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GMAW Fundamentals

Gas Metal Arc Welding

Metal Inert Gas


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Safety

Electrocution hazard

Skin burns from flying metal

Skin burns from direct light from arc

Skin burns from indirect light from arc

Cotton clothing and leather gloves

Helmet to protect eyes from light

Safety glasses when chipping slag

Ventilation to remove dangerous fumes Do not weld near water

Do not weld near combustible materials

Keep welding cables and jobs free grease

Protect bystanders from arc rays


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Introduction

GMAW is defined as arc welding using a

continuously fed consumable electrode and

a shielding gas.

GMAW is also known as MIG (Metal Inert

Gas).

Produces high-quality welds

Yields high productivity


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Advantages

Large gaps filled or bridged easily

Welding can be done in all positions

No slag removal required

High welding speeds

High weld qualityLess distortion of work piece


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Disadvantages

Hard to reach locations are less easily weldedbecause of bulky torch and cables

Wind or air drafts may compromise gas shielding

Reactive metals (i.e. titanium) may need specialshielding provisions

High heat may be uncomfortable to welders

Correct parameter selection learning needsdedicated training

Equipment is more complex and expensive thanthat of alternative processes


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Several tips must be consider in

selecting mode of transferType, intensity and polarity of welding

current

Electrode size

Electrode composition

Electrode extension

Shielding gas mix composition


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Types of Metal Transfer

The basic GMAW process includes three

distinctive process techniques:

1. Short Circuit (Short Arc)

2. Globular Transfer

3. Spray Arc Transfer


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Modes of GMAW Transfer


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Short Circuit (Short Arc)

Operates at low voltages and welding current

Small fast-freezing weld puddle obtained

Useful in joining thin materials in any position, aswell as thick materials in vertical and overhead

positions

Metal transfer occurs when an electrical short

circuit is established this cycle can repeat itself between 20 and as

much as 250 times per second.


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Short CircuitA - Electrode is short circuited to base metal. No

arc, and current is flowing through electrode wire

and base metal.

B - Resistance increases in electrode wire causing

it to heat, melt and neck down.

C - Electrode wire separates from weld puddle,creating an arc. Small portion of electrode wire

is deposited which forms a weld puddle.

D - Arc length and load voltage are at maximum.

Heat of arc is flattening the puddle and increasing

the diameter tip of electrode.

E - Wire feed speed overcomes heat of arc andwire approaches base metal again.

F - Arc is off and the short circuit cycle starts

again.


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Advantages

All-position capability, including flat, horizontal,

vertical-up, vertical-down and overhead.

Handles poor fit-up extremely well, and is capable

of root pass work on pipe applications.

Lower heat input reduces weldment distortion.

Higher operator appeal and ease of use. Higher electrode efficiencies, 93% or more.


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Limitations

Restricted to sheet metal thickness range and openroots of groove joints on heavier sections of basematerial.

Poor welding procedure control can result inincomplete fusion. Cold lap and cold shut areadditional terms that serve to describe incompletefusion defects.

Poor procedure control can result in excessivespatter, and will increase weldment cleanup cost.

To prevent the loss of shielding gas to the wind,welding outdoors may require the use of awindscreen(s).


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Globular Transfer

Welding current and wire speed are

increased above maximum for short arc

Droplets of metal have a greater diameter

than the wire being used

Spatter present

Welding is most effectively done in the flat

position when using globular transfer


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Globular transfer is often a

high voltage, highamperage, high wire feed

speed transfer, and is the

result of using CO2

shielding gas (or 75% AR-

25% CO2) with parameters

higher than the short-

circuiting range


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Advantages

Uses inexpensive CO2 shielding gas, but is

frequently used with argon/CO2 blends.

Is capable of making welds at very hightravel speeds.

Inexpensive solid or metal-cored electrodes.

Welding equipment is inexpensive.


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Limitations

Higher spatter levels result in costly cleanup.

Prone to cold lap or cold shut incomplete fusion defects,which results in costly repairs.

Weld bead shape is convex, and welds exhibit poor wettingat the toes.

High spatter level reduces electrode efficiency to a rangeof 8793%.

Less desirable weld appearance than spray arc transfer

Welding is limited to flat positions and horizontally filletwelds

Welding is limited to metal 1/8 inch (3 mm) or thicker


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Spray Arc Transfer

Occurs when the current and voltage settings areincreased higher than that used for GlobularTransfer

Used on thick sections of base material, bestsuited for flat position due to large weld puddle

Spatter is minimal to none

Uses 5% to 10% co2 mix with argon or oxygen.

>Forms very small droplets of metal>Very good stability>Very little spatter


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Spray arc transfer sprays a stream

of tiny molten droplets across thearc, from the electrode wire to the

base metal.

Spray arc transfer uses relatively

high voltage, wire feed speed andamperage values, compared to short

circuit transfer.


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Advantages

High deposition rates.

High electrode efficiency of 98% or more.

Employs a wide range of filler metal types in anequally wide range of electrode diameters.

Excellent weld bead appearance.

High operator appeal and ease of use.

Requires little post weld cleanup.

Absence of weld spatter.

Excellent weld fusion.

Lends itself to semiautomatic, robotic, and hard

automation applications.


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Limitations

Restricted to the flat and horizontal weldingpositions.

Welding fume generation is higher.

The higher-radiated heat and the generation of avery bright arc require extra welder and bystander

protection.

The use of axial spray transfer outdoors requires

the use of a windscreen(s). The shielding used to support axial spray transfer

costs more than 100% CO2.


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Pulse Spray Transfer

GMAW-P was developed for two demanding reasons:control of weld spatter and the elimination of incompletefusion defects common to globular and short-circuiting

transfer. The welding current alternates between a peak current and

a lower background current.

This faster-freezing weld puddle is what allows the pulsed-spray transfer to be used fort thinner metals,

better control on out-of-position work.

allows for larger wire sizes to be used on varied metalthicknesses.


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In pulse spray transfer (GMAW-P) the

welding power sources pulse controlpulses the welding output with

high peak currents (amperage) which are

set at levels which will cause the transfer

to go into a spray. The backgroundcurrent (amperage) is set at a level that

will maintain the arc,

but is too low for any metal transfer to

occur.


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Pulsed arc transfer


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Advantages Absent or very low levels of spatter.

More resistant to lack of fusion defects than other modesof GMAW metal transfer.

Excellent weld bead appearance and offers an engineeredsolution for the control of weld fume generation.

Reduced levels of heat induced distortion and tendency for

arc blow Ability to weld out-of-position and handles poor fit-up.

When compared to FCAW, SMAW, and GMAW-S,pulsed spray transfer provides a low cost high-electrodeefficiency of 98%.

Lends itself to robotic and hard automation applications.

Is combined for use with Tandem GMAW or othermultiple arc scenarios.

Capable of arc travel speeds greater than 50 inches perminute (1.2 M/min.).


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Limitations Equipment to support the process is more

expensive than traditional systems.

Blends of argon based shielding gas are

more expensive than carbon dioxide.

Higher arc energy requires the use of

additional safety protection for welders and

bystanders.

Adds complexity to welding.

Requires the use of windscreens outdoors.


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Manual GMAW Equipment

Three major elements are :

1.) Welding torch and accessories

2.) Welding control & Wire feed motor

3.) Power Source

GMAW equipment can be used either

manually or automatically


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GMAW Schematic Diagram


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WIRE CONTROL

&

WIRE FEED MOTOR

POWER SOURCE


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Welding Torch & Accessories

The welding torch guides the wire and

shielding gas to the weld zone.

Brings welding power to the wire also

Major components/parts of the torch are the

contact tip, shielding gas nozzle, gas

diffuser, and the wire conduit


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Others types of torch


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TRIGGER

INSTALLED

COMPONENTS

NOZZLE

CONTACT TIP

GAS DIFFUSER


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Welding Control & Wire

Feed MotorWelding control & Wire feed motor are

combined into one unit

Main function is to pull the wire from thespool and feed it to the arc

Controls wire feed speed and regulates the

starting and stopping of wire feed

Wire feed speed controls Amperage


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Types of Wire Feed Motor


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Types of WFM Roller


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Types of Wire Feeder

WIRE FEEDER


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WIRE FEEDER


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Power Source

Almost all GMAW is done with reversepolarity also known as DCEP

Positive (+) lead is connected to the torchNegative (-) lead is connected to the work

piece

Provides a relatively consistent voltage tothe arc

Arc Voltage is the voltage between the endof the wire and the work piece


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Contact Tip To Work Distance

In constant current,the CTWD (contact tip towork distance) determines the arc length.

As the CTWD increases the arc length increases, andas the

CTWD decreases the arc length decreases. Thispresented a

problem for semiautomatic welding because it is

difficult tomaintain the same CTWD. To compensate for this

problem an arc voltage controlled wire


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Constant voltage power source designs provide a specificarc voltage for a given pre-selected wire feed speed Thevolt-amp curve, or slope, is comparatively flat. As theCTWD increases with these types of power sources, thereis a decrease in the welding current. As the CTWDdecreases there is an increase in the welding current. Thearc in this case becomes a series circuit, and the CTWD

provides resistance to current. In either scenario, thevoltage remains the same and the arc length remains thesame


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POSITIVE

TERMINAL

NEGATIVE

TERMINAL


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Shielding Gases

Purpose of shielding gas is the protect theweld area from the contaminants in the

atmosphereGas can be Inert, Reactive, or Mixtures of

both

Gas flow rate is between 25-35 CFH

Argon, Helium, and Carbon Dioxide are themain three gases used in GMAW


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Properties of Gases

Affect the performance of the welding process

include:

1) Thermal properties at elevated temperatures. 2) Chemical reaction of the gas with the various

elements in the base plate and welding wire.

3) Effect of each gas on the mode of metal

transfer.


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Types of shielding gases

Inert Gas

1. Argon

2. Helium

3. Ar + He

Active Gas

1. Carbon Dioxide

2. Inert gas + any type of active gas


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Argonis an inert gas which is used both

singularly and in combination with other

gases to achieve desired arc characteristicsfor the welding of both ferrous and non-

ferrous metals.

Carbon DioxidePure carbon dioxide is not aninert gas, because the heat of the arc breaks down

the CO2 into carbon monoxide and free oxygen.

This oxygen will combine with elements

transferring across the arc to form oxides which are

released from the weld puddle in the form of slag

and scale.


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Heliumis an inert gas which is used on

weld applications requiring higher heat

input for improved bead wetting, deeper

penetration and higher travel speed. In

GMAW it does not produce as stable an arc

as argon. Compared to argon, helium has ahigher thermal conductivity and voltage

gradient


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FLOW METER

CYLCINDER

PRESSURE

GAUGE

CFH PRESSURE

ADJUSTMENT

KNOB


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Filler Wire


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Wire Chemistries


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GMAW Operation techniques

To setting WFS (Wire Feed Speed)


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Voltage-bead Change


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Electrode Stick-out


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ESO Setting

EOS


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EOS


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Gun angles and techniques


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Gun angles and techniques


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Direction of Travel


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