Gas Metal Arc Welding

(Metal Inert Gas )(MIG)

GMAW FUNDAMENTALS

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

ADVANTAGES Large gaps filled or bridged easily

Welding can be done in all positions

No slag removal required

High welding speeds

High weld quality

Less distortion of work piece

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

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, as well as thick materials in vertical and overhead positions

Metal transfer occurs when an electrical short circuit is established

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

SPRAY ARC TRANSFER

Occurs when the current and voltage settings are increased higher than that used for Globular Transfer

Used on thick sections of base material, best suited for flat position due to large weld puddle

Spatter is minimal to none

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

GMAW COMPONENTS • DC or Direct Current power supply

• Electrode or wire feed controller

• Wire drive roller assembly

• Shielding gas source (cylinder) & regulator

• Manually held Gun & ground clamps

• Wire reel

GMAW COMPONENT DIAGRAM

WIRE CONTROL&

WIRE FEED MOTOR

POWER SOURCE

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

TRIGGER

INSTALLED

COMPONENTS

NOZZLE

CONTACT TIP

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

POWER SOURCE

Almost all GMAW is done with reverse polarity also known as DCEP

Positive (+) lead is connected to the torchNegative (-) lead is connected to the work pieceProvides a relatively consistent voltage to the arcArc Voltage is the voltage between the end of the

wire and the work piece

POSITIVETERMINAL

NEGATIVETERMINAL

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 bothGas flow rate is between 25-35 CFHArgon, Helium, and Carbon Dioxide are the main

three gases used in GMAW

FLOW METER

CYLCINDERPRESSURE

GAUGE

CFH PRESSURE ADJUSTMENT

KNOB

SHIELDING GAS

• Air in the welding zone is displaced by inert gas to “Shield” the molten weld pool and prevent it from contamination from Oxygen, Nitrogen and Water present in the atmosphere.

• Insufficient gas flow will not displace the atmosphere resulting in “porosity” or voids in the deposited weld.

• Flow is measured in CFH (Cubic Feet per Hour).

INSUFFICIENT SHIELDING GAS COVERAGE

• Gas not turned on

• Flow rate not properly adjusted

• Leaks in the hose supplying the shielding gas to the machine

• GMAW / MIG Gun loose at wire drive connection

• Spatter buildup on gas cup

• Windy environment

EXCESSIVE GAS COVERAGE

• Will cause porosity.

• The turbulence caused by the rapid flow of shielding gas exiting from the gas cup will draw the surrounding atmosphere into the stream of gas.

• It will reduce weld pool temperatures causing decreased penetration.