Gas Metal Arc Welding GMAW Section 8 Unit 26

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Gas Metal Arc Welding (GMAW)Section 8Unit 26
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Introduction

• GMAW is an arc welding processes.
• Same three requirements
• Also known as Metal Inert Gas (MIG).
• Sometimes referred to as semiautomatic arc
  welding
• It was developed for production welding
• More difficult than SMAW to switch sizes of metal
  and types of metal.
• GMAW is best process for welding thin steel

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GMAW Safety

• Follow manufacturers recommendations
• Check all cables
• Wear appropriate PPE
• Never touch electrical or welding wire when the
  switch is on.
• Never weld in wet locations or when wet.
• Use pliers for hot metal
• Insure adequate ventilation.
• Have machine repaired by competent person.
• Turn off and safety store welder when done
  welding.

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Introduction

• The three requirements of fusion welding and how
  GMAW meets these requirements.

Heat
Shielding
Filler Material
Process
Outer Envelope
Manual Rod Or Base Metal
OFW
Flame
Electric Arc
Inert Gas from flux
Manual Electrode
SMAW
Inert Gas from cylinder
Continuous wire Electrode
Electric arc
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GMAW Process

• A continuous wire electrode from a spool or coil
  is pushed by the wire feeder through a flexible
  conduit to the welding gun.
• The wire exits the welding gun through the
  contact tip.
• A constant potential power supply supplies the
  current for an electrical arc that is hot enough
  to melt the base metal and the electrode.
• The wire melts and combines with the molten base
  metal to form a weld.
• Shielding is provided by an inert gas supplied by
  a cylinder
• Gas flow is controlled by a solenoid valve and is
  delivered to the nozzle on the gun.

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GMAW System Parts
Cylinder valve
Flow meter
Safety nut
Drive roll tension adjustment
Wire spool
Cylinder
Wire guide
Volt and/or amp meters
Wire spool Over run adjustment
Gun
Gas hose
Feed rolls
Welding current adjustment
Ground clamp
Welding current taps
Ground Lead
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GMAW Gun
Welding Wire
Insulation ring
Gas orifice
Liner
Nozzle
Gas diffuser
Contact tube
Trigger
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Flow meter

• GMAW systems use a flow meter to provide a
  constant flow rate (ft3/min) of shielding gas.
• Two types of flow meters are used based on the
  principle that to vary the flow rate of a gas you
  must either change the pressure, or change the
  metering orifice size.
• One piece the flow is regulated by using a fixed
  orifice and changing the pressure to change the
  flow rate.
• Two piece the flow is regulated by using a
  constant pressure regulator and changing the flow
  rate with an adjustable orifice (needle valve).

• The two piece flow meter does a better job of
  maintaining a constant flow rate.

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Methods of Controlling The Weld

• Semiautomatic
• Equipment controls the feeding of the electrode
  to a hand held welding gun.
• The operator sets the machine, positions the gun,
  starts and stops the process by a trigger on the
  gun, and controls the movement of the gun.
• Operator must maintain the correct stickout, gun
  alignment with the joint and welding speed.

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Methods--cont.

• Automatic
• Operator positions parts and programs machine.
• Robotic equipment positions the gun and controls
  movement.

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Methods--cont.

• Mechanized
• Gun is mounted in a carriage or other device that
  positions it in the weld and manipulates it.
• The operator is responsible for operating the
  controls that start and stop the weld and moving
  the gun along the joint.
• MIG gun may be stationary and the work moves pass
  beneath it.

• The machine controls the gun position, stickout
  and welding speed.

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Summary of Methods
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GMAW Processes

• The GMAW process provides four different metal
  transfer methods.
• Short circuiting
• Spray
• Globular
• Pulse spray

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Short Circuiting Transfer

• When the gun trigger is activated, the wire
  starts to move toward the metal.

When the wire touches the metal it shorts out.
The wire touches the metal and the process
continues
The high current flow melts the end of the wire
and a droplet is formed.
As the wire continues to move, the arc restarts.
The current pinches off the droplet and the arc
stops.

• Process is recognizable by its sharp buzzing or
  crackling sound
• Most common method

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Spray Transfer

• Requires high current. The higher current
  density of the spray process causes the wire to
  melt into small droplets (less than the diameter
  of the wire).
• The arc forces propels the droplets to the metal.
• The puddle is larger and more fluid.
• The high current density results in deeper
  penetration.

• High current results in high deposition rate.

• Process results in a hissing or whispering sound.
• Preferred process for thick metal in the flat
  position.

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Globular Transfer

• Low current density results erratic arc and
  globules of metal larger than the diameter of the
  wire.
• Minimum penetration.
• Excessive spatter
• Use is limited to submerged arc welding.

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Pulse Spray Transfer

• Process starts as short circuiting, but instead
  of the current peak staying on when the wire is
  shorted, the current is lowered after a period of
  time to a background current.
• Produces a heating and cooling effect like short
  circuiting.
• Cooling improves puddle control, spray period
  produces deep penetration.

A Peak Current B Background Current C Peak
Time D Pulses per Second
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Stickout

• An important function of all GMAW processes is
  the stickout.
• Stickout is the distance from the end of the
  contact tube to the weld surface.
• Insufficient stickout will cause the wire to fuse
  to the contact tube.
• Excessive stickout will cause the wire to
  overheat and it will melt into irregular pieces.

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FCAW
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Introduction

• FCAW (Flux Core Arc Welding) uses a hollow wire
  that is filled with flux.
• Same transfer methods as GMAW.
• Two types
• Externally shielded
• Self shielded

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External Shielded

• Requires the same gas equipment used in the GMAW
  process.
• Less cleaning is required when compared with self
  shielded wire.
• Produces a large volume of fumes
• Weld must be protected from air currents and
  drafts.
• Small diameter wires, all positions.
• The combination of flux core and external
  shielding allows welding engineers to customize
  the shielding environment for specialized metals.

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Self Shielded

• Shielding is provided by flux in the core of the
  wire.
• Produces a large volume of fumes.
• Can withstand more wind and draft than external
  shielded wire.
• Will have slag to remove.
• Does not require the use of a nozzle.
• Nozzle is usually left on to help maintain
  stickout.

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GMAW Equipment
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GMAW Equipment

• Power source
• Wire feeder and controls
• Welding gun and components
• Shielding gas delivery system
• Weld cleaning and finishing tools

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Power Supply

• GMAW and FCAW both use a constant voltage power
  supply.
• Many different types are available.
• Industrial rate are usually 100 duty cycle.
• Non industrial power supplies have sloping duty
  cycle.
• May have a built in heat sensitive overload
  switch.
• Install power supply so that the cooling air
  through the machine is not restricted.

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Selecting Power Supply

• Seven factors to consider when selecting a GMAW
  or FCAW power supply.
• Type of metal
• Metal thickness range
• Type of metal transfer
• Wire diameter range
• Primary power available
• Capacity of power source
• Control and adjustments

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Wire Feeding systems

• Function is to deliver the welding wire at a
  constant rate to the gun.
• Three systems
• Push
• Pull
• Push--pull
• Two types
• Constant speed
• Voltage sensing
• Two types of construction
• Internal
• External

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Wire Drive Systems

• The drive system is responsible for pulling the
  wire from the spool and then pushing the wire
  down the cable.

• The wire drive systems consisted of one or more
  pair of feed rolls and wire guides.
• The guides must be the proper size and adjusted
  close to the feed rolls and aligned with gap
  between the rolls.

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Feed Rolls

• For a GMAW system to work correctly, the feed
  rolls must be the correct type and size.
• The feed roll tension must also be set correctly.

U Groove
V Groove
Knurled V groove
Cogged U groove
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Welding Gun

• Function is to deliver the shielding gas, wire
  and welding current to the weld pool.
• MIG guns are rated on amperage capacity.
• The reason is heat.
• 400 amps are water cooled.
• Welding wire is delivered to the gun by the
  liner.
• Liner must sized to the welding wire.
• Small liner excessive friction which may cause
  the wire to seize.
• Large liner wire can buckle and spring forward
  causes erratic wire feed.
• Liner must be correct length.

• Cable
• Insulation ring
• Gas diffuser
• Contact tube
• Wire
• Nozzle

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Welding Gun--cont.

• Contact tube
• Sliding electrical connection.
• Must match the diameter of the wire
• Must be replaced frequently
• The nozzle directs the shielding gas towards the
  weld pool.

Nozzle
Contact tube
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Consumables

• Shielding gas
• Carbon dioxide
• Requires special valve
• Must use washer between valve and flow meter.
• Argon
• Mixture
• Welding wire
• Solid (Copper clad)
• Flux core
• Stainless steel
• Hard surfacing
• Aluminum

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Questions