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Gas Metal Arc Welding (GMAW)

Definition

GMAW: A semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are delivered to the weld through a welding gun.

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GMAW Hazards

1 Electrical shock

2 Hot metal

3 Harmful rays

4 Hot spatter

5 Fumes

6 Shielding gas

7 Pinch points

How is the risk of these

hazards managed?

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Introduction

GMAW has the same three requirements of all arc welding.

Shielding Heat Filler Material

GMAW

How does GMAW meet these requirements?

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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.

2. The high current flow melts the end of the wire and a droplet is formed.

3. The current pinches off the droplet and the arc stops.

1. When the wire touches the metal it shorts out.4. As the wire

continues to move, the arc restarts.

5. The wire touches the metal and the process continues

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

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Spray Transfer

Requires high current density.

Would the process work better for thin metal or thick metal?

What is current density?

The 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.

What are the characteristics of the spray transfer method?Why?

Is the spray transfer method recommended for all welding positions?

Why, Why not?

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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

In the pulse spray transfer the current alternates from the desired welding current to a background current.

The background current just maintains the arc.

The peak current, time of peak current, background current and time of background current are all adjustable.

A = Peak Current

B = Background Current

C = Peak Time

D = Pulses per Second

What are the advantages of the metal transfer method?

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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.

What is a potential problem of insufficient stickout?

What is a potential problem of Excessive stickout?

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GMAW Equipment

Equipment Requirements

1 Store and deliver gas to the weld at the correct rate.

2 Store and deliver welding wire to the weld at the correct rate.

3 Provide correct current.

4 Automatically or semi automatically control all of the features.

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GMAW

Components

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What is the function

of each part?

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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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GMAW System Parts

A. Hand wheelB. Flow meterC. Wire spoolD. WireE. Feed roll tension

F. Voltage meterG. Wire speed meterH. Wire speed adjustmentI. Voltage adjustmentJ. Power switchK. Gun cableL. GunM. Ground clampN. Ground cableO. Feed rollsP. Wire guideQ. Wire overrunR. Safety nut

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GMAW Gun

BC

H

EG

A

DF

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Flow meter

Two types of flow meters are available.

One piece: the gas flow rate is regulated by using a fixed orifice and changing the pressure to change the flow rate.

Two piece: the gas flow rate is adjusted by using a constant pressure regulator and changing the flow rate with an adjustable orifice (needle valve).

Which type will provide better control of the gas flow?

What is the purpose of the flow meter?

Why?

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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.

Gun end cutaway

1. Cable

2. Insulation ring

3. Gas diffuser

4. Contact tube

5. Wire

6. Nozzle

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Welding Gun--cont.

Contact tubeSliding electrical connection.

Must match the diameter of the wire

Must be replaced frequently

The nozzle directs the shielding gas towards the weld pool. Contact tube

Nozzle

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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.

1 Type of metal

2 Metal thickness range

3 Type of metal transfer

4 Wire diameter range

5 Primary power available

6 Capacity of power source

7 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 systemsPush

Pull

Push--pull

Two typesConstant speed

Voltage sensing

Two types of constructionInternal

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.

V GrooveKnurled V groove

U Groove

Cogged U groove

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Consumables—Shielding Gases

Carbon dioxide Requires special valve Must use washer between valve

and flow meter.

Argon Same valve as helium

Mixture Same valve as argon

Consumables—Welding wire

Solid (Copper clad)

Flux core

Stainless steel

Hard surfacing

Aluminum

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What three wire diameters are most popular?

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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 typesExternally shielded

Intershielded

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Self Shielded

• Shielding is provided by flux in the core of the wire.

• Produces a larger volume of fumes.

• Resistant to contamination caused by sidedrafts.

• Will have slag to remove.

Does this fume removal system meet safety codes?

Self Shielded--gun

Does not require the use of a nozzle.

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What is this?

GMAW gun FCAW gun

What are the advantages of not using a nozzle?

What are the disadvantages of not using a nozzle?

No nozzle?

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Questions