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Gas tungsten arc welding

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   Gas tungsten arc welding
   Enlarge
   Gas tungsten arc welding

   Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG)
   welding, is an arc welding process that uses a nonconsumable tungsten
   electrode to produce the weld. The weld area is protected from
   atmospheric contamination by a shielding gas (usually an inert gas such
   as argon), and a filler metal is normally used, though some welds,
   known as autogenous welds, do not require it. A constant-current
   welding power supply produces energy which is conducted across the arc
   through a column of highly ionized gas and metal vapors known as a
   plasma.

   GTAW is most commonly used to weld thin sections of stainless steel and
   light metals such as aluminium, magnesium, and copper alloys. The
   process grants the operator greater control over the weld than
   competing procedures such as shielded metal arc welding and gas metal
   arc welding, allowing for stronger, higher quality welds. However, GTAW
   is comparatively more complex and difficult to master, and furthermore,
   it is significantly slower than most other welding techniques. A
   related process, plasma arc welding, uses a slightly different welding
   torch to create a more focused welding arc and as a result is often
   automated.

Development

   After the discovery of the electric arc in 1800 by Humphry Davy, arc
   welding developed slowly. C. L. Coffin had the idea of welding in an
   inert gas atmosphere in 1890, but even in the early 1900s, welding
   non-ferrous materials like aluminium and magnesium remained difficult,
   because these metals reacted rapidly with the air, resulting in porous
   and dross-filled welds. Processes using flux covered electrodes did not
   satisfactorily protect the weld area from contamination. To solve the
   problem, bottled inert gases were used in the beginning of the 1930s. A
   few years later, a direct current, gas-shielded welding process emerged
   in the aircraft industry for welding magnesium.

   This process was perfected in 1941, and became known as heliarc or
   tungsten inert gas welding, because it utilized a tungsten electrode
   and helium as a shielding gas. Initially, the electrode overheated
   quickly, and in spite of tungsten's high melting temperature, particles
   of tungsten were transferred to the weld. To address this problem, the
   polarity of the electrode was changed from positive to negative, but
   this made it unsuitable for welding many non-ferrous materials.
   Finally, the development of alternating current made it possible to
   stabilize the arc and produce high quality aluminium and magnesium
   welds.

   Developments continued during the following decades. Linde Air Products
   developed water-cooled torches that helped to prevent overheating when
   welding with high currents. Additionally, during the 1950s, as the
   process continued to gain popularity, some users turned to carbon
   dioxide as an alternative to the more expensive welding atmospheres
   consisting of argon and helium. However, this proved unacceptable for
   welding aluminium and magnesium because it reduced weld quality, and as
   a result, it is rarely used with GTAW today.

   In 1953, a new process based on GTAW was developed, called plasma arc
   welding. It affords greater control and improves weld quality by using
   a nozzle to focus the electric arc, but is largely limited to automated
   systems, whereas GTAW remains primarily a manual, hand-held method.
   Development within the GTAW process has continued as well, and today a
   number of variations exist. Among the most popular are the
   pulsed-current, manual programmed, hot-wire, dabber, and increased
   penetration GTAW methods.

Operation

   GTAW weld area
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   GTAW weld area

   Manual gas tungsten arc welding is often considered the most difficult
   of all the welding processes commonly used in industry. Because the
   welder must maintain a short arc length, great care and skill are
   required to prevent contact between the electrode and the workpiece.
   Unlike other welding processes, GTAW normally requires two hands, since
   most applications require that the welder manually feed a filler metal
   into the weld area with one hand while manipulating the welding torch
   in the other. However, some welds combining thin materials (known as
   autogenous or fusion welds) can be accomplished without filler metal;
   most notably edge, corner and butt joints.

   To strike the welding arc, a high frequency generator provides a path
   for the welding current through the shielding gas, allowing the arc to
   be struck when the separation between the electrode and the workpiece
   is approximately 1.5-3 mm (0.06-0.12 in). Bringing the two into contact
   also serves to strike an arc, but this can cause contamination of the
   weld and electrode. Once the arc is struck, the welder moves the torch
   in a small circle to create a welding pool, the size of which depends
   on the size of the electrode and the current. While maintaining a
   constant separation between the electrode and the workpiece, the
   operator then moves the torch back slightly and tilts it backward about
   10-15 degrees from vertical. Filler metal is added manually to the
   front end of the weld pool as it is needed.

   Welders often develop a technique of rapidly alternating between moving
   the torch forward (to advance the weld pool) and adding filler metal.
   The filler rod is withdrawn from the weld pool each time the electrode
   advances, but it is never removed from the gas shield to prevent
   oxidation of its surface and contamination of the weld. Filler rods
   composed of metals with low melting temperature, such as aluminium,
   require that the operator maintain some distance from the arc while
   staying inside the gas shield. If held too close to the arc, the filler
   rod can melt before it makes contact with the weld puddle. As the weld
   nears completion, the arc current is often gradually reduced to prevent
   the formation of a crater at the end of the weld.

Safety

   Like other arc welding processes, GTAW can be dangerous if proper
   precautions are not taken. Welders wear protective clothing, including
   heavy leather gloves and protective long sleeve jackets, to avoid
   exposure to extreme heat and flames. Due to the absence of smoke in
   GTAW, the electric arc can seem brighter than in shielded metal arc
   welding, making operators especially susceptible to arc eye and skin
   irritations not unlike sunburn. Helmets with dark face plates are worn
   to prevent this exposure to ultraviolet light, sometimes featuring a
   liquid crystal-type face plate that self-darkens upon exposure to high
   amounts of UV light. Transparent welding curtains, made of a polyvinyl
   chloride plastic film, are often used to shield nearby workers and
   bystanders from exposure to the UV light from the electric arc.

   Welders are also often exposed to dangerous gases and particulate
   matter. Shielding gases can displace oxygen and lead to asphyxiation,
   and while smoke is not produced, the brightness of the arc in GTAW can
   cause surrounding air to break down and form ozone. Similarly, the
   brightness and heat can cause poisonous fumes to form from cleaning and
   degreasing materials. Cleaning operations using these agents should not
   be performed near the site of welding, and proper ventilation is
   necessary to protect the welder.

Applications

   While the aerospace industry is one of the primary users of gas
   tungsten arc welding, the process is used in a number of other areas.
   Many industries use GTAW for welding thin workpieces, especially
   nonferrous metals. It is used extensively in the manufacture of space
   vehicles, and is also frequently employed to weld small-diameter,
   thin-wall tubing. In addition, GTAW is often used to make root or first
   pass welds for piping of various sizes. In maintenance and repair work,
   the process is commonly used to repair tools and dies, especially
   components made of aluminium and magnesium. Because the welds it
   produces are highly resistant to corrosion and cracking over long time
   periods, GTAW is the welding procedure of choice for critical welding
   operations like sealing spent nuclear fuel canisters before burial.

Quality

   GTAW t-joint weld
   Enlarge
   GTAW t-joint weld

   Among arc welding process, GTAW ranks the highest in terms of the
   quality of weld produced. Maximum quality is assured by maintaining the
   cleanliness of the operation—all equipment and materials used must be
   free from oil, moisture, dirt and other impurities, as these cause weld
   porosity and consequently a decrease in weld strength and quality. To
   remove oil and grease, alcohol or similar commercial solvents may be
   used, while a stainless steel wire brush or chemical process can remove
   oxides from the surfaces of metals like aluminium. Rust on steels can
   be removed by first grit blasting the surface and then using a wire
   brush to remove any embedded grit. These steps are especially important
   when negative polarity direct current is used, because such a power
   supply provides no cleaning during the welding process, unlike positive
   polarity direct current or alternating current. To maintain a clean
   weld pool during welding, the shielding gas flow should be sufficient
   and consistent so that the gas covers the weld and blocks impurities in
   the atmosphere. GTA welding in windy or drafty environments increases
   the amount of shielding gas necessary to protect the weld, increasing
   the cost and making the process unpopular outdoors.

   Because of GTAW's relative difficulty and the importance of proper
   technique, skilled operators are employed for important applications.
   Low heat input, caused by low welding current or high welding speed,
   can limit penetration and cause the weld bead to lift away from the
   surface being welded. If there is too much heat input, however, the
   weld bead grows in width while the likelihood of excessive penetration
   and spatter increase. Additionally, if the welder holds the welding
   torch too far from the workpiece, shielding gas is wasted and the
   appearance of the weld worsens.

   If the amount of current used exceeds the capability of the electrode,
   tungsten inclusions in the weld may result. Known as tungsten spitting,
   it can be identified with radiography and prevented by changing the
   type of electrode or increasing the electrode diameter. In addition, if
   the electrode is not well protected by the gas shield or the operator
   accidentally allows it to contact the molten metal, it can become dirty
   or contaminated. This often causes the welding arc to become unstable,
   requiring that electrode be ground with a diamond abrasive to remove
   the impurity.

Equipment

   GTAW torch with various electrodes, cups, collets and gas diffusers
   Enlarge
   GTAW torch with various electrodes, cups, collets and gas diffusers
   GTAW torch, disassembled
   Enlarge
   GTAW torch, disassembled

   The equipment required for the gas tungsten arc welding operation
   includes a welding torch utilizing a nonconsumable tungsten electrode,
   a constant-current welding power supply, and a shielding gas source.

Welding torch

   GTAW welding torches are designed for either automatic or manual
   operation and are equipped with cooling systems using air or water. The
   automatic and manual torches are similar in construction, but the
   manual torch has a handle while the automatic torch normally comes with
   a mounting rack. The angle between the centerline of the handle and the
   centerline of the tungsten electrode, known as the head angle, can be
   varied on some manual torches according to the preference of the
   operator. Air cooling systems are most often used for low-current
   operations (up to about 200  A), while water cooling is required for
   high-current welding (up to about 600 A). The torches are connected
   with cables to the power supply and with hoses to the shielding gas
   source and where used, the water supply.

   The internal metal parts of a torch are made of hard alloys of copper
   or brass in order to transmit current and heat effectively. The
   tungsten electrode must be held firmly in the centre of the torch with
   an appropriately sized collet, and ports around the electrode provide a
   constant flow of shielding gas. The body of the torch is made of
   heat-resistant, insulating plastics covering the metal components,
   providing insulation from heat and electricity to protect the welder.

   The size of the welding torch nozzle depends on the size of the desired
   welding arc, and the inside diameter of the nozzle is normally at least
   three times the diameter of the electrode. The nozzle must be heat
   resistant and thus is normally made of alumina or a ceramic material,
   but fused quartz, a glass-like substance, offers greater visibility.
   Devices can be inserted into the nozzle for special applications, such
   as gas lenses or valves to control shielding gas flow and switches to
   control welding current.

Power supply

   Gas tungsten arc welding uses a constant current power source, meaning
   that the current (and thus the heat) remains relatively constant, even
   if the arc distance and voltage change. This is important because most
   applications of GTAW are manual or semiautomatic, requiring that an
   operator hold the torch. Maintaining a suitably steady arc distance is
   difficult if a constant voltage power source is used instead, since it
   can cause dramatic heat variations and make welding more difficult.
   GTAW power supply
   Enlarge
   GTAW power supply

   The preferred polarity of the GTAW system depends largely on the type
   of metal being welded. Direct current with a negatively charged
   electrode (DCEN) is often employed when welding steels, nickel,
   titanium, and other metals. It can also be used in automatic GTA
   welding of aluminium or magnesium when helium is used as a shielding
   gas. The negatively charged electrode generates heat by emitting
   electrons which travel across the arc, causing thermal ionization of
   the shielding gas and increasing the temperature of the base material.
   The ionized shielding gas flows toward the electrode, not the base
   material, and this can allow oxides to build on the surface of the
   weld. Direct current with a positively charged electrode (DCEP) is less
   common, and is used primarily for shallow welds since less heat is
   generated in the base material. Instead of flowing from the electrode
   to the base material, as in DCEN, electrons go the other direction,
   causing the electrode to reach very high temperatures. To help it
   maintain its shape and prevent softening, a larger electrode is often
   used. As the electrons flow toward the electrode, ionized shielding gas
   flows back toward the base material, cleaning the weld by removing
   oxides and other impurities and thereby improving its quality and
   appearance.

   Alternating current, commonly used when welding aluminium and magnesium
   manually or semi-automatically, combines the two direct currents by
   making the electrode and base material alternate between positive and
   negative charge. This causes the electron flow to switch directions
   constantly, preventing the tungsten electrode from overheating while
   maintaining the heat in the base material. This makes the ionized
   shielding gas constantly switch its direction of flow, causing
   impurities to be removed during a portion of the cycle. Some power
   supplies enable operators to use an unbalanced alternating current wave
   by modifying the exact percentage of time that the current spends in
   each state of polarity, giving them more control over the amount of
   heat and cleaning action supplied by the power source. In addition,
   operators must be wary of rectification, in which the arc fails to
   reignite as it passes from straight polarity (negative electrode) to
   reverse polarity (positive electrode). To remedy the problem, a square
   wave power supply can be used, as can high-frequency voltage to
   encourage ignition.

Electrode

                 ISO Class ISO Colour AWS Class AWS Colour          Alloy
                        WP      Green       EWP      Green            None
                      WC20       Gray    EWCe-2     Orange      ~2% CeO[2]
                      WL10      Black    EWLa-1      Black   ~1% La[2]O[3]
                      WL15       Gold  EWLa-1.5       Gold ~1.5% La[2]O[3]
                      WL20   Sky-blue    EWLa-2       Blue   ~2% La[2]O[3]
                      WT10     Yellow    EWTh-1     Yellow      ~1% ThO[2]
                      WT20        Red    EWTh-2        Red      ~2% ThO[2]
                      WT30     Violet                           ~3% ThO[2]
                      WT40     Orange                           ~4% ThO[2]
                      WY20       Blue                         ~2% Y[2]O[3]
                       WZ3      Brown    EWZr-1      Brown    ~0.3% ZrO[2]
                       WZ8      White                         ~0.8% ZrO[2]

   The electrode used in GTAW is made of tungsten or a tungsten alloy,
   because tungsten has the highest melting temperature among pure metals,
   at 3,422  °C (6,192 °F). As a result, the electrode is not consumed
   during welding, though some erosion (called burn-off) can occur.
   Electrodes can have either a clean finish or a ground finish—clean
   finish electrodes have been chemically cleaned, while ground finish
   electrodes have been ground to a uniform size and have a polished
   surface, making them optimal for heat conduction. The diameter of the
   electrode can vary between 0.5 millimeter and 6.4 millimeters
   (0.02–0.25  in), and their length can range from 75 to 610 millimeters
   (3–24 in).

   A number of tungsten alloys have been standardized by the International
   Organization for Standardization and the American Welding Society in
   ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes, and
   are summarized in the adjacent table. Pure tungsten electrodes
   (classified as WP or EWP) are general purpose and low cost electrodes.
   Cerium oxide (or ceria) as an alloying element improves arc stability
   and ease of starting while decreasing burn-off. Using an alloy of
   lanthanum oxide (or lanthana) has a similar effect. Thorium oxide (or
   thoria) alloy electrodes were designed for DC applications and can
   withstand somewhat higher temperatures while providing many of the
   benefits of other alloys. However, it is somewhat radioactive, and as a
   replacement, electrodes with larger concentrations of lanthanum oxide
   can be used. Electrodes containing zirconium oxide (or zirconia)
   increase the current capacity while improving arc stability and
   starting and increasing electrode life. In addition, electrode
   manufacturers may create alternative tungsten alloys with specified
   metal additions, and these are designated with the classification EWG
   under the AWS system.

   Filler metals are also used in nearly all applications of GTAW, the
   major exception being the welding of thin materials. Filler metals are
   available with different diameters and are made of a variety of
   materials. In most cases, the filler metal in the form of a rod is
   added to the weld pool manually, but some applications call for an
   automatically fed filler metal, which often is stored on spools or
   coils.

Shielding gas

   GTAW system setup
   Enlarge
   GTAW system setup

   As with other welding processes such as gas metal arc welding,
   shielding gases are necessary in GTAW to protect the welding area from
   atmospheric gases such as nitrogen and oxygen, which can cause fusion
   defects, porosity, and weld metal embrittlement if they come in contact
   with the electrode, the arc, or the welding metal. The gas also
   transfers heat from the tungsten electrode to the metal, and it helps
   start and maintain a stable arc.

   The selection of a shielding gas depends on several factors, including
   the type of material being welded, joint design, and desired final weld
   appearance. Argon is the most commonly used shielding gas for GTAW,
   since it helps prevent defects due to a varying arc length. When used
   with alternating current, the use of argon results in high weld quality
   and good appearance. Another common shielding gas, helium, is most
   often used to increase the weld penetration in a joint, to increase the
   welding speed, and to weld metals with high heat conductivity, such as
   copper and aluminium. A significant disadvantage is the difficulty of
   striking an arc with helium gas, and the decreased weld quality
   associated with a varying arc length.

   Argon-helium mixtures are also frequently utilized in GTAW, since they
   can increase control of the heat input while maintaining the benefits
   of using argon. Normally, the mixtures are made with primarily helium
   (often about 75% or higher) and a balance of argon. These mixtures
   increase the speed and quality of the AC welding of aluminium, and also
   make it easier to strike an arc. Another shielding gas mixture,
   argon-hydrogen, is used in the mechanized welding of light gauge
   stainless steel, but because hydrogen can cause porosity, its uses are
   limited. Similarly, nitrogen can sometimes be added to argon to help
   stabilize the austenite in austentitic stainless steels and increase
   penetration when welding copper. Due to porosity problems in ferritic
   steels and limited benefits, however, it is not a popular shielding gas
   additive.

Materials

   Gas tungsten arc welding is most commonly used to weld stainless steel
   and nonferrous materials, such as aluminium and magnesium, but it can
   be applied to nearly all metals, with notable exceptions being lead and
   zinc. Its applications involving carbon steels are limited not because
   of process restrictions, but because of the existence of more
   economical steel welding techniques, such as gas metal arc welding and
   shielded metal arc welding. Furthermore, GTAW can be performed in a
   variety of other-than-flat positions, depending on the skill of the
   welder and the materials being welded.

Aluminium and magnesium

   A TIG weld showing an accentuated AC etched zone
   Enlarge
   A TIG weld showing an accentuated AC etched zone
   Closeup view of an aluminium TIG weld AC etch zone
   Enlarge
   Closeup view of an aluminium TIG weld AC etch zone

   Aluminium and magnesium are most often welded using alternating
   current, but the use of direct current is also possible, depending on
   the properties desired. Before welding, the work area should be cleaned
   and may be preheated to 175 to 200 °C (350 to 400 °F) for aluminium or
   to a maximum of 150 °C (300 °F) for thick magnesium workpieces to
   improve penetration and increase travel speed. AC current can provide a
   self-cleaning effect, removing the thin, refractory aluminium oxide (
   sapphire) layer that forms on aluminium metal within minutes of
   exposure to air. This oxide layer must be removed for welding to occur.
   When alternating current is used, pure tungsten electrodes or
   zirconiated tungsten electrodes are preferred over thoriated
   electrodes, as the latter are more likely to "spit" electrode particles
   across the welding arc into the weld. Blunt electrode tips are
   preferred, and pure argon shielding gas should be employed for thin
   workpieces. Introducing helium allows for greater penetration in
   thicker workpieces, but can make arc starting difficult.

   Direct current of either polarity, positive or negative, can be used to
   weld aluminium and magnesium as well. Direct current with a negatively
   charged electrode (DCEN) allows for high penetration, and is most
   commonly used on joints with butting surfaces, such as square groove
   joints. Short arc length (generally less than 2 mm or 0.07 in) gives
   the best results, making the process better suited for automatic
   operation than manual operation. Shielding gases with high helium
   contents are most commonly used with DCEN, and thoriated electrodes are
   suitable. Direct current with a positively charged electrode (DCEP) is
   used primarily for shallow welds, especially those with a joint
   thickness of less than 1.6 millimeters (0.06 in). While still
   important, cleaning is less essential for DCEP than DCEN, since the
   electron flow from the workpiece to the electrode helps maintain a
   clean weld. A large, thoriated tungsten electrode is commonly used,
   along with a pure argon shielding gas.

Steels

   For GTA welding of carbon and stainless steels, the selection of a
   filler material is important to prevent excessive porosity. Oxides on
   the filler material and workpieces must be removed before welding to
   prevent contamination, and immediately prior to welding, alcohol or
   acetone should be used to clean the surface. Preheating is generally
   not necessary for mild steels less than one inch thick, but low alloy
   steels may require preheating to slow the cooling process and prevent
   the formation of martensite in the heat-affected zone. Tool steels
   should also be preheated to prevent cracking in the heat-affected zone.
   Austenitic stainless steels do not require preheating, but martensitic
   and ferritic chromium stainless steels do. A DCEN power source is
   normally used, and thoriated electrodes, tapered to a sharp point, are
   recommended. Pure argon is used for thin workpieces, but helium can be
   introduced as thickness increases.

Dissimilar metals

   Welding dissimilar metals often introduces new difficulties to GTA
   welding, because most materials do not easily fuse to form a strong
   bond. However, welds of dissimilar materials have numerous applications
   in manufacturing, repair work, and the prevention of corrosion and
   oxidation. In some joints, a compatible filler metal is chosen to help
   form the bond, and this filler metal can be the same as one of the base
   materials (for example, using a stainless steel filler metal with
   stainless steel and carbon steel as base materials), or a different
   metal (such as the use of a nickel filler metal for joining steel and
   cast iron). Very different materials may be coated or "buttered" with a
   material compatible with a particular filler metal, and then welded. In
   addition, GTAW can be used in cladding or overlaying dissimilar
   materials.

   When welding dissimilar metals, the joint must have an accurate fit,
   with proper gap dimensions and bevel angles. Care should be taken to
   avoid melting excessive base material. Pulsed current is particularly
   useful for these applications, as it helps limit the heat input. The
   filler metal should be added quickly, and a large weld pool should be
   avoided to prevent dilution of the base materials.

Process variations

Pulsed-current

   In the pulsed-current mode, the welding current rapidly alternates
   between two levels. The higher current state is known as the pulse
   current, while the lower current level is called the background
   current. During the period of pulse current, the weld area is heated
   and fusion occurs. Upon dropping to the background current, the weld
   area is allowed to cool and solidify. Pulsed-current GTAW has a number
   of advantages, including lower heat input and consequently a reduction
   in distortion and warpage in thin workpieces. In addition, it allows
   for greater control of the weld pool, and can increase weld
   penetration, welding speed, and quality. A similar method, manual
   programmed GTAW, allows the operator to program a specific rate and
   magnitude of current variations, making it useful for specialized
   applications.

Dabber

   The dabber variation is used to precisely place weld metal on thin
   edges. The automatic process replicates the motions of manual welding
   by feeding a cold filler wire into the weld area and dabbing (or
   oscillating) it into the welding arc. It can be used in conjunction
   with pulsed current, and is used to weld a variety of alloys, including
   titanium, nickel, and tool steels. Common applications include
   rebuilding seals in jet engines and building up saw blades, milling
   cutters, drill bits, and mower blades.
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