Gas Metal Arc Welding (GMAW)

• An arc welding process that uses an arc between a continuous filler metal electrode and the weld pool with shielding from an externally supplied gas and without the application of pressure

IntroductionGMAW is defined as arc welding using a continuously fed

consumable electrode and a shielding gas.

Arc can be shielded by an active (Co2) or inert (Ar, He).

If the inert gas is used then it is termed as MIG (Metal Inert Gas) and if shielding gas is active gas (like CO2 , mixture of CO2 +O2 or CO2+H2) GMAW is called as MAG (Metal Active Gas) welding.

Produces high-quality weldsYields high productivity

Vibrations in GMAW • Vibrations depending up on type of shielding gas

used, type of metal transfer, the type of metal welded ,and so on.

• MIG or MAG welding, fine wire welding, spray arc welding, pulse arc welding, CO2 welding, short-circuit arc welding and various trade names.

Principle of Operation• The gas metal arc welding uses the heat of an arc between a

continuously fed consumable electrode and the work to be welded.

• The heat of the arc melt the surface of the base metal and the end of the electrode.

• The metal melted off the electrode is transferred across the arc to the molten pool.

• An envelope of gas fed through the nozzle provides shielding of molten pool, the arc, and surrounding area.

PRINCIPLE OF OPERATION

It was developed for welding aluminum .Used for special metals for bearing surfaces and corrosion resistant surfaces.

PROCESS

• GMAW requires only that the operator guide the welding gun with proper position and orientation along the area being welded

• Keeping a consistent contact tip-to-work distance (the stickout distance) is important, because a long stickout distance can cause the electrode to overheat and will also waste shielding gas.

• Stickout distance varies for different GMAW weld processes and applications.

Short-circuit transfer = ¼” - ½” Spray transfer = ½”

• The orientation of the gun is also important • 45o for a fillet weld and • 90o degrees for welding a flat surface

The travel angle or lead angle is the angle of the torch with respect to the direction of travel, and it should generally remain approximately vertical.

GMAW Welds

AdvantagesHigh operating factor High deposition ratesHigh utilization of filler metalWelding can be done in all positionsAutomation is possibleNo slag removal required, No slag inclusionHigh welding speedsHigh weld qualityLess distortion of work piece

GMAW Vs SMAWPROCESS Heat Shielding Filler

MaterialPower Source

SMAW Electric Arc

Inert Gas

(Flux)

Stick Electrode

Constant Current

GMAW Electric Arc

Inert Gas Wire Electrode

Constant Voltage

GMAW can be used to weld the metal thickness from 0.13mm upward can be welded .

Types of Metal Transfer

1. Spray Arc Transfer2. Globular Transfer3. Short Circuit (Short Arc)4. Pulsed - Spray Arc Transfer

Methods of application

1. Semi-automatic

Welder provides manual travel and guidance of the welding gun

2. Automatic

Machine itself controls the travel and guidance of the welding gun

Position

1. Globular / CO2 process – Larger electrode – Flat & Horizontal

2. Spray arc process - Larger electrode - Flat &Horizontal

- Smaller electrode - Vertical & Over

head

3. Short – circuit and Pulsed spray - All positions

Weldable Metals & Thickness Range

• Almost all metals and alloys

• CO2 process is only for steels

• Almost all thicknesses

# 0.13mm onwards

JOINT DESIGN

• Almost all joint designs are possible

• As similar that of SMAW process

• But, low groove angle (45o instead of 60o)

GMAW EquipmentThree major elements are :

1) Welding torch and accessories2) Welding control & Wire feed motor3) Power Source4) Travel and guidance mechanism

GMAW equipment can be used either manually or automatically

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, and gas diffuser

Welding Control & Wire Feed Motor

Welding control & Wire feed motor are combined into one unit

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

Controls wire feed speed and regulates the starting and stopping of wire feed

Wire feed speed controls Amperage

WIRE FEEDER

WIRE CONTROL&

WIRE FEED MOTOR

POWER SOURCE

1. Constant Speed Feeder

Used only with a constant voltage (CV) power source

This type of feeder has a control cable that will connect to the power source

The control cable supplies power to the feeder and allows the capability of remote voltage control with certain power source/feeder combinations

The wire feed speed (WFS) is set on the feeder and will always be constant for a given preset value

2. Voltage-Sensing Feeder

Can be used with either a constant voltage (CV) or constant current (CC) - direct current (DC) power source

When set to (CV), the feeder is similar to a constant speed feeder

When set to (CC), the wire feed speed depends on the voltage present

The feeder changes the wire feed speed as the voltage changes

A voltage sensing feeder does not have the capability of remote voltage control

Power SourceAlmost all GMAW is done with reverse polarity

also known as DCEP

Provides a relatively consistent voltage to the arc Constant Voltage (CV) Power sources

Rectifier, Inverter or Generator

Current = 20 A – 750 A Voltage = 18 V – 50 V

POSITIVETERMINAL

NEGATIVETERMINAL

ELECTRODE

# Electrode selection is based primarily on

# the composition of the metal being welded

# the process variation being used

# thickness and joint design

# the material surface conditions

# Specifications / Service conditions

# All commercially available electrodes contain deoxidizing metals such as silicon, manganese, titanium, and aluminum in small percentages to help prevent oxygen porosity, and some contain denitriding metals such as titanium and zirconium to avoid nitrogen porosity

ELECTRODE CLASSIFICATION

E XX S - X

Electrodemin T.S (ksi)Solid electrode wireChemical Composition &

Shielding

E 70 S - 2

SFA – 5.18Carbon steel filler metals for Gas shielded arc welding

Chemical composition requirements for solid electrodes and rods

AWSClassification

C Mn Si Cu Ti Zr Al

ER70S-2 0.07 0.9-1.40 0.40-0.70 0.50 0.05-0.15 0.02-0.12 0.05-0.15

ER70S-3 0.06-0.15

0.90-1.40 0.45-0.75 0.50 …… ……. ……

ER70S-4 0.07-0.15

1.00-1.50 0.65-0.85 0.50 …… ……. ……

ER70S-5 0.07-0.19

0.90-1.40 0.30-0.60 0.50 ….. …… 0.50-0.90

ER70S-6 0.06-0.15

1.40-1.85 0.80-1.17 0.50 …… …….. ……..

ER70S-7 0.07-0.15

1.50-2.00 0.50-0.80 0.50 ……. …….. ……..

Shielding Gases

Purpose of shielding gas is the protect the weld area from the contaminants in the atmosphere

Gas can be Inert, Reactive, or Mixtures of both

Gas flow rate is between 25-35 CFH

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

FLOW METER

CYLCINDERPRESSUREGAUGE

CFH PRESSURE ADJUSTMENT KNOB

WELD PROFILES FOR DIFFERENT SHIELDING GASES

75% + 25%95% + 5% 25% + 75%

# Argon = Non – Ferrous metals & alloys# Helium = Non – Ferrous metals & alloys # CO2 = Steels

QUALITY OF WELD# High quality welds obtained than compared with SMAW

No flux melting required No / Low hydrogen process

# But it depends on,

Electrode wire and its cleanliness, Joint cleanliness Purity of the shielding gases Position of welding & Welding

procedure Presence of wind Skill of the welder

TIPS FOR USING THE PROCESS

# Semi-automatic welding using short-circuitmetal transfer is easy to learn and apply

# Stickout distance (or) Tip-to-Work distance should be controlled very properly

# Correct Nozzle-to-work distance 1 to 1.5 times inside dia of gas

nozzle

# Angle of gun nozzle Travel angle and Work angle

# Work angle = 45o - Fillet= 90o – Groove

# Travel angle may be “Push angle” or “Drag angle”

Push angle = Pure inert gas shielding Drag angle = CO2 shielding

# Equipment should be proper working condition

# For out of position welding, the weld pool should be remain as small as possible

# Polarity should be correct

# ULTIMATELY USE D.C power supply

COMPARISON BETWEEN THE VARIATIONS

Basic components of GMAW

1. Welding Power Source

2 . Electrode wire feed control unit

3 . Welding Torch

4 . Shielding Gas

5 .cooling system (water or air)

6 .Travel mechanism for automatic welding