Gas Metal Arc Welding (GMAW) Flux Cored Arc Welding (FCAW) (MIG) metal inert gas - incorrect because not always inert gases used (MAG) - metal active gas

An electric arc between a continuously fed metal electrode and the base metal produces heat. The arc is shielded by a gas.

Used in production and repair shops auto body, agriculture

Popularity: low cost used on all commercially important metals such as al, magnesium, stainless, carbon, alloys steels, copper etc.

FCAW main difference from GMAW - electrode is hollow shielding gas is created from the flux inside the tubular electrode as it melts also used to add alloying elements some FCAW still requires additional shielding gas to be supplied those that don't - called shelf shielding electrodes

Equipment: a constant voltage DC welder is used with this process shielding gases (carbon dioxide, argon, helium) metal electrode wire electrode feed device cable to carry: wire, current and shielding gas to the arc torch

SAFETY - same as for SMAW with addition of gas cylinder safety: cap should be on cylinder when being moved or stored cylinders should be chained or secured to rigid object - wall etc. don't strike arc on cylinder store and use in upright position

Advantages of GMAW and FCAW: 1. continuous electrode - no stopping to change electrodes 2. no electrode stub loss 3. each pound of wire used, 92-98% becomes deposited weld metal (SMAW 60-70%) 4. no slag, just a very thin glass like coating over the weld bead 5. deeper penetration possible than with SMAW 6. less training and skill required

Disadvantages: 1. equipment costs more than SMAW 2. some joints hard to reach with welding gun 3. rapid air movement (strong wind) can blow shielding gas away from weld

Two adjustments are made on the welding machine by the operator 1. voltage 2. wire feed speed arc length,current, transfer method, and shielding gas are determined by the setting of these two parameters

Metal transfer occurs in two ways 1.short circuiting method (short arc) 2 metal transfer across the arc which includes: a. globular b. spray

1. Short circuit transfer: electrode touches molten pool, arc is no longer present, surface tension of pool pulls molten metal on the end of the electrode into the pool. repeats itself 100’s of times per second

Characteristics of Short Circuit sounds like bacon frying done with 100% CO2 or 75% CO2 and 25% argon CO2 is a cold gas so get low heat input minimizes distortion for thin materials in all positions

2. Spray transfer increase the current setting above the current required for short circuit - - Very fine droplets of metal form and travel at high speed directly through the arc stream to the weld pool.(100's of droplets per second)

Characteristics of spray hotter - higher voltage arc makes a hissing sound it’s fast, pours the metal down - used in production it is wet and hot - not good for out of position welds for 1/8” and thicker material must be 80% Argon or more for spray to occur -

There is an additional type of transfer called globular transfer occurs at a point between short circuit and spray metal transfers across the arc as large irregularly shaped drops

Continued: does not occur in U.S. because we can get Argon gas relatively expensive about 6X more expensive than CO2 but in other countries, Argon is too expensive to use so they use CO2 and increase the weld current above that for short circuit

New technique called pulse used to reduce fumes in production areas sounds like a buzzing or humming when used properly

GMAW power sources: AC generator or alternator with a DC rectifier AC transformer with a DC rectifier

DCEP most common DCEN seldom used - is used with only one electrode called an emissive electrode welding machine tries to maintain a CONSTANT VOLTAGE during welding while current varies widely AC is never used

Selecting electrode wire determined by: 1. base metal 2. shielding gas 3. metal transfer method 4. welding position electrodes designed for all positions are designated as (E71T-X) #1 indicates all positions

wire diameters: .030" to .0625" (1/16") large dia. wire - not used for out of position welds soft electrodes like al. and magnesium with small dia cannot be pushed through cable easily, must use push pull wire feeder - also use a Teflon liner.

Wire (electrode) Drive Unit Wire drive unit: pulls electrode from spool and pushes it to welding gun two mated rollers are located in the wire drive unit. one roller is driven by a electric motor

Drive unit continued: Amount of pressure - only enough to advance the wire without the rollers slipping. Too much force could flatten soft wire or crush flux cored wire. alignment of the wire guides alignment of the drive rolls

Birds nest

Cleaning electrode cable can use graphite powder in cable as lubrication

Shielding gasses: inert: Argon (Ar)(most common), Helium (He) reactive: Nitrogen (N2), Oxygen (O2), carbon dioxide (CO2) they will mix with other chemicals such as the metal in the weld joint reactive gasses gases are not used alone as shielding gases with exception of CO2

Gas selection consider: 1. type and thickness of base metal welded 2. amount of penetration 2. metal transfer method to be used 3. welding speed

Shielding gas flow rates: see table 13-31 for suggested flow rates too little gas flow = weld is not protected - will have a popping sound, spatter will occur, weld will have porosity

Shielding Gasses Cont. heavier shielding gases like CO2 and Argon will tend to drop away from the weld area when welding out of position so the rate must be increased may use a gas mixer or gasses can come premixed each shielding gas has a different effect on shape of the bead and penetration

Gas nozzles and contact tubes: gas nozzle - usually made of copper - come in various shapes and sizes electrode contact tube (inside gas nozzle) - made in many diameters for wire must fit tight enough to make a good sliding contact with electrode

Gas Nozzles Cont. gas nozzle and contact tube should be about EVEN end of torch gas nozzle becomes spattered, the flow of shielding gas will becomes turbulent causes contamination use a cleaning reamer or anti-stick compounds

Welding procedures

Preparing metal surface: mechanically or chemically groove angle for GMAW (30-45 deg) can be smaller than for SMAW (60-75 deg) (for of two reasons): 1. wire dia are smaller 2. GMAW penetrates better than SMAW

Electrode Extension: amount the end of the electrode wire sticks out beyond the end of the contact tube. maintain 1/4” to 1/2” extension for short circuit transfer method too much extension = increase in resistance and cause current to heat along the extended wire - causes spatter, shallow penetration and low weld bead shape

Tack weld or place in fixtures prior to welding

Welding speed determined by bead width and penetration

Torch angle 1. Use forehand torch position about 15 deg away from direction of travel

To start the arc, wire and gas, squeeze the trigger - no striking is necessary as in SMAW as the arc pool reaches the proper width, welder moves electrode forward

Torch Motion For MIG, don’t use whipping motion - moves out of weld pool

If welding without a runoff tab, the welder can move the electrode to the end of the weld then back over the completed bead about 1/2" to fill the crater

Glass like slag is produced from MIG can be cleaned with a wire brush

WHISKERS: lengths of electrode wire stick through the root side of a groove weld electrode wire is advancing ahead of the weld pool to fix reduce the wire feed or slow the welding speed

Automatic and semiautomatic GMAW and FCAW: semi - when operator holds and moves the gun and the electrode wire fed automatically full automatic: when the gun is moved by a machine or robot

GMAW SPOT welding: done on metals under 1/16" machine must be equipped with special controls