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TIPS AND TECHNIQUESSelecting electrodes, shielding gas, and filler metal fo rGTAWThe r ight choices m ake the best weldsWhen weld quality is most important, gas tungsten arc welding (GTAW) is the preferred welding processfor stainless steel, low alloy steel, maraging steel, nickel, cobalt, titanium, aluminum, copper, andmagnesium.GTAW is popular because of its versatility---it can be used in all positions and produces clean welddeposits, avoiding grinding and post-weld finishing. It can also be applied by manual, semiautomatic,mechanized or fully automatic methods.Industries that most typically use GTAW include industrial piping, nuclear power facilities, shipbuilding,aerospace, transportation, pressure vessels, boilers, heat exchangers, food grade processing equipment,etc.The variety of GTAW applications makes the appropriate selection of electrodes, shielding gas, and fillermetal crucial for its success. GTAW uses an electrode that is considered to be nonconsumable, plus afiller metal rod if any is needed. Filler metal may or may not be needed, depending on the specific weldingapplication. The shielding gas is important for GTAW because it is necessary to shield the electrode andmolten weld puddle from the surrounding atmosphere.ElectrodesGTAW uses an electrode that is considered to be nonconsumable. These electrodes are made oftungsten or tungsten alloys that melt in the range of 6,170 degrees Fahrenheit (3,410 degrees Celsius),the highest melting point of all metals.It is virtually impossible to melt a tungsten electrode during welding, provided the electrode is used within

its current-carrying capacity range with the proper inert shielding gas. Tungsten retains its hardness, evenat red heat temperature.There are several types of GTAW electrodes. These are made of pure tungsten or alloyed with thoria,zirconia, ceria, lanthana, or a combination of oxides. The welding electrodes are classified by chemicalcomposition and are identified by colored markings in the form of bands, dots, etc., on the surface of theelectrode.Tungsten electrodes usually come in lengths of 3 to 24 inches (76 to 610 millimeters), with 7-inch beingmost common, and in diameters from .01 inch (0.25 millimeters) to 1/4-inch (6.4 millimeters). Pure tungsten electrodes are generally used on applications with alternating current (AC). They have a

moderate current-carrying capacity and a low contamination resistance, but provide good arc stability withconventional AC. These are identified by a green marking.The tungsten electrodes alloyed with 1% (yellow marking) or 2% (red marking) thoria have severaladvantages over pure tungsten electrodes, with both direct current (DC) and some AC applications.These electrodes have higher current-carrying capacities, longer life, higher electron emissivity, andgreater contamination resistance. Thoriated tungsten electrodes also provide easier arc starting and amore stable arc.

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Ceriated tungsten electrodes (orange marking) contain cerium oxide and exhibit a reduced rate ofvaporization or burn-off, as compared with pure tungsten electrodes.Abbreviations used with electrodes are as follows:F= ElectrodeT = TungstenL = LanthanumZr = ZirconiaG = Rare-earthThe EWLs (black marking) electrodes contain lanthanum oxide and are very similar to the ceriatedtungsten electrodes.EWZr (brown marking) electrodes contain a small amount of zirconium oxide. Their weldingcharacteristics generally fall between those of pure and thoriated tungsten, but have a higher resistanceto contamination.The EWG (gray marking) electrodes contain an unspecified addition of oxides (rare earth or others) that

affect the characteristics of the arc.Shielding GasesArgon, helium, or a mixture of argon and helium are the most widely used shielding gases for GTAW. Thecharacteristics most desirable for shielding purposes are chemical inertness and an ability to producesmooth arc action at high current densities.Argon and helium are both inert, which means they do not form compounds with other elements. Inertshielding gas is used because it protects the tungsten electrode and the molten weld metal fromcontamination. Additions of hydrogen and nitrogen can be used for special applications.Gas purity may have a considerable effect on welding. For the best results, welding grade gas with arating of 99.99+ percent should be used. Titanium and zirconium have a very low tolerance to impuritiesin the shielding gas, and only the very purest should ever be used. ArgonArgon is a heavy gas that is obtained from the atmosphere by the liquidization of air. Depending on thevolume of use, argon may be supplied as a compressed gas or as a liquid. This gas can be purchased atmuch lower prices in the bulk liquid form compared to the compressed gas form and is the most widelyused type of shielding gas for GTAW.Argon has several advantages over helium:1. Quieter and smoother arc action.2. Easier arc starting.3. Lower arc voltage for current settings and arc lengths, which is particularly good on thin metals.4. Good cleaning action, which is preferred for the welding of aluminum and magnesium.

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5. Lower flow rates (argon is heavier than air and helium) are required for good shielding.6. Lower cost and more availability.7. Better resistance to cross-drafts.8. Better for welding dissimilar metals.9. Better weld puddle control in the overhead and vertical positions.HeliumHelium is a light gas that is obtained by separation from natural gas. It may be distributed as a liquid, butis most often used as compressed gas in cylinders.Since helium is lighter than air, it leaves the welding area more quickly and requires higher flow rates foradequate coverage. Another disadvantage is that it is more expensive and less available than argon. Helium does have several advantages over argon shielding gas: 1. Generates a smaller heat-affected zone ((HAZ))2. Produces higher arc voltages for given current settings and arc lengths, which is particularly good onthicker metals and metals with high conductivity3. Welds better at higher speeds4. Gives better coverage in vertical and overhead positions5. Penetrates more deeply due to higher heat input6. Tends to flatten out the root pass of the weld bead when used as a backing gas Argon-Helium MixturesArgon-helium mixtures are used for applications that require better control of argon and deeperpenetration of helium. Common mixtures of these gases by volume are 75% helium/25% argon or 80%helium/20% argon. A variety of mixtures is available. Combinations of argon and helium are widely usedfor automatic welding.Argon-Hydrogen MixturesMixtures of argon and hydrogen are often used when welding metals such as austenitic stainless steel.INCONEL, and MONEL, and when porosity is a problem. In some cases, argon-hydrogen mixtures areused when no other shielding gas can prevent porosity. The purposes of argon-hydrogen mixtures are to increase the welding heat and help control the weldbead profile. The argon-hydrogen mixtures give the weld puddle better wetting action, a more uniformweld bead, and higher travel speeds. This gas mixture is not completely inert. Argon-hydrogen mixtures should not be used for welding aluminum, plain carbon, or low-alloy steels.Austenitic stainless steel can be welded with argon-hydrogen mixtures with the hydrogen percentage up

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to 15%; typically though, the argon-hydrogen mixture is 95 to 98% argon with the remaining balancebeing hydrogen.NitrogenAlthough rarely used, nitrogen can also be applied as shielding gas. It produces higher currents because

of the higher voltage nitrogen generates. The efficiency of heat transfer is higher than that of either heliumor argon, making nitrogen ideal for welding copper and copper alloys.However, nitrogen will reduce arc stability and contaminate electrodes because it is not an inert gas. Ifthoriated electrodes are used, nitrogen causes negligible contamination.Filler MetalsGTAW is used to weld a wide variety of metals, resulting in the need for various filler metals.Selecting the proper filler metal depends primarily on the chemical composition of the base metal beingwelded. Filler metals are often similar, although not necessarily identical, to the base metal being welded.Filler metals are generally produced with closer control on chemistry, purity, and quality than are basemetals. The choice of a filler metal for a given application depends on the suitability for the intendedapplication. The tensile strength, impact toughness, electrical conductivity, thermal conductivity, corrosionresistance, and weld appearance needed for a specific weldment are important considerations.Deoxidizers are sometimes added to the filler metals to provide better weld soundness.ClassificationAWS developed the classification system for filler metal used with GTAW. In this system, designations forfiller metal wires consist of the letters ER (E=electrode, R=rod) and an alloy number in most cases. Thedifference between an electrode and a rod is that an electrode carries welding current and the metal istransferred across the arc, but a filler rod is added directly to the weld puddle generally without

electricityrunning through it.Since GTAW filler rods are typically chosen on the basis of chemical composition, they are classifiedaccording to their chemical composition. Carbon and low-alloy steel welding rods differ in that they areclassified according to mechanical properties and chemical compositions.An example of a classification is an ER4043 aluminum welding rod. The ER indicates that the wire can beused as either an electrode or a filler wire and the 4043 indicates the chemical composition. Other nonferrous metals and stainless steels are classified in a similar way. Magnesium classificationscan be found by referring to AWS A.519-92, and for copper and copper alloys, to AWS A5.7-84. Refer toAWS A5.14-89 for chemical compositions of filler wire and rods used for welding nickel and nickel alloys.SizingFiller wires come in either straight cut lengths that are usually 36 inches (914 millimeters) long for manualwelding, or in continuous spooled wire for mechanized welding. The diameters of filler wire range fromabout .020 inches (.50 millimeters) for delicate or fine work, to about 1/4 inch (6.4 millimeters) for highcurrent welding and surfacing.Selection of Filler Metal

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The type of metal being welded and the specific mechanical and chemical properties desired are themajor factors in determining the choice of a filler metal. Identifing the base metal is absolutely required toselect the proper filler metal.If the type of base metal is not known, tests can be done based on appearance, weight, a magneticcheck, chisel tests, flame tests, fracture tests, spark tests, and chemistry tests. Scientific analysis can be

conducted with spectrograph or eddy current units.

Selecting the proper filler metal for a specific application is quite involved but is based on the followingfactors:1. BaseMetal Strength Properties.A filler metal is chosen to match the tensile strength of the basemetal. This is usually most important with steels.2. Base Metal Composition. The chemical composition of the base metal must be known. Matching thechemical composition is not as important for mild steels as it is for low-alloy steels, stainless steels, andnonferrous metals. Close matching of the filler metal to the base metal is needed when corrosionresistance, creep strength, toughness, or color match are important considerations.3. Thickness and Shape of Base Metal Weldments. This may include thick sections orcomplex shapesthat may require maximum ductility toavoid weld cracking. Filler metal types that give best ductility arerecommended.4. Service Conditions and/or Specifications. When weldments are subjected to severe serviceconditions such as low temperatures, high temperatures, or shock loading, a filler metal that closelymatches the base metal composition, ductility, and impact resistance properties should be used.GTAW will remain a popular process because of the variety of its applications. Proper selection ofelectrodes, shielding gas, and filler metal will help ensure the best weld in a safe and cost-effectivemanner.