, crimp or snap locks

Adhesive bonding

Brazing and Soldering

Base metal does not fuse.

Molten filler drawn into close-fit joints by capillary

action (surface tension forces).

Brazing filler melts >450˚C, solder <450˚C

Welding

Weld

A joint produced by heat or pressure or both

so there is continuity of material.

Filler (if used) has a melting temperature

similar to the base material

Welding processes

Fusion welding

Welding in the liquid state with no pressure

Union is by molten metal bridging

Solid phase welding

Carried out below the melting point without filler

additions

Pressure often used

Allied processes

Thermal cutting

Oxyfuel gas, plasma, laser cutting

Gouging

Air-arc, plasma, oxyfuel gas

Surfacing

Powder and arc spray coating

Clad welding, hardfacing

Solid phase welding

Hot processes

Forge welding

Friction welding

Diffusion bonding

Cold processes

Ultrasonic welding

Explosive welding

Fusion welding

Intense energy source melts base metal locally

Energy density 0.001 W/cm2 to 1 MW/cm2

Energy source may be stationary or move at a

constant speed

Filler metal

From electrode

Independently added filler

No filler (autogenous welding)

Fusion welding heat sources

Power beams

Laser

Electron beam

Spot, seam and

projection welding

Electroslag

Electric arc Chemical reaction Electric resistance

Oxyfuel gas

welding

Thermit welding

MMAW

GMAW

GTAW

FCAW

SAW

The electric arc

+

- Cathode

drop zone

Anode

drop zone

Peak

temperatures

18,000 K

Electric discharge between 2

electrodes through a gas

10 to 2000 amps at 10 to 500 V arc

voltage

Column of ionised gas at high

temperature

Forces stiffen the arc column

Transfer of molten metal from

electrode to workpiece

Can have a cleaning action,

breaking up oxides on workpiece

WELDING ARC The cathode drops the electrical connection between the

arc column and the negative pole (cathode). There is a

relatively large temperature and this is the point at

which the electrons are emitted through the arc column.

The stability of the arc depends on the smoothness of

the flow of electrons at this point.

Tungsten and carbon provide thermionic emissions

since both are good emitters of electrons.

They have high melting temperatures, are practically

nonconsumable, and are therefore used for welding

electrodes. Tungsten has the highest melting point of

any metal.

The anode drop occurs at the other end of the arc and

is the electrical connection between the positive pole

and the arc column. The temperature changes from the

arc column to the anode is considerably lower

Arc Heat Input

yxEfficiencv

EIQ 06.0

Q = arc heat input in kJ/mm

E = arc voltage

I = current in amps

v = travel speed in mm/min

Efficiency

SMAW = 75%

MIG/MAG = 90%

SAW = 90%

TIG = 80%

High arc heat • Large weld pool size

• Low cooling rate

• Increased solidification

cracking risk

• Low ductility and strength

• Precipitation of unwanted phases

(corrosion and ductility)

Low arc heat • Small weld pool size

• Incomplete fusion

• High cooling rate

• Unwanted phase transformations

• Hydrogen cracking

Some arc welding processes

MMAW - Manual Metal Arc Welding

SAW - Submerged Arc Welding

GTAW - Gas Tungsten Arc Welding (TIG)

GMAW - Gas-Metal Arc Welding (MIG, MAG)

FCAW - Flux Cored Arc Welding

Manual Metal Arc Welding

MMAW, SMAW, Stick welding

MMAW Process

Work

Lead

+

-

Electrode

lead

Power Source

DCEP Shown

Base material

Electrode

Coating

Core

wire

Weld pool

Slag

Weld metal

Minimum equipment

Power source (ac or dc, engine driven or

mains transformer)

Electrode holder and leads

May carry up to 300 amps

Head shield with lens protects face & eyes

Chipping hammer to remove slag

Welding gloves protect hands from arc

radiation, hot material and electric shock

Process features

Simple portable equipment

Widely practiced skills

Applicable to wide range of materials, joints,

positions

About 1kg per hour of weld deposited

Portable and versatile

Properties can be excellent

Benchmark process

Covered electrodes

Core wire

Solid or tubular

2mm to 8mm diameter, 250

to 450mm long

Coating

Extruded as paste, dried to

strengthen

Dipped into slurry and dried

(rare)

Wound with paper or chord

(obsolete)

Functions of coating

Slag protects weld pool from oxidation

Gas shielding also protects weld pool

Surface tension (fluxing)

Arc stabilising (ionising)

Alloying and deoxidation

Some ingredients aid manufacture (binder

and extrusion aids)

Typical coating constituents

Organic materials (Cellulose)

Titanium dioxide (rutile)

Silica, alumino-silicates

Sodium and potassium silicate binders

Calcium carbonate and fluoride

Iron powder, ferro-alloys

Electrodes for C-Mn Steel

E6010, E6011 - cellulosic

Punchy, penetrating arc

E6012, E6013 - rutile

Smooth arc, general purpose

E7024 - iron powder (rutile)

Thick coating, high deposition

E7016, E7018, E7028 - Basic low hydrogen

High toughness, low cracking risk

Classification

E xx yz - nHmR

Useable positions (y) 1=all positions

2=flat + horizontal

4=vertical down

Rm/10 (xx) 60 = 60000 psi min

Flux type (z) 20 = acid (iron oxide)

10, 11 = cellulosic

12, 13 = rutile

24 = rutile iron powder

27 = acid iron powder

16 = basic

18, 28 = basic iron powder

Impact properties (n) 0 = 47J at 0°C

2 = 47J at -20°C

3 = 47J at -30°C

4 = 47J at -40°C

Hydrogen level (HmR) H5 = 5 ml / 100g of WM

R = low moisture pick-up

AC vs. DC

DC

Heat Concentrated at

Work piece

Forceful, Digging Arc

Medium to Deep

Penetration

AC

Heat Concentrated at

Electrode

Lower Penetration

Increased Deposition

Rates

(used for welding

thin metal)

Approximate Amperage Settings

Approximate Electrode Amperage Settings

Fast Freeze Fill Freeze Fast Fill Low Hydrogen

E6010 - E6011 E6013 - E7014 E7024 - E7028 E7018

Diameter of Current Setting Current Setting Current Setting Current Setting

Electrode

Inches(Millimeters) Amperes Amperes Amperes Amperes

3/32 in (2.4 mm) 40 - 90 75 - 105 85 - 155 70 - 140

1/8 in (3.2 mm) 75 - 130 100 - 165 100 - 175 90 - 185

5/32 in (4.0 mm) 80 - 160 135 - 225 160 - 270 140 - 230

3/16 in (4.8 mm) 110 - 225 185 - 280 220 - 330 210 - 300

7/32 in (5.6 mm) 200 - 260 235 - 340 270 - 410 230 - 380

1/4 in (6.4 mm) 220 - 325 260 - 425 315 - 520 290 - 440

Applications

Wide range of welded products:

Handyman & light structure

Heavy steel structures, workshop and site

High integrity (nuclear reactors, pressure

equipment)

Ideal where access is difficult - construction

site, inside vessels, underwater

Joins a wide range of materials

Limitations of MMAW

Low productivity

Low power

Low duty cycle (frequent electrode changes)

Hydrogen from flux coatings

Electrode live all the time

Arc strike, stray current and electric shock risks

Submerged Arc Welding

SAW, subarc

Submerged arc welding

Power

Source

Work Lead

-

+

Workpiece travel

Granular

Flux

Flux

Hopper

Unfused flux

Electrode Wire from Reel

Drive rolls

Contact tip

Slag

Weld metal

Weld pool

Arc cavity

SAW features

High productivity

2 to 10 kg/hour

Up to 2m/min

Bulky, expensive and

heavy equipment

Flat and horizontal

positions only

Thicker sections (6mm

and above)

Mostly ferrous materials

(also Ni alloys)

Equipment

Power source

Welding head and

control box

Welding head travel

Flux recovery system

(optional)

Positioners and Fixtures

Consumables

Solid or cored wires

Granular fluxes

Agglomerated, fused or sintered

Alloying activity

Contribution to weld metal chemistry from flux

Basicity

Acid fluxes made from manganese oxide, silica, rutile

are easy to use

Basic fluxes (MgO, CaO, CaF2, Al2O3) provide excellent

toughness welds

Applications of SAW

Long straight welds in heavier material

Vessel longitudinal and circumferential welds

Flange to web joints of I beams

Flat or horizontal position

Flux has to be supported

Access has to be good

Gas shielded arc processes

Gas metal arc welding (GMAW)

Gas tungsten arc welding (GTAW)

Gas Tungsten Arc Welding

Alternative names -

GTAW,TIG (Tungsten

Inert Gas), Argonarc

Heat source is an electric

arc between a non-

consumable electrode and

the workpiece

Filler metal is not added or

is added independently

GTAW process outline

Ceramic

shroud

Torch

Gas lens

(optional)

Inert

gas

Power

source

Torch

lead (-)

Work

lead (+)

Filler Arc

Weld metal

Weld pool

Collet

Tungsten

electrode

Process features

Excellent control

Stable arc at low power (80A at 11V)

Independently added filler

Ideal for intricate welds eg root runs in pipe or thin sheet

Low productivity 0.5kg/h manual

High quality

Clean process, no slag

Low oxygen and nitrogen weld metal

Defect free, excellent profile even for single sided welds

Equipment for GTAW

Welding power source with constant current

characteristic

DC for most metals, AC for Al

Arc starting by high frequency (5000V, 0.05A)

Sequence timers for arc starting, arc finishing & gas

control

Water- or gas-cooled torch with tungsten

electrode

Electrode may contain thoria or zirconia, etc

Characteristics of Current Types for Gas Tungsten Arc Welding

Shielding gases

Torch is fed with an inert or reducing gas

Pure argon - widespread applications

Argon-helium - Higher arc voltage, inert

Argon-2% hydrogen - Cu alloys & austenitic steel

Torch gas must not contain oxygen or CO2

Backing (or purge) gas

Used for all single-sided welds except in carbon steel

Argon, nitrogen, former gas (N2 + H2)

Supplementary shielding

Reactive metals: Ti, etc

Gas filled chambers or additional gas supply devices

Filler metals

Autogenous welding (no filler)

Filler wire or rod of matching composition

C-Mn & low alloy steel

Stainless Steel

Al, Mg, Ti

Cu & Ni

Consumable inserts - filler preplaced in joint

GMAW and FCAW

Gas metal arc welding

(MIG, MAG, CO2 welding)

Flux cored arc welding

GMAW & FCAW processes

A continuous solid wire, small diameter

GMAW uses solid wire, no flux

FCAW uses flux-filled wire

Fed through the gun to the arc by wire

feeder.

The weld pool may be protected from

oxidation by shielding gas.

High productivity 3 kg/h or more

Direct current (DCEP mostly)

GMAW and FCAW outline

Base material Return Lead

+

_

Power

source

Torch gas

Weld Metal

Weld pool

Wire

feed

Wire feeder

GMAW & FCAW equipment

Welding power source

Wire feeder mechanism

May be in power source cabinet

Gun with gas supply & trigger switch

Manual (semiautomatic) guns

Automatic torches available

Can be fitted to robot etc

Consumables

Solid Wires (GMAW)

A wide variety of alloys are available

Flux cored arc welding (FCAW)

Gas shielded flux cored wires

Self-shielded flux cored wires

Used outdoors

Metal cored wires

Light flux cover

Torch gas mixtures

Inert gases (MIG)

Argon or helium or mixtures of these

Active base metals, Al, Mg, Ti

Active gases (MAG and FCAW)

Carbon dioxide

Argon plus oxygen and/or carbon dioxide

Nitrogen, hydrogen