Gas Metal-Arc Welding (GMAW)
The gas metal-arc welding process (GMAW), often called MIG, has revolutionized arc welding. In this process, a consumable electrode (in the form of wire) is fed from a spool through the torch (welding gun) at a preset controlled speed. As the wire passes through the contact tube of the gun, it picks up the welding current. The consumable wire electrode serves two functions: it maintains the arc and provides filler metal to the joint. The method of delivery of the filler metal allows GMAW welding to be basically a one-handed operation which does not require the same degree of skill as GTAW. Figure 8-23 shows the basic principle of gas metal-arc welding.
Figure 8-23.—GMA welding process.
An important factor in the GMA welding process is the high rate at which metal can be deposited. This high rate of metal deposition and high speed of welding results in minimum distortion and a narrow heat-affected zone. When you are deciding whether to use GTA or GMA welding, the thickness of the material should be a deciding factor. GMAW is often chosen for welding thicker material.
Like GTA welding, gas metal-arc welding also uses a shielding gas to protect the weld zone during welding. The inert gas is fed through the gun into the weld zone and prevents atmospheric contamination.
GMAW offers many of the advantages of GTAW. Since there is no flux, GMA welds are clean and there is no slag to remove. GMAW enables you to produce sound welds in all positions quickly. Now let’s take a look at the equipment you will use for GMA welding.
GMA Welding Equipment and Supplies
Gas metal-arc welding equipment basically consists of four units: the power supply, the wire feeding mechanism, the welding gun (also referred to as the torch), and the gas supply. Figure 8-24 shows atypical GMA welding outfit.
Figure 8-24.—Diagram of GMA welding equipment.
When you use a conventional type of welding ma-chine for GMA welding, the voltage varies depending on the length of the arc. Whenever the nozzle-to-work distance changes, the arc length and the voltage changes. The only way to produce uniform welds with this type of power source is to maintain the arc length and voltage at a constant value. Besides producing non-uniform welds, this inconsistent voltage can cause the wire to burn back to the nozzle.
A constant voltage (CV) power source was developed to overcome the inconsistent voltage characteristics of a conventional welding machine, (See fig. 8-25). It can be either a dc rectifier or motor generator that supplies current with normal limits of 200 to 250 amperes.
Figure 8-25.—Constant voltage (CV) power unit.
The CV type power source has a nearly flat volt-ampere characteristic. This means that the machine maintains the same voltage regardless of the amount of current used. With this type of power source, you can change the wire-feed speed over a considerable range without causing the wire to burn back to the nozzle. When the wire-feed speed is set at a specific rate, a proportionate amount of current is automatically drawn. In other words, the current selection is based on the wire-feed speed. When the wire is fed faster, the current increases; when it is fed slower, the current decreases. With this type of power supply, variations in the nozzle-to-work distance will not change the arc length and burn back is virtually eliminated
In gas metal-arc welding, direct-current reverse polarity (DCRP) is recommended. You should recall from the previous section that DCRP produces excellent cleaning action and allows for deeper penetration.
Wire Feed Drive Motor
The wire feed drive motor is used to automatically drive the electrode wire from the wire spool through the gun up to the arc point. You can vary the speed of the wire feed by adjusting the controls on the wire-feed control panel. The wire feeder can be mounted on the power unit or it can be separate from the welding machine.
The function of the welding gun is to carry the electrode wire, the welding current, and the shielding gas to the arc area. The gun has a trigger switch that controls the wire feed and arc as well as the shielding gas. The welding operator directs the arc and controls the weld with the welding gun. GMA welding guns are available in many different styles, some of which are shown in figure 8-26. When using these guns, the wire is fed to the torch by an automatic wire feeding machine which pushes the wire through a flexible tube to the arc point.
Figure 8-26.—GMA welding guns.
Figure 8-27 shows another type of GMA welding gun that welders could use. This model incorporates the drive motor and a small spool of wire inside the gun. This type of gun is attached directly to the welding unit and gas supply, eliminating the need for a separate control unit and wire drive assembly.
Figure 8-27.—GMA welding torch with wire feed motor and wire spool inside.
As with the GTA welding torch, the torch nozzle must be kept clean at all times. Also, you should clean the tube through which the electrode wire passes each time the electrode reel is changed.
In gas metal-arc welding, as with gas tungsten-arc welding, the shielding gas can have a major effect on the properties of the base metal. Some of the shielding gases commonly used with the GMA process are pure argon, argon-helium, argon-oxygen, argon-carbon dioxide, and carbon dioxide. Refer to table 8-4 for a selection of shielding gases recommended for various metals for both the GMA and GTA welding processes. The smoothness of operation, weld appearance, weld quality, and welding speeds are affected indifferent ways with each type of metal, thickness, and gas mixture.
ARGON.— Earlier in this chapter, we said that argon provides greater cleaning action than other gases. Because it is heavier than air, argon blankets the weld from contamination. Also, when you are using argon as a shielding gas, the welding arc tends to be more stable. For this reason, argon is often used in combination with other gases for arc shielding. Argon reduces spatter by producing a quiet arc and reducing arc voltage that results in lower power in the arc and thus lower penetration. The combination of lower penetration and reduced spatter makes argon desirable when welding sheet metal.
Pure argon is seldom used for arc shielding except in welding such metals as aluminum, copper, nickel, and titanium. The use of pure argon to weld steel usually results in undercutting, poor bead contour, and the penetration is somewhat shallow.
ARGON-OXYGEN.— Small amounts of oxygen added to argon can produce excellent results. Normally oxygen is added in amounts of 1, 2, or 5 percent. When oxygen is added to argon, it improves the penetration pattern. It also improves the bead contour and eliminates the undercut at the edge of the weld. You use argon-oxygen mixtures in welding alloy steels, carbon steels, and stainless steel.
HELIUM.— Helium, like argon, is an inert gas. But there are few similarities between the two gases. Argon is heavier than air and helium is lighter than air. Helium has a high-voltage change as the arc length changes. When you use helium for GMA welding, more arc energy is lost in the arc itself and is not transmitted to the work In the section on GTA welding, we said that helium produces good penetration and fast welding speeds. For GMA welding, the opposite is true. In GMA welding, helium produces a broader weld bead, but shallower penetration.
Because of its high cost, helium is primarily used for special welding tasks and for welding nonferrous metals, such as aluminum, magnesium, and copper. It is also used in combination with other gases.
CARBON DIOXIDE (CO2 ).— Argon and helium gases are composed of single atoms. Carbon dioxide, on the other hand, consists of molecules. Each molecule contains one carbon atom and two oxygen atoms. At normal temperatures carbon dioxide is essentially an inert gas; however, at high temperatures it decomposes into carbon monoxide (CO) and oxygen (O2). Because the excess oxygen atoms can combine with carbon or iron in the weld metal, wires used with this gas must contain deoxidizing elements. A deoxidizing element has a great affinity for the oxygen and readily combines with it. Some of the more common deoxidizers used in wire electrodes are manganese, silicon, and aluminum.
Carbon dioxide is used primarily for the GMA welding of mild steel. Because of its low cost, CO2 is often used in combination with other shielding gases for welding different types of metals. Direct-current reverse polarity (DCRP) is generally used with CO2. The current setting is about 25 percent higher with CO2 than with other shielding gases.
Carbon dioxide produces a broad, deep penetration pattern. It also produces good bead contour and there is no tendency toward undercutting. The only problem with CO2 gas is the tendency for the arc to be violent. This can lead to spatter problems; however, for most applications this is not a problem and the advantages of CO2 far outweigh the disadvantages.
You should use the same type of regulator and flowmeter for gas metal-arc welding that you use for gas tungsten-arc welding. The gas flow rates vary, depending on the types and thicknesses of the material and the joint design. At times it is necessary to connect two or more gas cylinders (manifold) together to maintain higher gas flow.
For most welding conditions, the gas flow rate is approximately 35 cubic feet per hour (cfh). This flow rate may be increased or decreased, depending upon the particular welding application. Final adjustments usually are made on a trial-and-error basis. The proper amount of gas shielding results in a rapidly crackling or sizzling arc sound. Inadequate gas shielding produces a popping arc sound and results in weld discoloration, porosity, and spatter.
The composition of the filler wire used for GMA welding must match the base metal. For mild steel, you should select mild steel wire; for aluminum, you should select aluminum wire. Additionally, you should try to select electrode wire that matches the composition of the various metals you are welding. For instance, when you are welding Type 308 aluminum, you should use an ER-308L filler wire.
Wires are available in spools of several different sizes. The wire varies in diameter from .020 to 1/8 of an inch. You should select the proper diameter of wire based on the thickness of the metal you are welding as well as the position in which you are welding. Wires of 0.020,0.030, and 0.035 of an inch are generally used for welding thin materials. You also can use them for welding low- and medium-carbon steels and low-alloy/high-strength steels of medium thicknesses. (See table 8-6.) Medium thicknesses of metals are normally welded with 0.045-inch or 1/16-inch diameter wires. For thicker metals, larger diameter electrodes may be required.
Table 8-6.—Recommended Wire Diameters for GMA Welding Using Welding Grade CO2
and a Wire Stick-out of 1/4 to 3/8 of an Inch
As you learned earlier, the position of welding is a factor that must be considered. For instance, when you are welding in the vertical or overhead positions, you normally use smaller diameter electrodes.
Special attention must be given to ensure the wire is clean. Unsound welds result from the use of wire that is contaminated by oil, grease, dust, or shop fumes. You can obtain the best welding results with wire that has just been taken out of its carton. Wire should be stored in a hot locker or in a warm dry area, and should be kept covered. If welding is stopped for a long period of time, you should remove the wire and place it in its original carton to prevent contamination.
WIRE STICK-OUT.— In gas metal-arc welding, wire stick-out refers to the distance the wire extends from the nozzle of the gun (fig. 8-28). The correct amount of wire stick-out is important because it influences the welding current of the power source. Since the power source is self-regulating, the current output is automatically decreased when the wire stick-out increases.
Figure 8-28.—Correct wire stick-out.
Conversely, when the stick-out decreases, the power source is forced to furnish more current. Too little stick-out causes the wire to fuse to the nozzle tip, which decreases the tip life.
For most GMA welding, the wire stick-out should measure from 3/8 to 3/4 of an inch. For smaller (micro) wires, the stick-out should be between 1/4 and 3/8 of an inch.
WIRE-FEED SPEED.— As we stated earlier, you can adjust the wire-feed drive motor to vary the wire-feed speed. This adjustment is limited to a definite range, depending on the welding current used. (See table 8-6). The wire-feed speed is measured in inches per minute (ipm). For a specific amperage setting, a high wire-feed speed results in a short arc, whereas a low speed produces a long arc. You use higher speeds for overhead welding than with flat-position welding.
Personal Protective Equipment
As with any other welding process, SAFETY is extremely important. A welding hood like the one used in shielded metal-arc welding should be used for gas metal-arc welding. The correct shade of lens depends on the intensity of the arc. Eye fatigue indicates you should use a different shade of lens or there is leakage around the protective filter glass.
In addition to the welding hood, protective clothing, such as gloves and an apron, should be worn. Bare skin should never be exposed to the rays of the welding arc because it could result in painful burns.
Types of GMA Welding
When using the GMA welding process, metal is transferred by one of three methods: spray transfer, globular transfer, or short-circuiting transfer. The type of metal transfer depends on the arc voltage, current setting, electrode wire size, and shielding gas.
Spray-arc transfer is a high-current range method that produces a rapid disposition of weld metal. This type of transfer is effective for welding heavy-gauge metals because it produces deep weld penetration. The use of argon or a mixture of argon and oxygen are necessary for spray transfer. Argon produces a pinching effect on the molten tip of the electrode, permitting only small droplets to form and transfer during the welding process. Spray transfer is useful when welding aluminum; however, it is not practical for welding light-gauge metal.
Globular transfer occurs when the welding current is low. Because of the low current, only a few drops are transferred per second, whereas many small drops are transferred with a higher current setting. In this type of transfer, the ball at the tip of the electrode grows in size before it is transferred to the workpiece. This globule tends to reconnect with the electrode and the workpiece, causing the arc to go out periodically. This results in poor arc stability, poor penetration, and excessive spatter.
Globular transfer is not effective for GMA welding. When it is used, it is generally restricted to thin materials where low heat input is desired.
Short-Circuiting Arc Transfer
Short-circuiting arc transfer is also known as short arc. Short arc was developed to eliminate distortion, burn-through, and spatter when welding thin-gauge metals. It can be used for welding in all positions, especially vertical and overhead where puddle control is more difficult. In most cases, it is used with current levels below 200 amperes and wire of 0.045 of an inch or less in diameter. Small wire produces weld puddles that are small and easily manageable.
The shielding gas mixture for short-arc welding is 75% carbon dioxide and 25% argon. The carbon dioxide provides for increased heat and higher speeds, while the argon controls the spatter. Straight CO2 is now being used for short-arc welding; however, it does not produce the excellent bead contour that the argon mixture does.
GMA Welding Preparation
Preparation is the key to producing quality weldments with the gas metal-arc welding process. As in GTA welding, the equipment is expensive; therefore, you should make every effort to follow the manufacturer’s instruction manuals when preparing to use GMA welding equipment.
For the most part, the same joint designs recommended for other arc welding processes can be used for gas metal-arc welding. There are some minor modifications that should be considered due to the welding characteristics of the GMA process. Since the arc in GMA welding is more penetrating and narrower than the arc for shielded metal-arc welding, groove joints can have smaller root faces and root openings. Also, since the nozzle does not have to be placed within the groove, less beveling of the plates is required. GMA welding can actually lower material costs, since you use less weld metal in the joint.
The following suggestions are general and can be applied to any GMA welding operation: Check all hose and cable connections to make sure they are in good condition and are properly connected.
- Check to see that the nozzle is clean and the correct size for the particular wire diameter used.
- Make sure that the guide tube is clean and that the wire is properly threaded through the gun.
- Determine the correct wire-feed speed and adjust the feeder control accordingly. During welding, the wire-speed rate may have to be varied to correct for too little or too much heat input.
- Make sure the shielding gas and water coolant sources are on and adjusted properly.
- Check the wire stick-out.
GMA Welding Procedures
As with any other type of welding, the GMA welding procedure consists of certain variables that you must understand and follow. Many of the variables have already been discussed. This section applies some of these variables to the actual welding procedure.
Starting the Arc
For a good arc start, the electrode must make good electrical contact with the work For the best results, you should clean the metal of all impurities. The wire stick-out must be set correctly because as the wire stick-out increases, the arc initiation becomes increasingly difficult When preparing to start the arc, hold the torch at an angle between 5 and 20 degrees. Support the weight of the welding cable and gas hose across your shoulder to ensure free movement of the welding torch. Hold the torch close to, but not touching, the workpiece. Lower your helmet and squeeze the torch trigger. Squeezing the trigger starts the flow of shielding gas and energizes the welding circuit. The wire-feed motor does not energize until the wire electrode comes in contact with the workpiece.
Move the torch toward the work, touching the wire electrode to the work with a sideways scratching motion, as shown in figure 8-29. To prevent sticking, you should pull the torch back quickly, about 1/2 of an inch—the instant contact is made between the wire electrode and the workpiece. The arc strikes as soon as contact is made and the wire-feed motor feeds the wire automatically as long as the trigger is held.
Figure 8-29.—Striking the arc (GMAW).
A properly established arc has a soft, sizzling sound. Adjustment of the wire-feed control dial or the welding machine itself is necessary when the arc does not sound right. For example, a loud, crackling sound indicates that the arc is too short and that the wire-feed speed is too fast. You may correct this problem by moving the wire-feed dial slightly counterclockwise. This decreases the wire-feed speed and increases the arc length. A clockwise movement of the dial has the opposite effect. With experience, you can recognize the sound of the proper length of arc to use.
To break the arc, you simply release the trigger. This breaks the welding circuit and de-energizes the wire-feed motor. Should the wire electrode stick to the work when striking the arc or during welding, release the trigger and clip the wire with a pair of side cutters.
In gas metal-arc welding, the proper position of the welding torch and weldment are important. The position of the torch in relation to the plate is called the work and travel angle. Work and travel angles are shown in figure 8-30. If the parts are equal in thickness, the work angle should normally be on the center line of the joint; however, if the pieces are unequal in thickness, the torch should angle toward the thicker piece.
Figure 8-30.—Travel and work angle for GMA welding.
The travel angle refers to the angle in which welding takes place. This angle should be between 5 and 25 degrees. The travel angle may be either a push angle or a drag angle, depending on the position of the torch. When the torch is ahead of the weld, it is known as pulling (or dragging) the weld. When the torch is behind the weld, it is referred to as pushing the metal (fig. 8-31).
Figure 8-31.—Pulling and pushing travel angle techniques.
The pulling or drag technique is for heavy-gauge metals. Usually the drag technique produces greater penetration than the pushing technique. Also, since the welder can see the weld crater more easily, better quality welds can consistently be made. The pushing technique is normally used for light-gauge metals. Welds made with this technique are less penetrating and wider be-cause the welding speed is faster.
For the best results, you should position the weldment in the flat position. ‘This position improves the molten metal flow, bead contour, and gives better shielding gas protection.
After you have learned to weld in the flat position, you should be able to use your acquired skill and knowledge to weld out of position. These positions include horizontal, vertical-up, vertical-down, and overhead welds. The only difference in welding out of position from the fiat position is a 10-percent reduction in amperage.
When welding heavier thicknesses of metal with the GMA welding process, you should use the multipass technique (discussed in chapter 3). This is accomplished by overlapping single small beads or making larger beads, using the weaving technique. Various multipass welding sequences are shown in figure 8-32. The numbers refer to the sequences in which you make the passes.
Figure 8-32.—Multipass welding.
Common Weld Defects
Once you get the feel of welding with GMA equipment, you will probably find that the techniques are less difficult to master than many of the other welding processes; however, as with any other welding process, GMA welding does have some pitfalls. To produce good quality welds, you must learn to recognize and correct possible welding defects. The following are a few of the more common defects you may encounter along with corrective actions that you can take.
SURFACE POROSITY.— Surface porosity usually results from atmospheric contamination. It can be caused by a clogged nozzle, shielding gas set too low or too high, or welding in a windy area. To avoid surface porosity, you should keep the nozzle clean of spatter, use the correct gas pressure, and use a protective wind shield when welding in a windy area.
CRATER POROSITY.— Crater porosity usually results from pulling the torch and gas shield away before the crater has solidified. To correct this problem, you should reduce the travel speed at the end of the joint. You also may try reducing the tip-to-work distance.
COLD LAP.— Cold laps often result when the arc does not melt the base metal sufficiently. When cold lap occurs, the molten puddle flows into an unwelded base metal. Often this results when the puddle is allowed to become too large. To correct this problem, you should keep the arc at the leading edge of the puddle. Also, reduce the size of the puddle by increasing the travel speed or reducing the wire-feed speed. You also may use a slight whip motion.
LACK OF PENETRATION.— Lack of penetration usually results from too little heat input in the weld zone. If the heat input is too low, increase the wire-feed speed to get a higher amperage. Also, you may try reducing the wire stickout.
BURN-THROUGH.— Burn-through (too much penetration) is caused by having too much heat input in the weld zone. You can correct this problem by reducing the wire-feed speed, which, in turn lowers the welding amperage. Also you can increase the travel speed. Burn-through can also result from having an excessive amount of root opening. To correct this problem, you increase the wire stick-out and oscillate the torch slightly.
WHISKERS.— Whiskers are short pieces of electrode wire sticking through the root side of the weld joint. This is caused by pushing the wire past the leading edge of the weld puddle. To prevent this problem, you should cut off the ball on the end of the wire with side cutters before pulling the trigger. Also, reduce the travel speed and, if necessary, use a whipping motion.
GMA Welding Common Metals
You can use the welding equipment and techniques for gas metal-arc welding to join all types of metals; however, as we discussed in the GTAW process, each of the metals requires a unique welding method. In this section, we discuss some of the welding methods associated with a few of the more commonly welded metals.
The majority of welding by all methods is done on carbon steels. When you are using GMA to weld carbon steels, both the spray-arc and short-arc methods may be applied. For spray-arc welding, a mixture of 5-percent oxygen with argon is recommended. As we mentioned earlier, this mixture provides a more stable arc. Also you may use a mixture of argon and CO2 or straight CO2. Straight CO2 is often used for high-speed production welding; however, with CO2 the arc is not a true spray arc. For short-arc welding, a 25-percent CO2 and 75-per-cent argon mixture is preferred.
For GMA welding of thin materials (0.035 inch to 1/8 inch), no edge preparation is needed and a root opening of 1/16 of an inch or less is recommended. For production of adequate welds on thicker material, some beveling is normally required. When welding plates 1/4 of an inch or greater in thickness, you should prepare a single or double-V groove with 50- to 60-degree included angle(s).
The joint design for aluminum is similar to that of steel; however, aluminum requires a narrower joint spacing and lower welding current setting.
The short-arc welding method is normally used for out-of-position welding or when welding thin materials because short-arc produces a cooler arc than the spray type arc. When welding thinner material (up to 1 inch in thickness), you should use pure argon.
The spray-arc welding method is recommended for welding thicker materials. With spray arc, more heat is produced to melt the wire and base metal. When you are welding thicker material (between 1 and 2 inches) a mixture of 90-percent argon and 10-percent helium is recommended. The helium provides more heat input and the argon provides good cleaning action.
DCRP with a 1- or 2-percent oxygen with argon mixture is recommended for most stainless steel welding. In general, you weld stainless steel with the spray-arc welding method and a pushing technique. When welding stainless steel up to 1/16 of an inch in thickness, you should use a copper backup strip. For welding thin materials in the overhead or vertical positions, the short-arc method produces better results.