TIPS AND TECHNIQUES
Selecting electrodes, shielding gas, and filler metal for GTAW
The right choices make the best welds
When weld quality is most important, gas tungsten arc welding (GTAW) is the preferred welding process for stainless steel, low alloy steel, maraging steel, nickel, cobalt, titanium, aluminum, copper, and magnesium.
GTAW is popular because of its versatility---it can be used in all positions and produces clean weld deposits, 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 filler metal crucial for its success. GTAW uses an electrode that is considered to be nonconsumable, plus a filler metal rod if any is needed. Filler metal may or may not be needed, depending on the specific welding application. The shielding gas is important for GTAW because it is necessary to shield the electrode and molten weld puddle from the surrounding atmosphere.
GTAW uses an electrode that is considered to be nonconsumable. These electrodes are made of tungsten 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, even at 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 chemical composition and are identified by colored markings in the form of bands, dots, etc., on the surface of the electrode.
Tungsten electrodes usually come in lengths of 3 to 24 inches (76 to 610 millimeters), with 7-inch being most 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 with conventional AC. These are identified by a green marking.
The tungsten electrodes alloyed with 1% (yellow marking) or 2% (red marking) thoria have several advantages 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, and greater contamination resistance. Thoriated tungsten electrodes also provide easier arc starting and a more stable arc.
Ceriated tungsten electrodes (orange marking) contain cerium oxide and exhibit a reduced rate of vaporization or burn-off, as compared with pure tungsten electrodes.
Abbreviations used with electrodes are as follows:
T = Tungsten
L = Lanthanum
Zr = Zirconia
G = Rare-earth
The EWLs (black marking) electrodes contain lanthanum oxide and are very similar to the ceriated tungsten electrodes.
EWZr (brown marking) electrodes contain a small amount of zirconium oxide. Their welding characteristics generally fall between those of pure and thoriated tungsten, but have a higher resistance to contamination.
The EWG (gray marking) electrodes contain an unspecified addition of oxides (rare earth or others) that affect the characteristics of the arc.
Argon, helium, or a mixture of argon and helium are the most widely used shielding gases for GTAW. The characteristics most desirable for shielding purposes are chemical inertness and an ability to produce smooth arc action at high current densities.
Argon and helium are both inert, which means they do not form compounds with other elements. Inert shielding gas is used because it protects the tungsten electrode and the molten weld metal from contamination. 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 a rating of 99.99+ percent should be used. Titanium and zirconium have a very low tolerance to impurities in the shielding gas, and only the very purest should ever be used.
Argon is a heavy gas that is obtained from the atmosphere by the liquidization of air. Depending on the volume of use, argon may be supplied as a compressed gas or as a liquid. This gas can be purchased at much lower prices in the bulk liquid form compared to the compressed gas form and is the most widely used 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.
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.
Helium is a light gas that is obtained by separation from natural gas. It may be distributed as a liquid, but is 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 for adequate 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 on thicker metals and metals with high conductivity
3. Welds better at higher speeds
4. Gives better coverage in vertical and overhead positions
5. Penetrates more deeply due to higher heat input
6. Tends to flatten out the root pass of the weld bead when used as a backing gas
Argon-helium mixtures are used for applications that require better control of argon and deeper penetration 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 used for automatic welding.
Mixtures 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 are used when no other shielding gas can prevent porosity.
The purposes of argon-hydrogen mixtures are to increase the welding heat and help control the weld bead profile. The argon-hydrogen mixtures give the weld puddle better wetting action, a more uniform weld 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 to 15%; typically though, the argon-hydrogen mixture is 95 to 98% argon with the remaining balance being hydrogen.
Although 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 helium or 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. If thoriated electrodes are used, nitrogen causes negligible contamination.
GTAW 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 being welded. 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 base metals. The choice of a filler metal for a given application depends on the suitability for the intended application. The tensile strength, impact toughness, electrical conductivity, thermal conductivity, corrosion resistance, and weld appearance needed for a specific weldment are important considerations. Deoxidizers are sometimes added to the filler metals to provide better weld soundness.
AWS developed the classification system for filler metal used with GTAW. In this system, designations for filler metal wires consist of the letters ER (E=electrode, R=rod) and an alloy number in most cases. The difference between an electrode and a rod is that an electrode carries welding current and the metal is transferred across the arc, but a filler rod is added directly to the weld puddle generally without electricity running through it.
Since GTAW filler rods are typically chosen on the basis of chemical composition, they are classified according to their chemical composition. Carbon and low-alloy steel welding rods differ in that they are classified 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 be used 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 classifications can be found by referring to AWS A.519-92, and for copper and copper alloys, to AWS A5.7-84. Refer to AWS A5.14-89 for chemical compositions of filler wire and rods used for welding nickel and nickel alloys.
Filler wires come in either straight cut lengths that are usually 36 inches (914 millimeters) long for manual welding, or in continuous spooled wire for mechanized welding. The diameters of filler wire range from about .020 inches (.50 millimeters) for delicate or fine work, to about 1/4 inch (6.4 millimeters) for high current welding and surfacing.
Selection of Filler Metal
The type of metal being welded and the specific mechanical and chemical properties desired are the major factors in determining the choice of a filler metal. Identifing the base metal is absolutely required to select the proper filler metal.
If the type of base metal is not known, tests can be done based on appearance, weight, a magnetic check, 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 following factors:
1. Base Metal Strength Properties. A filler metal is chosen to match the tensile strength of the base metal. This is usually most important with steels.
2. Base Metal Composition. The chemical composition of the base metal must be known. Matching the chemical composition is not as important for mild steels as it is for low-alloy steels, stainless steels, and nonferrous metals. Close matching of the filler metal to the base metal is needed when corrosion resistance, creep strength, toughness, or color match are important considerations.
3. Thickness and Shape of Base Metal Weldments. This may include thick sections or complex shapes that may require maximum ductility to avoid weld cracking. Filler metal types that give best ductility are recommended.
4. Service Conditions and/or Specifications. When weldments are subjected to severe service conditions such as low temperatures, high temperatures, or shock loading, a filler metal that closely matches 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 of electrodes, shielding gas, and filler metal will help ensure the best weld in a safe and cost-effective manner.
This article is based on information from the "Gas Tungsten Arc Welding Technical Guide" published by Hobart Institute of Welding Technology, 400 Trade Square E., Troy, Ohio, phone 937-332-5000, fax 937-332-5200. Larry J. Barley, Applications Engineer with ITW/Hobart Brothers Company, edited and updated the information for Practical Welding Today.
Taken from Practical Welding Today : July/August 1998 Volume 2 Number 4