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Chapter 10
Gas Metal Arc Welding
Equipment, Setup, and Operation
© 2012 Delmar, Cengage Learning
Objectives
• List the various terms used to describe gas metal
arc welding
• Discuss the various methods of metal transfer
including the axial spray metal transfer process,
globular transfer, pulsed-arc metal transfer,
buried-arc transfer, and short-circuiting transfer
GMAW-S
• List shielding gases used for short-circuiting,
spray, and pulsed-spray transfer
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Describe the more commonly used GMA welding
filler metals
• Define deposition efficiency, and tell how a welder
can control the deposition rate
• Define voltage, electrical potential, amperage, and
electrical current as related to GMA welding
• Tell how wire-feed speed is determined and what
it affects
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Discuss how the GMAW molten weld pool can be
controlled by varying the shielding gas, power
settings, weave pattern, travel speed, electrode
extension, and gun angle
• Describe the backhand and forehand welding
techniques
• List and describe the basic GMAW equipment
• Explain how the arc spot weld produced by
GMAW differs from electric resistance spot
welding and the advantages of GMA spot welding
© 2012 Delmar, Cengage Learning
Introduction
• 1920s: metal arc welding process using an
unshielded wire was being used
• 1948: first inert gas metal arc (GMA) welding
process developed
– Used to weld aluminum using argon gas for
shielding
• Later, carbon dioxide and dioxide were used as
shielding gases
• May be semiautomatic, machine, or automatic
© 2012 Delmar, Cengage Learning
Weld Metal Transfer Methods
• Several modes of transferring filler metal
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Short-circuiting transfer (GMAW-S)
Axial spray transfer
Globular transfer
Pulsed-arc transfer (GMAW-P)
• Selecting the mode depends on:
– Welding power source and type of shielding gas
– Wire electrode size and material type and thickness
– Best welding position
© 2012 Delmar, Cengage Learning
Short-Circuiting Transfer GMAW-S
• Low currents allow liquid metal at electrode tip to
be transferred
– Direct contact with molten weld pool
• Most common process used with GMA welding:
– On thin or properly prepared thick sections of
material
– On a combination of thick to thin materials
– With a wide range of electrode diameters
– With a wide range of shielding gases
© 2012 Delmar, Cengage Learning
Globular Transfer
• Generally used on thin materials and at a very low
current rang
– Transfers molten ball metal across arc
– Little control over where metal lands
• Rarely used alone
• Used in combination with pulsed-spray transfer
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Axial Spray Metal Transfer
• Wire tip projects very small drops
– Projected axially across arc gap to molten weld
pool
• Hundreds of drops per second
• Drops are propelled by arc forces at high velocity
• Spray transfer process conditions
– Argon shielding
– DCEP polarity
– Transition current
© 2012 Delmar, Cengage Learning
FIGURE 10-5 Axial spray metal transfer. Note the pinch effect
of filler wire and the symmetrical metal transfer column.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Pulsed-Arc Metal Transfer
• Dual pulsed current
– Pulse of high current: axial spray transfer mode
• Lower pulse of current: should not transfer any weld
metal
• Advantage
– Ease of controlling the weld
• Synergic systems
– Interlock power supply and wire feeder
© 2012 Delmar, Cengage Learning
Pulsed-Arc Metal Transfer
Current Cycle
FIGURE 10-7 Mechanism of pulsed-arc spray transfer at a low average current.
© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Pulsed-Arc Metal Transfer
Current Cycle (cont’d.)
• Components
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Ramp up
Overshoot
High pulse current
High pulse time
Ramp down
Step-off current
Background current
Pulse width
• Advantages
– Lower average currents
– All position
– Less distortion
– Reduced spatter
– High-quality welds
– Several others
• Disadvantages
• Cost and complexity
© 2012 Delmar, Cengage Learning
Shielded Gases for Spray
or Pulsed-Spray Transfer
• Axial spray transfer
– Required: shielded gas containing argon
• Helium/argon mixtures may contain as much as 80%
helium
• Adding small amounts of oxygen
– Provides a stable site for the arc
• Amount of oxygen needed to stabilize arcs in steel
varies with the alloy
© 2012 Delmar, Cengage Learning
Buried-Arc Transfer
• Carbon dioxide is very forceful
– Wire tip can be driven below surface of molten weld
pool
– Spatter produced by the arc: trapped in the cavity
• Useful for high/speed mechanized welding of thin
sections
– Compressor domes for hermetic air-conditioning
and refrigeration equipment or automotive
components
© 2012 Delmar, Cengage Learning
GMAW Filler Metal Specifications
• Key points
– GMA welding filler metals: available for a variety of
metals
– Some steel wire electrodes have a thin copper
coating
• Protects electrode from rusting
• Improves electrical contact
• Burns off or is diluted into weld pool
© 2012 Delmar, Cengage Learning
Wire Melting and Deposition Rates
• Wire melting rates, deposition rates, and wire feed
speeds
– Affected by the same variables
• Wire melting rate: measured in inches per minute or
pounds per hour
• Deposition rate: nearly always less than melting rate
• Deposition efficiency: ratio of amount of weld metal
deposited to wire used
© 2012 Delmar, Cengage Learning
Welding Power Supplies
• Important terms
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Voltage
Electrical potential
Amperage
Electrical current
• GMAW power supplies
– Constant-voltage, constant potential-type machines
• SMAW power supplies
– Constant-current-type machines
© 2012 Delmar, Cengage Learning
Speed of the Wire Electrode
• Selected in inches per minute (ipm)
– Wire speed control dial: used to control ipm
• To accurately measure wire-feed ipm:
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Snip off wire at contact tube
Squeeze trigger for six seconds
Release and snip off the wire electrode
Measure number of inches of wire that was fed
Multiply its total length by ten
© 2012 Delmar, Cengage Learning
Power Supplies for Short-Circuiting
Transfer
• There is a slight decrease in voltage as amperage
increases
– Rate of decrease is called slope
• Voltage decrease per 100-ampere increase
– Slope is called volt-ampere curve
• Machine slope is affected by circuit resistance
• Slope increases: short-circuit current and pinch effect
are reduced
© 2012 Delmar, Cengage Learning
Molten Weld Pool Control
• GMAW molten weld pool can be controlled by
varying several factors
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Shielding gas
Power settings
Weave pattern
Travel speed
Electrode extension
Gun angle
© 2012 Delmar, Cengage Learning
Shielding Gas
• Shielding gas selected affects the weld
– Method of metal transfer, speed, weld contour, etc.
– Also consider metal to be welded
• Commonly used shielding gases
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Argon
Argon gas blends
Helium
Carbon dioxide
Nitrogen
© 2012 Delmar, Cengage Learning
FIGURE 10-15 Effect of shielding gas on weld bead shape.
© Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Power Settings
• Weld bead is affected by several factors
– Power settings
– Voltage
– Amperage
• Welds require a balance of voltage and amperage
– Wire-feed speed affects amperage
– Increasing voltage changes arc length
© 2012 Delmar, Cengage Learning
Weave Pattern
• GMA welding process
– Greatly affected by electrode tip and weld pool
location
• Keep arc and electrode tip directed into molten weld
pool
• Most of the SMAW weave pattern can be used for
GMA welds
© 2012 Delmar, Cengage Learning
Travel Speed
• Location of arc inside molten weld pool is
important
– Speed cannot exceed ability of arc to melt base
metal
• Too high: results in overrunning of weld pool
• Too low: can restrict fusion to base plate
© 2012 Delmar, Cengage Learning
Electrode Extension
• Distance from contact tube to arc
– Measured along the wire
• Adjustments in this distance
– Cause a change in resulting wire bead
• GMA welding currents are relatively high
– Length of wire increases: voltage increases
© 2012 Delmar, Cengage Learning
FIGURE 10-16 Electrode-to-work distances. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning
Gun Angle
• Relation of the gun to the work surface
– Can be used to control the weld pool.
• Forehand/perpendicular/backhand welding
– Forehand technique: pushing the weld bead
– Backhand welding: dragging the weld bead
– Perpendicular: gun angle is at approximately 90°
to work surface
© 2012 Delmar, Cengage Learning
Equipment
• Basic GMAW equipment
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Gun
Electrode feed unit
Electrode supply
Power source
Shielded gas supply
Control circuit
Related hoses, liners, and cables
© 2012 Delmar, Cengage Learning
Power Source
• May be transformer-rectifier or generator type
– Transformers
• Stationary
• Commonly require a three-phase power source.
– Engine generators
• Ideal for portable use or where sufficient power is not
available
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit
• Push-type feed system
– Wire rollers clamped securely against the wire to
push it through the conduit
– Rollers have smooth or knurled U-shaped or Vshaped grooves
– Soft wires are easily damaged by knurled rollers
– Difference between push-type and pull-type: size
and location of drive rollers
– Electrode must have enough strength to be pushed
through the conduit without kinking
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Pull-type feed system
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Smaller higher-speed motor located in the gun
Wire moves through conduit
Possible to move soft wire over great distances
Gun is heavier and more difficult to use
Rethreading wire takes more time
Operating life of motor is shorter
Not commonly used
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Push-pull-type feed system
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Synchronized system
Motors located at both ends of electrode conduit
Can be used to move any type of wire
Ability to move wire over long distances
Faster rethreading
Increased motor life
System is more expensive
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Linear electrode feed system
– Does not have gears or conventional-type rollers
– Uses a small motor with a hollow armature shaft
– Changing roller pinch changes speed at which the
wire is moved
– Bulky system of gears is eliminated
– Motor operates at a constant high speed
– Wire may become twisted as it is moved through
the feeder
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Spool gun
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Compact, self-contained system
Allows welder to move freely
Major control system is mounted on welder
Feed rollers and motor are found in the gun
Very soft wires can be used
Small spools of wire required: often expensive
Guns are small but feel heavy
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Electrode conduit
– Guides welding wire from feed rollers to the gun
– Power cable and gun switch circuit wires are
contained in a conduit
– Steel conduit may have a nylon or Teflon liner
– Failure to attach conduit can cause misalignment
© 2012 Delmar, Cengage Learning
Electrode (Wire) Feed Unit (cont’d.)
• Welding gun
– Attaches to end of power cable, electrode conduit,
and shielded gas hose
– Trigger switch starts and stops weld cycle
– Contact tube transfers welding current to electrode
– Gas nozzle directs shielded gas onto weld
© 2012 Delmar, Cengage Learning
GMA Spot Welding
• GMAW spot weld
– Starts on one surface of one member and burns
through the other
• Fusion occurs and small nugget is left on metal
surface
• Allows welds to be made:
– In thin-to-thick materials
– When only one side of the materials to be welded is
accessible
– When there is paint on the interfacing surfaces
© 2012 Delmar, Cengage Learning
Summary
• Keys to producing quality GMA welds
– Equipment, setup, and adjustments
• Advantage of GMA welding process
– Ability to produce long, uninterrupted welds
• Selecting proper method of metal transfer
– Normally done by shop foreman or supervisor
• Welder must be proficient with each method of
metal transfer
– Practice and develop skills
© 2012 Delmar, Cengage Learning