GMAW welding.pdf

feeders has made GMAW more useful. Welding usingGMAW is easy to learn, especially if a welder has alreadylearned to weld using a different process. GMAW equip-ment is relatively low in cost. Also, this process depositsmore weld metal in lbs./hr. (kg/hr) than the shieldedmetal arc or gas tungsten arc welding processes. The lowpurchase cost, the ability to weld continuously, and theability to deposit weld metal faster, make GMAW anattractive choice for welding.

GMAW can be used to produce high-quality welds onall commercially important metals such as aluminum,magnesium, stainless steels, carbon and alloy steels,copper, and others. GMAW may also be done easily in allwelding positions.

9.1 Gas Metal Arc Welding Principles

Gas metal arc welding is generally used because ofits high productivity. GMAW is done using solid wideelectrodes. FCAW uses flux cored wire electrodes. SeeFigure 9-1. A shielding gas or gas mixture must be usedwith GMAW.

GMAW is done using DCEP (DCRP). Alternatingcurrent is never used. DCEN (DCSP) is rarely used forGMA welding, but has found very limited use forsurfacing. DCEN (DCSP) is used with only one specialelectrode, called an emissive electrode. (AWS designationE70U-1).

For every pound of solid electrode wire used, 92%-98% becomes deposited weld metal. Flux cored arcwelding wire is deposited with a wire efficiency of 82%-92%. As a comparison, shielded metal arc welding(SMAW) deposits 60%-70% of the electrode wire as weldmetal. Some spatter does occur in the GMAW and FCAWprocesses. Very little stub loss occurs when continuouslyfed wire is used.

There is a very thin glass-like coating over the weldbead after GMA welding. No heavy slag is developedbecause the weld area is shielded by a gas. When FCAW, aslag covering is present. Some of the flux in the FCAW

233

Learning Objectives

After studying this chapter, you will be able to:� Contrast the various GMAW metal transfer

methods, considering arc characteristics, weldcharacteristics, and the possibility of performingout-of-position welds.

� Select the proper arc welding machine, wirefeeder, shielding gas, flow rate, contact tube,nozzle size, and electrode wire type to producean acceptable GMA weld.

� Contrast the various types of shielding gasesused when GMAW, and how they affect theshape and penetration of the completed welds.

� Properly assemble and adjust all the equipmentrequired to produce an acceptable GMA andFCA weld.

� Correctly prepare metals for welding, andperform acceptable welds on all types of joints inall positions using GMAW and FCAW.

� Identify the potential safety hazards involved inthe GMAW and FCAW process in a workingenvironment; be able to describe ways of safelydealing with these hazards.

� Be able to pass a safety test on the proper use ofthe GMAW and FCAW process.

The gas metal arc welding (GMAW) process uses asolid wire electrode that is continuously fed into the weldpool. The wire electrode is consumed and becomes thefiller metal. Flux cored arc welding (FCAW) is very similarto gas metal arc welding. One big difference is that FCAWuses an electrode wire with flux inside the wire. For anoverview of these processes, refer to Headings 4.3 and 4.4and also to Figures 4-3 and 4-4.

The growth in the use of GMAW is the result ofseveral events. The continuous development and refine-ment of constant voltage arc power sources and wire

Chapter 9Gas Metal Arc Welding

forms a gas around the weld area. Some of the flux formsa slag, covering the weld. Shielding gas may or may not beused when FCAW. More welder time can be spent on thewelding task with a continuously fed wire process. Thisimproves the cost efficiency of GMA and FCA welding.

The GMAW process can be adapted to a variety of jobrequirements by choosing the correct shielding gas, elec-trode size, and welding parameters. Welding parametersinclude the voltage, travel speed, and wire feed rate. Thearc voltage and wire feed rate will determine the fillermetal transfer method.

Metal transfer occurs in two ways. One is by the shortcircuiting method. The second is to transfer metal acrossthe arc. Methods of transferring metal across the arcinclude:

• Globular transfer.• Spray transfer.• Pulsed spray transfer.

9.1.1 Short Circuit GMAWShort circuit gas metal arc welding (GMAW-S) is

used with relatively low welding currents. It also uses elec-trode wire sizes under 0.045″ (1.1mm). This process isparticularly useful on thin metal sections in all positions.All position welds are made easily because there is nometal transfer across the arc. The weld pool cools andsolidifies rapidly using the short circuiting arc. Shortcircuiting transfer has a low heat input into the base metal.

Since short circuit gas metal arc welding has a lowheat input, it is also used to weld thick sections in the over-head or vertical welding position. It is very effective infilling the large gaps of poorly fitted parts.

Refer to Figure 9-2 to see how the short circuiting arcmethod deposits metal. When the electrode touches themolten weld pool, the arc is no longer present. The surface

234 Modern Welding

Shieldinggas

Contacttube

Wiremotion

Electrode

Gasnozzle

(GMAW)Gas metal Arc welding

Wiremotion

Shieldinggas(if used)

Contacttube

Gas nozzle(if used)

(FCAW)

Flux cored arc welding

Flux coredelectrode

Figure 9-1. Schematic views of GMAW and FCAW gas nozzles and electrodes. Shielding gas is not always used with FCAW.If shielding gas is not used, no nozzle is required.

Arcreignites

Wire nearsanother short

circuit

Metal shortcircuits toweld pool

Pinch forcesqueezingoff droplet

Pinchforce

Shieldinggasenvelope

Figure 9-2. The sequence of metal transfer during the shortcircuit GMAW method.

This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.

tension of the pool pulls the molten metal from the end ofthe electrode into the pool. The pinch force around theelectrode squeezes the molten end of the electrode. Thecombined effects of surface tension and the pinch forceseparate the molten metal and the electrode. The arc thenreestablishes itself. The continuously fed electrode againtouches the molten pool and the process repeats. Thedroplet transfer or short circuiting process repeats itselfabout 20 to 200 times per second. The strength of the pinchforce depends on the arc voltage, the slope of the powersource or welding machine, and the circuit resistance.These factors — voltage, slope, and resistance — affect thewelding current. The frequency of the pinch force and theformation of droplets is controlled by the inductance of thepower source.

If a 150A current is set on the arc welding machine,the amperage may rise rapidly to the maximum output ofthe machine when the electrode short-circuits. This couldbe 500A or more. To control and slow down this possiblerapid rise in current, an inductance circuit is built into thearc welding machine.

Inductance is the property in an electric circuit thatslows down the rate of the current change. Some arcwelding machines have an electric coil built in near thewelding current transformer coils. See Heading 5.2.1 for adiscussion of inductance. The current traveling through aninductance coil creates a magnetic field. This magneticfield creates a current in the welding circuit that is in oppo-sition to the welding current. Increasing inductance in awelding machine will slow down the increase of thewelding current. Decreasing the inductance will increasethe rate of change of the welding current.

When too little inductance is used, the current rises toorapidly. The pinch force is so great that the molten metal atthe end of the electrode literally explodes. A great deal ofspatter occurs in this case. When too much inductance isused, the current will not rise fast enough. The molten endon the electrode is not heated sufficiently.

By properly balancing the inductance and slope, anideal droplet transfer rate and pinch force can be obtained.See Figure 9-3 for the metal deposition rate for the shortcircuiting transfer method. Shielding gas also has an effecton short circuiting transfer. Inert gases must be used on allnonferrous base metals. Nonferrous base metals are thosethat do not contain iron as the main element. Thisgrouping includes everything except steels, steel alloys,

and cast irons. Adding helium to argon will increase thepenetration. Argon and helium mixtures are used only onnonferrous base metals.

Carbon dioxide (CO2) may be used as a shielding gaswhen GMA welding carbon and low-alloy steels. CO2 willproduce greater penetration, but will create more spatterthan an inert gas used for shielding. Mixtures of argon andCO2 are often used. They provide a good combination ofimproved penetration with minimal spatter. Stainless steelusually requires a mix of three gases. A typical mixture is90% helium, 7 1/2% argon, and 2 1/2% CO2.

9.1.2 Globular TransferGlobular metal transfer gas metal arc welding occurs

when the welding current is set slightly above the rangeused for short circuiting metal transfer. In the globularmetal transfer process, the metal transfers across the arc aslarge, irregularly shaped drops. See Figure 9-4. The dropsare usually larger than the electrode diameter.

Chapter 9 Gas Metal Arc Welding 235

GMAWMethod

Metal Depositedlbs/hr kg/hr

Short circuitingGlobularSprayPulsed spray

2-64-76-122-6

0.9-2.71.8-3.22.7-5.40.9-2.7

Figure 9-3. The approximate rate at which filler metal isdeposited with various GMAW methods. (American Welding Society)

Arc


Spatter

Deepweldpool

Irregular large dropletforming

Droplet may shortcircuit when it falls

Droplet may fall erraticallyand cause spatter

Buried arc helps to containdroplet to reduce spatter

Figure 9-4. GMAW globular metal transfer. Drops may fallerratically and cause spatter. Note that the buried arc mayhelp contain the drops to reduce spatter.

Drops form on the end of the electrode. Each dropgrows so large that it falls from the electrode due to its ownweight. When a high percentage of inert gas is used forshielding, the drops will fall into the weld pool. When a high percentage of carbon dioxide is used, the dropstravel across the arc in random patterns, creating spatter.To minimize spatter, a shorter arc length can be used.However, a short arc will allow large drops to short to thework. The drops will explode, still creating a lot of spatter.

One way to minimize spatter when using CO2 is toincrease the current slightly. This will create a deep weldpool that is below the metal surface. This is referred to as aburied arc or submerged arc. Using a buried arc, much ofthe spatter is contained within the deep weld pool. With aburied arc, a combination of globular and short circuitingtransfer occurs. Deeper penetration occurs when using aburied arc.

Welds of sufficient quality for many applications canbe produced with this process. When using globulartransfer, welding can be done only in the flat welding posi-tion, because the molten metal falls into the weld pool.Welds may be made faster with this process than with theshort circuiting transfer method. See Figure 9-3 for the rateat which metal is deposited with this method.

9.1.3 Spray TransferSpray transfer gas metal arc welding will occur when

the current and voltage settings are increased above thoserequired for globular transfer. When spray transfer occurs,very fine droplets of metal form. These droplets travel at ahigh rate of speed directly through the arc stream to theweld pool. Figure 9-5 illustrates the spray arc metaltransfer method.

Spray transfer will only take place when a highpercentage of argon is used. When welding nonferrousmetals and alloys, 100% argon shielding gas is used. Whenusing spray transfer on carbon or low-alloy steels or stain-less steels, a shielding gas mixture containing at least 90%argon is used

Before spray transfer can occur, a current settingabove the transition current level must be made on thewelding machine. The transition current varies with theelectrode diameter, its composition, and the amount ofelectrode extension. A higher transition current is requiredfor steel than aluminum. The transition current increaseswith the electrode diameter. It decreases as the electrodeextends farther from the contact tube. See Figure 9-40.Until the transition current is exceeded, the metal transfersas large globules. Above the transition current level, thepinch force becomes great enough to squeeze the metal offthe tip of the electrode as fine droplets. See Figure 9-6 forvarious transition current levels.

The droplets are squeezed off cleanly and transferredacross the arc gap in a straight path. Spray transfer occursonly when at least 90% argon is used as the shielding gas.Common shielding gas mixtures for carbon and low-alloysteels are: 98% Ar plus 2% O2; 95% Ar plus 5% O2; 95% Arplus 5% CO2 ; and 90% Ar plus 10% CO2. The spraytransfer method produces deep penetration. The arc can be

directed easily by the welder. This is because the arc andmetal spray pattern are stable and concentrated. Spraytransfer is best done in the flat or horizontal welding posi-tion, and on metal over 1/8″ (3mm) thick. See Figure 9-3for the metal deposition rate.

9.1.4 Pulsed Spray TransferThe pulsed spray transfer gas metal arc welding

method is similar to the spray transfer method. See Figure9-7. The current level for pulsed spray must be above thetransition current level. Special circuits within the powersource (welding machine) cause the current to pulse. Alow-level current in the globular transfer range is used tomaintain the arc. This current is called the backgroundcurrent. The current is increased at a regular frequency tothe peak current. The peak current is above the transitioncurrent level. Since the background current is on for only ashort time, no globular transfer actually occurs. During thepeak current time period, spray transfer occurs. In this

236 Modern Welding

Stage 1

Stage 2

Metaldroplets

Wirenecksdown


Arc

Figure 9-5. Spray transfer method. Note how the dropletsare concentrated in the center of the arc. Spray transfer willoccur only when a high percentage of argon gas is used.

method, no necking down of the wire occurs. The metalleaves the electrode in a spray of small droplets.

Spray transfer does not occur continually, therefore,the name pulsed spray transfer. The rate of metal transferincreases and the droplet size decreases as the pulsefrequency increases. Basic welding machines with pulsecapabilities allow the welder to select pulse frequencies of60 or 120 pulses per second. Some machines allow the userto adjust the pulse frequencies. Pulse frequencies can gomuch higher than 120 pulses per second. The coolest spraytransfer occurs at 60 pulses per second.

A lower average current level is used in pulsed spraythan in spray transfer. This lower average current levelmakes it possible to weld out of position. Thin metalsections may also be welded more easily with the pulsedspray. This method creates very little metal spatter.

The pulsed spray transfer method can use larger-diameter electrode wire. This is an advantage. Larger-diameter electrodes are cheaper. Also, nonferrous wires oflarger diameter can be fed through the wire drive unitmore easily without kinking.

See Figure 9-3 for the metal deposition rate for thepulsed spray transfer method. Pulsed spray is also used toweld parts with silicon bronze filler wire. This process issometimes called MIG brazing. Light steel parts in autorepair shops can be welded with very low heat inputs.This reduces the problems of distortion and melt-through.

9.2 GMAW Power Sources

Welding power sources for GMAW provide aconstant voltage. The most common types of power


Wire electrodetype

Mild steelMild steelMild steelMild steelStainless steelStainless steelStainless steelAluminumAluminumAluminumDeoxidized copperDeoxidized copperDeoxidized copperSilicon bronzeSilicon bronzeSilicon bronze

Wire electrodediameter

Shielding gasMinimumspray arccurrent, A

0.760.891.141.590.891.141.590.761.141.590.891.141.590.891.141.59

98% argon-2% oxygen98% argon-2% oxygen98% argon-2% oxygen98% argon-2% oxygen99% argon-1% oxygen99% argon-1% oxygen99% argon-1% oxygenargonargonargonargonargonargonargonargonargon

150165220275170225285 95135180180210310165205270

Note: Spray transfer will only occur when high percentage of argon are used.

in. mm0.0300.0350.0450.0620.0350.0450.0620.0300.0450.0620.0350.0450.0620.0350.0450.062

Figure 9-6. Approximate transition current levels to obtain spray transfer for various metals. (American Welding Society)

Dropletsform ata constantrate


Dropletformswithoutneckingof wire

Figure 9-7. Pulsed spray metal transfer method. Spraytransfer only occurs during peak current.

sources are transformer-rectifier machines. GMAW is doneusing DCEP. DCEN can be used in special applications. Acis not used for gas metal arc welding.

Inverter power sources are much smaller and lighterthan traditional transformer-rectifier machines. This typeof power source is gaining in popularity. Quite often, aninverter arc welding machine will provide a choice ofconstant current or constant voltage from the samemachine. The welder must select the constant voltagemode when GMAW. Performance of an inverter machineis very similar to a transformer-rectifier constant voltagemachine.

Machines used for GMAW may have a wire feederbuilt into the power supply. The wire feeder may be anexternal unit, as seen in Figure 9-8.

Inverter machine controls are very similar to thoseon a transformer-rectifier machine. If the invertermachine has the ability to do multiple processes, selectGMAW or the constant voltage setting. After making thisselection, the main control setting on the invertermachine is used to set the voltage. The wire feed speedadjustment sets the wire feed speed and also sets theappropriate current. The output and performance of aninverter are the same as those of a transformer-rectifiertype machine. Refer to Heading 5.2.3 for more informa-tion on inverter machines.

9.3 Setting Up the GMAW Station

Figure 9-9 illustrates a complete GMAW outfit. Thesame equipment may be used for flux cored arc welding.Remember, self-shielding FCAW does not require anyshielding gas.

To prepare a GMAW or FCAW outfit for welding, thefollowing steps should be taken:1. Connect a separate wire feed unit to the welding

power source, if required. The manufacturer’sinstructions should be followed to make theseconnections. Usually, a single cable assembly isenough to electrically connect the wire feeder to thewelding machine. The welding lead from the positiveterminal of the welding machine is usually connectedto the wire feeder. Connecting the positive lead to thewire feeder will provide DCEP current. A shieldinggas hose may also need to be connected.

2. Mount the desired electrode wire reel (spool) ontothe wire feeder.

3. Determine what shielding gas is required. SeeHeading 9.3.3. No shielding gas is required whenusing self-shielded FCAW. Properly secure thecylinder (if used) to prevent it from being knockedover. Check that the regulator and gas flowmeter areattached properly. Connect a hose from the flowmeterto the welding machine or wire feeder as required.The hose and fittings should be checked to make surethere are no leaks.

4. Connect the welding gun to the correct place on thewelding machine or wire feeder. Quite often there aretwo connections to be made. See Figure 9-8. One is themain cable, that is connected where the electrode wireexits the wire feeder. Attach this part of the cableassembly to its proper place. The second connection isfor the electrical control circuit. This part of the cableassembly also must be connected to its proper place.A water-cooled gun will have additional connections.

5. Connect the workpiece lead to the welding machine.Both the welding gun cable assembly and the work-piece lead should be checked for any signs of wear orcuts. Such wear or cuts on the outside may indicatedamage to the leads.

6. The workpiece clamp should be checked. The clampshould be clean so it can make a good electricalconnection.

238 Modern Welding

Remote contactorcable connection

Cable towelding gun

Figure 9-8. An inverter power source with a separate wirefeeder mounted on top of the welder. (Miller Electric Mfg. Co.)

Transformer-rectifier machines are designed tocontrol voltage. Voltage is one of the two important vari-ables used to set the welding parameters for GMAW. Onthe front of the machine, there is a control that is used toset the voltage.

The second important variable is the wire feed speed.This control will be on the welding power source if the wire feeder is built into the power source. If the wirefeeder is not in the power source, the wire feed speed is seton the external wire feeder. When the welder sets the wirefeed speed, the wire feed speed and the appropriatewelding current are being adjusted. A higher wire feedspeed requires a higher current to melt the electrode wirefaster. A slower wire feed speed requires less current tomelt the electrode wire.

7. If a water cooler is used, connect it to the weldingmachine or to the gun according to the manufactures’recommendations. Usually, the welding machine,wire feeder, or welding gun is connected to the outleton the water cooler. This way, cool water flows fromthe cooler to the gun. Warm water returning from thegun is connected to the inlet on the water cooler.When setting up the GMAW station, look for all

potential safety problems. Spatter from GMAW or FCAWcan cause a fire. All flammable materials must be removedfrom the welding area.

9.3.1 Setting Up the GMAW Power SourceProperly setting up of a GMAW power source is

necessary to obtain the desired transfer method. Beforesetting up the power source, the following informationneeds to be known:

• The type of base metal to be welded.• Base metal thickness.• The type of transfer method to be used.• The type of shielding gas to be used.• The type and diameter of electrode wire.Once these are known, the welding machine can be

properly set up. Only a few controls must be set prior towelding. Figure 9-10 shows a welding machine and itscontrols.

Two switches must be set. One is to allow the voltageto be set on the panel or remote. The second switch allowsa choice between a remote or panel contactor. Once thesetwo switches are set, they are rarely changed. Once theremote or panel voltage and contactor switch are set, the

only adjustments that need to be made are the voltage andwire feed speed.

The next adjustment is the voltage setting. Thevoltage determines the arc length and helps determine theelectrode transfer method. Other factors also affect thetransfer method as discussed in Headings 9.1.1 through9.1.4.

Welding machines with a wire feeder built in willhave the wire feed speed adjustment on the machine itself.If the wire feeder is a separate unit, the wire feed speedadjustment is on the wire feeder. When adjusting the wire


Wire speed control

Gas out

Switch Wire reel

Gas in

Flowmeter

Shieldinggassource

Regulator

Voltagecontrol

Wire feeddrive motor

Contactor control

110V supply Welding power source

Contactor cable

Manuallyheld gun

Work

Figure 9-9. Diagram of a complete gas metal arc welding (GMAW) outfit.

Remote/panelselector

Voltageselector

Volts

Amps

Poweron/off

Figure 9-10. A cc/cv welding power source. (Miller Electric Mfg. Co.)

feed speed, the welder is adjusting the nominal amperageof the welding machine.

The following figures list the voltage and amperagesettings for welding different base metals using both shortcircuiting transfer and spray transfer.

Metal FigureMetal Transfer Method No.Mild and low- Short circuit 9-11

alloy steel Spray transfer 9-12 Stainless steel Short circuit 9-13

(300 series) Spray transfer 9-14Aluminum and Short circuit 9-15

aluminum alloys Spray transfer 9-16

Globular transfer voltages and amperages will fall inthe range between those shown for short circuiting andspray transfer. Pulsed spray background voltage settingswill be slightly higher than the values shown for shortcircuiting transfer. The peak current must be above thetransition current.

Figure 9-17 shows a welding machine on whichwelding variables are set and stored in electronic memory.The welding machine has a microprocessor inside. Amicroprocessor can be considered a small computer. The

240 Modern Welding

Electrodediameter

0.0300.0350.045

Arcvoltage

Amperage range

in. mm

0.760.891.14

15-2116-2217-22

70-130 80-190100-225

Note: The values shown are based on the use of CO2 for mild steel and argon CO2 for low-alloy steel.

Figure 9-11. Approximate machine settings for short circuiting metal transfer on mild and low-alloy steel.

Electrodediameter

0.0300.0350.0451/163/32

Arcvoltage

Amperage range

in. mm

0.760.891.141.592.38

24-2824-2824-3024-3224-33

150-265175-290200-315275-500350-600

Note: The values shown are based on the use of argon with 2%-5% oxygen for mild and low-alloy steel.

Figure 9-12. Approximate machine settings for spraytransfer on mild or low-alloy steel.

Electrodediameter

0.0300.0350.045

Arcvoltage

Amperage range

in. mm

0.760.891.14

17-2217-2217-22

50-145 65-175100-210

Note: The values shown are based on a mixture of 90% helium; 7 1/2% argon; 2 1/2% CO2. The flow rates were about 20 cfh (9.44 L/min.).

Figure 9-13. Approximate machine settings for short circuiting transfer on 300 series stainless steel.

Electrodediameter

0.0300.0350.0451/163/32

Arcvoltage

Amperage range

in. mm

0.760.891.141.592.38

24-2824-2924-3024-3224-32

160-210180-255200-300215-325225-375

Note: The values shown are based on the use of argon-oxygen shielding gas. The oxygen percentage varies from 1-5%.

Figure 9-14. Approximate machine settings for spraytransfer on 300 series stainless steel.

Electrodediameter

0.0300.0350.047(3/64)

Arcvoltage

Amperage range

in. mm

0.760.891.19

15-1817-1916-20

45-12050-15060-175

Note: The values shown are based on the use of argon shielding gas.

Figure 9-15. Approximate machine settings for short circuiting transfer on aluminum and aluminum alloys.

Electrodediameter

0.0300.0350.047(3/64)1/163/32

Arcvoltage

Amperage range

in. mm

0.760.891.19

1.592.38

22-2822-2822-28

24-3024-32

90-150100-175120-210

160-300220-450

Note: The values shown are based on the use of argon as the shielding gas.

Figure 9-16. Approximate machine settings for spraytransfer on aluminum and aluminum alloys.

microprocessor is programmed by the manufacturer witha set of welding values. Based on a set of input data abouta weld, the microprocessor determines the best settings orparameters for the welding application. Since the weldingmachine has determined the welding parameters, the typeof transfer is also determined by the welding machine.

A welder using a microprocessor-equipped weldingmachine enters the following type information about theweld to be made: electrode wire type, wire diameter, typeof shielding gas, and metal thickness. The microprocessorsets the voltage, wire feed speed, and possibly the slope,thus determining the type of metal transfer that will beused. Standard welding values are preprogrammed intothe welding machine. Special welding parameters canalso be saved as a program in the machine. This program,or any preset values, can be recalled at any time in thefuture. Because the welding values are stored electroni-cally, the welding machine will be set up exactly the sameway each time.

Some power sources allow the slope to be changed.Many machines have a preset value for the slope. Heading7.10 discusses slope.

Power sources that have the ability to pulse weldhave additional controls to set up. These controls includean on-off switch, a background voltage adjustment, apeak amperage adjustment, and sometimes a “pulsesper second” adjustment. The background voltage is setrelatively low, in the globular transfer range. The peak

amperage is fairly high to cause spray transfer to occur.This peak amperage must be above the transition current.The pulses per second adjustment is used to set thenumber of times per second the current will pulse from thelow value to the high value. Figure 9-18 shows a GMAWmachine with a power supply, wire feeder, and shieldinggas cylinder.

9.3.2 Setting Up the Wire FeederMost wire feeders use a 115V ac motor; however,

24V dc motors are becoming very popular. Figure 9-19shows a complete wire drive unit. Two mated gears arelocated in the wire drive unit. One gear is driven by anelectric variable-speed motor. A roll is attached to each gear.Figure 9-20 illustrates a two-drive-roll wire drive system.

The lower roll on the wire drive unit shown in Figure 9-20 is adjustable in and out. The lower drive gearhas spring washers behind it. By turning the adjustmentbolt in the center of this gear, the gear and drive roll can bemoved inward or outward. This adjustment is provided toalign the groove in the wire drive roll with the center of thewire. Figure 9-21 illustrates the adjustment of the wiredrive rolls.


Figure 9-17. A microprocessor-controlled power sourceand wire feeder with digital displays. (Lincoln Electric)

Wire feedspeed

Amp/voltdisplayselector

On/off

Welding processselector

Arc powerinductancecontrol

Pulsed GMAWscheduleselector

Figure 9-18. A portable GMAW power source. The controlpanel includes a pulse schedule selector. (Hobart Brothers Co.)

guide should be adjusted as close to the drive rolls aspossible without touching them. After the wire guide is set,the securing bolt is tightened to hold the guide in place.

One problem that occurs occasionally during wirefeeder operation is the wire getting jammed and forminga bird’s nest. A bird’s nest is a tangle of electrode wire thatdid not feed properly through the rolls and into the guidetube. Figure 9-24 shows such a bird’s nest.

242 Modern Welding

Wire spoolspindle

Selectorswitch for 2 presetwire feedconditions

Digital voltagereadout

Digital amperagereadout

Figure 9-19. A wire feeder with multiple feed controlprograms. (Thermal Arc, a Thermadyne Company)

Pressureroll

Driveroll

In/outadjustmentbolt

Well-adjustedpressure roll:

down far enough and in alignment

Wire too loose:adjust pressureroll downward

Rolls misaligned:adjust drive roll outward

Figure 9-21. Adjusting drive rolls. The pressure (upper) rollis adjusted up and down by means of the pressure-adjusting knob, as in Figure 9-20. The lower drive roll isadjusted in and out by means of an adjustment screw.

Maindrive gear

Figure 9-22. A four-wheel wire drive system. The parts aresimilar to a two-wheel wire drive. The main drive gear is inthe center below the two lower rolls. The main gear drivesthe gears behind the lower rolls. (Lincoln Electric Co.)

Upper gear swings upto disengage gears

Outer wireguide

In and outadjustment

for wireguide

In and outadjustmentfor drive roll

Drive housing bolt

Drivehousing

Drive roll withdrive gear behind it

Wire pressure adjusting knob

Inlet wireguide

Pressure rollwith gearbehind it

Figure 9-22 shows a wire drive unit with four driverolls. This drive unit is similar to the two drive roll unitshown in Figure 9-20. The unit shown in Figure 9-22 has three wire guides. Wire guides must be in alignmentwith each other and with the center of the drive rolls.Figure 9-23 illustrates properly and improperly adjustedwire guides. The alignment of the wire guides is made atthe factory. In time, an adjustment may be necessary. Seedrive units in Figures 9-20 and 9-22. The end of each wire

Figure 9-20. A two-drive roll wire drive system. The upperpressure roll is pivoted out of the way when the wing nutis loosened and the gear cover lifted up. (Miller Electric Mfg. Co.)

To eliminate the bird’s nest and continue welding, thefollowing steps should be taken:1. Turn off the power source and the wire feeder.2. Raise the upper pressure roll.3. Cut the bird’s nest wire at the outlet of the inlet wire

guide and at the entry of the outlet wire guide. SeeFigure 9-24.

4. Remove the electrode wire from the cable assemblygoing to the welding gun. This has eliminated thebird’s nest.

5. Feed new wire into the cable assembly and lower theupper pressure roller.

6. Feed the electrode wire to the gun by pulling thetrigger on the gun, or by pressing the inch switch.To load a spool of electrode wire, place the spool onto

the hub on the wire feeder. Secure the spool using themethod available on the type of wire feeder being used.

Pressure rolls have one or two grooves cut in them.Select and install pressure rolls that have the same groovediameter as the diameter of the electrode wire being used.Remember to change the pressure rolls if the electrodediameter changes.

The final adjustment is to apply the proper force fromthe rolls to the electrode. Adjustment is made using aspring-loaded wing nut or knob. Tighten the knob toapply only enough force to drive the wire without slip-page. Too much force on the rolls and wire may cause thesolid wire to flatten (especially if the wire is aluminum).Flux cored electrodes may be crushed. If the wire isdamaged, it will not feed through the wire cable and torchproperly. If not enough force is applied to the wire, therolls will slip and not drive the wire consistently.

Once the adjustments discussed so far have beenmade, the wire feeder is ready to feed wire continuously.Only the wire feed speed needs to be adjusted to meet therequirements for each welding job. Adjust the feed speedto obtain the amperage and transfer method desired.


A bird’s nest can be caused by the followingconditions:

• Stubbing the electrode onto the base metal. Thisis caused by holding the gun too close to thework, using too-low a voltage or by using too-high a wire feed speed.

• Misaligned guide tubes and rolls.• A blockage in the cable or liner.To correct the cause of a bird’s nest, use the correct

contact tube-to-work distance (see Figure 9-40) or adjustthe settings on the welding machine or wire feeder. If thisdoes not solve the problem, determine if there is amisalignment of the guide tubes and rolls or if there is ablockage in the cable or liner. Correcting these problemswill eliminate the cause of the bird’s nesting.

Wire

Inlet wireguide

Outlet wireguide

Drive rollDrivehousing

Drive housingattachingbolt(s)

Good Wire bent down Wire bent up

Drive housing andwire guides too low.To correct: raisedrive housing

Drive housing and wire guides too high.To correct: lowerdrive housing

Drive rolls and wireguides properly aligned

Figure 9-23. Properly and improperly aligned wire guides. If the wire bends going through the drive rolls, adjust the drivehousing up or down. Loosen the drive housing bolts, align, and tighten the bolts.

Figure 9-24. Removing a bird’s nest by cutting the elec-trode wire behind the inlet wire guide and before the outletwire guide.

Figures 9-28 and 9-29 list shielding gases to be usedwith different metals and transfer methods. The shieldinggases listed for short circuiting transfer are usually alsoused for globular transfer. Those gases listed for spraytransfer are also used for pulsed spray transfer.

Inert gases, such as argon and helium, are chemicallyinactive and do not unite with other chemical elements.Nitrogen, oxygen, and carbon dioxide are reactive gases.They will mix or react with metals in a weld. With theexception of CO2, reactive gases are not used alone asshielding gases. Nitrogen gas is used in Europe to weldcopper.

As noted earlier, each gas and gas mixture has aneffect on the type of metal transfer, and on the bead size,penetration, welding speed, and undercutting tendencies.Each of the important gases or gas mixtures is discussed inthe following paragraphs. Also refer to Heading 7.12.

ArgonThis gas causes a squeezing (constricting) of the arc.

The results are a high current density (concentration) arc,deep penetration, a narrow bead, and almost no spatter.Argon ionizes more easily than helium and it conductssome electricity. Therefore, lower arc voltages are requiredfor a given arc length. Argon conducts heat through thearc more slowly than helium. Argon has a lower thermal(heat) conductivity. It is an excellent choice for use on thinmetal. It is also good for out-of-position welds because ofthe low voltages required.

Argon is the most common inert gas used for weldingnonferrous metals. It is used for all types of metal transfer.When welding steel and steel alloys using spray transfer,high percentages of argon, 90% or greater, must be used.

Pure argon used on carbon steel will cause undercut-ting using the spray transfer method. Because this under-cutting is not acceptable, argon is usually mixed withsmall amounts of oxygen or carbon dioxide. Argon is

244 Modern Welding

Figure 9-26. Two gas mixers. The one on the left mixes upto 50% CO2 with argon. The one on the right mixes up to10% oxygen with argon. (Thermco Instrument Corp.)

Other features on wire feeders often include an inchswitch or jog switch. This switch is used to feed wire to thegun at a relatively slow speed to prevent kinking the wire.Another switch is the purge switch. This is used to allowthe shielding gas to flow, so that the shielding gas will fillthe hose and remove (purge) all air.

Some wire feeders have a display that shows eitherthe set or actual voltage or wire speed. Figure 9-25 showssuch a wire feeder.

9.3.3 Inert Gases and Gas Mixtures Used forGMAW

The inert shielding gases and other gases used inshielding gas mixtures for GMAW are argon (Ar), helium(He), oxygen (O2), carbon dioxide (CO2), and nitrogen(N2).

Inert gases used should be of a welding grade. Carbondioxide gas is generally supplied 100% pure. Gasmixtures can be purchased from a welding gas distributoror can be mixed using a gas mixer like the one shown inFigure 9-26.

Each shielding gas and mixture of gases will have adifferent effect on the shape of the bead and the penetra-tion. See Figure 9-27.

Factors that must be considered when choosing ashielding gas are:

• The type of metal transfer desired: shortcircuiting, globular, spray, or pulsed spraytransfer.

• The desired bead shape, width, and weld pene-tration.

• The required welding speed.• The undercutting tendencies of the gas.

Wire speeddisplay

Voltage display

Figure 9-25. A wire drive unit. The visual displays willshow set or actual wire speed and voltage. (Lincoln ElectricCo.)

heavier than helium, therefore, less gas is needed toprotect a weld.

HeliumThe inert gas helium (He) has a high heat-conducting

ability. It transfers heat through the arc better than argon.

Helium is used to weld thick metal sections. This gas is alsoused to weld metals that conduct heat well. Such metals asaluminum, magnesium, and copper will conduct heataway from the weld zone rapidly. More heat must be putinto the metal, therefore, helium gas is the best choice. Thearc voltages required for helium are higher and spatter isgreater. Helium will allow filler metal to be deposited at a

Chapter 9 Gas Metal Arc Welding 245Chapter 9 Gas Metal Arc Welding 245

Argon &oxygen

Argon Helium &argon

Helium

Arc

Figure 9-27. Bead contours and penetration shapes that occur with various gases using DCEP polarity.

Metal Shielding gas

Advantages

Aluminum,copper,magnesium,nickel, andtheir alloys

Steel,carbon

Steel,low-alloy

Steel,stainless

argon andargon-helium

argon-20-25% CO2

argon-50% CO2

CO2a

60-70% helium-25-35% argon-4-5% CO2

argon-20-25% CO2

90% helium-7.5% argon-2.5% CO2

Argon satisfactory on sheet metal; argon-heliumpreferred on thicker sheet metal.

Less than 1/8″ (3mm) thick; high welding speedswithout melt-through; minimum distortion and spatter;good penetration.

Greater than 1/8″ (3mm) thick; minimum spatter; clean weld appearances; good weld pool controlin vertical and overhead positions.

Deeper penetration; faster welding speeds; minimumcost.

Minimum reactivity; good toughness; excellent arcstability, wetting characteristics, and bead contour;little spatter.

Fair toughness; excellent arc stability; wetting characteristics,and bead contour; little spatter.

No effect on corrosion resistance; small heat-affected zone; no undercutting; minimum distortion; good arc stability.

a - CO2 is used with globular transfer also.

Figure 9-28. Suggested gases and gas mixtures for use in GMAW short circuiting transfer.

faster rate than is possible with argon. This gas is oftenused on nonferrous metals. It produces welds with widerbead reinforcements. Helium is lighter than argon and willrequire a greater gas flow to protect a weld as well as argon.In addition to requiring a greater flow rate that uses moreshielding gas, helium is about 10% more expensive thanargon. Even though the cost for helium may be greater thanthat for argon, the benefits of helium for the right weldingapplication make helium an excellent choice.

Carbon dioxideThis gas has a higher thermal (heat) conductivity than

argon. It requires a higher voltage than argon. Since carbondioxide (CO2) is heavy, it covers the weld well. Therefore,less gas is needed.

CO2 costs about 80% less than argon. This price differ-ence will vary from location to location. Beads made withCO2 have a very good contour. The beads are wide andhave deep penetration and no undercutting. The arc in a

CO2 atmosphere is unstable and a great deal of spatteringoccurs. This is reduced by holding a short arc. Deoxidizerslike aluminum, manganese, or silicon are often added tothe filler metal. The deoxidizers remove the oxygen fromthe weld metal. Good ventilation is required when usingpure CO2. About 7%-12% of the CO2 becomes dangerousCO (carbon monoxide) in the arc. The amount of COincreases with the arc length.

NitrogenIn Europe, nitrogen (N2) is used where helium is not

readily available. Mixtures containing nitrogen have beenused to weld copper and copper alloys. One mixture usedcontains 70% argon and 30% nitrogen.

Argon-heliumMixtures of argon and helium help to produce welds

and welding conditions that are a balance between deeppenetration and a stable arc. A mixture of 25% argon and

246 Modern Welding

Metal Shielding gas

Advantages

Aluminum

Copper, nickel,& their alloys

Magnesium

Reactive metals (titanium, zirconium, tantalum)

argon

75% helium-25% argon90% helium-10% argon

argon

helium-argon

argon

argon

argon-2-5% oxygen

0.1″-1″ (0.25mm-25mm) thick; best metal transfer and arcstability; least spatter.1-3″ (25-76mm) thick; higher heat input than argon.

3″ (76mm) thick; highest heat input; minimizes porosity.

Provides good wetting; good control of weld pool for thicknessup to 1/8″ (3mm).

Higher heat inputs of 50% and 75% helium mixtures offset high heat conductivity of heavier gages.

Excellent cleaning action.

Good arc stability; minimum weld contamination. Inert gasbacking is required to prevent air contamination on backof weld area.

Good arc stability; produces a more fluid and controllable weld pool; good coalescence and bead contour, minimizes undercutting; permits higher speeds, compared with argon.

Steel,carbon

argon-2% oxygen

Minimizes undercutting; provides good toughness.Steel,low-alloy

argon-1% oxygen

Good arc stability; produces a more fluid and controllableweld pool, good coalescence and bead contour, minimizesundercutting on heavier stainless steels.

Steel,stainless

argon-2% oxygen

Provides better arc stability, coalescence, and welding speedthan 1% oxygen mixture for thinner stainless steel materials.

Figure 9-29. Suggested gases and gas mixtures for use in GMAW spray transfer.

75% helium will give deeper penetration with the arcstability of a 100% argon gas. Spatter is almost zero whena 75% helium mixture is used. Argon-helium mixtures areused on thick nonferrous sections.

Argon-carbon dioxideMixing CO2 in argon makes the molten metal in the

weld pool more fluid. This helps to eliminate undercuttingwhen GMA welding carbon steels using spray transfer.CO2 also stabilizes the arc, reduces spatter, and promotes astraight-line (axial) metal transfer through the arc.

Argon-oxygenArgon-oxygen gas mixtures are used on low-alloy,

carbon, and stainless steels. A 1%-5% oxygen mixture willproduce beads with penetration that is wider and lessfinger-shaped. Oxygen also improves the weld contour,makes the weld pool more fluid, and eliminates undercut-ting. Oxygen seems to stabilize the arc and reduce spatter.The use of oxygen will cause the metal surface to oxidizeslightly. This oxidization will generally not reduce thestrength or appearance of the weld to an unacceptablelevel. If more than 2% oxygen is used with low-alloy steel,a more expensive electrode wire with additional deoxi-dizers must be used.

Helium-argon-carbon dioxideThis shielding gas mixture is used to weld austentic

stainless steel, using the short circuiting transfer method.The following mixture is often used and produces a lowbead: 90% He; 7 1/2% Ar; 2 1/2% CO2.

Metal transfer methodsThe various GMAW metal transfer methods, and the

gases suggested for use with them follow.

Short circuiting transferPure argon or helium, or argon and helium mixtures,

are used on aluminum and other nonferrous metals andtheir alloys. For carbon steels, pure CO2 or a mixture of75% argon and 25% CO2 is often used. A mixture ofhelium, argon, and CO2 is used to weld stainless steel.

Globular transferArgon with high percentages of CO2, or pure CO2, are

used to weld low-carbon steels with globular transfer.With CO2, the globules leave the wire in a random wayand spatter is high. When argon or a high argonpercentage gas mixture is used, the metal is squeezed offthe wire and travels in a straighter line to the metal.

Spray and pulsed spray transferThe spray transfer method will occur only in an

atmosphere that has a high argon percentage. Pure argonor an argon-helium mixture is used on nonferrous metals.

The following argon mixtures are used when welding low-carbon steels: argon with 2%-5% oxygen (O2), and alsoargon with 5%-10% CO2.

Small amounts of oxygen lower the transition current.Oxygen appears to decrease the surface tension of themolten metal on the wire. This allows the molten metaldroplets to leave the electrode more easily. Oxygen makesthe weld pool more fluid and reduces undercutting. It alsoacts to stabilize the arc.

Figure 9-30 lists shielding gas selections for GMAWon a number of metals.

9.3.4 Selecting the Proper Shielding Gas FlowRate for GMAW

Enough gas must flow to create a straight line(laminar) flow. If too much gas comes out of the nozzle, thegas may become turbulent. See Figure 9-31. If it becomesturbulent, the shielding gas will mix with the atmospherearound the nozzle area. This will cause the weld to becomecontaminated. To create a steady laminar gas flow, a gaslens may be used. See Figure 7-23.

When too little gas flows, the weld area is not prop-erly protected. The weld will become contaminated andporosity will occur. The recommended rate of flow for agiven nozzle is generally provided by the manufacturer.Once the correct flow rate is known, it can be used at allwire speeds. Too little gas will give a popping sound.Spatter will occur, the weld will have porosity showing,and the bead will be discolored. Refer to Figure 9-32 forsome suggested gas flow rates for use with various metalsand thicknesses. Set the proper flow rate, using theflowmeter.

The heavier shielding gases like CO2 and argon willtend to drop away from the weld area when welding outof position. Therefore, the gas flow rates must be increasedas the position moves from the flat to the horizontal,vertical, and overhead welding positions.

When a gas mixture is used, it may be necessary touse a double- or triple-unit gas mixer. Such units have aseparate pressure regulator and flowmeter for each gas.See Figure 9-26. Premixed gas mixtures can be purchasedfrom welding gas suppliers in cylinders, just like pureargon or oxygen.

9.3.5 Selecting the Correct Gas Nozzles andContact Tubes

The gas nozzle is located at the end of the GMAWgun. See Figures 9-33 and 9-34A and B. It is designed todeliver the shielding gas to the weld area in a smooth,unrestricted manner. The gas nozzle is usually made ofcopper, which is a very good heat conductor. A coppernozzle will resist melting when exposed to the heat gener-ated in the welding operation. GMAW nozzles and FCAWnozzles (if used) are the same. The construction of thenozzle end of a GMAW gun is shown in Figure 9-34.

Nozzles are made with different exit diameters. Gunmanufacturers usually provide information on the correctnozzle to use for various applications. A general-purpose


nozzle is often used and will work well for most applica-tions. A variety of nozzles is shown in Figure 9-35. It can beseen that some nozzles thread onto the gun. Other nozzlesare designed to slip onto or over a nozzle adaptor and areheld by tension. Special nozzle shapes are also manufac-tured, as illustrated in Figure 9-36.

Under the nozzle lies the electrode contact tube. Acontact tube makes the electrical connection between thewelding gun and the electrode.

The contact tube is threaded into a part of the guncalled a diffuser or an adaptor. See Figures 7-49 and 9-35.One end of the diffuser or adaptor threads into thewelding gun. The other end has threads for installing thecontact tube. Diffusers have holes around them to allowshielding gas to escape into the nozzle. Shielding gas exitsthe end of the nozzle to protect the weld area.

Contact tubes, also called contact tips, are made with avariety of inside diameters (ID) and lengths. The contact

248 Modern Welding

Metals Gases % Uses and results

Aluminum Ar Good transfer, stable arc, little spatter. Removes oxides

50%Ar-50%He Hot arc - 3/8″ to 3/4″ (10mm to 19mm) thickness. Remove oxides.25%Ar-75%HeHe

Hot arc, less porosity, removes oxides - 1/2″ to 1″ (13mm to 25mm)Hotter, more gas; 1/2″ (13mm) and up. Removes oxides.

Magnesium Ar75%He-25%Ar

Good cleaning.Hotter, less porosity, removes oxides.

Copper (deox.) 75%He-25%ArAr

Preferred. Good wetting, hot.For thinner materials.

Carbon steel CO2 Short circuiting arc: high quality, low current, out-of-position, medium spatter

Ar-5% O2

Ar-2% O2

Fast, stable, good bead shape,little undercut, fluid weld pool.

75%Ar-25%CO2 Short circuiting arc: fast, no melt-through, little distortion and spatter.

50%Ar-50%CO2 Short circuiting arc: deep penetration, low spatter.

Low-alloy steel Ar-2% O2 Removes oxides, eliminates undercut, good properties.

High-strength steels 60%He-35%Ar-5%CO2

Short circuiting arc: stable arc, good wetting and bead contour,little spatter. Good impacts.

75%Ar-25%CO2 Short circuiting arc: same except low impact.

Stainless steel Ar-1% O2 No undercutting. Stable arc, fluid weld, good shape.

Ar-5% O2 More stable arc .

90%He-7 1/2% Ar-2 1/2% CO2

Short circuiting arc: small heat-affected zone, no undercut, little warping.

Nickel, monel Ar Good wetting - decreases fluidity.

Inconel Ar-He Stable arc on thinner material.

Globular arc: fast, cheap, spattery, deep penetration.

Figure 9-30. Some shielding gas selections for GMAW of various metals.


Contacttube

Nozzle

Laminar(straightline)Gas flow

A B

Turbulentgas flow

Figure 9-31. Effects of gas flow rate. A—Laminar gas flow is the result of the proper gas flow rate. B—Turbulence occurswhen too much gas is used.

Metal

Aluminum andaluminum

alloys

Typejoint

ThicknessWeldposition

Argon flow

in. mm

Stainlesssteel

Nickel and nickel alloys

Magnesium

All

ButtButt

60˚ Bevel

60˚ Double Bevel

Lap, 90˚ Fillet

1/163/321/83/161/4

3/8

3/4

1.592.383.184.766.35

9.53

19.05

FF,H,V,OF,H,V,OF,H,V,OFH,VOFH,VOFH,V,O

25303023-274045605055806080

11.8014.1614.1610.85-12.7418.8821.2428.3223.6025.9637.7628.3237.76

ft3/hr. L/min

1/161/8-3/16

1.593.18-4.76

30(98Ar-2O2)35

14.16

16.521/4-1/2 6.35-12.7 35 16.52

1/2-5/8 12.7-15.88 35 16.52

1/8-5/16 3.18-7.94 35 16.52

All Up to 3/8 Up to 9.53 25 11.80

Butt 0.025-0.1900.250-1.000

0.64-4.836.35-25.4

40-6050-80

18.88-28.3223.60-37.76

Figure 9-32. Suggested gas flow rates for various metals and thicknesses.

tube must be designed for the diameter of electrode wirebeing used. A good sliding electrical contact must be madewith the electrode wire. Each time the wire diameter ischanged, the contact tube must be changed so that the IDmatches the diameter of the wire.

Most manufacturers of contact tubes make them indifferent lengths. Different lengths are used to help obtaindifferent transfer methods. The longest tubes for a gun areusually used for short circuiting transfer. When usingshort circuiting transfer, the contact tube should be flushwith the end of the nozzle or should stick out about 1/16(1.6mm) beyond the end of the nozzle. With a long contacttube, minimal resistance heating of the wire takes place.See Figure 9-37.

Resistance heating of the electrode takes place afterthe electrode wire exits from the contact tube. The elec-trode extension distance, shown in Figure 9-40, is thedistance over which the electrode is heated. The longer thisdistance, the more heating takes place. Figure 9-37 showsthat a long contact tube minimizes the electrode extensionand reduces the resistance heating of the electrode wire.

A medium-length contact tube is used for spraytransfer. A medium-length contact tube usually keeps theend of the contact tube inside the end of the nozzle. Thisallows the welding current to preheat the wire more thanwhen using a long contact tube.

Short contact tubes are used for flux cored arcwelding. A flux cored electrode must be heated to a highertemperature than a solid electrode so that some of the fluxwill vaporize and create a shielding atmosphere aroundthe weld. A shorter contact tube allows the electrode to beheated to a higher temperature. See Figure 9-37.

Contact tubes will wear and must be changed regu-larly. Eight hours of continuous welding with a steel elec-trode can excessively wear a contact tube. Regularreplacement of the contact tube will ensure a continuousgood electrical contact with the electrode wire.

Look at the contact tube occasionally. If the roundhole is becoming elongated or if the arc appears to be fluc-tuating while welding, it is time to replace the tube. A fluc-tuating arc may be due to a worn contact tube not makingconsistent contact with the electrode.

While arc welding, the inside and outside of the noz-zle and the outside of the contact tube can becomespattered. This spatter can be kept from sticking byspraying or dipping the nozzle with a special proprietaryanti-stick compound. If the inside of the nozzle becomes

250 Modern Welding

Guntube

Electrodecable

Nozzleadaptor

Insulator

Nozzle

MetalO-rings

Electrode

Electrodecontacttube

Contacttube adaptoror diffuser

Figure 9-34. A—A schematic drawing of the nozzle end ofa GMAW or FCAW torch. B—An exploded view of a gascooled GMAW gun showing the parts. (Beech & Associates)

Handle

Torchtube

Difusser

Nozzle holder

Electrode contact tube

Nozzle

Shieldinggas

passage

Figure 9-33. A GMAW gun. The nozzle on this gun is heldin place by tension. (Miller Electric Mfg. Co.)

A

B

spattered, the flow of shielding gas will become turbulent.Gas turbulence may cause weld contamination. Toremove the spatter from the nozzle, a special cleaningreamer is used.

9.3.6 Selecting and Installing a LinerThe electrode wire travels from the wire feeder to the

welding gun in a cable. Inside the cable a liner, also calleda conduit, is installed. The liner protects the cable from thecontinuous wear of the electrode wire. The liner alsoprevents the electrode wire from getting tangled or stuckwhile traveling through the cable.

There are two types of liners. One is a hardened steelwire wound in a tight coil to form a tube. This wound steelliner is used for hard materials like steel and stainless steelwires. See Figure 9-38.

The second type liner is made of Teflon®. Teflon is atype of plastic. It is much softer than the wound steel liner

material. Teflon liners are used with softer materials, espe-cially aluminum wires.

Fine metal filings can accumulate in a coiled liner. It isa good idea to occasionally blow compressed air throughthis type of liner to remove these very fine particles. Theelectrode must not be in the liner when it is being blownout. When blowing out the liner, always point the openend of the liner toward the floor or a trash can. Neverallow the open end to point toward yourself or any otherperson.


Nozzles Contact tubeadaptors Nozzle adaptor

Contacttubes

Figure 9-35. A number of different GMAW nozzles, contact tubes, and contact tube adaptors. (American Torch Tip Co.)

Figure 9-36. Special GMAW nozzle for spot or tackwelding.

LongcontacttubeNozzle

Short contacttube

Electrodeextension

Visibleextension Electrode

extension

Figure 9-37. The length of the contact tube and the amountof electrode extension affects the amount of heating thatthe electrode wire receives. The heating takes place in thelength of electrode wire that extends from the contact tube.With a long contact tube, there is less extension and thus,less heating.

Occasionally, liners get worn or become clogged withfine metal particles. A problem also can occur if the linerever gets kinked. When any of these happen, the electrodewire will not feed smoothly. The liner must be replaced.

To replace a liner, or to change from one type to theother, disconnect the gun cable from the wire feeder.Remove the nozzle, contact tip, diffuser, and any setscrewsused to keep the liner in place. Then, remove the liner fromthe gun and cable. Install the new liner. Push it firmly untilit bottoms out against the far end of the cable. Secure theliner in place. Most liners are made slightly long and mustbe trimmed to a specific length. Each manufacturer hasdirections to follow. Reassemble the welding gun. Attachthe cable to the wire feeder, then refeed the wire throughthe cable and liner to the gun.

9.3.7 Selecting the Correct GMAW or FCAWElectrode

Smaller-diameter wire usually costs more than larger-diameter wire. The rate at which filler metal is depositedwhen using small-diameter wire makes up for its addedcost. Because of the small diameter and the high currentsgenerally used in GMAW and FCAW, small-diameter elec-trode wire is melted more rapidly than larger-diameterwire. Small-diameter wire is thus deposited at a muchhigher rate.

The electrode wire used must match, or be compatiblewith, the base metal being welded. When CO2 or O2 isused on steel-based metals, it causes oxidation of the weldmetal. Deoxidizer types of electrode wires must be used toneutralize this oxidation. Manganese, silicon, andaluminum are used as deoxidizers in steel electrode wires.Titanium, silicon, and phosphorus are the deoxidizers incopper electrodes.

For more information regarding GMAW electrodes,see Heading 7.14. Also refer to Figure 7-51 for carbon steelelectrodes and Figure 7-52 for low-alloy electrodes. Moreinformation about FCAW electrodes may be found inHeading 7.18 and Figure 7-60.

Once the correct electrode is selected it should beloaded in the wire feeder as stated in Heading 9.3.2. Thecorrect-size drive wheels must be used in the wire feeder.The wire should be fed through the electrode cable usingthe inch switch until about 2 to 3 (50mm to 75mm)extend beyond the nozzle. Cut the electrode wire so that itsextension is correct for the type of welding being done.Refer to Heading 9.5.

9.4 Preparing Metal for Welding

Metal surfaces usually may be cleaned mechanicallyor chemically. Abrasive cloth or wire brushing may beused. On severely corroded areas, grinding may be used.Welding may be done on an oxidized (rusted) carbon steelor low-alloy steel surface without cleaning. However, ifthe surface is rusty, a deoxidizing electrode wire should beused. This reduces oxidation and weld porosity.

Joint designs for GMAW and FCAW are similar tothose used for SMAW. The groove angle used whenGMAW or FCAW may be smaller than the angle usedwhen SMAW. See Figure 9-39. This narrower angle ispossible for two reasons. The wire diameters used aresmaller and GMAW penetrates better than SMAW. A 45°groove angle will take less filler metal to fill than a 75°groove angle. Welding time will also be less. Therefore,savings in filler metal and welder's time are possible.

9.5 Electrode Extension

Electrode extension is the amount that the end of theelectrode wire sticks out beyond the end of the contacttube. See Figure 9-40. This distance is sometimes referredto as stickout.

A good extension for use with the short circuitGMAW transfer method is about 1/4″ to 1/2″ (6mm to13mm). The correct electrode extension for all othertransfer methods varies between 1/2″ and 1″ (13mm and25mm). An electrode extension used for gas-shieldedFCAW may vary from 1/2″ to 1 1/2″ (13mm to 38mm).The suggested electrode extension for use with self-shielding FCAW is 3/4″ to 3 3/4″ (19mm to 95mm).

Contact tips are made in different lengths. Thedifferent lengths help to establish the correct electrodeextension. Longer tips are used for short circuitingtransfer; shorter tips are used for spray transfer andFCAW. Heading 9.3.5 discusses different contact tiplengths.

As the electrode extension increases, the resistanceheating of the electrode increases. Resistance causes thecurrent to heat the wire along the electrode extension dis-tance. A long extension may cause too much filler metal tobe deposited with low heating by the arc. This may causespatter, shallow penetration, and a low weld bead shape.

252 Modern Welding

Trigger switchconnection

Coiled wire liner

Combinationgun cable

Figure 9-38. A coiled wire liner may be used in a GMAWcable to guide the electrode wire. The wire liner stickingout of this welding gun cable is replaced when worn.

9.6 Welding Procedures

Before beginning to weld, the welding stationshould be checked for safety. All electrical, gas, andwater connections must be checked for tightness.

Weldments should be tack welded or placed intofixtures prior to welding. When complete joint penetrationis required, backing is often recommended. Backing isused to control the penetration and may be in the form ofa backing plate, strip, ring, or other design.

Most arc welding processes require the welder tocontrol the arc length, welding speed, and torch or gunangle to obtain a good weld. In GMAW and FCAW, the arclength will remain constant and is determined by the arcvoltage. The welder doing GMAW must watch and controlthe distance from the nozzle or contact tube to the work.See Figure 9-40. By controlling the nozzle-to-workdistance, the welder will control the electrode extensiondistance. Heading 9.3.5 explains the importance of elec-trode extension.


A

B

60˚-75˚

30˚-45˚

SMAW

GMAW

Figure 9-39. Arc welding beads compared. A—Typical groove angle and weld bead for SMAW. B—Typical groove angleand weld bead for GMAW and FCAW. Notice that less filler metal is required to fill the groove at B. Welding time will alsobe less.

Nozzle-to-workdistance

Electrodeextensiondistance

Contact tube-to-workdistance

Base metal

Exit diameter

Arc length

Electrodewire

Contacttube

Nozzle

Figure 9-40. Electrode extension distance. Other distances important in GMAW and FCAW are also shown.

The welding speed will be determined by the appear-ance of the bead width and penetration. Torch angle willalso affect the bead width and penetration. The terms fore-hand, backhand, and perpendicular welding are used.

In forehand welding, the tip of the electrode points inthe direction of travel. When backhand welding, the elec-trode tip points away from the direction of travel.Perpendicular welding is done with the electrode at 90° tothe base metal. Figure 9-41 shows the effects of thesevarious methods.

The backhand method will give the best penetration.A 25° angle forward of perpendicular will give the bestpenetration in the flat welding position, as shown inFigure 9-41C. For the best control of the weld pool, anangle of 5°-15° forward of perpendicular is preferred for allpositions.

To start welding, tip the top of the gun 5°-15° in thedirection of travel and place the helmet down over youreyes. To start the arc, the wire feeder and the gas, squeezethe trigger on the gun. The wire will arc as soon as it feedsout far enough to touch the metal. No striking or up-and-down motion is required to start the arc as required withSMAW.

As the weld pool reaches the proper width, whichoccurs rapidly, the welder moves the welding gunforward. Continue to move the gun along the weld,watching the width of the weld pool to maintain a uniformsize. Continue this procedure until the end of the weld isreached. A run-off tab may be used to ensure a full-width

bead to the end of the weld. Without a run-off tab, the endof the weld may have a crater (depression). This depres-sion can be reduced by moving the electrode to the end ofthe weld and then back over the completed bead about1/2 (13mm). At the end of this reverse travel, thecontactor switch is released. To shield the end of the weld,hold the gun in position to allow the gas postflow toprotect the weld until it cools.

More than one pass may be required to fill a weldgroove. Each pass should be cleaned before the next passis laid. This is generally done with a wire brush or wheel.The glass-like coating on some gas metal arc welds iseasily removed. The slag layer on a flux cored arc weld isheavier and requires more effort to remove.

Out-of-position welds require that leathers beworn. Molten base metal, filler metal, and spatter mayfall on the welder. Therefore, a cap, coat, cape, and chapsshould be worn to protect against burns.

9.7 Shutting Down the Station

When welding is stopped for an extended period, thestation should be shut down. To shut down the station,proceed as follows:1. Return the wire speed to zero.2. Turn off the wire drive unit.3. Turn off the shielding gas cylinder(s).

254 Modern Welding

A - Forehand B - Perpendicular C - Backhand

Directionof travel

Directionof travel

Directionof travel

25˚ forward ofperpendicular

Figure 9-41. Effects of the welding method on the bead. A—Forehand. B—Perpendicular. C—Backhand. Notice that thebackhand method gives the deepest penetration.

4. Squeeze the gun trigger and hold it in for a fewseconds to bleed the gas lines.

5. Turn the flowmeter adjusting knob(s) in to close it.6. Turn off the power switch on the arc welding power

source.7. Hang the gun on an insulated hook.8. Turn out the pressure adjusting knob on the flow-

meter regulator, if an adjustment knob is provided.

9.8 Welding Joints in the Flat Welding Position

The face of a weld made in the flat welding positionshould be horizontal or nearly horizontal. The weld axis isalso horizontal. See Figure 9-42. Any of the metal transfermethods may be used in the flat welding position. Themethod used will depend on the metal thickness and otherfactors. Figure 9-43 shows a welder practicing running abead in the flat welding position.

The electrode should point more toward the surface if theedge begins to melt too quickly. The electrode and gunshould be held between 5°-15° forward from a vertical lineto the metal surface.

A C-shaped weld pool will form, as when GTAW.When the end of the weld is reached, reverse the directionfor about 1/2″ (13mm). This movement will help reducethe crater which occurs if the weld is stopped at the end ofthe joint. No matter what type weld is made, this samefinish movement can be made. A run-off tab will totallyeliminate the crater at the end of a weld.

9.8.2 Fillet Weld on an Inside Corner JointFillet welds may be made on metal up to 3/8″ (10mm)

thick without edge groove preparation. This can be donebecause of the deep penetration possible with the spraytransfer method. The centerline of the electrode should beheld at 45° to each metal surface. If the backhand weldingprocedure is used, the electrode and gun are held between5° and 15° forward of vertical. See Figure 9-44.

GMAW can generally weld 1/4″ (6mm) beads oneach pass. If the weld size is more than 1/4″ (6mm) thick,two or more weld passes will be required.


Directionof travel

Weldfacehorizontal

Weld axishorizontal

5˚-15˚Direction of travel

45˚ to surface

Figure 9-42. A fillet weld on a lap joint in the flat weldingposition. Note the angles used and the deep penetration ofthe weld. Also, notice that the weld face and axis are horizontal or near-horizontal.

9.8.1 Fillet Weld on a Lap JointThe metal should be set up as shown in Figure 9-42. It

should be tack welded about every 3″ (75mm). This willhold it in position while the weld is made.

To make the fillet weld, the centerline of the electrodeshould be held at about 45° to the edge and metal surface.

Figure 9-43. A welder running a bead in the flat weldingposition using an FCAW gun. (American Welding Society)

9.8.3 Groove Weld on a Butt JointSquare-groove welds can be made on metal up to

3/8″ (10mm) thick without edge shaping. Groove weldswith shaped edges of any thickness can be made with theGMAW process. The groove angle on a V-groove butt weldcan be narrower than is used with SMAW. Because of thepenetration possible with spray transfer methods, the rootface can be larger. The root opening can be smaller withGMAW than the opening used for SMAW.

The centerline of the electrode should be directly overthe axis of the weld. An angle of between 5° and 15°forward of vertical is correct for the backhand weldingmethod. See Figure 9-45.

A keyhole in the weld pool will indicate that completepenetration is occurring. One problem that may occur in agroove weld made with GMAW is whiskers. Whiskers arelengths of electrode wire that stick through the root side ofa groove weld. Whiskers occur when the electrode wire isadvanced ahead of the weld pool.

The wire goes through the weld root and burns off.The burned-off length is left stuck in the weld. Whiskerscan be prevented by slowing the welding speed. They mayalso be prevented by reducing the wire feed speed. A smallweaving motion may be used to keep the wire fromgetting ahead of the weld pool.

9.8.4 Groove Weld on an Outside Corner JointThe outside corner joint is set up as shown in Figure

9-46. A square- or prepared-groove weld may be used. Theelectrode angles are the same as those used for weldsmade on a butt joint. Since groove welds are made on theoutside corner joint, whiskers can occur.

256 Modern Welding

Directionof travel


45˚ to surface

Figure 9-44. A fillet weld on an inside corner joint in theflat welding position. The electrode is 45 from each metalsurface. It is also tipped 5 -15 forward in the direction oftravel.

Directionof travel


Figure 9-45. A V-groove weld on a butt joint in the flatwelding position. Note the narrow (45 ) groove possiblewith GMAW.

Directionof travel


Figure 9-46. A bevel-groove weld on an outside cornerjoint in the flat welding position.

9.9 Welding Joints in the HorizontalWelding Position

The face of a weld made in the horizontal weldingposition is in the vertical or near-vertical position. In thehorizontal welding position, the centerline of weld axisruns in a horizontal or near-horizontal line. See Figure 9-47.

more. It does not need bead width and reinforcement tostrengthen the weld.

The electrode should be held at 45° to each metalsurface as seen in Figure 9-48. Aiming the wire moretoward the vertical surface may improve the bead shape.This will help compensate for the molten metal sag. Inclinethe gun and the electrode about 5°-15° forward ofvertical. See Figure 9-49.


Directionof motion

5˚-15˚Direction of travel 45˚ to surface

Tack weldWeld faceis verticalor near-vertical

Centerline ofweld axis isnear-horizontal

Figure 9-47. A fillet weld on a lap joint in the horizontalwelding position. In the horizontal welding position, theweld axis is near-horizontal and the face of the weld near-vertical.

Short circuiting, globular, spray, or pulsed spraytransfer methods may be used when welding horizontalfillet welds. Horizontal butt welds are limited to shortcircuiting and pulsed spray transfer. The weld pool isoften too large and fluid when using globular or spraytransfer. Also, metal transfer in globular transfer will notfall into the weld pool.

9.9.1 Fillet Weld on a Lap JointFor practice welds, the metal should be set up and

tack welded as shown in Figure 9-47. The centerline of theelectrode should be about 45° to the edge and metalsurface. It may point more toward the surface if the edgemelts too quickly. The electrode or gun should tip about5°-15° forward of vertical in the direction of travel. Thetypical C-shaped weld pool will indicate that both theedge and surface are melting properly.

9.9.2 Fillet Weld on an Inside Corner or T-JointSquare or prepared-groove welds may be made in the

horizontal welding position. The use of a V-, bevel-, U-, J-type prepared groove will depend on the metal thicknessand joint design. The bead width used in GMAW does nothave to be as wide for the same thickness as when doingSMAW. This is because the gas metal arc weld penetrates

Figure 9-48. This welder is making a horizontal weld on a T-joint using the GMAW process. (American Welding Society)

Directionof travel


45˚ to surface

Tack weld

Figure 9-49. A fillet weld on a T-joint in the horizontalwelding position. Note the angles from the metal and inthe direction of travel.

9.9.3 Groove Weld on a Butt Joint or OutsideCorner Joint

A square- or prepared-groove weld may be used.Figure 9-50 shows a U-groove weld in progress. The elec-trode centerline should be directly over the weld line. Forbest weld pool control, the electrode should tip 5°-15° inthe direction of travel. The gun and electrode should alsopoint upward slightly to keep the molten metal fromsagging. Short circuiting transfer and pulsed spraytransfer allow the molten weld pool to cool slightly.

weld pool. This can be avoided if the welder can keep theweld pool ahead of the molten flux.

The centerline of the electrode should be tipped 5°-15° in the direction of travel, as in other position welds.This angle will permit the easiest weld pool control. Theweld pool remains relatively cool when the short circuitingmethod of metal transfer is used. A properly adjustedpulsed spray arc will allow time between pulses for theweld pool to cool. Spray arc transfer can be used in someapplications, but the weld pool must be kept small. Tomaintain a small weld pool, a higher travel speed must beused. The short circuiting method of metal transfer keepsthe weld pool coolest.

9.10.1 Fillet Weld on a Lap JointFigure 9-53 illustrates a fillet weld being made in the

vertical welding position. The angles of the electrode andgun are the same as for other positions. The electrode

258 Modern Welding

Directionof travel


Keyhole

Line perpendicularto the basemetal surface

Figure 9-50. A U-groove weld on a butt joint in the hori-zontal welding position. Note the keyhole at the root of theweld.

To ensure complete penetration, watch for a contin-uous keyhole through the root pass. More than one pass isnecessary on thicknesses above 3/16″ (5mm). Tocompletely fill the groove, an electrode weaving motionmay be required. Figure 9-51 shows a horizontal butt jointbeing welded.

9.10 Welding Joints in the VerticalWelding Position

GMAW in the vertical welding position is done usingthe short circuiting or pulsed spray transfer method. Spraytransfer may also be used, but only with small-diameterwire and a small molten weld pool. In the vertical weldingposition, the weld axis and the weld face are both vertical.Figure 9-52 shows a vertical weld in progress.

GMAW may be made uphill (from the bottom up) ordownhill (from the top down). Downhill welding is moredifficult with FCAW. The flux material might flow into the

Figure 9-51. A mechanized GMAW machine mounted ona track. The track guides the GMAW gun along the circularbutt joint on this large tank. (Bug-O Systems, Inc.)

Figure 9-52. This farmer is using FCAW to make equipmentrepairs in the field. (Lincoln Electric Co.)

should tip about 5°-15° in the direction of motion. Thecenterline of the electrode should be at about 45° to theedge and the flat surface. If the edge of the metal melts toorapidly, point the electrode more toward the flat surface.Be certain that both the edge and surface are meltingcompletely as the filler metal is added. The appearance ofa C-shaped molten weld pool will indicate good fusion.

9.10.2 Fillet Weld on an Inside Corner JointThe centerline of the electrode should be held at 45° to

each surface. It should be tipped at 5°-15° in the directionof motion. A C-shaped weld pool will indicate good fusionis occurring. Short circuiting and pulsed spray transfer arebest suited for vertical welding. Spray arc can be used witha weaving motion in some applications.

9.10.3 Groove Weld on a Butt or Outside Corner Joint

A V-groove butt weld in progress is shown in Figure9-54. The electrode centerline should be directly above theweld line. The electrode and torch should be inclined(tipped) 5°-15° in the direction of travel. A keyhole at theroot of the weld will indicate complete penetration.

9.11 Welding Joints in the OverheadWelding Position

The short circuiting and pulsed spray metal transfermethods are recommended for overhead welding. Whenoverhead welding, it is strongly suggested that a cap,coat, cape, and possibly chaps be worn. This is necessaryto protect the welder from falling molten metal.

The angle of the electrode to the joint surfaces is thesame as for other welding positions. The electrode shouldbe held more vertically when overhead welding. An angleof between 5° and 10° is suggested. The weld pool in shortcircuiting and pulsed arc transfer is relatively cool. Aweaving motion is not required for the purpose of coolingthe weld pool. As the weld pool increases in size, thepossibility of the metal falling out or sagging increases.The use of several narrower beads, rather than a weavingmotion, is recommended.

9.11.1 Fillet WeldsSee Figures 9-55 and 9-56 for examples of the angles

used. The centerline of the electrode should be 45° fromeach metal surface. It should be tipped about 5°-10° in thedirection of travel.

9.11.2 Groove WeldsFigures 9-57 and 9-58 show the angles used to weld a

butt joint and an outside corner joint in the overheadwelding position.


Weld axis

Weld face

Figure 9-53. A fillet weld on a lap joint in the verticalwelding position. Two passes are being used on the weld.Notice that the weld axis and bead face are vertical.

Directionof travel

Figure 9-54. A V-groove weld on a butt joint. The root passis in progress.

9.12 Automatic GMAW and FCAW

Gas metal arc welding and flux cored arc weldingmay be semiautomatic or fully automatic processes. In thesemiautomatic process, the welder must direct and movethe arc welding gun while the electrode wire feeds auto-matically into the weld pool.

Both GMAW and FCAW guns may be mounted on amotor-driven carriage or robot. Figure 9-59 shows a robotmoving a GMAW gun. When the welding gun is con-trolled by a machine with feedback controls, the processbecomes fully automatic. Refer to Chapter 25 for informa-tion on robots and other automatic welding equipment.

260 Modern Welding

Directionof travel

5˚-10˚Directionof travel

Figure 9-55. A fillet weld on a lap joint in the overheadwelding position. In this joint, two passes will be made.This is done to keep the weld pool size small and easy tomanage.

Directionof travel

45˚ tosurface

Figure 9-56. A fillet weld on an inside corner joint. Theelectrode and gun are tipped 5 -10 in the direction oftravel.

Directionof travel


Cover passRoot pass

Figure 9-57. Bevel-groove weld on a butt joint in an over-head welding position using two passes.


Directionof travel

Figure 9-58. A J-groove weld in an outside corner joint inthe overhead welding position.

Figure 9-59. A GMA welding gun mounted on a robot. Thepart is held in a fixture. (Motoman, Inc.)

9.13 Gas Metal Arc Spot Welding

The gas metal arc welding power source and arcwelding gun can be used to produce a weld in one smallspot. Metals commonly welded with the gas metal arc spotwelding process are low-carbon steel, stainless steel,aluminum, copper-based metals, and magnesium. Gasmetal arc spot welding is generally done on metals under1/16″ (1.6mm) thick. Metals up to 3/16″(5mm) thick can bewelded.

Small tack welds can be made on lap and cornerjoints. See Figure 9-60. A completed spot weld is shown inFigure 9-61. A spot weld is a weld made on overlappingpieces with the weld away from the edges. The gas metalarc welding power source must be equipped with specialcontrols to do spot welding. The arc welding gun must be fitted with a special nozzle for spot welding. See Figure 9-36.

Several welding variables must be controlled to makegas metal arc spot welds. These variables are:

• Arc voltage.• Welding current.• Welding time.• Electrode size and composition.• Electrode extension.• Shielding gas.Voltage settings are made on the arc welding power

source in the same way as when GMAW. If the voltage isincreased, the arc length will increase. However, the pene-

tration and the weld reinforcement (buildup) will decreaseslightly.

Welding current is DCEP (DCRP). Current iscontrolled by varying the wire feed speed. Weldingcurrent greatly affects the spot weld penetration. Highercurrents create greater penetration.

Welding time is controlled by timers in the arcwelding equipment. Timers allow the same quality weldto be made each time. Spot welding times are usuallyabout one second. Longer times are necessary whenwelding thicker metals. Penetration increases as thewelding time is lengthened. The diameter of the weld areaalso increases as the welding time is increased.


Tack welds on a lap joint

Tack welds on anoutside corner joint

Spot welds on two overlapping pieces Tack welds on an

inside corner joint

Figure 9-60. Tack welds on lap, inside corner, and outside corner joints. Several spot welds are also shown. Notice thedepth of penetration shown in section.

Figure 9-61. A completed gas metal arc spot weld. (ESAB Welding and Cutting Products)

The same size and type of solid wire used for weldingmay be used for spot welding a particular metal. Electrodeextension, Figure 9-40, must remain constant during theGMA spot welding process. The extension distance is keptconstant by using a special nozzle. Several GMA spotwelding nozzle designs are shown in Figure 9-36. The endof the contact tube is set back from the end of the nozzle.This is done to keep the contact tube out of the weld. Thissetback will also reduce the possibility of the electrodemelting up into the contact tube at the end of the weldcycle. The shielding gas used may be the same gas or gasmixture used for welding beads.

GMA spot welds on thin metals may be made in anyposition. As the metal thickness increases, GMA spotwelding is limited to the flat welding position. Weldquality and uniformity is not as good as that possible withresistance spot welding. The big advantage of GMA spotwelding over resistance spot welding is that access to onlyone side of the parts is required.

The GMA spot welding controls found on various gasmetal arc welding machines differ. Figure 9-62 shows aGMAW power source. Some controls typically found on aGMA spot welding control panel are:

• Control switch. The switch used to change thegas metal arc welding machine from a regularwelder to a spot welder.

• Weld timer. This control is for setting thewelding time. The entire spot welding operationtakes place in one or two seconds.

• Burn-back adjustment. Some machines have aburn-back adjustment. This control allows thecurrent to flow for a short time after the wirefeed stops. The continued current flow preventsthe wire from sticking in the weld pool. If theburn-back time is set too high, the electrode wiremay burn back into the contact tube. If it is notset high enough, the wire will stick in the weldpool at the end of the welding time.

9.14 GMAW Troubleshooting Guide

Figure 9-63 is a chart that describes many typical trou-bles which may occur when making a gas metal arc weld.Steps to take to correct each problem are listed. The causesare shown along with methods for correcting eachproblem.

9.15 GMAW and FCAW Safety

The safety precautions for arc welding covered inChapter 1 and in other chapters of this book also apply toGMAW and FCAW.

Adequate eye protection must always be worn. Ifwelding for long periods, flash goggles with a #2 lensshade should be worn under the arc helmet. A #11 lens isrecommended for nonferrous GMAW and a #12 forferrous GMAW. Lens shades up to #14 may be worn asrequired for comfort. Figure 9-64 shows a welder withproper hood and an electronic quick-changing lensinstalled. All welding should be done in booths or in areasshielded by curtains. This is done to protect others in theweld area from arc flashes.

Suitable dark clothing must be worn. This is done toprotect all parts of the body from radiation or hot metalburns. Leather clothing offers the best protection fromburns.

It is suggested that all welding should be done inwell-ventilated areas. Ventilation and/or filtering equip-ment should be provided, as necessary, to keep the atmos-phere around the welder clean. Carbon monoxide gas isgenerated when using CO2 as a shielding gas whiledoing GMAW and FCAW. Ozone is also produced whendoing GMAW and FCAW. Ozone is a highly toxic gas.Metals still covered with chlorinated hydrocarbonsolvents will form toxic (poisonous) phosgene gas whenwelded.

Protect arc cables from damage. Do not touch unin-sulated electrode holders with bare skin or wet gloves. Afatal shock could result. Welding in wet or damp areas isnot recommended.

Shielding gas cylinders must be handled with greatcaution. Refer to Chapters 1, 12, and 13 for a review of howto handle high-pressure cylinders. Chapters 12 and 13should also be referred to for instructions on how to attachregulators and other gas equipment.

262 Modern Welding

Wirespeed

Heatselector

Spot, continuousor stitch modeselector

Weldtime

Stitch offtime

Figure 9-62. This GMAW outfit can make continuous, spot,or stitch-type welds. The wire feeder is under the top cover.(Century Mfg. Co.)


Trouble Possible causes How to correctDifficult arc start Polarity wrong

Insufficient shielding gasPoor groundOpen circuit to start switch

Check polarity, try reversingCheck valves, increase flowCheck ground—return circuitRepair

Irregular wire feed, burn back

Insufficient drive roll pressureWire feed too slowContact tube pluggedArcing in contact tubePower circuit fluctuationsPolarity wrongTorch overheatingKinked electrode wireConduit liner dirty or wornDrive rolls jammedConduit too long

Increase drive roll pressureCheck, adjust wire feed speedClean, replace contact tubeClean, replace contact tubeCheck line voltageCheck polarity, try reversingReplace with higher amp gunCut out, replace spoolClean, replaceClean drive case, clean electrode wireShorten, install push-pull drive

Welding cables overheating

Cables too smallCable connections looseCables too long

Check current requirements, replaceCheck, tightenCheck current-carrying capacity

Unstable arc Cable connections looseWeld joint area dirty

Check, tightenClean chemically or mechanically

Arc blow Magnetic field in DC causes arc to wander

Rearrange or split ground connectionUse brass or copper backing barsCounteract “blow” by direction of weldReplace magnetic work-bench

Undercut Current too highWelding speed too highImproper manipulation of gunArc length too long

Use lower current settingSlow downChange angle to fill undercutShorten arc length

Excessively wide bead Current too highWelding speed too slowArc length too long

Use lower current settingSpeed upShorten arc length

Incomplete penetration Faulty joint design

Welding speed too rapidWelding current too lowArc length too longImproper welding angle

Check root opening, root face dimensions, including angleSlow down welding speedIncrease welding currentShorten arc lengthCorrect faults, change gun angle

Incomplete fusion Faulty joint preparation

Arc length too longDirty joint

Check root opening, root face dimensions, included angleShorten arc lengthClean chemically or mechanically

Dirty welds Inadequate gas shielding

Dirty electrode wire

Dirty base metal

Hold nozzle closer to workIncrease gas flowDecrease gun angleCheck gun and cables for air and water leaksShield arc from draftsCenter contact tube in nozzleReplace damaged nozzleKeep wire spool on welder coveredKeep unused wire in shipping containersClean wire as it enters wire driveClean chemically or mechanically

Porosity

See above, Dirty welds

Dirty electrode wireDirty base metalInadequate gas shielding



Cracked welds Improper technique

Faulty designFaulty electrode

Shape of bead

Travel speed too fastImproper techniqueRigidity of joint

Change angle of gun to improve shielding

Check edge preparation and root spacingCheck electrode wire for compatibility with base metal

Change travel speed or shielding gas to obtain more convex beadSlow downChange angle of gun to improve depositionRedesign joint, preheat and postheat, weave bead

Figure 9-63. A troubleshooting guide for problems that might occur when GMAW. (Welding and Fabricating Data Book)

264 Modern Welding

Figure 9-64. This welder is wearing a quick change filterlens in the welding helmet. The lens will darken to aprotective shade in a fraction of a second after the arc isstruck. (Jackson Products, Inc.)

Test Your Knowledge

Write your answers on a separate sheet of paper. Donot write in this book.1. Name three benefits of using GMAW.2. The polarity used for almost all GMAW and FCAW is

DCE _____ or DC _____ _____.3. Name three metal transfer methods.4. _____ is the property in an electric circuit that slows

down the rate of current change.5. Spray transfer will only occur when the current is set

above the _____ current.6. Spray transfer will only occur when at least _____ %

argon is used.7. Which welding procedure gives the deepest penetra-

tion — forehand, perpendicular, or backhand?8. How many pounds and kilograms of filler metal can

be deposited per hour with the spray transfermethod?

9. Other than setting switches, what are the two mainvariables made on the welding machine or wire feeder prior to welding?

10. A GMAW power source used for pulsed spraytransfer must have what additional controls?

11. Using spray arc transfer, _____ volts and _____amperes are used with a 0.045″ (1.1mm) electrode toweld stainless steel.

12. _____ volts and _____ amps are used to weld mildsteel using short circuiting transfer and 0.035″(0.9mm) diameter wire.

13. On the wire drive unit shown in Figure 9-20, wireguides and drive rolls are aligned by loosening the_____ _____ securing bolts and moving the __________ up or down.

14. To feed the electrode wire through the electrode cableto the arc welding gun, the _____ switch is operated.

15. What factors must be considered when choosing ashielding gas?

16. Argon has a _____ thermal conductivity than helium,so _____ is used to weld thick aluminum or coppersections.

17. Why is good ventilation important when using CO2 gas?

18. What effect does oxygen (O2) have on the arc whenmixed with argon?

19. Which gases are suggested for use with pulsed spraytransfer?

20. What argon flow rate in ft3/hr. and L/min should beused to weld 0.150 (3.8mm) thick magnesium in abutt joint?

21. What part of the gas metal arc welding gun contactsthe electrode wire and passes electricity to the electrode?

22. How can metal spatter be kept from sticking to thenozzle?

23. Electrode extension is the distance from the end of the_____ _____ to the end of the _____ .

24. The suggested angle for the electrode and gun for bestweld pool control with backhand welding in mostpositions is _____ ° to _____° forward of vertical.

25. Metals still covered with chlorinated hydrocarbonsolvents will form a toxic _____ gas when welded.


This welder is using GMA to make a downhill weld on an outside corner joint. (Hornell Speedglas, Inc.)

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