GAS METAL ARC WELDING
The term Gas Metal Arc Welding is the American Welding Society’s preferred name for this semi-automatic welding process that uses a wire feeder to deliver the filler metal to a hand operated gun to produce the weld.
The process is also widely known by the shop name MIG (Metal Inert Gas). The name Metal Inert Gas was used when the process was first developed to weld Aluminum using an inert (chemically non reactive) gas supply. The process has evolved to become a favorite choice for welding steel with gases that are not inert.
When compared with Stick welding, the mig welding process is faster, easier, and requires little cleanup of welds. This makes Mig welding cost effective for production welding in fabrication shops. The wire fed welding arc is capable of joining thin sections and bridging gaps in poor fit up situations.
THE MIG WELDING CIRCUIT
Welding is done by using a constant voltage welding machine to supply the power, a wire feed unit with an attached gun to feed the filler wire to the arc, and a gas supply system to shield the weld area.
MAKING THE WELD
The voltage, wire speed and gas flow are set by the welder according to recommended ranges for the application before welding.
After positioning the gun, the welder pulls and holds the trigger to start the gas flow and the arc. The welder then controls the nozzle distance from the work, the angle of the gun, and rate of travel speed across the joint. At the end of the joint the trigger is released to stop the wire feed, gas flow, and break the arc.
TYPICAL MIG GUN
MAJOR EQUIPMENT FOR MIG WELDING
In addition to the safety clothing and hand tools generally used in the welding trade the major parts of the Mig welding process are:
1. THE WELDING MACHINE
2. THE WIRE FEED UNIT
3. WELDING GUN
4. SHIELDING GAS SUPPLY
1. THE WELDING MACHINE
Although there are many styles and designs of MIG welding machines produced by manufacturers, they serve the same basic function. The welding machine is classified as a constant voltage Machine. This means the voltage remains relatively constant as set on the machine, while the amperage increases or decreases according to the arc length, in other words the distance of the nozzle and wire from the work.
The Amperage in Mig welding is controlled by the wire speed setting on the wire feed unit. The welding machine is usually set to provide DC reverse polarity current.
Some Mig machines are designed with an enclosed wire feed unit, while others have a separate wire feed unit attached.
TYPICAL MIG MACHINE WITH WIRE FEED ENCLOSED
Smaller portable units are available for light shop work or home use. These units typically use a small one pound wire spool.
The Miller Dimension 400 Combination Mig, Stick and Tig Machine used at the School is shown below with a description of the controls used for mig welding.
1. ON: BLACK PUSH BUTTON.
This is a push button that when depressed starts the machine.
2. OFF: RED PUSH BUTTON.
This push button stops the machine.
3. CURRENT SELECTOR.
This lever selects either CC. Constant Current used for Stick and Tig Welding or CV Constant Voltage used for Mig Welding. Make sure the lever is pushed up for CV when Mig Welding.
4. ARC CONTROL.
The Arc Control setting is not used for mig welding and is disabled when in the CV mode for Mig Welding.
5. VOLTS OR AMPERAGE DIAL.
This Dial shows amperage intervals in regular numbers and Voltage settings in the white squares. For example: 150 Amps or approximately 18 - 20 Volts. Remember when mig Welding to read the numbers in the white square.
6. VOLT AND AMP METERS.
These meters show the amperage and voltage during welding.
7. REMOTE CONTACTOR SWITCH.
The remote contactor switch should be set to ON to enable the Mig Gun trigger as the remote contactor.
8. REMOTE CONTROL SWITCH.
The remote control switch is used for Tig Welding with a Foot pedal, and should be in the OFF position for Mig Welding.
9. MIG FEEDER CONNECTION.
The wire feed unit is connected to this receptacle for Mig Welding.
2. THE WIRE FEED UNIT
The wire feed unit may be placed on the machine, located close to the machine or built in to the machine depending on manufactures style and type of machine. Although styles may differ they serve the same basic function. The wire feed unit supplies a constant and smooth rate of filler wire from a spool mounted on the back of the unit. A gas cylinder with attached regulators is hooked up to the wire feed units solenoid to supply the gas shielding. The filler wire and gas are fed to the Arc by the attached gun.
The wire feed unit used with the Dimension 400 Welding machine is the Miller model S-52 located on the top of the machine. This unit is used to describe the controls on a typical wire feed unit.
The pictures below show the controls of the Miller S-52 wire feed unit.
1. WIRE SPOOL
The wire spool shown holds about 30 pounds of wire when full and sits on the wire spool hub at the rear of the wire feed unit. When the gun trigger is pulled, the wire is drawn from the underside of the spool through the guide rolls and the gun to the weld zone.
The Type of wire shown, and used at the school, is classified as AWS ER-70S-6 used for welding mild steel structural shapes. The wire size is 0.035.
3. PRESSURE SPRING
The pressure spring clamps the wire between the guide rolls to feed the wire through the gun. The pressure can be increased or decreased using the thumb screw on the clamp bridge. Too little pressure may cause the wire feed to slip or skip, while too much pressure may flatten the wire or cause it to freeze up.
4. GUIDE ROLLS
The Guide Rolls, part of the wire feed system, are a set of upper and lower rollers with grooves that pull the wire from the spool and drive it through the gun. The rollers are sized to match the wire used. The guide rolls and wire used above are size 0.035. To change the guide rolls: The adjusting spring is released and the gear cover flipped back to access the guide rolls retaining screws, the screws are removed and the rolls are pulled out. Once the new rolls are installed the retaining screws are replaced.
The gun plugs into the wire feed system to accept the wire and gas supply and deliver them to the welding arc.
6. TRIGGER PLUG
The trigger plug is attached to the trigger receptacle to enable the gun to remotely activate the wire feed and gas flow.
7. SLOW-FAST WIRE START SWITCH
The Slow-Fast Wire Start Switch is used to set the initial wire flow start as either a slow start or fast start when starting the Arc. After the Arc is started the wire speed reverts to the speed set on the Wire Speed Control.
8. GAS PURGE BUTTON
The Gas Purge Button is used to purge the gas from the gun when the gas supply has been turned off. Pressing and holding this button allows the gas flow to be adjusted without starting the wire flow or the arc.
The Fuse protects the Wire Feed Unit from overload or an internal short.
The Breaker is a Circuit Breaker that protects the Motor of the wire feed unit from overload.
11. ON-OFF SWITCH
This Switch is used to activate the wire feed unit and turn it off at the end off the class session. The switch must be off when switching to Constant Current mode and Stick welding to prevent the gun from inadvertently arcing if it comes into contact with the work surface.
12. TRIGGER PLUG FRONT VIEW
Another view of the Trigger Plug attached to the trigger receptacle
13. WIRE SPEED CONTROL
The Wire Speed Control is used to set the rate of wire feed through the gun for welding. The wire feed control is marked in graduations of ten and ranges from zero to 100. The appropriate setting depends on; the metal thickness, size of wire, and welding position among other variables. The Wire Speed is co-coordinated with the voltage setting to produce a smooth arc.
14. WIRE JOG BUTTON
The Wire Jog button when pressed and held advances the wire through the gun without energizing the gas shielding. The wire jog button is often used to advance the wire when changing wire spools.
15. REMOTE SWITCH
The remote switch has two settings Remote and Standard. For normal operations using an attached Mig gun the switch is left in the Standard position. When a remote device is attached to the remote receptacle the switch is in the Remote position.
16. REMOTE RECEPTACLE
The Remote Receptacle is used to attach certain remote devices and the remote switch located above the receptacle is set to the remote position to activate the remote device.
17. GAS HOSE
The gas hose attached to the wire feed unit allows the shielding gas to be fed to the attached Mig gun.
3. WELDING GUN
The welding gun is attached to the wire feed unit to deliver the filler wire and shielding gas to the welding arc. The gun assembly is made up by the gun head, welding lead, power connector and wire feed connector.
TYPICAL WELDING GUN
The welding lead cable is constructed to allow electrical flow, conduct the shielding gas and feed the filler wire through the gun to the arc. In addition to the electrical wiring, the welding lead has a gas hose running through it to carry the gas supply and a liner to conduct the wire.
GUN HEAD AND PARTS
The typical mig gun has a handle with a trigger, a neck shaped like a goose neck, and a head consisting of a gas diffuser, contact tip and nozzle. The gun has a power connector that plugs into either the machine or the wire feed unit depending on style and design. In the case of the units with a separate wire feeder, the gun also has a wire feed connection.
BERNARD MIG GUN HEAD AND PARTS
4. SHIELDING GAS SUPPLY
The gas is usually supplied from a high pressure cylinder chained or secured on, or close to the welding machine. The safety rules for handling and using cylinders from the SAFETY SECTION must be understood and followed.
The principal gases used in MIG welding either alone or in a mixture are: ARGON, HELIUM and CARBON DIOXIDE.
Both Argon and Helium are inert gases, which mean they are chemically inactive and do not directly affect the Base metal or Weld metal.
Argon is 1.4 times heavier than air and blankets the weld zone to protect the weld zone from the air.
Argon may be used when welding non-ferrous metals (metals that contain little or no iron) such as; Aluminum, Magnesium, or copper and its alloys.
Argon is used in mixture with other gases because of its ability to stabilize the arc and reduce spatter.
Helium is used for similar applications to Argon, but since Helium is lighter than air it requires higher flow rates than when using Argon.
Helium produces a hotter arc and may be desirable for welding applications that require a higher heat input such as; for welding thicker sections, or for automated welding applications.
Helium provides deeper penetration characteristics than argon but with a less stable Arc. Helium and Argon may be used in gas mixtures to combine the benefits of each component part.
Carbon Dioxide is a chemical compound of one part Carbon and two parts Oxygen. Although not an Inert gas, Carbon Dioxide may be used for welding carbon steels when the filler wire with the appropriate deoxidizers are used.
Carbon Dioxide is often used for its relatively low cost and good penetration characteristics. When pure Carbon Dioxide is used as the shielding gas the welds that have a rougher appearance, more spatter, and a less stable arc than when used in a mixture with Argon.
There are a variety of gas mixtures available for use with the MIG welding process that combines the advantages of their component parts. It is critical to select the gas shielding that provides the best weld characteristics for the metal being welded. Before welding consult Gas suppliers, Engineering, Supervision, or shielding gas charts.
At Delta School Of Trades the MIG welding process is used to weld mild steel in all positions using a popular gas mixture of 75% Argon and 25% Carbon Dioxide. This gas is commonly referred to as simply 75-25.
FILLER WIRE (MIG WIRE)
The filler wire used at Delta School Of Trades is one of the series designed for welding Low Carbon Steel and is supplied on 30 pound spools that are attached to the back of the wire feed unit. The wire size used is 0.35 commonly referred to as just 035 (0 thirty five). The AWS (American Welding Society) has developed a system of numbers and letters to designate filler wire type and usage. The filler wire used for welding mild steel at the school is the AWS ER-70S-6. The letter and number designation is as follows:
ER – 70 S 6
The E designates Electric Welding
The R designates Filler Rod
The 70 designates the Tensile Strength
The two or three numbers following the hyphen designate the Tensile Strength (the ability of the weld metal to withstand forces acting to pull it apart) in thousands of pounds per square inch. In the above the Tensile Strength is 70,000 pounds per square inch.
The S designates Solid Wire.
If the filler is designed for Flux Core the letter may be a T for tubular wire.
The 6 is used to designate the usability and characteristics of the wire.
This wire has good penetration and deoxidizers added among other weld metal characteristics. Other numbers may be used in place of the 6 to designate other desirable weld metal characteristics. In some cases a letter may be added at the end to designate other materials have been added.
A selection of filler wires are available to Mig weld other metals and Alloys and information regarding their selection can be obtained from Welding Supply Outlets.
THE MIG WELDING ARC
The Mig Welding arc is usually Direct Current Electrode Positive (DCEP) or Reverse Polarity with Constant Voltage.
Some multiple process welding machines, like the Miller Dimension 400 used at the School, have a separate DCEP terminal marked CV for attaching a lead wire that runs from the machine to the wire feed unit that energizes the attached Mig gun. Some machines designed for Mig welding are set for DCEP polarity only.
METAL TRANSFER ACROSS THE ARC
There are three methods of metal transfer from the wire spool through the arc to the weld puddle possible with the Mig welding Process.
1. SHORT CIRCUITING TRANSFER
2. GLOBULAR TRANSFER
3. SPRAY TRANSFER
SHORT CIRCUITING TRANSFER
Short circuiting transfer as the name implies creates a short circuiting effect when the energized wire touches the workpiece and melts into the weld puddle. The short circuiting effect is a simplified way of describing an action that happens approximately 200 times per second and is not readily apparent to the welder.
The short circuiting transfer usually occurs when the voltage is relatively low, approximately 17v to 21 volts. This type of transfer results in a smooth weld with shallow penetration that easily bridges gaps or poor fit-up.
Short circuiting or short arc is used extensively for welding sheet metal or angles etc that are fabricated and do not require a great deal of strength. For example; A welding table, window guards, or Ladder rack.
Short Circuiting may be used in all positions and the arc makes a sound like frying bacon.
Globular Transfer as the name implies is a metal transfer where the tip of the wire forms globules or large droplets that melt and are forced across the Arc into the weld puddle.
Globular Transfer occurs with higher voltage settings than Short Circuiting Transfer and results in deeper penetration with a flatter weld bead.
Globular Transfer may be used to weld in all positions and the arc makes a hissing sound.
Spray Transfer as the name implies is a form of metal transfer that sprays small globules or droplets of metal across the arc into the weld puddle.
Spray transfer occurs at higher voltages and above certain critical transition currents determined by the wire size. Spray transfer usually requires the use of two percent oxygen or more for the spraying effect to occur.
Spray Transfer results in weld beads with a minimum of spatter, deep penetration and smooth weld beads.
The Spray Transfer is a hotter arc and is best suited to thicker materials in the flat and horizontal positions. The arc makes a harsh sound like high pressure water forced through a small opening.
USING THE MIG WELDING PROCESS
Before using the Mig process to weld a specific application, the welding parameters must be considered to achieve a sound weld with an acceptable appearance. In some instances Management may inform the welder of the specific parameters or a Welding Procedure Specification (WPS) may be provided to set the welding parameters. In other cases the Welder is responsible for determining the welding parameters. The following should be considered when the welder selects the welding parameters;
1. Determine the Filler Wire
The metal composition and classification must be considered when selecting the filler wire. In general the filler wire should match or be compatible with the base metal being welded.
Filler wire guides and information for selecting the correct filler wire for a given application are available from most welding supply stores.
Keep in mind a change in filler wire may require a change in gas shielding.
Filler wire with a T in the classification or inner shield are for FLUX CORE ARC WELDING and may require a change in Polarity. Flux Core filler metal may or may not require a shielding gas as a backup gas.
2. Determine the Gas Shielding
As mentioned above there are a variety of gas types and mixtures designed for specific applications. Consult gas suppliers for recommendations for the specific welding application. Gas flow should be carefully considered, too much gas flow may cause swirling and result in porosity (Air Holes), too little and the weld will be of poor quality with porosity.
Welding Non-ferrous metals or applications that require a certain metal transfer may require a change in gas supply.
3. Determine the voltage and wire speed settings
For most Mig welding applications, but not all, the current should be Direct Current Reverse Polarity (DCEN). If you are not sure check on the box the wire spool came in for the current type and range or contact your welding supply store.
The Mig process uses a constant voltage machine that means the voltage stays relatively while the amperage varies with the stick out distance (gun nozzle tip to work distance).
The amperage is controlled by the wire speed setting. The wire speed and voltage must be coordinated to produce a sound weld with good appearance. Changing the wire speed or voltage changes the way the weld bead runs and looks.
To produces good welds, always set the machine by practicing setting on scrap metal in the same configuration, thickness, and position you are going to weld.
The following are a few tips for setting the current;
Which metal transfer is taking place: Lower voltage will be Short Circuiting; Higher voltages will be Globular; Higher voltage and 2% or more oxygen will produce a hot arc and spray transfer.
The weld bead should be well formed with a gradual transition to the base metal at the corners of the weld.
If the wire is hitting the base metal and popping or skipping, turn down the wire speed or increase the voltage.
If the wire is melting in blobs without forming a good weld turn up the wire speed or decrease the voltage.
Note: Contact tips and nozzles must be cleaned of dirt and spatter frequently. A dirty tip may also cause popping or the wire to hang up and not melt properly. Check the tip and clean the nozzle before changing setting, especially if the setting were working.
A slight push angle with the gun will produce a smoother flatter weld bead than when using a drag angle.