Gmaw Thumb Rule
Transcript of Gmaw Thumb Rule
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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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THANK YOU