T
he gas metal arc welding (GMAW) process was first conceived in the 1920s, but it was not available for commercial use until the late 1940s. The primary application was the welding of aluminum. During the early 1950s, it was discovered that reactive gases, such as carbon dioxide (CO2), and mixtures of inert and reactive gases could be used to join a wider range of materials with increased weld purity and production efficiency. The development of more versatile continuous consumable wire electrodes (welding wires) increased the popularity of GMAW. Today, GMAW is used extensively in the automotive industry and in a variety of other manufacturing and fabrication environments.
GAS METAL ARC WELDING Gas metal arc welding (GMAW) is an arc welding process that uses an arc between a continuous wire electrode and the weld pool. GMAW equipment consists of a welding power source, a welding gun cable and gun assembly, electrode wire (welding wire), a wire feeder, shielding gas, and a workpiece lead with a workpiece connection. The continuous wire electrode (welding wire) is fed through the wire feeder and the welding gun cable and gun assembly at a preset speed (the wire feed speed, or wfs). The molten welding wire transfers across the arc where it fuses with the base metal to form the weld. A shielding gas or a combination of shielding gases supplied from an external source is also fed through the welding gun cable and gun assembly. The shielding gas completely covers and protects the weld pool. Metal transfer is the manner in which molten metal transfers from the end of the electrode across the welding arc to the weld pool. GMAW is capable of producing three modes of metal transfer: short circuiting transfer, globular transfer, and spray transfer, as well as a
OBJECTIVES • • • • • •
Describe gas metal arc welding (GMAW). Describe types of power sources used for GMAW. Describe welding guns and their use in GMAW. Explain how wire feeders work. Identify common types of shielding gas used for GMAW. Identify considerations in selecting GMAW welding wire.
variation of spray transfer called pulsed spray transfer. The type of metal transfer used is determined by wire feed speed (which controls the amperage or current), arc voltage, welding wire diameter and composition, and type of shielding gas. A GMAW weld can be applied by the semiautomatic, mechanized, or automatic methods. When semiautomatic welding is used, the wire feed speed, voltage setting, and gas flow rate are preset, but the welding gun is manually operated. The welder directs the welding gun along the weld joint to complete the weld. A constant-voltage welding machine with direct current electrode positive is most commonly used when welding with GMAW.
In mechanized GMAW, the welding operator sets the welding parameters and monitors the welding operation while a mechanical device controls the welding gun along the joint. In automatic GMAW, the welding parameters and welding gun movements are programmed into a computer, and all aspects of the process are controlled by the equipment, such as in a robotic cell in a manufacturing environment. 217
GMAW-Welding Machine
GMAW WELDING POWER SOURCES GMAW uses a direct current (DC) welding power source capable of producing constant voltage (CV). CV welding power sources are also referred to as constant-potential (CP) machines. Unlike constant-current (CC) power sources that have a steep volt-ampere (V-A) curve, CV power sources have a slightly sloping V-A curve. See Figure 19-1. CC and CV Volt-Ampere Characteristics
VOLTAGE
Media Clip
GMAW produces high-quality welds on a wide variety of ferrous and nonferrous metals at relatively low cost. It can be used to join a wide range of material thicknesses. The process allows higher deposition rates, faster travel speeds, less electrode waste, and it is easier to use than manual welding processes like SMAW, GTAW and OAW. There is very little if any post-weld cleaning required with GMAW because welds are slag-free. The process can also be easily adapted to mechanized and robotic applications. However, the process is less portable than SMAW because it requires a supply of external shielding gas. Another disadvantage is that GMAW is not well-suited to outdoor applications, because wind can blow shielding gas away from the weld zone, exposing the molten weld pool to the atmosphere.
GMAW CURRENT SELECTION
DCEP provides deep penetration and excellent cleaning action.
218 Welding Skills
CURRENT
CONSTANT CURRENT (CC) V-A CURVE
VOLTAGE
The most common current selected for GMAW is direct current electrode positive (DCEP). DCEP is the most efficient current because it produces deep penetration. DCEP also provides greater surface cleaning, which is important for removing the oxide layer on metals. A wide range of current values can be used for GMAW. In GMAW, current is a function of wire feed speed. Current is limited by metal thickness, welding wire diameter, and the type of shielding gas. The correct wire feed speed for a particular joint must often be determined by trial and error. The wire feed speed should be high enough to allow the desired penetration, but low enough to prevent undercut, overlap, or excessive melt-through. Once the wire feed speed is set, it remains constant. DCEN should not be used for GMAW because weld penetration is shallow and wide, there is excessive spatter, and no surface cleaning occurs. DCEN is also ineffective because metal transfer is erratic and globular. AC current should not be used with GMAW since electrode melting rates (burnoff rates) are unequal for each half-cycle.
CURRENT
CONSTANT VOLTAGE (CV) V-A CURVE Figure 19-1. Constant-current (CC) power sources have a steep volt-ampere (V-A) curve, while constant-voltage (CV) power sources have a slightly sloping V-A curve.
CV welding power sources for GMAW can be transformer-rectifiers or inverters. They range in size from small, compact units for light-duty work, to heavy-duty machine for high-volume production and fabrication work. Duty cycle is the percentage of time during a 10 min period that a power source can operate at rated power. Welding power sources rated at 350 A with 100% duty cycles are adequate for most commercial applications.
With CV power sources, welding current provides the energy necessary to melt the electrode wire. The higher the wire feed speed, the higher the welding current. An increase in current produces an increase in depth of penetration, weld metal deposition rate, and weld bead size. Voltage is set in relation to wire feed speed and is affected by the type and diameter of the welding wire, the type of base metal, and the type of shielding gas. Voltage is directly related to arc length. The higher the welding current, the higher the voltage necessary to maintain a particular arc length. When the wire feed speed and voltage are set correctly in relation to each other, the arc length will remain constant. From a particular voltage setting, an increase in voltage will produce a flatter, wider weld bead. If the voltage is set too high in relation to wire feed speed, the arc length becomes too long and may cause porosity, increased spatter, and undercut. Excessive voltage can cause problems with burnback. If the voltage is set too low in relation to wire feed speed, the weld bead becomes narrow with a high crown
and deeper penetration. If the voltage is excessively low, it may cause the electrode to stub into the base metal. Arc Length and Electrode Extension CV power sources are designed to maintain selected arc voltage. They will maintain this preset voltage even with changes in electrode extension. Electrode extension is the distance from the contact tip to the end of the welding wire. An increase in electrode extension results in a slight increase in voltage. The increase in the electrode extension also increases the resistance in the wire, which results in a decrease in the welding current. Similarly, a decrease in electrode extension decreases voltage slightly. The decrease in electrode extension reduces the resistance in the wire and results in an increase in the current. In either case, the arc length and voltage remain constant with changes in the electrode extension. See Figure 19-2.
CV Power Source—Arc Length and Electrode Extension NORMAL EXTENSION
SHORTER EXTENSION
VOLTAGE
LONGER EXTENSION
CURRENT
ELECTRODE EXTENSION
VISIBLE STICKOUT
ARC LENGTH
LONGER EXTENSION
NORMAL EXTENSION
SHORTER EXTENSION
Figure 19-2. CV power sources are self-correcting, which means that arc length remains constant with changes in electrode extension.
CHAPTER 19— GMAW – Equipment 219
In mechanized and automatic applications, the term contact tip-to-work distance is used in place of electrode extension. Contact tip-to-work distance (CTWD) is the distance from the end of the contact tip to the work and includes arc length.
GMAW EQUIPMENT In addition to a DC/CV welding power source, GMAW equipment includes a welding gun cable and gun assembly, a wire feeder, shielding gas, and a workpiece lead with a workpiece connection. See Figure 19-3. Additional equipment may be added to automate the system. Welding Gun Cable and Gun Assembly A welding gun cable conducts the welding wire, shielding gas, and welding current to
the welding gun. The welding gun cable contains a separate gas line for shielding gas, a current conductor to energize the electrode, and a gun cable liner that serves as a conduit for the welding wire. If the torch is water cooled, the gun cable will also contain a water line to the gun and a return line to the water circulator. The welding gun cable must not become kinked or damaged, as restricted flow of welding wire or shielding gas may occur. Welding gun components include a handle with a conductor tube and trigger, a contact tip, a gas nozzle, a gas diffuser, and an insulator. See Figure 19-4. The handle and conductor tube allow easy positioning of the gun by the operator. Welding guns are available with curved or straight conductor tubes. Curved conductor tubes are typically used for semiautomatic applications. Straight conductor tubes are used for mechanized and automatic applications. See Figure 19-5.
GMAW Equipment FLOWMETER
SHIELDING GAS REGULATOR SHIELDING GAS WORKPIECE CLAMP WORKPIECE LEAD
WELDING MACHINE ON
OFF POWER
WELDING GUN
TRIGGER GAS NOZZLE
WELDING GUN CABLE (SUPPLIES WELDING WIRE AND SHIELDING GAS)
Figure 19-3. GMAW equipment includes a welding gun cable and gun assembly, a wire feeder, shielding gas, and a workpiece lead with a workpiece connection.
220 Welding Skills
GMAW Gun Assembly INSULATOR
GAS DIFFUSER
CONDUCTOR TUBE
CONTACT TIP
TRIGGER GAS NOZZLE
HANDLE
The Lincoln Electric Company
Figure 19-4. Welding gun components include a handle with a conductor tube and trigger, a contact tip, a gas nozzle, a gas diffuser, and an insulator.
GMAW Welding Guns
CURVED Miller Electric Manufacturing Company
STRAIGHT Bernard Welding Equipment Company
Figure 19-5. Welding guns are available with curved or straight conductor tubes.
The contact tip conducts current from the welding gun to the welding wire. Contact tips are available with different hole sizes. The diameter of the hole in the contact tip should match the diameter of the welding wire.
The gas diffuser distributes shielding gas evenly around the welding wire and contact tip, and the gas nozzle directs the flow of shielding gas to the molten weld. The insulator prevents the gas nozzle from becoming energized. It also reduces heat transfer from the gas nozzle to the conductor tube. Gas nozzle size and shape may vary. Spatter buildup on the gas nozzle, contact tip, and gas diffuser can restrict the flow of shielding gas. These components should be checked and cleaned regularly. Damaged gas nozzles should be replaced because they can cause turbulence in the flow of shielding gas. Contact tips should be checked regularly for wear and replaced if necessary. A worn or damaged contact tip may cause problems with arc starting because of poor electrical contact with the welding wire. Gas diffusers should be replaced if the gas ports become blocked with spatter. The hand-operated trigger on the welding gun energizes the welding wire, starts the flow of shielding gas, and activates the wire feeder. Cooling of the welding gun is required to prevent overheating. Cooling is provided by the shielding gas or by shielding gas and water circulating through the gun. The welding gun cable should be kept as straight as possible to prevent kinking or flattening of the liner, which could impede the welding wire. A damaged liner can also cause burnback or bird nesting. Burnback is a condition that occurs when welding wire is restricted and fuses to the end of a contact tip. Bird nesting is a tangle of wire that forms in a wire feeder when welding wire is restricted in the liner or by a burnback condition. Too much drive roll tension can also cause bird nesting. A dirty gun cable liner can also restrict welding wire, causing problems with burnback and bird nesting. Residue from the welding wire can combine with dust to create a buildup of dirt in the liner. To keep the gun liner clean, shop air should be blown through it each time a new spool of welding wire is installed.
For GMAW, a constant-voltage welding machine with a nearly flat voltampere characteristic maintains a constant, preset voltage level during welding.
CHAPTER 19— GMAW – Equipment 221
There are two types of gun cable liners, helical steel and Teflon® lined. Helical steel gun cable liners are used with steel welding wired. Teflon gun cable liners are used for soft welding wires. They keep soft welding wire like aluminum feeding smoothly. There are several welding guns available for GMAW, but they can be categorized into two groups: welding guns designed for semiautomatic applications and welding guns designed for automatic applications.
Ensure that the wire feed speed is set for the current to be used for welding.
222 Welding Skills
Semiautomatic Welding Guns. A semiautomatic welding gun allows the welder to manually control and direct welding wire to the joint. Semiautomatic welding guns are manufactured in many shapes and sizes. A variety of factors, including current requirements, determine the correct semiautomatic welding gun to use for a particular welding task. A welding gun and welding gun cable must be capable of providing sufficient current for the welding task. Semiautomatic welding guns are rated to operate between 100 A and 750 A. A semiautomatic welding gun generally has a curved conductor tube. The curved conductor tube is used for most welding positions and provides easy access to intricate joints and difficult-to-weld patterns. Gas nozzles are commonly made of copper because copper can conduct away the intense heat that builds up near the arc. Gas nozzles are available with orifice diameters from ³\,″ to ⁷\,″. Gas nozzle size depends on the size of the weld pool, the gas shielding required, and the weld joint design. A semiautomatic welding gun attaches to the welding gun cable, which contains the power cable, gun cable liner, and hose for shielding gas. Semiautomatic welding guns can be air cooled or water cooled. Air-cooled welding guns use shielding gas for cooling. A water-cooled welding gun has two additional connections for “Water In” and “Water Out” to control water flow. Water lines that run through the welding gun cable provide water to the welding gun for cooling.
The trigger on a semiautomatic welding gun starts the wire feeder, the arc, and the flow of shielding gas. When the trigger is released, the wire feeder, arc, and shielding gas stop immediately. Preflow and postflow timers are included on some equipment to permit shielding gas to flow before and after welding to better protect the weld zone. Automatic Welding Guns. Automatic welding guns have a design similar to semiautomatic welding guns, but the gun is usually mounted to a fixture directly below the wire feeder. The fixture may move the welding gun, the worktable, or both. An automatic welding gun does not usually have a trigger. The welding gun is energized from a control panel or a remote power control. Automatic welding guns may be rated up to 1200 A. An air-cooled welding gun is used for welding at low currents, while a water-cooled welding gun is used for welding at high currents. Automatic welding guns are typically water-cooled because of the high currents and duty cycles at which they operate. Wire Feeders A wire feeder automatically advances the welding wire from the wire spool, through the welding gun cable liner and welding gun, to the arc. The wire feeder must be selected to match the power source used for the GMAW application. Constant-speed wire feeders are typically used with CV welding machines. The wire feeder may be portable, mounted on the welding machine, or mounted elsewhere to facilitate welding in a large area. See Figure 19-6. Wire feeders are designed for use with a wide range of solid and metal cored welding wire from 0.023″ to Z\zn″. The wire feed speed control on the wire feeder can be adjusted to vary the wire feed speed. The wire feeder does more than feed welding wire. It also supplies welding current to the welding gun cable and contains a solenoid that activates the flow
of shielding gas. Most wire feeders have a speed control, a voltage control, a purge button, and a jog button. Wire feeders designed for high wire feed rates may also have a burnback control.
WIRE SPOOL
Miller Electric Manufacturing Company
Figure 19-6. The wire feeder may be portable, mounted on the welding machine, or mounted elsewhere to facilitate welding in a large area.
Wire Feed Speed Control. The wire feed speed control adjusts the speed with which welding wire feeds into the arc. Wire feed speed determines the welding current. Wire feed speed is measured in inches per minute (ipm). Most wire feeders can be adjusted from 70 ipm to 800 ipm. A welding procedure specification (WPS) frequently provides the wire feed speed, as well as the voltage for the specified welding wire and shielding gas. Voltage Control. The voltage control allows the welder to set the optimum arc length. The arc voltage required for a particular application depends on the type and diameter of welding wire, the wire feed speed, and the type of shielding gas used. Purge Button. The purge button allows the welder to set the shielding gas flow rate without using the gun trigger. Using the gun trigger instead of the purge button is not recommended, because the gun trigger not only starts the flow of gas, it also starts the wire feeder and energizes
the welding wire. This not only wastes welding wire, it can also cause the wire to arc if it touches a metal surface. The purge button is also used to purge the gas line in the welding gun cable before welding. The line should be purged if the welding machine has been idle for an extended period. Jog Button. The jog button advances the welding wire without using the gun trigger. The jog button is used to advance the welding wire through the liner when installing a new spool. It can also be used to feed wire without energizing the contact tip. Burnback Control. The burnback control prevents the welding wire from freezing to the base metal by maintaining the arc briefly after the trigger is released. At low feed rates, the welding arc and the welding wire stop when the gun trigger is released. However, at high feed rates, the arc stops when the trigger is released, but the momentum of the wire can cause it to stub into the weld pool. The burnback control maintains the arc until the wire stops to keep the wire from sticking to the work. The burnback control should not be confused with the type of burnback that occurs when the wire freezes to the contact tip due to a blockage in the liner, or to the voltage being set too high in relation to wire feed speed. The wire feed mechanism that drives the welding wire consists of a variable speed electric motor connected to drive rolls. The drive rolls push the welding wire through the gun liner and gun. Wire feeders for light-duty applications have two drive rolls. Wire feeders for industrial applications typically have four drive rolls. Drive rolls for GMAW are grooved, and the tension on the drive rolls can be adjusted. However, too much drive roll tension can cause bird nesting. Grooves can be U-shaped, V-shaped, or knurled depending on the type of welding wire. U-shaped grooves are used with soft wire such as aluminum, V-shaped grooves are used with steel wires, and knurled grooves are used with metal cored and flux cored wires. See Figure 19-7.
Media Clip Wire Feeder Drive Roll
CHAPTER 19— GMAW – Equipment 223
Drive Rolls
U-GROOVE
V-GROOVE
KNURLED
Figure 19-7. Drive roll grooves can be U-shaped, V-shaped, or knurled depending on the type of welding wire.
Visible stickout is the distance the welding wire projects from the end of the nozzle of the welding gun.
The proper nozzle-towork distance must be maintained to ensure adequate shielding gas coverage.
224 Welding Skills
The drive rolls and the welding gun liner must be properly sized to match the diameter of the welding wire. The wire outlet guide must be aligned closely with the groove in the drive rolls without touching it. See Figure 19-8. Misalignment of the liner and the drive rolls can impede the welding wire, causing problems with burnback and bird nesting. The wire feeder can be a push type, a pull type, or a push-pull type, depending on the location of the drive rolls. Spool gun wire feeders are also available for small spools of welding wire. Push Type Wire Feeder. The most common wire feeder for steel welding wire is the push type wire feeder. A push type wire feeder has drive rolls that push the welding wire through the gun cable liner to the welding gun. The gun cable liner can be up to 15′ for steel wire or 6′ for aluminum wire. The push type wire feeder can handle large-diameter welding wire for ferrous metals in welding conditions where current is over 250 A.
WIRE OUTLET GUIDE
DRIVE ROLLS
Figure 19-8. The wire outlet guide must be aligned closely with the groove in the drive rolls, without touching it.
Pull Type Wire Feeder. In the pull type wire feeder, the welding wire is fed through the liner and pulled by drive rolls located on the welding gun. A pull type wire feeder is often used for mechanized and automatic welding. The drive rolls are built into the welding gun and pull the welding wire from the wire feeder. A pull type wire feeder works best with soft welding wire and small-diameter steel welding
wire up to about 0.045″ in diameter. A pull type wire feeder can be used with any semiautomatic welding gun. Push-Pull Type Wire Feeder. The pushpull type wire feeder is used for driving welding wire long distances and for lowstrength welding wires. The push-pull wire feeder has synchronous drive motors in the wire feeder and the welding gun. The drive rolls in the wire feeder push the welding wire from the wire feeder through the liner, while the drive rolls in the gun pull it. Spool Gun Type. Spool gun wire feeders are designed for small spools of welding wire between 1 lb and 2 lb. The spool is mounted at the back of the welding gun. A spool gun includes a drive motor, drive rolls, and a wire feed speed control. Typically, spool guns are used for small-diameter aluminum welding wire. The type of wire feeder used is determined by the characteristics of the welding wire. Small-diameter, soft aluminum welding wire must be pulled through the gun cable or fed from a spool gun. Large-diameter electrodes often require a push-pull wire feeder for a consistent flow of wire. Shielding Gas Shielding gas affects the properties of the weld deposit. The air in the weld area is displaced by the shielding gas to prevent it from contacting the weld pool. The arc is then started under a blanket of shielding gas and welding can be performed. Since the weld pool is exposed only to the shielding gas, it is not contaminated, and strong, dense weld deposits are obtained. The nozzle-to-work distance of the welding gun must be maintained to ensure an adequate shielding gas cover. Air is made up of 21% oxygen, 78% nitrogen, 0.94% argon, and 0.04% other gases (primarily carbon dioxide). The atmosphere will also contain a certain amount of water in the form of hydrogen
depending on its humidity. The elements of air that cause difficulties when welding are oxygen, nitrogen, and hydrogen. The effects of oxygen, nitrogen, and hydrogen on the weld make it essential that they be excluded from the weld area during welding. Oxygen (O2) is a highly reactive gas and readily combines with other elements in molten metal to form oxides and gases. The oxide-forming characteristic of oxygen can be overcome by using deoxidizers in the filler metal. Deoxidizers, such as manganese and silicon, combine with oxygen and float to the top of the weld pool, forming deposits called silica islands on the surface of the finished weld or along the toes of the weld. If deoxidizers are not provided in the filler metal, oxygen combines with the molten metal, causing porosity and other problems that affect the mechanical properties of the metal. Nitrogen (N) causes serious problems when welding steel. When the molten weld pool is exposed to nitrogen, it forms nitrides as it cools. Nitrides increase hardness and decrease ductility and impact resistance. The loss of ductility often leads to cracking in the weld and in the HAZ. In excessive amounts, nitrogen can also cause porosity in the weld. Hydrogen (H) is harmful because even small amounts in the weld pool can cause an erratic arc. Hydrogen can also become trapped in the solidifying metal and HAZ, causing small cracks in the weld as well as underbead cracking. Atmospheric gases can be excluded by using an inert gas for shielding. Inert gases do not react readily with other materials, making them useful as shielding gases for arc welding. Argon and helium are inert gases that are commonly used for shielding. See Figure 19-9. A mixture of 75% Ar and 25% CO2 produces shallower penetration when welding sheet metal and less spatter for autobody applications as compared to 100% CO2. CHAPTER 19— GMAW – Equipment 225
GMAW SHIELDING GASES Material
Mild Carbon Steel and Low-Alloy Steel
Gas 100% CO2
Produces deep, broad penetration and excessive spatter; unsuitable for sheet metal and autobody applications due to deep penetration and spatter
75% Argon (Ar) + 25% CO2
Produces shallower penetration with narrower penetration finger and less spatter than 100% CO2; good for sheet metal and autobody
98% Ar + 2% O2
Stainless Steels
Remarks
90% Helium (He) + 7.5% Ar + 2.5% CO2
Removes oxidation; gas mixtures containing over 80% Ar produce spray transfer; welds are high quality and low spatter, but limited to the flat position; oxygen minimizes undercut Popular blend for welding stainless steels with short circuiting transfer; high thermal conductivity of He produces flat bead with excellent fusion; promotes high travel speeds
99% Ar + 1% O2
Minimizes problems with undercut
95% Ar + 5% O2
Oxygen improves arc stability
Aluminum Alloys
100% Ar
Good cleaning action for removal of aluminum oxide coating
Aluminum Bronze
100% Ar
Less penetration of base metal; commonly used as a surfacing material
Magnesium
100% Ar
Good cleaning action for removal of magnesium oxide coating
Nickel
100% Ar
Monel
100% Ar
Inconel
100% Ar
Titanium
100% Ar
Reduces heat-affected zone (HAZ); improves metal transfer
Silicon Bronze
100% Ar
Reduces crack sensitivity
Magnesium Aluminum Alloys
75% He + 25% Ar
Higher heat input reduces risk of porosity; good cleaning action to remove oxide coating
Copper (deoxidized)
75% He + 25% Ar
Good wetting; increased heat input to counteract high thermal conductivity
Good wetting; decreases fluidity of weld pool
Figure 19-9. Shielding gas selection for GMAW is based on the type and thickness of metal to be welded as well as the type of metal transfer desired.
Although it is not inert, carbon dioxide (CO2) can also be used for shielding the weld area if compensation is made for its oxidizing tendencies. CO2, argon, or helium can be used alone or mixed for different applications. All shielding gases should be welding grade. Weldinggrade shielding gases are over 99% pure. They provide the best protection and produce the best results. Different shielding gases produce different weld bead contour and penetration characteristics. See Figure 19-10. The use of CO2 as a shielding gas is most effective and least expensive when welding steel. 226 Welding Skills
Carbon Dioxide (CO2). Although it may be used in other shielding-gas mixtures, CO2 is used primarily for welding mild steel. At normal temperatures, CO2 is essentially an inert gas. However,
when subjected to high temperatures, CO2 dissociates into carbon monoxide and oxygen. Because of the oxidizing characteristic of CO2 gas, the welding wire used with CO2 must contain deoxidizing elements. The deoxidizing elements readily combine with oxygen, preventing it from causing porosity and other problems in the weld. The most common deoxidizers used in welding wire are manganese, silicon, aluminum, titanium, and vanadium. CO2 produces a wide, deep-penetrating weld bead. Bead contour with CO2 is good and there is less tendency toward undercutting. Another advantage is its relatively low cost compared to other shielding gases.
Effects of Different Shielding Gas on Penetration
ARGON
ARGON-OXYGEN
ARGON-HELIUM
HELIUM
ARGON-CO2
CO2
CO2
Figure 19-10. Different shielding gases produce different weld bead contour and penetration characteristics.
While manufacturers generally use color codes to identify gas cylinders, colors may not be consistent between suppliers. Always check the cylinder for contents before attaching and using a gas cylinder.
A drawback of using CO2 for shielding is that it produces a somewhat violent arc. This can cause excessive spatter. For many applications, spatter is not a major problem, and the excellent penetration characteristics of CO2 outweigh its disadvantages. Also, antispatter sprays are available to prevent spatter from sticking to the base metal, gas nozzle, contact tip, and gas diffuser. However, 100% CO2 produces too much penetration for sheet metal welding and too much spatter for autobody applications. A mixture of 75% Ar and 25% CO2 reduces penetration sufficiently for sheet metal welding. An argon-CO2 mixture also minimizes spatter for autobody work. Many GMAW welding guns may be used at 100% duty cycle with CO2 as the shielding gas at a particular current setting; however, using the same welding gun with argon as the shielding gas, a lower current setting must typically be used for a 100% duty cycle.
Argon (Ar). Argon is the most commonly used inert gas. Argon has a relatively low ionization potential. This means that the welding arc is easier to start, tends to be more stable, and produces little or no spatter. Since argon has a low ionization potential, the arc voltage is reduced when an argon mixture is used as a shielding gas. This results in lower power in the arc and shallower penetration. The combination of shallower penetration and reduced spatter makes the use of an argon-CO2 mixture desirable when welding sheet metal. Straight argon is seldom used as a shielding gas except when welding metals such as aluminum, copper, nickel, and titanium. When welding steel, the use of straight argon leads to undercutting and poor bead contour. Additionally, penetration with straight argon is shallow at the bead edges and deep at the center of the weld, which can lead to lack of fusion at the root of the weld. Argon is often mixed with other gases to improve their stability.
Argon as a shield gas produces the most effective results when welding aluminum.
Helium (He). Helium is lighter than air and requires high flow rates to produce adequate coverage. Helium has a higher ionization potential than argon, which allows for higher arc voltage. Ionization potential refers to the amount of voltage CHAPTER 19— GMAW – Equipment 227
For most welding, the gas ow rate is approximately 20 cfh to 35 cfh.
required to ionize the gas column so it can conduct the arc. A shielding gas with a high ionization potential makes the arc harder to start. Helium produces a deep, broad, parabolic weld with a low bead profile. Because of its high cost, helium is used primarily for special welding tasks and for nonferrous metals such as aluminum, magnesium, and copper. It is most commonly used in combination with other shielding gases. Argonhelium mixtures are commonly used for welding aluminum greater that 1″ thick. Argon-helium mixtures are also used on stainless steel instead of argon-CO2 mixtures because CO2 can adversely affect the mechanical properties in the weld and in the HAZ.
Argon-CO2. For some mild steel welding applications like sheet metal and autobody, welding-grade CO2 does not provide the required arc characteristics. This is usually evident in the form of excessive penetration or excessive spatter. Using an argon-CO2 mixture minimizes these problems. A mixture of 75% Ar and 25% CO2 is commonly used for welding mild and low-alloy steels with short-circuiting transfer. A 75/25 mixture produces shallower penetration, faster travel speeds, a smoother and more focused arc, and less spatter than 100% CO2, making it ideal for autobody applications. The most commonly used argon-CO2 mixture for metal-cored electrode wire is 90% Ar/10% CO2.
Argon-Oxygen. Oxygen is added to argon when welding mild steel to improve bead contour and penetration. A small amount of oxygen improves penetration by broadening the deep penetration finger at the center of the weld bead. It also improves bead contour and eliminates the undercutting at the edge of the weld that occurs with pure argon. Normally, oxygen is added in amounts of 1%, 2%, or 5%. Adding oxygen in amounts greater than 5% may lead to porosity in the weld as well as underbead cracking. Argon-oxygen mixtures are common when welding alloy steel, carbon steel, and stainless steel.
Argon-Helium-CO2. An argon-heliumCO2 shielding-gas mixture is used for welding austenitic, martensitic, and ferritic stainless steels. This combination of gases provides a unique characteristic to the weld. With it, it is possible to make a weld with very little buildup of the top bead profile. An argon-helium-CO2 mixture is used for applications where a high-crowned weld is detrimental.
Information on a gas cylinder label typically includes the type of gas or mixture of gas contained, and the manufacturer or supplier name.
228 Welding Skills
Gas Flow Rates. A regulator and flowmeter assembly deliver a steady preset flow of shielding gas to the weld area. The regulator reduces cylinder pressure to working pressure. The flowmeter indicates the rate of flow of shielding gas to the weld in cubic feet per hour (cfh). The amount of shielding gas required is determined by the type of welding gun, weld joint design, base metal, type of metal transfer, and conditions in the weld area. For example, welding outdoors in breezy conditions requires higher shielding-gas flow rates to provide adequate coverage than when welding in the shop. For most welding, the gas flow rate is approximately 20 cfh to 35 cfh. A WPS typically specifies the type of shielding gas along with recommended gas flow rates. Final adjustments must often be made on a trial-and-error basis. The correct gas flow rate is determined by the type and thickness of metal to be welded,
the type and diameter of welding wire, the welding position and type of joint, the shielding gas used, and the type of metal transfer used. Proper gas shielding usually results in a rapid, crackling or sizzling sound for short circuiting transfer, and a hissing sound for spray transfer. Inadequate gas shielding produces a popping sound and results in porosity, spatter, and a discolored weld. Gas drift may occur with high travel speeds or in drafty conditions around the weld area. Gas drift commonly results in inadequate gas shielding. The welding area should be protected with windbreaks to eliminate breezes. It may also be necessary to increase the gas flow to provide better coverage. Some welding guns also have adjustable nozzles that can be positioned to improve gas coverage. See Figure 19-11. The distance from the work to the gas nozzle is determined by the nature of the weld. The gas nozzle is usually placed up to Z\₂″ from the work. Too much space between the gas nozzle and the work reduces the effectiveness of the gas shield, while too little space may result in excessive weld spatter, which collects on the gas nozzle, contact tip and gas diffuser and shortens component life.
GMAW WELDING WIRE
DIRECTION OF WIND
DIRECTION OF WELDING
GAS DRIFT
WINDY CONDITIONS CAUSE SHIELDING GAS TO DRIFT
DIRECTION OF WIND WINDBREAK
DIRECTION OF WELDING
PROPER GAS SHIELD
The correct diameter wire must be used to ensure a quality weld. Check the wire manufacturer recommendations for correct wire diameters.
WINDBREAK AND NOZZLE ADJUSTMENT PREVENT DRIFT
Figure 19-11. The welding area should be protected with windbreaks to prevent gas drifts.
Welding wire for GMAW should be similar in composition to the base metal. The copper coating on GMAW electrode wire aids in arc starting, improves wire feeding, and prolongs contact tip life. Welding wire for GMAW is classified by a letter and number system. See Figure 19-12. Metal cored welding wire is used extensively in high-production applications because it has 30% faster travel speeds than solid wires. It handles contaminants well, bridges gaps due to poor fit-up without excessive melt-through, and can be used to weld a variety of metal thicknesses.
Miller Electric Manufacturing Company
Welding wire is selected to match the composition of the metal to be welded. Welding wire designations are based on AWS classifications.
CHAPTER 19— GMAW – Equipment 229
ELECTRODE ROD TENSILE STRENGTH OF DEPOSITED WELD IN 1000 psi WIRE TYPE (S = SOLID C = COMPOSITE) CHEMICAL COMPOSITION (X = 2, 3, 4, 5, 6, OR 7 G = MANUFACTURER SPECIFIED)
Figure 19-12. The AWS classification system for carbon steel electrodes and rods for GMAW consists of a series of letters and numbers.
The E stands for electrode. The R stands for rod and indicates that it can be used as a non-current-carrying filler rod, as in GTAW. The number specifies the tensile strength of the deposited weld in thousands of pounds per square inch, for example, 70 = 70,000 psi. The S indicates a solid wire. The letter C in this same position would indicate composite (metal cored) wire. The digit following the dash indicates the chemical composition of the welding wire. Using this system enables a welder to choose the correct steel welding wire base on AWS specifications. See Figure 19-13. Basic welding wire diameters include 0.023″, 0.030″, 0.035″, 0.045″, 0.052″, Z\zn″, and Z\,″. Generally, welding wire of 0.023″, 0.030″, or 0.035″ is best for welding thin metal, although it can be used to weld low- and mediumcarbon steel and medium-thickness, high-strength/low-alloy (HSLA) steel. Medium-thickness metal normally requires 0.045″ or Z\zn″ diameter welding wire. See Figure 19-14. Welding position is a factor that must be considered when selecting welding wire. For vertical and overhead welding, 230 Welding Skills
small-diameter wire is preferred because it produces a smaller, more manageable weld pool. Metal cored welding wires are often preferred to solid wires for high-production GMAW applications. Metal cored welding wires are composite electrode wires consisting of a metal sheath (tube) filled with metallic powders. Like solid GMAW wire, metal cored wires produce slag-free welds that require little or no cleanup. Metal cored welding wires produce a wide arc with a broader penetration profile and better fusion into the toes of the weld than solid welding wires. However, metal cored welding wires are limited to the flat position and to horizontal fillet welds because of the fluidity of the weld pool. Metal cored wires handle contaminants such as rust and mill scale (a surface layer of ferrous oxide) well and bridge gaps due to poor fit-up without excessive melt-through. They can also be used to weld single and multiple-pass welds. Metal cored welding wires can weld a variety of metal thicknesses, including thin-gauge metal used in automobile exhaust systems.
WELDING WIRE FOR GMAW AWS Classification
Remarks
AWS A5.18/A5.18M
Specification for Carbon Steel Electrodes and Rods for Gas Shielded Arc Welding
ER70S-2
High in silicon and manganese deoxidizers; also contains aluminum, titanium, and zirconium deoxidizers; can be used for single and multiple pass welds, as well as root passes on carbon steel pipe; deoxidizer content makes it suitable for use on steels with moderate levels of mill scale
ER70S-3
Most commonly used welding wire; contains medium levels of silicon and manganese deoxidizers; can be used for single or multiple pass welds, with 100% CO2, Ar/CO2 mixtures, or Ar/O2 mixtures
ER70S-4
Higher in silicon and manganese that ER70S-3; can be used for single- or multiple-pass welds
ER70S-5
Contains aluminum; can be used for single- or multiple-pass welds on rimmed, semi killed, or killed mild steel; suitable for welding on rusty or dirty surfaces; normally used with CO2
ER70S-6
Higher in silicon and manganese that ER70S-4; can be used for single or multiple pass welds; good for welding rusty base metal or metal with moderate to high mill scale; high deoxidizer content produces a fluid weld pool with excellent fusion into the toes and a flat bead contour
ER70S-7
Higher in manganese that ER70S-6, but lower in silicon; can be used with Ar/CO2 mixtures; produces hardness levels between ER70S-3 and ER70S-6
ER70C-6M
Composite (metal cored) wire high in silicon and manganese deoxidizers; better than solid wire for welding on base metal high in mill scale; 30% faster travel speeds than solid wire; commonly used for mechanized and automatic applications; excellent penetration and fusion characteristics on a wide range of thicknesses; limited to flat position and horizontal fillet welds due to fluidity of weld pool
AWS A5.28/A5.28M
Specification for Low-Alloy Steel Electrodes and Rods for Gas Shielded Arc Welding
ER80S-Ni1
High in silicon and manganese with 1% nickel (corrosion resistance); also contains small amounts of chromium, molybdenum, and vanadium for higher strength and impact resistance
ER80S-D2
Higher than ER80S-Ni1 in silicon and manganese; contains 0.50% molybdenum for improved strength and toughness of weld metal
AWS A5.10/A5.10M
Specification for Bare Aluminum and Aluminum-Alloy Welding Electrodes and Rods
ER1100 ER4043 ER5183
Weld aluminum with similar composition welding wire
ER5554, 5556 ER5654 AWS A5.9/A5.9M ER308L ER308L-Si
Specification for Bare Stainless Steel Welding Electrodes and Rods For welding types 304, 308, 321, and 347 For welding types 301 and 304
ER309
For welding types 309 and straight chromium grades when heat treatment is not possible; also for welding types 304-clad
ER310
For welding types 310, 304-clad, and hardenable steels
ER316
For welding types 316
ER347
For welding types 321 and 347 where maximum corrosion resistance is required
AWS A5.7/A5.7M
Specification for Copper and Copper-Alloy Bare Wire Welding Rods and Electrodes
ECuSi (Silicon Bronze) ECuAl-A1 (Aluminum Bronze) ECu (Deoxidized Copper)
Special wires for welding copper and copper-based alloys
ECuAl-A2 (Aluminum Bronze) ECuAl-B (Aluminum Bronze) Figure 19-13. AWS has developed specifications for electrodes and rods used for welding a variety of metals, including carbon steel, stainless steel, aluminum, and copper.
CHAPTER 19— GMAW – Equipment 231
WELDING WIRE DIAMETERS Metal Thickness*
Wire Size*
.025 .031 .037 .050 .062 .078 .125 .125 .187 .187 .250 .250
.030 .030 .035 .035 .035 .035 .035 .045 .035 .045 .045 .052
Welding Conditions (DCEP) (arc volts) (amperes) 15 – 17 15 – 17 15 – 17 17 – 19 17 – 19 18 – 20 19 – 21 20 – 23 19 – 21 20 – 23 19 – 21 20 – 23
30 40 65 80 90 110 140 180 140 180 140 180
– 50 – 60 – 85 – 100 – 110 – 130 – 160 – 200 – 160 – 200 – 160 – 200
Gas Flow†
Travel Speed‡
15 – 20 15 – 20 15 – 20 15 – 20 20 – 25 20 – 25 20 – 25 20 – 25 25 – 30 25 – 30 30 – 35 30 – 35
15 – 20 18 – 22 35 – 40 35 – 40 30 – 35 25 – 30 20 – 25 27 – 32 14 – 19 18 – 22 10 – 15 12 – 18
NOTE: Gas flow rates will vary from values shown based on the type of metal welded Shielding gas CO2, welding grade Wire stickout—¹⁄₄″ to ³⁄₈″ * in in. † in cubic feet per hour (cfh) ‡ in in./min
Figure 19-14. Welding position is a factor that must be considered when selecting welding wire.
Metal cored electrode wires are noted for their high deposition rates. They are used extensively in high-production manufacturing and fabrication applications because they allow travel speeds 30% faster than solid wire and require little or no post-weld cleaning. The high travel speeds associated with metal cored wires make them well-suited to mechanized and automated applications. The most commonly used shielding
232 Welding Skills
gas for metal-cored electrodes is 90% Ar/10% CO2. Metal cored welding wires have replaced flux cored wires in some environments because they produce less smoke and fumes. They are also less sensitive to changes in welding gun angles and in welding wire extension. They also reduce top-toe undercut on horizontal fillet welds. This makes them extremely well-suited to semiautomatic applications as well.
Points to Remember • DCEP provides deep penetration and excellent cleaning action. • For GMAW, a constant-voltage welding machine with a nearly flat volt-ampere characteristic maintains a constant, preset voltage level during welding. • Ensure that the wire feed speed is set for the current that is to be used for welding. • Visible stickout is the distance the welding wire projects from the end of the nozzle of the welding gun. • The proper nozzle-to-work distance must be maintained to ensure an adequate shielding gas cover. • The use of CO2 as a shielding gas is most effective and least expensive when welding steel. • Argon produces the most effective results when welding aluminum. • For most welding, the gas flow rate is approximately 20 cfh to 35 cfh. • The correct diameter wire must be used to ensure a quality weld. Check the wire manufacturer recommendations for correct wire diameters.
CHAPTER 19— GMAW – Equipment 233
Questions for Study and Discussion 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
What are some of the specific advantages of GMAW? Why is DCEP current used for GMAW? What results can be expected if DCEN current is used? How does a constant-voltage (CV) welding machine differ from a constant-current (CC) welding machine? What is the advantage of using a CV welding machine for GMAW? What are the elements that make up air? Why is oxygen generally a harmful element in welding? Why does nitrogen cause the most serious problems in welding? When is argon or an argon-oxygen mixture considered the ideal gas for shielding? When is CO2 better for shielding than an inert gas? How is it possible to determine the proper gas flow for shielding? What happens if the gas flow is allowed to drift from the weld area? What factors must be taken into consideration in selecting the correct diameter welding wire? How is the welding wire fed to the welding gun? What determines the rate at which the wire feed should be set? Why is the correct electrode extension important?
Refer to Chapter 19 in the Welding Skills Workbook for additional exercises.
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234 Welding Skills