SHIELDING GASES

SELECTION MANUAL

Introduction 1

Properties of Electric Arcs and Cases 3Basic Electrical Concepts 3Physical Properties of Gases 8Basic Gas Properties 10

Gas Tungsten Arc Welding (GTAW) 13Process Description 13Gas Flow Rate 14Preflow and Postflow 15Backup Shielding and Trailing Shields 15Shielding Gases for GTAW 15

Plasma Arc Processes (PAW and PAC) 19Plasma Arc Welding (PAW) 19

Process Description 19 Application of PAW 21 Shielding Gases for PAW 22

Plasma Arc Cutting (PAC) 24 Process Description 24 Gas Flow Rates 27 Shielding and Cutting Gases for PAC 27

Gas Metal Arc Welding (GMAW) 29Process Description 29Metal Transfer Modes in GMAW 30Metal-Cored Electrodes 34Shielding Gases for GMAW 35

T A B L E O F C O N T E N T S

Chapter Topic Page

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1

2

3

4

Flux-Cored Arc Welding (FCAW) 43Process Description 43Flux Effects 44Metal Transfer 44Shielding Gases for FCAW 45

The Economics of Gas Selection for GMAW 47Labor and Overhead 47Effect of Shielding Gas on Welding Speed 48Effect of Shielding Gas on Duty Cycle and Cleanup 48Consumable Costs 50Total Cost of a Shielding Gas 51Gas Supply 53

Gas Supply Systems 55Cylinder Storage Systems 55High Pressure Cylinders 56Identification of Gases in Cylinders 56Carbon Dioxide Supply 56Gas Mixing 57Pre-Blended Mixtures 57Bulk Storage Systems 57Inert Gas Distribution Systems 58

Precautions and Safe Practices 59Welding and Cutting 59Shielding Gases 62Material Safety Data Sheets (MSDS) 63

Glossary 64

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Chapter Topic Page

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7

8

a-z

T A B L E O F C O N T E N T S

During any arc welding process, oxygen andother atmospheric gases can react with themolten metal, causing defects that weaken theweld. The primary function of a shielding gasis to protect the molten weld metal fromatmospheric contamination and the resultingimperfections. In addition to its shieldingfunction, each gas or gas blend has uniquephysical properties that can have a majoreffect on welding speed, penetration, mecha-nical properties, weld appearance and shape,fume generation, and arc stability. A changein the shielding gas composition is usuallyconsidered an essential variable in mostqualified welding procedures.

The primary gases used for electric weldingand cutting are argon (Ar), helium (He),hydrogen (H2), nitrogen (N2), oxygen (O2),and carbon dioxide (CO2). The compositionand purity of the gas or gas mixture should betailored to meet the process, material, andapplication requirements. Shielding gases areused in either pure form or in blends of vary-ing components. Therefore, the selection of agas or gas mixture can become quite complexdue to the many combinations available.

Selection of the most economical shieldinggas or blend must be based on a knowledgeof the gases available, volume requirements,their applications, and the overall effect theyhave on the welding process. PraxairsShielding Gas Selection Manual describes thearc plasma characteristics and basic propertiesof shielding gases, as well as their applica-tions in the welding and cutting processesmost frequently used by industry today.The processes discussed are Gas TungstenArc Welding (GTAW), Plasma Arc Welding(PAW), Plasma Arc Cutting (PAC), Gas MetalArc Welding (GMAW), and Flux-Cored ArcWelding (FCAW). The shielding gasespresented are Praxairs Star Gases (puregases) and Star Gas Blends, includingStarGold, Stargon, HeliStar

Mig Mix Gold and HydroStar blends.

Additional sections of this manual explain theeconomics of gas selection, various methodsof gas supply, and the precautions andpractices recommended to ensure a safeworking environment.

I N T R O D U C T I O N

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Shielding Gases Selection Manual

1

History

The history of shielding gas developmentbegan late in the nineteenth century whenCharles Lewis Coffin replaced the air ina box placed over a welding joint with a non-oxidizing atmosphere. Interest in the use ofinert, non-oxidizing gases for welding wassporadic for the next forty years. Then in1930, two U.S. patents were issued which areconsidered to be the first descriptions of theuse of inert shielding gases for welding.

The first patent, issued to H. M. Hobart,described the use of helium with carbon ormetallic arcs. The other, issued to P. K.Devers, described the use of argon and itsmixtures for arc processes.

Early in the 1940s, Northrup Aircraft Com-pany, Inc. first used an inert gas shieldedwelding process with a nonconsumabletungsten electrode. The process was develop-ed specifically for the welding of magnesiumfor aircraft fabrication using helium as theshielding gas. Recognizing its possibilities,Praxair acquired the rights to the invention in1942 and started an extensive research anddevelopment program to expand the use of theprocess. Introduced commercially in 1946 asthe Heliarc process, it is today also known asGas Tungsten Arc Welding (GTAW) orTungsten Inert Gas (TIG).

A significant portion of this developmentwork was devoted to the evaluation of argonand helium utilized as shielding gases, and tostudies that identified the importance of gaspurity in producing quality welds. Therefore,it was not until the postwar era, after Praxairpioneered the economical production of high-purity argon on a commercial scale, thatGTAW became a practical reality.

In 1950, a patent was issued to Air ReductionCo. Inc. (Airco) covering a process laterknown as Gas Metal Arc Welding (GMAW),Metal Inert Gas (MIG) and Metal ActiveGas (MAG). The initial application was forspray arc welding of aluminum in a heliumatmosphere. At that time, the process wascalled SIGMA (Shielded Inert Gas Metal Arc).Extensive development work by Praxair,Airco, and others followed during the 1950s.The effort was dedicated to developing gasmixtures, wire chemistry, and equipmentsystems to improve and expand the applica-tion range of the process. This work led tothe rapid growth of GMAW during the nextthirty years and to its wide use today. It alsoprovided the groundwork for the invention ofFlux-Cored Arc Welding (FCAW) by ArthurBernard in the late 1950s. His patent wasassigned to the National Cylinder Gas Co.(NCG) in 1957, where the process was furtherdeveloped and introduced for industrial use.

Also during the 1950s, Praxair developed,patented, and commercialized Plasma ArcCutting and Welding Systems. Praxairresearchers discovered that the properties ofthe open arc could be altered by directing thearc through a nozzle located between theelectrode and the workpiece. This proceduregreatly increased arc temperature and voltage,producing a highly constricted jet that wascapable, depending upon its velocity, of eitherplasma arc cutting or welding. The basicpatents covering this discovery were issuedin April 1957.

All of these processes, which rely on shielding(or cutting) gases for effectiveness, are now inuse all over the world and play a major role inthe fabrication of products for the automotive,aerospace, railroad, trucking, and constructionindustries, to name a few.

2

The unique properties of a gas or gas blendcan have a dramatic influence on arc charac-teristics, heat input, and overall process per-formance. A basic understanding of the key

Electrons

It is convenient to think of electrons as nega-tive charges that can move about freely in acircuit. They can be thought of simplisticallyas being piled up at the negative end of acircuit, waiting to flow to the other (positive)end. The positive terminal does not haveenough electrons, the negative terminal hastoo many. When the two terminals are con-nected to each other by wires, the negativecharges travel to the positive terminal(figure 1).

Welding power supplies can be thought ofas a source of more electrons. As long as anelectrical power supply is connected to acircuit, a negative terminal can never use upits surplus of electrons, and a positive terminalcan never receive too many electrons.

BasicElectricalConcepts

Figure 1

Electron flow

C H A P T E R 1

1

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Properties of Electric Arcs and Gases

electrical concepts and the fundamentalphysical properties of gases is necessary inorder to select shielding gases wisely.

3

Voltage

Voltage is the unit of pressure or electromo-tive force that pushes current, or electrons,through a circuit. One volt will push one amp-ere through a resistance of one ohm. Weldingvoltages typically range from 14 to 35 volts.

Ampere

An ampere is the unit of current, or the mea-sure of the number of electrons that flow pasta point in a circuit every second. The quantityof electrons is expressed in coulombs. Onecoulomb equals 6.25 billion electrons. Oneampere is equal to one coulomb per second.Open arc welding amperages typically rangefrom 15 to 400 amps.

Ohm

An ohm is the unit of resistance to currentflow. One ohm is the quantity of resistancewhich produces a drop of one volt in a circuitcarrying one ampere.

In a garden hose, the water pressure is similarto voltage and the quantity of water flowing issimilar to amperage. Any restrictions in thehose, such as a kink, would produce resistanceto the flow of water. The relationship of thesevariables is expressed by the equation:

V (volts) = I (amperes) x R (ohms).

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Welding Polarity

The direction of current flow influencesthe melting efficiency of the welding arc.Consequently, the control of the polarity in awelding system is very important. In GMAW,there are two welding polarity connectionsutilized: straight polarity and reverse polarity.

When the direct current (DC) straight polarityor the direct current electrode negative(DCEN) connection is used, the electrode isthe nega-tive pole and the workpiece is thepositive pole of the welding arc. See figure 2.

When the DC reverse polarity or the directcurrent electrode positive (DCEP) connectionis used, the electrode is the positive pole andthe workpiece is the negative pole (figure 3).

When welding with direct current electrodenegative (DCEN), the direction of electronflow is to the base metal, striking the weldarea at high speed. At the same time, thepositive argon ions flow to the welding elect-rode, breaking up surface oxides on their way.

In GMAW, by changing polarity to directcurrent electrode positive (DCEP), the heatingeffect of the electron flow is concentrated atthe tip of the electrode. This contributes to theefficient melting of the wire electrode whichprovides molten metal and reinforcement forthe weld. Welding penetration and productiv-ity are increased with DCEP.

Another important characteristic of DCEPoperation is the surface cleaning action itproduces. This is especially true when weld-ing aluminum and magnesium which have aninvisible refractory oxide coating that mustbe removed in some matter to permit soundwelds to be made. It is useful to remove thisoxide prior to welding, as it has a highermelting point than the base material.

The surface cleaning action obtained withDCEP operation is probably the result of ionbombardment, which suggests that the flowof heavy gas ions produces a sand-blastingeffect on the oxide film. This theory issupported by the fact that argon ions, whichare about ten times heavier than helium ions,yield a greater degree of surface cleaning.

Figure 2

DC Electrode Negative (DCEN) connection

Figure 3

DC Electrode Positive (DCEP) connection

4

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Workpiece

Electrons

ElectrodeWeldingPowerSupply

Workpiece

Electrons

ElectrodeWeldingPowerSupply

Applying these basic electrical concepts to awelding arc is complicated by the introduc-tion of a shielding gas. In a normal electricalcircuit, electrons (current) are pushed (byvoltage) through a solid conductor, such asa copper wire, which has some degree of re-sistance (ohm). In a welding circuit, there isa break or gap between the electrode and theworkpiece where there is no solid wire con-ductor. The supply of electrons, therefore,must come from the gas.

To provide the necessary electrons, theatomic structure of the gas must be physicallychanged to a state that will allow it to conductelectricity. The temperature of the welding arcfacilitates this process.

In order to select the most appropriateshielding gas, it is important to understand afew of the fundamental characteristics thatapply to these gases at the elevated tempera-tures present in welding arcs. The characteris-tics that have a major influence on gasselection criteria, such as heat input to theworkpiece, penetration profile, the effect onmetal transfer, arc starting and arc stabilityare of particular interest.

The Welding Arc

Conceptually, a welding arc can be thought ofas a conversion device that changes electricalenergy into heat (figure 4).

The amount of heat that an arc produces fromelectricity depends upon many factors. One ofthe most important is arc current. When thearc current is increased, the amount of heat thearc produces is increased; the reverse is alsotrue. Another factor which controls arc heatis arc length. Changes in the length of anarc will cause changes in the amount of heatavailable from the arc. Consequently, success-ful welding depends upon control of both thearc current and arc length. Most of the time,when considering manual welding, the arccurrent is controlled by the welding powersupply and the arc length is controlled by thewelder.

A welding arc has been defined as A control-led electrical discharge between the electrodeand the workpiece that is formed and sus-tained by the establishment of a gaseousconductive medium, called an arc plasma.

An arc, or electrical flame, emits bright light,as well as ultraviolet and infrared radiation.Depending on the current level, arc tempera-tures may be very high, relative to the basemetal melting point, producing enough heat tomelt any known material. The characteristicsof the arc depend on the shielding gas presentin the arc environment because it affects theanode, cathode, and plasma regions foundthere. These regions are shown in figure 5and described on the next page.

Figure 4

An (GMAW) arc is a (DCEP)

conversion device

5

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Electricity

Arc

Heat

Light

The AnodeThe positive end of an arc is called the anode.It can be either the electrode or the workpiece,depending upon polarity of the system. Theanode is the very thin, intensely bright area onthe electrode or workpiece where the arc isattached. The anode voltage does not changewhen the electrode and workpiece are movedcloser together or farther apart. The anodevoltage depends on the composition of thefiller material and the gas that surrounds it andis essentially constant for any set of materials.

The CathodeThe negative end of the arc is called thecathode. It also can be either the electrode orthe workpiece, depending upon the polarityselected. The cathode is also a very thin brightarea where the arc is attached. In aluminumwelding, the cathode can appear as rapidlymoving flashes of light at the edge of the arcattachment point to the aluminum surface.Cathode voltage is similar to anode voltage.It depends on materials in the immediatesurroundings of the cathode zone and isessentially constant for any set of materials.

Arc PlasmaAn arc plasma is A gas that has been heatedby an electric arc to at least a partially ionizedcondition, enabling it to conduct an electriccurrent. It is the visible electrical flame, orarc, in the gap between the anode and cathode.

The arc voltage is the sum of the anode andcathode voltages plus the voltage across thearc plasma. With most welding processes,this voltage increases when the electrode andworkpiece move farther apart and decreasesas they move closer together. Arc voltage isalso impacted by the electrical conductivityof the shielding gas selected.

Ionization Potential

The ionization potential is the energy, express-ed in electron volts, necessary to remove anelectron from a gas atom, making it an ion, oran electrically charged gas atom. All otherfactors held constant, the ionization potentialdecreases as the molecular weight of the gasincreases. The significance of this is illus-trated in figure 6, which shows a simplifiedatomic structure of argon and helium. Simplis-tically speaking, ionization potential can bedescribed as a measurement of electricalconductivity of the arc shielding gas.

Argon, with eighteen electrons, is muchheavier than helium, which has only twoelectrons. The force of attraction holdingthe outer electrons in their orbit is inverselyproportional to the square of the distance fromthe nucleus. Simply stated, the energy requir-ed to remove an electron from the argon atomis significantly less than that required forhelium. Specifically, it takes 15.7 electronvolts to remove the first electron in argoncompared to 24.5 electron volts in helium.At these energy levels, ionization of the gasbegins in the arc gap, which creates the freeelectrons necessary to support current flowacross the gap, forming the plasma.6

Figure 5

The regions of

an arc

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Plasma

Anode

Cathode

Base Material

Although other factors are involved insustaining the plasma, these respectiveenergy levels must be maintained for thisto be accomplished.

From this relationship, it becomes readilyapparent that, for equivalent arc lengths andwelding currents (see figure 7), the voltage

Figure 6

Atomic structures of argon and helium

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Figure 7

Voltage-current relationship

(alternative current, AC)

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7

Figure 8

Increase of heat input with pure argon

and argon/helium mixtures.

obtained with a helium enhanced mixture isappreciably higher than it is with argon.

Since heat in the arc is roughly measured bythe product of current and voltage (arcpower), the use of helium yields a muchhigher available heat than does argon. This isone reason that helium is commonly referredto as a hotter gas (see figure 8).

See Table 1, page 11 for specific values ofmolecular weight and ionization potential forthe six pure shielding gases.

Arc starting and arc stability are also largelydependent on the ionization potential of theshielding gas selected. Gases with relativelylow ionization potential, such as argon, giveup electrons more easily, helping to initiateand maintain the arc in a stable operatingmode.

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Electrons ()

ArgonHelium

30

25

20

15

10

5

0

0 100 250 400

Arc Current (Amperes)

Arc

(v)

Gas Tungsten Arc Welding, Aluminum

Helium

Argon

35030020015050

Arc Length

0.08 Inch

0.16 Inch

ShieldingCup

TungstenElectrode

Arc Length1/8 (3.18 mm)

Amps (DCEN) = 300Arc Voltage = 11

Watts = 3300

Amps (DCEN) = 300Arc Voltage = 15

Watts = 4500

Helistar A-25Argon

Electromagnetic Pinch Force

Electromagnetic pinch force has a strongeffect on metal transfer. This pinch force isproduced by current flow in a wire or elec-trode (see figure 9). Every current-carryingconductor is squeezed by the magnetic fieldthat surrounds it. When the GMAW electrodeis heated close to its melting point and is inits eutectic state, the electromagnetic pinchforce squeezes off drops of metal and assistsin the transfer of the filler metal into theweld pool.

Thermal Conductivity

The thermal conductivity of a gas is relatedto its ability to conduct heat. It influences theradial heat loss from the center to the periph-ery of the arc column. Pure argon, when usedas a shielding gas has mild thermal conductiv-ity, and produces an arc which has two zones:a narrow hot core and a considerably coolerouter zone (see figure 10). As a result, thepenetration profile of a typical weld fusionarea is characterized by a narrow finger atthe root and a wider top. A gas with highthermal conductivity conducts more of the

Figure 10

Comparison of arcs in argon

and helium atmospheres

Figure 9

Pinch Force

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8

PhysicalPropertiesof Gases

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heat outward from the arc core. This resultsin a broader, hotter arc. This type of heatdistribution, which occurs with helium, argon-hydrogen, and argon/carbon dioxide mixturesis more uniform and produces a generallywider profile throughout the fusion zone.

Dissociation and Recombination

When two or more atoms combine they forma molecule. Shielding gases such as carbondioxide, hydrogen, and oxygen are molecules.For example, carbon dioxide is made up ofone atom of carbon and two atoms of oxygen.

When heated to the high temperatures presentin the arc plasma (12-15,000 F), these gasesbreak down, or dissociate into their separateatoms. They are then at least partially ionized,producing free electrons and improved currentflow. As the dissociated gas comes in contactwith the relatively cool work surface, theatoms recombine, and it releases energy tothe base material in the form of heat. Thisprocess does not occur with gases such asargon, which consists of a single atom. There-fore, for the same arc temperature, the heatgenerated at the work surface can be greaterwith gases such as carbon dioxide, hydrogen,and oxygen.

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Pinch EffectForce

Electrode

.......

Current (A)

Electrode

Workpiece Workpiece

Penetration Profile

Wide ArcCore

Narrow ArcEnvelope

Wide ArcEnvelope

NarrowArc Core

Argon Helium

Reactivity

Reactivity, as it applies to shielding gases, isa comparative measurement of how readily agiven shielding gas (at arc temperatures) willreact with the elements in the puddle.

Argon and helium are completely non-reactive, or inert, and therefore have nochemical effect on the weld metal. Nitrogen isan inert gas, but, at the temperatures commonto welding, it may react and have an adverseeffect on weld chemistry.

Oxygen and carbon dioxide fall into acategory of reactive gases known as oxidizers.These gases will react to form oxides withthe molten metal in the arc and in the weldpuddle. This property may contribute tocausing welding fume.

Hydrogen is a reactive gas, but is reducingin the nature. Hydrogen will (preferentially)react with oxidizing agents, thereby helping toprevent the formation of oxides in the moltenweld metal. However, hydrogen can producedetrimental effects such as underbead crack-ing, when used on some high strength andlow alloy steels.

Surface Tension

In any liquid there is an attractive forceexerted by the molecules below the surfaceupon those at the surface. An inward pull, orinternal pressure is created, which tends torestrain the liquid from flowing. Its strengthvaries with the chemical nature of the liquid.

In welding, the surface tension betweenmolten steel and its surrounding atmospherehas a pronounced influence on bead shape.If surface tension is high, a convex, irregularbead will result. Lower values promote flatterbeads with minimum susceptibility forundercutting.

Pure argon shielding when used with GMAWis usually associated with high interfacialenergy, producing a sluggish puddle and ahigh-crowned bead when mild steel welding isconsidered. This is partially attributed to thehigh surface tension of liquid iron in an inertatmosphere. For this reason, it is not recom-mended for use in MIG welding of mild steel.Iron oxides, however, have a considerablylower surface tension and thus promote goodwetting to the parent metal. Therefore, theaddition of small percentages of oxygen orcarbon dioxide to argon when performingGMAW result in a more fluid weld puddle.

Gas Purity

Some base metals, such as carbon steel havea relatively high tolerance for contaminants.Others, such as aluminum, copper andmagnesium, are fairly sensitive to impurities.Still others, such as titanium and zirconium,have extremely low tolerances for anyimpurity in the shielding gas.

Depending on the metal being welded and thewelding process used, even minuscule gasimpurities can have a detrimental impact onwelding speed, weld surface appearance, weldbead coalescence, and porosity levels. Theseimpurities can appear in several ways.

9

It is always possible that the gas can be con-taminated as delivered; but it is more likelythat it becomes contaminated somewhere bet-ween the supply and the end-use points.Praxair is equipped with the analytical equip-ment to determine purity levels anywhere inthe gas supply system, and can assist inidentifying the cause of gas purity problemsand their solution.

See Table 2, page 12 for standard industryminimum purity levels for welding gradegases.

Gas Density

Gas density is the weight of the gas per unitvolume. It is usually expressed in pounds percubic feet. Gas density is one of the chieffactors influencing shielding effectiveness.Basically, gases heavier than air require lowerflow rates than gases that are lighter than airto achieve equivalent weld puddle protection.See Table 1, page 11 for specific values.

The properties of the gases commonly usedin welding and cutting and how they functionin metal fabrication applications are discussedbelow.

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Argon (Ar)

Slightly less than one percent of the earthsatmosphere is composed of argon, which iscolorless, odorless, tasteless, and nontoxic.As an inert gas, argon does not react withother compounds or elements. Argon is about1.4 times heavier than air and cannot sustainlife. The inert properties of argon make it idealas a shield against atmospheric contamination,thus it is used in many welding processes.

Argon promotes good arc starting character-istics and arc stability due to its low ionizationpotential.

Carbon Dioxide (CO2)

Carbon dioxide (CO2), a reactive gas, is about1.5 times heavier than air. It is an odorless,colorless gas with a slightly pungent, acidtaste and is slightly toxic. It will not sustainlife or support combustion. Differing fromother reactive gases such as oxygen, CO2can be used alone for GMAW shielding gasapplications. Its relatively high oxidizingpotential can be countered by the use ofGMAW or FCAW wires higher in alloyingelements, such as silicon and manganese.

Carbon dioxide is commonly mixed withargon to improve productivity and penetrationin GMAW.

Helium (He)

Helium is the second lightest element, afterhydrogen, and is lighter than air. Like argon,it is chemically inert and will not sustain life.Due to its high thermal conductivity andhigh ionization potential, helium is used as ashielding gas for welding applications whenincreased heat input is desired, and lowtolerance for oxidizing elements exist suchas with aluminum and magnesium welding.

Hydrogen (H2)

Hydrogen, the lightest known element, is aflammable gas. Explosive mixtures can beformed when certain concentrations of hydro-gen are mixed with oxygen, air, or otheroxidizers. Hydrogen is not life sustaining.Small quantities are useful in gas blends forplasma cutting and some welding applicationsbecause of its high thermal conductivity andreactive nature. It is very useful when GMAWand GTAW 300 series austenitic stainlesssteels.

Basic GasProperties

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11

Nitrogen (N2)

Nitrogen is a colorless, odorless, and tastelessgas which forms 78 percent of the earthsatmosphere (by volume). It is nonflammable,does not support combustion, and is slightlylighter than air. Nitrogen is inert except at arcwelding temperatures, where it will react withsome metals, such as aluminum, magnesium,and titanium. It is not recommended as aprimary shielding gas with GMAW, but iscommonly applied as an assist gas with lasercutting on stainless steels. It can be used incombination with other gases for some weld-ing applications and is also widely used inplasma and laser cutting.

Oxygen (O2)

Fifty percent of the earths crust and approxi-mately 21 percent of the earths atmosphere(by volume) is oxygen. Oxygen combineswith almost all known elements except rare orinert gases, and it vigorously supports com-bustion. Because of its highly oxidizing andcombustion-supporting properties, oxygen isan ideal gas for increasing flame temperaturesand improving performance for oxyfuel weld-ing and cutting. Small amounts of oxygen maybe added to argon for GMAW to increase arcstability and improve the wetting and shape ofthe weld bead when working with mild orstainless steels. It is also used to enhance cut-ting speeds with plasma and laser processes.

Table 1

Shielding

gas data

Argon Carbon Helium Hydrogen Nitrogen OxygenDioxide

Chemical Symbol Ar CO2 He H2 N2 O2Atomic Number 18 _ 2 1 7 8

Molecular Weight 39.95 44.01 4.00 2.016 28.01 32.00

Specific Gravity, 1.38 1.53 0.1368 0.0695 0.967 1.105Air = 1

Density (lb/cu ft) at 0.1114 0.1235 0.0111 0.0056 0.0782 0.08920 C, 1 atmosphere

Ionization Potential 15.7 14.4 24.5 13.5 14.5 13.2(ev)Thermal Conductivity 9.69 8.62 85.78 97.22 13.93 14.05(10-3 x Btu/hr-ft- F) (32 F) (32 F) (32 F) (32 F) (32 F) (32 F)Cubic ft/lb 9.67 8.73 96.71 192 13.8 12.08

Cubic ft/gal 113.2 74.0 100.6 103.7 93.2 115.0

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Product Minimum Maximum Approximate Dewpoint atState Purity Moisture* Maximum Moisture Content

(percent) (ppm) F CAir Liquid 99.98 120 - 40 - 40

Argon Gas 99.995 10 - 77 - 60

Liquid 99.997 6 - 83 - 64

Carbon Dioxide Gas 99.5 34 - 60 - 51

Liquid 99.8 13 - 73 - 58

Helium Gas 99.95 32 - 61 - 51

Liquid 99.995** 3 - 92 - 69

Hydrogen Gas 99.95 8 - 80 - 63

Liquid 99.995*** 5 - 86 - 65

Nitrogen Gas 99.7 32 - 61 - 51

Liquid 99.997 5 - 86 - 65

Oxygen Industrial 99.5 50 - 54 - 48

Liquid 99.5 6 - 83 - 64

* Moisture specifications are measured at full cylinder pressure.

** Including neon

*** Including helium

Based solely on oxygen content. Minute traces of other inert gases (such as argon, neon, helium, etc.) which remainafter oxygen removal are considered as nitrogen. (This is standard practice in the compressed gas industry.)

Table 2

Gas purity and

moisture content

(welding grade)

12s

Gas Tungsten Arc Welding (GTAW) is definedas an open arc welding process that producescoalescence of metals by heating them withan electric arc between a tungsten electrode(nonconsumable) and the workpiece. Themolten weld pool is protected by an externallysupplied shielding gas. Pressure may or maynot be used, and filler metal may or may notbe used. GTAW is also commonly referred toas TIG (Tungsten Inert Gas) or Heliarc weld-ing although the American Welding Societyrefers to it as GTAW. Figure 11 illustrates theessentials of GTAW.

The use of a nonconsumable tungsten elec-trode and inert shielding gases produces thehighest quality welds of any open arc weldingprocess. Welds are bright and shiny, with noslag or spatter, and require little or no post-weld cleaning. GTAW is easily used in allwelding positions and provides excellent weldpuddle control, especially on thin and intricateparts. It has found extensive use in theaircraft, aerospace, power generation, chemi-cal, and petroleum industries.

Although usually thought of as a manualprocess, GTAW is often automated with orwithout filler wire for high-productionapplications. In 1969, Praxair introduced avariation to the process called Hot Wire.With this process, the filler wire is indepen-dently pre-heated to a molten state as it entersthe weld puddle. This feature allows arc heatto be fully concentrated on melting the work-piece, not the wire (see figure 12). The HotWire process expands the versatility of auto-mated GTAW by increasing deposition ratesand travel speeds.

ProcessDescription

Figure 11

Essentials of GTAW

Figure 12

Diagram of Gas Tungsten Arc

hot wire system 13

C H A P T E R 2

2

s

Gas Tungsten Arc Welding (GTAW)

s

s

Arc

DC TIGPower

TIG Torch

Heated Wire

Workpiece

AC HotWirePower

Weld

Contact Tube

High SpeedWire Feeder

Travel

GroundConnection

ACHF, DCENor DCEP

Power Supply

TungstenElectrode

Workpiece

Welds madewith or without

filler metal

GasCup

GasEnvelope

ElectricArc

Gas flow rate, which can range from a fewcubic feet per hour (cfh) to more than 60 cfh,depends on the current developed, the torchsize, the shielding gas composition and thesurrounding environment (drafts, etc.).

Gas FlowRate

Figure 13

Shielding effectiveness of gas density (GTAW)

Figure 14

Relationship of flow requirements

to cross-draft velocity

14

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In general, a higher current will require alarger torch and higher flow rates. In addi-tion, gas density, or the weight of the gasrelative to air, has a major influence on theminimum flow rate required to effectivelyshield the weld.

Argon is approximately 1.4 times as heavy asair and ten times as heavy as helium. The sig-nificance of these differences in gas densityrelative to air is shown in figure 13.

Argon shielding gas is delivered to the arczone by the torch nozzle. Its function is toprovide a contaminant free blanket of shield-ing gas over the weld area. Because helium ismuch lighter than argon, to produce equiva-lent shielding effectiveness when welding inthe flat position, the flow of helium must betwo to two and one half times that of argon.The same general relationship is true formixtures of argon and helium, particularlythose high in helium content, although asargon content is increased, shielding gas flowis typically decreased. It should be noted thatfor overhead welding flow rates with heliummixtures can be reduced as the specificgravity of the gas is less than that of air.

Figure 14 shows the relationship of flowrequirements to the cross-draft velocity forpure argon and for a 25% argon/75% heliummixture using two different nozzle-to-workpiece distances.

Gas flow rate must be selected with care.It is not productive or economical to use moregas than necessary to achieve good shielding.High gas flows can pull air into the weldingarc, often causing porosity in the weld.To avoid wasting gas and contaminating theweld, use of an inexpensive critical flowdevice that restricts gas flow to an optimalrange is often recommended.

s

s

2 to 2 1/2 timesas much gas flow

Argon Helium

7

6

5

4

3

2

1

0

0 16 40

Minimum Shielding Gas Flow(Cubic Feet Per Hour)

Air

Vel

oci

ty (

Feet

Per

Sec

ond)

Tungsten Arc 5/8 Inch Diameter Cup

75% He25% Ar

Ar

564832248

Nozzle-To-WorkDistance

3/16 Inch

9/16 Inch

Ar

75% He25% Ar

When welding materials that are sensitiveto oxidation (such as copper, aluminum andstainless steel), gas preflow and postflow willminimize contamination of the weld zone andelectrode. A pre-flow of shielding gas removesmoisture which may have entered the systemand blankets the weld zone for optimum start-ing conditions. Changes in room temperaturecan cause air to move in and out of the end ofa torch while not in use; moisture in the aircondenses on the inside of the torch. A pre-flow of shielding gas for a period of timebefore the arc is initiated will remove themoisture.

Postflow works to minimize contamination ofthe weld pool in a different way. When the arcis turned off, the weld metal begins to cool.For a few moments, the weld metal remainshot enough to be contaminated by the sur-rounding air. To prevent this, the shielding gasis allowed to flow for several seconds after thearc is extinguished. The length of time varieson the size and temperature of the weld but arule of thumb is one second for every tenamps of current. This will provide shieldingto allow the weld to cool. The postflow ofgas also protects the hot electrode fromcontamination.

PreflowandPostflow

It is sometimes necessary to use shielding gason the underside of a weld to prevent oxida-tion of the hot weld bottom. As an example,backup shielding gas is used to purge the airfrom the interior of piping prior to and duringwelding. This procedure prevents contamina-tion of the backside of the weld while the pipeis being welded from the outside.

The same gas may be used for backup andwelding, but it is possible to use a gas blendfor welding and another gas, such as pure

argon, nitrogen, or an argon/hydrogen,nitrogen/hydrogen blend for the backup gas,depending on the workpiece material.

In some instances, the welding travel speedmay be too great for the shielding gas toprotect the weld until it has cooled. As the arcmoves on, the solidified weld metal remainshot and oxidizes. A trailing gas shield can beused to prevent oxidation on the surface of theweld bead from occurring.

BackupShieldingand TrailingShields

Argon

Argon, an inert rare gas that makes upapproximately 1% of the earths atmosphere,is the most commonly used shielding gas forGTAW. Its low thermal conductivity producesa narrow, constricted arc column and excellentelectrical conductivity which allow greatervariations in arc length with minimal influ-ence on arc power and weld bead shape. Thischaracteristic makes it the preferred choice formanual welding. In addition, argon providesgood arc starting due to its low ionizationpotential.

For AC welding applications, high purityargon offers superior cleaning action, arcstability, and weld appearance.

While pure argon may be used for mechanizedapplications, argon/helium or argon/hydrogenblends are frequently selected to promotehigher welding travel speeds. The hotter arccharacteristics of these blends also make themmore suitable for welding metals with highthermal conductivity, such as copper orstainless steel. Argon/hydrogen blends shouldonly be used for welding austenitic stainlesssteels.

ShieldingGases forGTAW

s

s

s

15

Helium

Helium, also an inert gas, has high thermalconductivity and high ionization potential,which require higher arc voltages than argonfor a given current setting and arc length.(See Chapter 1, figure 7, Voltage-currentrelationship.) This produces a hotter andbroader arc which improves the depth ofpenetration and weld bead width.

The use of helium is generally favored overargon at the higher current levels which areused for welding of thicker materials, espe-cially those having high thermal conductivityor relatively high melting temperatures. It isalso often used in high-speed mechanizedapplications, although an addition of argonwill improve arc initiation and cleaningaction.

Although argon is widely used for ACwelding of aluminum, helium has beensuccessfully used for DCEN mechanized andhigh current AC welding of this material.It produces greater penetration and highertravel speeds. However, surface oxides mustbe cleaned from the weld joint to obtainacceptable results.

The physical properties of helium definitelyoffer advantages in some applications. How-ever, due to it high ionization potential, it alsoproduces a less stable arc and a less desirablearc starting characteristic than argon. Its high-er cost and higher flow rates are also factorsto be considered. In some cases, an argonmixture is used for igniting the arc and purehelium is used for welding. This techniqueis used for mechanized DCEN-GTAWwelding of heavy aluminum.

Argon/Helium Blends

Praxairs HeliStar BlendsEach of these gases (argon and helium),as explained above, has specific advantages.Praxairs HeliStar blends (argon/heliumblends) are used to increase the heat input tothe base metal while maintaining the favor-able characteristics of argon, such as arcstability and superior arc starting.

Praxairs HeliStar A-75This blend is sometimes used for DC weldingwhen it is desirable to obtain higher heatinput while maintaining the good arc startingbehavior of argon. It is a favorite choice whenMIG welding thick aluminum (> 1/2").

Praxairs HeliStar A-50This blend is used primarily for high-speedmechanized welding of nonferrous materialunder 3/4 inch thick.

Praxairs HeliStar A-25The speed and quality of AC and DC weldingof aluminum, copper and stainless steels canbe improved with this blend. It is sometimesused for manual welding of aluminum pipeand mechanized welding of butt joints inaluminum sheet and plate. Praxairs HeliStarA-25 blend is also used for many of theGTAW hot wire applications to increasethe energy input and accommodate the highfiller metal deposition rates of the process.

16

Argon/Hydrogen Blends

Praxairs HydroStar BlendsHydrogen is often added to argon to enhanceits thermal properties. Hydrogens reducingcharacteristics also improve weld puddlewetting and produce cleaner weld surfacesdue to reduced surface oxidation. Hydrogenenhanced blends are commonly selected toweld 300 series stainless steels.

The higher arc voltage associated withhydrogen increases the difficulty of startingthe arc. For this reason, the smallest additionof hydrogen consistent with the desired resultis recommended. Additions up to 5% formanual welding and up to 10% for mecha-nized welding are typical. Ratios beyond thislevel typically cause porosity in GTAW.

Argon/hydrogen blends are primarily usedon austenitic stainless steel, nickel, and nickelalloys. Hydrogen is not used to weld carbonor low-alloy steel, copper, aluminum, ortitanium alloys since cracking or porositywill result from the absorption of hydrogen.

WarningSpecial safety precautions are requiredwhen mixing argon and hydrogen. DO NOTattempt to mix argon and hydrogen fromseparate cylinders. See the Safety section inthis handbook for more information.

Higher ratios of hydrogen may be mixed withargon (up to 35%) depending on the processselected. For example Praxairs HydroStarH-35 (65%Ar/35% H2) is commonly used inplasma gouging.

Praxairs HydroStar blends are hydrogen-enhanced argon-based blends which areideally suited for general purpose manual andmechanized GTAW of most commercially

available austenetic stainless steels. It maybe substituted for pure argon in manyapplications.

Praxairs HydroStar H-2 and H-5These blends are used for manual GTAWapplications on 300 series stainless steels.HydroStar H-5 blend is preferred on materialthicknesses above 1/16 inch. These blends arealso used for back purging on stainless pipe.

Praxairs HydroStar H-10This blend is preferred for high-speedmechanized applications. It is used with 300series stainless steels.

Praxairs HydroStar H-15This blend, which contains 15% hydrogen,is used most often for welding butt jointsin stainless steel (300 series) at speeds com-parable to helium, and is typically 50 percentfaster when compared with argon. HydroStarH-15 blend is frequently used to increase thewelding speeds in stainless steel tube mills.It can be used on all thicknesses of stainlesssteel, although concentrations greater than15% may cause weld metal porosity.

Praxairs HydroStar H-35This blend, which contains 35% hydrogen, isused most often for conventional plasma arccutting and gouging of stainless steel.

Oxygen and Carbon Dioxide

These gases are chemically reactive andshould not be used with GTAW. Their highoxidation potential can cause severe erosionof the tungsten electrode at arc temperatures.

See Table 3, page 18 for the GTAW ShieldingGases Selection Guide.

17

Table 3

Shielding Gases

Selection Guide

for GTAW

Material Weld Type Recommended DescriptionShielding Gas

Mild Steel Spot Argon Long electrode life; better weld nuggetcontour; easiest arc starting

Manual Argon Best puddle control, especially forout-of-position welding

Mechanized Argon/Helium High speeds; lower gas flows than withpure helium

Helium Higher speeds than obtained with argon;improved penetration

Aluminum Manual Argon Best arc starting, good cleaning action andand weld quality; lower gas consumptionMagnesium Argon/Helium Higher welding speeds, greater weld

penetration than argonMechanized Argon/Helium Good weld quality, lower gas flow than requir-

ed with straight helium, improved penetrationHelium (dcsp) Deepest weld penetration and greatest weld

speeds; can provide cleaning action foraluminum and magnesium welding

Stainless Spot Argon Excellent control of penetration on lightSteel gauge materials

Argon/Helium Higher heat input for heavier gauge materials;faster travel speeds, improved weld puddlefluidity

Manual Argon Excellent puddle control,controlled penetration

Mechanized Argon Excellent control of penetration onlight gauge materials

Argon/Helium Higher heat input, higher weldingspeeds possible

Argon/ Minimizes undercutting; produces desirableHydrogen weld contour at low current levels, requires

lower gas flows, ideal as a back purge gas on300 series stainless steel

Nitrogen/ Suitable for back purging on 300 seriesHydrogen stainless alloys

Copper, Argon Excellent puddle control, penetration and beadNickel and contour on thin gauge metalCu-Ni Alloys Argon/Helium Higher heat input to offset high heat

conductivity of heavier gauges,faster travelspeeds

Helium Highest heat input for sufficient weldingspeed on heavy metal sections

Titanium Argon High gas density provides better shieldingArgon/Helium Better penetration for manual welding of thick

sections (inert gas backing required to shieldback of weld against contamination)

Silicon Bronze Argon Reduces cracking of this hot short metal

Aluminum Bronze Argon Controlled penetration of base metal

Note:Argon/helium blendsusually require a watercooled torch and largertungsten diameter due toincreases in arc voltage.

18s

Process Description

Plasma Arc Welding (PAW) is an evolutionarystep in the overall development of GTAW.Basically, the process uses an open, unrestrict-ed gas tungsten arc that is squeezed througha copper nozzle. The result is a constrictedarc that is longer, thinner, and more focusedthan a GTAW arc. Figure 15 illustrates theessential difference between the GTAW andPAW processes.

The constriction process greatly increasesarc voltage and the amount of ionization thattakes place. In addition to raising arc tem-perature, the hottest area of the plasma isextended farther down toward the worksurface (figure 16). The overall result is amore concentrated heat source at a higherarc temperatures that greatly increases heattransfer efficiency; this promotes fastercutting and welding speeds.

PAW is defined as an arc welding processthat uses a constricted arc between a noncon-sumable electrode and the weld pool (trans-ferred arc) or between the electrode and theconstricting nozzle (nontransferred arc),see figure 17. Shielding is obtained from theionized gas supplied to the torch, which maybe supplemented by an auxiliary source ofshielding gas. The process is used without theapplication of pressure.

Plasma ArcWelding (PAW)

Figure 15

Comparison of

GTAW with PAW

Figure 16

Arc temperature profile

19

C H A P T E R 3

3

s

Plasma Arc Processes (PAW and PAC)

s

s

PlasmaGas

GTAW Open Arc PAW Constricted Arc

ShieldingGas

ShieldingGas

TungstenElectrode

Temperatures, K

10,000 - 14,000

14,000 - 18,000

18,000 - 24,000

24,000 and up

GTAW PAW

The plasma process can produce two types ofarcs. If the constricted plasma arc is formedbetween the electrode and the workpiece,it is said to have a transferred arc. If thearc is produced between the electrode andthe constricting nozzle, it is called nontrans-ferred arc. See figure 17.

Plasma arcs have an extremely wide range ofoperation. The nontransferred arc is used inspecial welding applications where it is notdesirable to make the workpiece part of theelectric circuit. It is also used for fusing non-metallic materials, such as ceramics andcertain types of glass. Operating currentsrange from 2 to 300 amps.

With the transferred arc, two basic weldingmethods are used: the Melt-in mode, (whichcan be used with or without filler metal), andKeyhole mode. See figure 18 for illustrationsof these methods.

Although similar to GTAW, the Melt-inmethod has some advantages due to its longer,more constricted arc shape. These includeimproved arc stability (particularly at lowcurrent levels), less distortion of the work-piece, higher potential welding speed, andgreater tolerance to changes in torch-to-workdistance. As shown in figure 19, the change inarc plasma area with a change in stand-offdistance, is much greater with GTAW thanwith PAW. This has a major effect on heatingof the work and, subsequently, on penetrationand weld shape.

Figure 18

Plasma arc welding modes

Figure 17

The two plasma arcs

20

s

s

PlasmaGas

Non-Transferred

ShieldingGas

PlasmaGas

Transferred

ShieldingGas

Melt-in Weldingwithout filler metal

Melt-in Weldingusing filler metal

Keyhole Welding

In Keyhole welding, the workpiece is fusedthrough its entire thickness. The plasma jetpierces through the molten metal giving 100%penetration and forms a welding eyelet (seefigure 20) which moves together with the arcin the direction of welding. Behind the plasmajet, the molten metal flows together again (asa result of surface tension), solidifies, andforms the completed weld.

Application of PAW

High-quality welds can be made with nickel,nickel-copper, nickel-iron-chromium, copper,heat resisting titanium, refractory alloy steels,and in nickel-chromium alloys up to approx-imately 0.3 inches thick. The process showsits greatest advantage when the keyholeapproach is used in the thickness range of0.062 to 0.0312 inches. The high-quality weldproduced by the single pass keyhole techniqueis illustrated in figure 21.

Figure 21

Cross-section of Keyhole weld

21

Figure 20

PAW Keyhole welding

Figure 19

Variation of heating effect

with stand off distances

s

s s

Torch

Trav

el

Keyhole

Stand-offDistance (L)

ArcLength

ArcPlasma

Area

ArcPlasma

GTAW PAW

Material Thickness Keyhole Melt-in

Aluminum Under 1/16" Keyhole tech. not recommended Argon or HeliumOver 1/16" Helium Helium

Carbon Steel Under 1/16" Keyhole tech. not recommended Argon, Helium or HeliStar A-75(Al. killed) Over 1/16" Argon or HeliStar A-25 Argon or HeliStar A-25

Low Alloy Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5

Steel Over 1/16" Argon or HeliStar A-25 Argon or Helium

Stainless Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5

Steel Over 1/16" Argon, HeliStar A-25 Argon, Helium, HydroStar H-5HydroStar H-2 or H-5

Copper Under 1/16" Keyhole tech. not recommended Helium or HeliStar A-75Over 1/16" Helium or HeliStar A-25 Helium

Nickel Under 1/16" Keyhole tech. not recommended Argon, Helium,HydroStar H-2 or H-5

Alloys Over 1/16" Argon, HeliStar A-25 Argon, Helium, HydroStar H-5

Reactive Under 1/16" Keyhole tech. not recommended ArgonMaterials Over 1/16" Argon, Helium or HeliStar A-25 HeliStar A-75

Shielding Gases for PAW

The physical configuration of PAW requiresthe use of two gases, a plasma or orifice gasand a shielding gas. The primary role of theplasma gas, which exits the torch through thecenter orifice, is to control arc characteristicsand shield the electrode. It also effects theheat transfer properties to the base metal. Theshielding gas, introduced around the peripheryof the arc, shields or protects the weld. Inmany applications, the shielding gas is alsopartially ionized to enhance the plasma gasperformance.

Low current (< 100 amps)Argon is the preferred orifice gas becauseits low ionization potential ensures easy andreliable starting. Argon/helium and argon/hydrogen mixtures are also used for applica-tions requiring higher heat input.

The choice of shielding gas is dependent onthe type and thickness of the base material.When welding aluminum, carbon steel, andcopper, the gases commonly used are argon,helium, and argon/helium mixtures. It isgenerally recommended that the percentageof helium be increased as the base-platethickness increases. When welding low alloysteels, stainless steels, and nickel alloys, theaforementioned gases in addition to argon-hydrogen mixtures are used. See Table 4,below for low-current gas selection.

High Current (> 100 amps)The choice of gas used when performing highcurrent plasma arc welding also depends onthe composition of the material to be welded.In all but a few cases, the shielding gas is thesame as the orifice gas.

Table 4

Low-Current

Plasma Arc

Welding Gas

Selection Guide

22

s

Argon

Argon is suitable as the orifice and shieldinggas for welding all metals, but it does notnecessarily produce optimum welding results.In the Melt-in mode, additions of hydrogento argon produce a hotter arc and offer moreefficient heat transfer to the work. Limits onthe percentage of hydrogen are related to itspotential to cause cracking and porosity.However, when using the Keyhole technique,a given material thickness can be welded withhigher percentages of hydrogen. This may beassociated with the Keyhole effect and thedifferent solidification pattern it produces.

Argon is used for welding carbon steel, highstrength steel, and reactive metals such astitanium and zirconium alloys. Even minutequantities of hydrogen in the gas used to weldthese materials may result in porosity, crack-ing, or reduced mechanical properties.

Argon/Helium Blends

Praxairs HeliStar BlendsHelium additions to argon produce a hotterarc for a given arc current. Argon/heliummixtures containing between 50% and 75%helium are generally used to make keyholewelds in heavier titanium sections and for filland capping passes on all materials when theadditional heat and wider heat pattern ofthese mixtures prove desirable.

Argon-Hydrogen Blends

Praxairs HydroStar BlendsArgon/hydrogen mixtures are used as theorifice and shielding gases for makingkeyhole welds in stainless steel, Inconel,nickel, and copper-nickel alloys. Permissiblehydrogen percentages vary from 5% to 15%.See Table 5, below for high-current gasselection.

Material Thickness Keyhole Melt-in

Aluminum Under 1/4" Argon Argon or HeliStar A-25Over 1/4" Helium Helium or HeliStar A-25

Carbon Steel Under 1/8" Argon Argon(Al. killed) Over 1/8" Argon HeliStar A-25

Low Alloy Under 1/8" Argon ArgonSteel Over 1/8" Argon HeliStar A-25

Stainless Under 1/8" Argon or HydroStar H-5 ArgonSteel Over 1/8" Argon or HydroStar H-5 HeliStar A-25

Copper Under 3/32" Argon Helium or HeliStar A-25Over 3/32" Keyhole tech. not recommended* Helium

Nickel Under 1/8" HydroStar H-5 ArgonAlloys Over 1/8" HydroStar H-5 HeliStar A-25

Reactive Below 1/16" Argon ArgonMaterials Above 1/16" Argon or Argon/Helium HeliStar A-25

23

Table 5

High-Current

Plasma Arc

Welding Gas

Selection Guide

* The underbead will not form correctly. However, on Cu-Zn alloys a keyhole technique can be used.

s

Note:Gas selections shownare for both the orificeand shielding gas.

Process Description

Plasma Arc Cutting is defined as an arccutting process that severs metal by melting alocalized area with a constricted arc whichremoves the molten material with a highvelocity jet of hot, ionized gas issuing fromthe constricting orifice.

The major difference between PAC and PAWis the velocity of the orifice gas. In somecases, a shielding gas as well as a cutting, ororifice gas may be used (the shielding gasprevents oxidation of the cut surface.) Thehigher velocity gas used in PAC removes orblows away the molten material. The PACprocess can be used to cut any electricallyconductive metal if its thickness and shapepermit full penetration by the plasma jet.Because the PAC process can be used to cutnonferrous materials, and is faster than oxy-fuel cutting in the less-than-two-inch thick-ness range for ferrous materials, it is ideal formany industrial applications.

Plasma ArcCutting (PAC)

24

Figure 22

Conventional Plasma Arc Cutting

s

Since PAC was introduced by Praxair in 1954,many process refinements, gas developments,and equipment improvements have occurred.The following sections describe the processvariations that are in use today.

Conventional Plasma Arc CuttingIn conventional Plasma Arc Cutting, the arcis constricted by a nozzle only; no shieldinggas is added. Generally the cutting gas(usually nitrogen or air) is tangentially in-jected around the electrode (see figure 22).

The swirling action of the gas causes thecooler (heavier) portions of the gas to moveradially outward, forming a protective bound-ary layer on the inside of the nozzle bore. Thishelps prevent nozzle damage and extends itslife. Electrode life is also improved since thearc attachment point (cathode spot) is forcedto move about and distribute its heat loadmore uniformly. Until the introduction ofWater Injection Plasma Arc Cutting in 1970(see page 26) conventional Plasma Arc Cut-ting was the most popular technique. It is stillthe best method for cutting thicker stainlessand aluminum plate.

Air Plasma Arc CuttingAir Plasma Arc Cutting was introduced in theearly 1960s for cutting mild steel. Oxygen inthe air provides additional energy by creatingan exothermic reaction with molten steel,boosting cutting speeds about 25 percent.Although this process can also be usedto cut stainless steel and aluminum, the cutsurface will be heavily oxidized and is oftenunacceptable for many applications. Electrodeand tip life are also reduced when comparedto the use of nitrogen as the plasma gas.

s

PlasmaJet

Electrode

Nozzle

Workpiece

Oxygen Plasma Arc Cutting

In Oxygen Plasma Arc Cutting, oxygen isused as the plasma (orifice) gas in place ofnitrogen or air. The oxygen in the plasmastream has a similar effect on steel as withoxyfuel cutting; it produces an exothermicreaction which increases cutting speed. It ispossible to achieve cutting speeds similar tonitrogen at much lower currents. Oxygenplasma cutting is used primarily on mild steel.

Limitations of Oxygen Plasma Arc CuttingThe conventional PAC process (with nitrogen)uses tungsten electrodes which cannot be usedin an oxygen environment. Halfnium is sub-stituted as the electrode material for oxygencutting. The halfnium must be kept cool andthe current capacity of the torch limited toensure longer life (see figure 23).

Dual-Flow Plasma Arc CuttingDual-Flow Plasma Arc Cutting is a slightmodification of conventional Plasma ArcCutting (see figure 24). It incorporatesmost of the features of conventional PlasmaArc Cutting, but adds a secondary shieldinggas around the nozzle.

Usually the cutting gas is nitrogen and theshielding gas is selected according to themetal to be cut. Cutting speeds are slightlybetter than conventional plasma arc cuttingon mild steel; however, cut quality is notacceptable for many applications. Cuttingspeed and quality on stainless steel andaluminum are essentially the same as inconventional Plasma Arc Cutting.

Figure 23

Oxygen PAC nozzle

Figure 24

Dual-flow Plasma Arc Cutting

25

s

s

Nozzle

Electrode Plasma Gas Inlet

ShieldingGas Inlet

Workpiece

Shield

Plasma Jet

Electrode

Nozzle

Workpiece

Shield Cup

Shield Gas orWater Shield

Water Injection Plasma Arc CuttingIn Water Injection Plasma Arc Cutting, wateris introduced inside the nozzle to provideadditional arc constriction (see figure 25)and nozzle cooling.

Two modes of water injection have beendeveloped: Radial Injection (the waterimpinges the arc with no swirl component),and Swirl Injection (the water is introduced asa vortex swirling in the same direction as thecutting gas).

The increased arc constriction provided by thewater improves cut squareness and increasescutting speed. The water also protects thenozzle since it provides cooling at the pointof arc constriction. The water completelyprotects the bottom half of the nozzle fromintense radiation, allowing complete insula-tion of the nozzle; hence, resistance to damageis greatly improved. This approach ensurescomponent durability, superior cut qualityand high cutting speeds.

Underwater Plasma Arc CuttingUnderwater Plasma Arc Cutting is ideallysuited to numerically-controlled shape cutting

and produces a comfortable noise level of85 dBA or less under normal operatingconditions. (Conventional Plasma Arc Cuttingtypically produces noise levels in the rangeof 105 to 115 dBA.) Underwater cuttingvirtually eliminates the ultraviolet radiationand fumes associated with conventionalPlasma Arc Cutting.

In underwater PAC, the steel plate being cutis supported on a cutting table with the topsurface of the plate two to three inches be-neath the surface of the water. A device thatlocates the submerged top surface of the metalis critical to this fully-automated process.Accurate height control is maintained by asensor that monitors arc voltage. Cuttingspeed and quality are comparable to thoseattained with plasma arc cutting by waterinjection.

WarningIt is hazardous to cut aluminum under-water. Hydrogen generated by the processcan be trapped under the plate creating thepotential for explosion.

Precision Plasma Arc CuttingPrecision Plasma Arc Cutting utilizes an im-proved nozzle design to increase arc constric-tion and dramatically increase energy density.Because of the higher energy density, the edgequality and squareness of the cut is improved,particularly on thinner material (3/8").

Recent developments in plasma torch designallow the operator to drag the nozzle on thematerial surface without the arcing problemsnormally associated with other PAC processvariations (see figure 26).

The Precision Plasma Arc Cutting process isemployed in cutting sheet in the range of 20gauge to 3/8". Conventional plasma can cut upto 2" thicknesses.

Figure 25

Water

Injection

Plasma Arc

Cutting

26

s

Electrode

Nozzle

Workpiece

Water Injection(Radial or Swirl)

Plasma Jet

Ceramic

Gas Flow Rates

The orifice gas will often have a lower flowrate than the shielding gas, but both will varyas changes in cutting current are made toaccommodate different base metals and thick-nesses. Most PAC equipment use only anorifice gas with no shielding gas.

Gas flow with most PAC equipment iscontrolled by a gas pressure regulator and aflowmeter. The range of gas flow can varybetween 1.0 and 100 standard cubic feet perhour (scfh) for the orifice gas and 8.0 and 200scfh for the shielding gas, as determined bythe cutting requirements. Because PAC equip-ment design can vary significantly betweenmodels, specific flow rates are not listed here.

Shielding and Cutting Gases for PAC

Inert gases, such as argon, helium, andnitrogen (except at elevated temperatures) areused with tungsten electrodes. Air may beused as the cutting gas when special elec-

trodes, made from water-cooled copper withhigh temperature resistant inserts of metalslike hafnium, are used. Recently, PAC unitsshielded by compressed air have beendeveloped to cut thin gauge materials.

Virtually all plasma cutting of mild steel isdone with one of four gas types: (1) Air,(2) Nitrogen with carbon dioxide shielding orwater injection (mechanized), (3) Nitrogen/oxygen or air, and (4) Argon/hydrogen andnitrogen/hydrogen mixtures. The first twohave become the standard for high-speedmechanized applications. Argon/hydrogen andnitrogen/hydrogen (20% to 35% hydrogen)are occasionally used for manual cutting, butdross formation is a problem with the argonblend. Dross is a tenacious deposit of re-solidified metal attached at the bottom of thecut. A possible explanation for the heavier,more tenacious dross formed in argon is thegreater surface tension of the molten metal.The surface tension of liquid steel is 30 per-cent higher in an argon atmosphere than innitrogen. Air cutting gives a dross similar tothat formed in a nitrogen atmosphere.

During cutting, the plasma jet tends to removemore metal from the upper part of the work-piece than from the lower part. This results incuts with non-parallel cut surfaces which aregenerally wider at the top than at the bottom.The use of argon/hydrogen, because of itsuniform heat pattern or the injection of waterinto the torch nozzle (mechanized only), canproduce cuts that are square on one side andbeveled on the other side. For base metalover three inches thick, argon/hydrogen isfrequently used without water injection. Airis used as a low cost plasma gas, but specificprecautions must be taken to ensure that it ismoisture and oil free. Table 6, page 28 liststhe combinations of orifice and shieldinggases that may be used with PAC.Figure 26

Precision PAC nozzle

27

s

High FlowVortex Nozzle

Electrode Plasma Gas Inlet

ShieldingGas Inlet

Workpiece

PlasmaGas Vent

Thickness Range1/4" 1/2" 1" 2" 3" 4" 5" 6"

Air Orifice

Auxiliary

Nitrogen* Orifice

Auxiliary

Oxygen Orifice

Auxiliary

Carbon Dioxide Orifice

Auxiliary

HydroStar OrificeH-35

Auxiliary

Argon/Nitrogen Orifice

Auxiliary

Table 6

Gas Selection

Guide for

Plasma Arc

Cutting

* For Water Injection Plasma Cutting, nitrogen is the preferred plasma gas.Notes Depending upon equipment type the following applies:(1) An orifice gas is often used with no auxiliary gas.(2) When multiple auxiliary gases are shown for a single orifice gas, only one auxiliary gas applies for a given application.(3) Cutting speed and quality can vary with gas selection.(4) This table is a composite based on gas requirements for currently available PAC equipment.Use manufacturers recommendations for selecting gases.

Key

Carbon Steel

Stainless Steeland Nickel Alloys

Aluminum

s

28

Gas Metal Arc Welding is defined as anelectric arc welding process that producescoalescence of metals by heating them withan arc between a continuous filler metalelectrode and the workpiece. Shielding isobtained entirely from an externally suppliedgas. Figure 27 shows the essential elementsof a basic GMAW welding process.

GMAW is used to weld all commerciallyimportant metals, including steel, aluminum,copper, and stainless steel. The process canbe used to weld in any position, including flat,vertical, horizontal, and overhead. It is usuallyconnected to use direct current electrodepositive (DCEP). It is an arc welding processwhich incorporates the automatic feeding ofa continuous, consumable electrode that isshielded by an externally supplied gas (seefigure 28). Since the equipment provides forautomatic control of the arc, the only manualcontrols required by the welder for semiauto-matic operation are gun positioning, guidanceand travel speed.

ProcessDescription

Figure 27

Basic GMAW welding

Figure 28

Basic GMAW system29

C H A P T E R 4

4

s

Gas Metal Arc Welding (GMAW)

s

s

Arc

PowerSupply

Torch orGun

Wirefeeder

Spool ofWire

Workpiece

ElectrodeWire Feed

Workpiece

GasEnvelope

Molten Weld

SolidifiedWeld Metal

WeldingTorch

ConsumableElectrode

ElectricArc

The GMAW process has five distinctive metaltransfer modes:

Short circuiting Globular Spray Pulsed spray High-current density (rotational and nonrotational) spray.

The metal transfer mode is determined bymany factors, including operating current,wire diameter, arc length or voltage, powersupply characteristics, and shielding gas.

Short-Circuit Gas Metal Arc Welding(GMAW-S)

GMAW-S is defined as a gas metal arcwelding process variation in which theconsumable electrode is deposited duringrepeated short circuits.

In the short-circuiting mode, metal transferoccurs when the electrode is in direct contactwith the weld pool. In this mode of metaltransfer, the relationship between the electrodemelt rate and its feed rate into the weld zonedetermines the intermittent establishment ofan arc and the short circuiting of the electrodeto the workpiece.

Specifically, the electrode is fed at a constantspeed at a rate that exceeds the melt rate.When it contacts the molten pool a shortcircuit occurs, at which time there is no arc.The current then begins to rise and heats thewire to a plastic state. At the same time, thewire begins to deform or neck down due to anelectromagnetic pinch force. Eventually, thecurrent value and resulting pinch force causesa drop of metal to transfer into the weldpuddle. At this point, an arc is established.This sequence repeats itself approximately50 to 250 times per second (see figure 29).

Since there is less arc on time establishedduring the short circuit, the overall heat inputis low, and the depth of fusion is relativelyshallow; thus, care must be exercised inselecting the procedure and weld technique toassure complete fusion when welding thickermaterials. Due to its low heat input character-istics, the process produces a small, fast-freezing weld puddle which make it ideal forwelding in all positions. Short-circuitingtransfer is also particularly adaptable to weld-ing sheet metal with minimum distortion andfor filling gapped or poorly fitted parts withless tendency for burn-through of the partbeing welded.

MetalTransferModes inGMAW

30

s

Figure 29

Short-circuiting

transfer

s

MeltRate

Arc No Arc New Arc

Drop

Before Transfer During Short Circuit After Transfer

FeedRate

Globular Transfer

Globular Transfer is characterized by thetransfer of molten metal in large drops acrossthe arc. This transfer mode takes place whenthe current and arc voltage are between theshort-circuiting and spray transfer current and

31

voltage levels; it occurs with all types ofshielding gas. Carbon dioxide yields this typeof transfer at all usable welding currentsabove the short circuiting range. Globulartransfer is characterized by a drop sizeapproximately two to four times greater thanthe diameter of the electrode (see figure 30).

With carbon dioxide, the droplet is notpropelled across the arc, due to the repellingforces acting upward toward the wire tip.These forces tend to hold the droplet on theend of the wire. During this time the dropgrows in size and eventually either transfersby gravity due to its weight, or short circuitsacross the arc gap.

Spray Transfer

In Spray Transfer, the molten metal ispropelled axially across the arc in smalldroplets. In a gas blend of at least 80% argon(see table 7, page 39), when combined withthe proper operating conditions, the electrodemetal transfer changes from globular to aspray or spray-like mode. The minimumcurrent and voltage levels required vary forany given electrode diameter. The changetakes place at a value called the globular-spraytransition current. Spray transfer in argon ischaracterized by a constricted arc column andpointed electrode tip (see figure 31).

Molten metal transfers across the arc as smalldroplets equal to or less than the electrodediameter. The metal transfer is axially directedto the workpiece. Since the metal droplets aresmall, the transfer rate can be as high asseveral hundred droplets per second. Due topuddle fluidity, spray transfer is limited to theflat or horizontal welding position.

Figure 31

Spray transfer

Figure 30

Globular transfer

s

s

Pulsed Gas Metal Arc Welding(GMAW-P)

In this variation, the power source providestwo output levels: a steady background level,too low in magnitude to produce any transfer,but able to maintain an arc; and a pulsed,high-output level which causes melting ofdroplets from the electrode which are thentransferred across the arc. This pulsed highoutput (peak) occurs at regular controlledintervals. The current can be cycled between ahigh and low value at up to several hundredcycles per second. The net result is to producea spray arc with average current levels muchbelow the transition current required for aparticular diameter and type of electrode.

In pulsed spray welding the shielding gasmust be able to support spray transfer. Metalis transferred to the workpiece only during thehigh current pulse. Ideally, one droplet istransferred during each pulse (see figure 32).

The pulsing rate can be varied depending onthe base material, thickness, wire diameterand weld position. The control of backgroundcurrent maintains the arc and heat input. Theresulting lower average current level allowsthe joining of base metals less than 1/8 inchthick with a spray type metal transfer. Pulsedspray welding may be used in all positions.Welding fume levels are the lowest obtainablewith solid wire GMAW.

32

Figure 32

Pulsed spray transfer

s

No WireNecking

DropletTransfers

WithPulse

High Current Density Metal Transfer

High current density metal transfer is a namegiven to a GMAW process having specificcharacteristics created by a unique com-bination of wire feed speed, wire extension,and shielding gas. Weld metal deposition ratescan range between 10 and 30 pounds per hour,whereas most GMAW Spray Arc is in the 8 to12 pound/hr range. The arc characteristics ofhigh density metal transfer are further dividedinto rotational spray transfer and nonrotationalspray transfer.

33

When using a solid carbon steel wire, a highwire feed speed is combined with a longelectrode extension and an argon/carbondioxide/oxygen shielding gas to create an arcphenomenon known as rotational spray arctransfer. The long electrode extension createshigh resistance heating of the wire electrodecausing the electrode end to become molten.The electro-mechanical forces generatedby the current flow in the wire cause themolten wire end to rotate in a helical path(see figure 33).

The shielding gas affects the rotationaltransition current by changing the surfacetension at the molten electrode end. PraxairsStargon and StarGold O-5 blends (see pages36-38) produce rotational spray transfer atdeposition rates of 10 to 30 pounds per hourwith 0.035 and 0.045 diameter wires usingcontact-tip to workpiece distances (ESO) of7/8 inch to 1 1/2 inch.

Nonrotational spray high current densitytransfer is produced when the molten wire enddoes not rotate. This also develops a deposi-tion rate range of 10 to 30 pounds per hour.Rotation is suppressed when the thermalconductivity of the shielding gas increases andthe surface tension of the molten electrode endincreases. The droplet rate decreases resultingin larger droplets across the arc. Shieldinggases with carbon dioxide or helium additions,such as Praxairs Stargon and HeliStar CSshielding gases (see page 38), will raise therotational spray transition current and sup-press the tendency for the arc to rotate. Thearc appears elongated and diffuse but lookssimilar to conventional spray transfer. Theplasma stream is axial and narrower thanrotational spray transfer. Because the heatsource is more concentrated, the depth offusion is greater than rotational spray transferat the same welding current.

Figure 33

Rotational spray transfer

s

Metal-cored wire welding is considered avariation of GMAW. A metal-cored wireoperates like a solid wire, has generally lowfume levels, no slag and a high depositionefficiency (95 percent or better) despite itscored-type construction.

A metal-cored wire is a composite filler metalelectrode consisting of a metal tube filledwith alloying materials. These metal powdersprovide arc stabilization and fluxing of oxides.Metal-cored wires provide high depositionrates with excellent deposition efficiency, andcan be used to weld in all positions. They areused successfully in applications where fit-upis poor.

Metal-cored wires are designed to give qualitywelds over some rust and mill scale usingargon-based shielding gases. They generallyhave a higher level of deoxidizers whichprovides a good bead profile with excellentpuddle control. This type of wire combines thehigh deposition rate of flux-cored wires withthe approximate deposition efficiency andfume levels of a solid wire. The weld metalmechanical properties are comparable tocarbon steel solid wires and, as with solidwires, little slag is formed on the weld beadsurface.

The advantages of metal-cored wires are:

High deposition rate High deposition efficiency (95 percent or better) Quality welds over light rust and mill scale Low spatter levels Little slag clean-up Easy to use All position welding capability Low fume levels Greater resistance to undercut Improved performance with poor base metal fit up Large variety of alloys available

Metal-cored carbon steel wires operate bestin an argon/carbon dioxide blend (8 to 20%carbon dioxide) or an argon/carbon dioxide/oxygen blend, while stainless wires operatewell with a argon/oxygen (1 to 2% oxygen)or an argon/CO2 blends (2 to 10%).

34

Metal-CoredElectrodes

s

Argon

Argon is used on nonferrous base metals suchas aluminum, nickel, copper, magnesiumalloys, and reactive metals, such as zirconiumand titanium. Argon provides excellent arcwelding stability, penetration, and bead profileon these base metals. When welding ferrous-based metals, argon is usually mixed withother gases, such as oxygen, helium, carbondioxide, or hydrogen.

The low ionization potential (good electricalconductivity) of argon helps create an excel-lent current path and superior arc stability.Argon produces a constricted arc column withhigh current density which causes the arcenergy to be concentrated over a small surfacearea. The result is a penetration profile, havinga distinct finger like shape, as shown infigure 34.

Carbon Dioxide

Carbon dioxide, a reactive gas, dissociatesinto carbon monoxide and free oxygen in theheat of the arc. Oxygen then combines withelements transferring across the arc to formoxide in the form of slag and scale, and alsohelps to generate a great deal of fumes.Although carbon dioxide is an active gas andproduces oxidation of the weld material,sound welds can be consistently achievedwith careful filler metal selection.

Carbon dioxide is often used for weldingcarbon steel because it is readily availableand produces good welds at low cost. How-ever, the low cost per unit of gas does notalways translate to the lowest cost per foot ofdeposited weld. Other factors, such as lowerdeposition efficiency due to spatter loss,high levels of welding fume, poor weldbead profile and reduced tensile strength caninfluence the final weld cost and should becarefully considered.

Carbon dioxide will not support spray transfer.Metal transfer is restricted to the short circuit-ing and globular modes. A major disadvantageof carbon dioxide is harsh globular metal trans-fer with its characteristic spatter (see figure 35).

The weld surface resulting from use of carbondioxide shielding is usually heavily oxidized.An electrode with higher amounts of deoxi-dizing elements is needed to compensate forthe loss of alloying elements across the arc. 35

Shielding Gasesfor GMAW

Figure 35

Typical spatter levels with

two common shielding gases

Figure 34

GMAW weld bead profiles

with several shielding gases

s

s

s

12

10

8

6

4

2

0 100 200 300

Welding Current (Amps DGEP)

% S

patt

er (

by w

eight)

Dep

osi

tion E

ffic

iency

%

88

90

92

94

96

98

Spatter and Deposition Efficiency

To approximate spatter for 0.062 inches (1.6 mm) wire add 50A to welding current.To approximate spatter for 0.035 inches (0.89 mm) wire subtract 50A to welding current.

CO2

STARGON

.045 (1.1 mm)Carbon and Low AlloySteel Wires

Argon/O2 Argon Helium/Argon Helium

Welded parts may require a cleaning operationprior to painting which can more than offsetthe lower cost of CO2 shielding gas. Theadvantages of carbon dioxide are good depthand width of fusion and the achievement ofacceptable mechanical properties.

Helium

Helium is a chemically inert gas that is usedfor welding applications requiring higher heatinputs. Helium may improve wetting action,depth of fusion, and travel speed. It does notproduce the stable arc provided by argon.Helium has greater thermal conductivity thanargon and produces a wider arc column.The higher voltage gradient needed for stableoperation generates a higher heat input thanargon, promoting greater weld pool fluidityand better wetting action. This is an advantagewhen welding aluminum, magnesium, andcopper alloys.

Argon/Oxygen Blends

Praxairs StarGold BlendsThe addition of small amounts of oxygen toargon greatly stabilizes the welding arc, in-creases the metal transfer droplet rate, lowersthe spray transition current, and enhancesbead shape. The weld pool is more fluid andstays molten longer, allowing the metal toflow out towards the weld toes. Welding fumemay be reduced with these mixtures.

Praxairs StarGold O-1Primarily used for spray transfer on stainlesssteels, one percent oxygen is usually sufficientto stabilize the arc and improve the dropletrate and bead appearance.

Praxairs StarGold O-2This blend is used for spray arc welding ofcarbon steels, low-alloy steels and stainlesssteels. It provides better wetting action thanthe 1% oxygen mixture. Weld mechanical

properties and corrosion resistance of weldsmade with 1% and 2% oxygen additions aresimilar. However, bead appearance will bedarker and more oxidized for the 2% blendswith stainless steels.

Praxairs StarGold O-5This blend provides a more fluid but control-lable weld pool. It is the most commonlyused argon/oxygen mixture for general carbonsteel welding. The additional oxygen permitshigher travel speeds.

Argon/Carbon Dioxide Blends

Praxairs StarGold andMig Mix Gold BlendsArgon/carbon dioxide blends are used withcarbon, low-alloy and some stainless steels.Greater amounts of carbon dioxide whenadded to argon and used at higher currentlevels, increase spatter.

In conventional GMAW, slightly highercurrent levels must be exceeded when usingargon/carbon dioxide in order to establish andmaintain stable spray transfer. Above approxi-mately 20% carbon dioxide, spray transferbecomes unstable and periodic short-circuit-ing and globular transfer occurs.

Praxairs StarGold C-5Used for pulsed spray transfer and conven-tional spray transfer with a variety of materialthicknesses. A 5% mixture may be used forGMAW-P of low alloy steels for out-of-position welding. This blend provides goodarc stability when welding over mill scale anda more controllable puddle than a argon/oxygen blend.

Praxairs Mig Mix GoldThis blend performs similarly to C-5, but itsincreased heat input provides a wider, morefluid weld puddle in either short-circuit, sprayor pulsed spray transfer.36

Praxairs StarGold C-10This blend performs similarly to PraxairsMig Mix Gold but its additional CO2 contentprovides a wider, more fluid weld puddle.This blend is frequently recommended foruse with metal-cored wires.

Praxairs StarGold C-15Used for a variety of applications on carbonand low-alloy steel. In the short-circuitingmode, maximum productivity on thin gaugemetals can be achieved with this blend. Thisis done by minimizing the excessive melt-through tendency of higher carbon dioxidemixes, while increasing deposition rates andtravel speeds. As the carbon dioxide percent-ages are lowered from the 20% range (maxi-mum spray arc levels), improvements indeposition efficiency occur due to decreasingspatter loss. This blend will support the sprayarc mode of transfer.

Praxairs StarGold C-20May be used for short circuiting or spraytransfer welding of carbon steel.

Praxairs StarGold C-25Commonly used for GMAW with short-circuiting transfer on carbon steel. It wasformulated to provide optimum dropletfrequency on short-circuiting transfer using

.035 and .045 diameter wire. PraxairsStarGold C-25 blend operates well in highcurrent applications on heavy base metal. Itpromotes good arc stability, weld pool control,and weld bead appearance. This blend will notsupport spray type metal transfer. StarGoldC-25 can also be used with flux-cored wires(see manufacturers recommendations).

Praxairs StarGold C-40This mixture is recommended for use withsome flux cored wires where improved arcstability, reduced spatter levels and improvedperformance over light surface contaminationare desirable.

Praxairs StarGold C-50This mixture is used for short arc welding ofpipe, particularly when contaminants arepresent on the surfaces to be welded.

Argon/Helium Blends

Praxairs HeliStar BlendsHelium is often mixed with argon to obtainthe advantages of both gases. Argon providesgood arc stability and cleaning action, whilehelium promotes wetting with a greater widthof fusion.

Argon/helium blends are used primarily fornonferrous base metals, such as aluminum,copper, nickel alloys, magnesium alloys, andreactive metals. Helium additions to an argon-based gas increase the effective heat input.Generally, the thicker the base metal, the high-er the percentage of helium. Small percent-ages of helium, as low as 20%, will affect thearc. As the helium percentage increases, therequired arc voltage, spatter, and weld widthto depth ratio increase, while porosity isminimized (see Figure 36). The argon per-centage must be at least 20% when mixedwith helium to produce and maintain a stablespray transfer.

37

Figure 36

Effect of argon and helium shielding gases on

weld profile when welding aluminum (DCEP)

s

Argon Helium

Praxairs HeliStar A-25This blend is used for welding nonferrousbase metals when an increase in heat input isneeded and weld bead appearance is ofprimary importance. It is ideal for bothGMAW and GTAW of aluminum alloys.

Praxairs HeliStar A-50Helistar A-50 blend is used primarily forhigh-speed mechanized welding of nonferrousmaterials under 3/4 inch thick.

Praxairs HeliStar A-75This blend is used for mechanized weldingof aluminum greater than 3/4" in the flatposition. It increases heat input and reducesporosity of welds made in copper and copperalloys.

Argon/Oxygen/Carbon Dioxide Blends

Praxairs Stargon BlendsThese three component mixtures provideversatility due to their ability to operate inshort-circuiting, globular, spray, pulsed orhigh-density transfer modes. Several ternarycompositions are available and their usedepends on the desired metal transfer modeand welding position.

The advantage of this blend is its ability toshield carbon steel and low-alloy steel of allthicknesses using any metal transfer modeapplicable. Praxairs Stargon blend producesstable welding characteristics and mechanicalproperties on carbon and low-alloy steels andsome stainless steels. On thin gauge basemetals, the oxygen constituent promotes arcstability at very low current levels (30 to 60amps) permitting the arc to be kept short andcontrollable. This helps minimize excessivemelt-through and distortion by lowering thetotal heat input to the base material. Stargon isgenerally used for spray arc welding, provid-ing high deposition rates and often highertravel speeds than carbon dioxide.

Argon/Helium/Carbon Dioxide Blends

Praxairs HeliStar BlendsHelium and carbon dioxide additions to argonincrease the heat input to the weld, whichimproves wetting,