GAS TUNGSTEN ARC WELDING
The American Welding Society’s preferred name for this arc welding process is Gas Tungsten Arc Welding As the name implies the process uses an external Gas supply and a Tungsten electrode to produce an arc that melts and fuses the metal to be welded with or without the use of a filler wire.
The term TIG WELDING is a common shop term that is derived from the name TUNGSTEN INERT GAS WELDING. This name describes the same process and highlights the fact that a Tungsten electrode and an external inert gas are used to produce a weld.
In this text the term GTAW (Gas Tungsten Arc Welding) will be used to discuss this process.
The Gas Tungsten Arc Welding process may be used to weld most metals and alloys in any position with or without the use of filler wire. Because of the smaller heat zone and weld puddle with the excellent shielding effect of the gases used, the welds produced are often stronger than welds made with other processes.
Although GTAW is slower and produces smaller weld beads than SMAW or GMAW it is often the process of choice for welding thinner sections, Aluminum, specialty metals and Stainless steels.
GTAW EQUIPMENT SET UP
Gas Tungsten Arc Welding is done by setting up a Torch to a Constant Current Welding Machine and an external Gas Supply to shield the weld area. An optional Foot Pedal may be used to remotely control the amperage during welding. Filler Wire may be added as necessary or welding may be done by fusing the parts with the molten weld puddle.
THE WELDING CIRCUIT
The major component parts of the welding circuit described below are:
1. The GTAW or TIG Torch
2. cONSTANT CURRENT MACHINE
3. A Gas Supply
4. A Foot Pedal
5. Filler Wire
1. The GTAW Torch
The GTAW torch is an assembly of parts that provides the heat or electric arc for welding and delivers the shielding gas to protect the weld. The picture below shows a Typical GTAW torch used to make the weld.
TORCH USED TO MAKE A WELD
The GTAW Torch used to establish and maintain an arc as well as direct the gas flow to the weld zone, is made up of the Torch Body, Gas Diffuser, Collet, Tungsten Electrode and Back Cap. The pictures below show the typical Torch Assembly.
The Tungsten Electrode used in GTAW and shown above is different to electrodes used in SMAW (STICK WELDING) in that it is used as the heat source to melt the base metal, but does not melt and become a part of the weld. When additional weld metal is required it is fed into the arc in the form of a filler wire.
There are a variety of types of tungsten electrodes used for GTAW the most common ones being 2% Thoriated for Steel and Stainless and Pure for Aluminum.
Check with your weld supply company to select the appropriate Tungsten Electrode for the given application.
The electrodes used for steel should have a sharpened point and the pure for aluminum a balled end. There are tools designed to form the point for welding, but a less expensive method is to use a bench grinder and prepare the point like sharpening a pencil.
The sharper and longer the point the smaller and finer the arc puddle. A shorter point results in a wider weld puddle and bead.
For AC Welding Aluminum prepare a small point and the heat of the arc will ball the end for proper welding puddle.
Since the Tungsten Electrode does not melt and become filler metal it is referred to as a Non-Consumable Electrode. The tungsten may get contaminated by touching to the weld puddle or hot filler wire and will need to be re-sharpened.
When shaping the tungsten point, be sure the point is even or a deflection of the arc may occur.
There are a variety of styles of Torch used for GTAW. The one illustrated earlier is an Air Cooled style that has a gas valve incorporated. The gas valve is used to start or stop gas flow when the optional foot pedal is not used. When a foot pedal is used the flow of gas is automatically controlled by starting or stopping the arc with the foot pedal.
Another popular style of torch is a Water Cooled torch used for welding with higher amperage. The water circulates internally around the torch to keep it cool. When using this torch a water cooling unit is added to the GTAW circuit.
WATER COOLED TORCHES
2 CONSTANT CURRENT WELDING MACHINE
The power source used for GTAW is a Constant Current Welding Machine. These machines provide constant amperage as set by the welder, while the voltage rises or lowers with changes in the arc length. If the arc length or distance from the tungsten to the work increases slightly, the voltage increases to maintain the amperage level. Conversely if the arc length decreases the voltage decreases to maintain the same amperage. The Constant Current classification of Welding machines is also used for STICK welding (SMAW).
Any Stick or Multiple Process Welding Machine may be used for GTAW as well as some machines developed especially for GTAW applications.
Machines developed with GTAW applications in mind have added features such as a built in high frequency capability. A built in high frequency capability provides easier arc starting by allowing the arc to be initiated by a high frequency current without striking an arc or touching the tungsten to the work. This is especially useful when welding Aluminum that can easily contaminate the Tungsten Electrode.
Some of these GTAW machines such as a Square wave or Synchrowave have a method of changing the characteristics of the current to benefit from altered electrical flow.
When the application or job requires an extensive use of the GTAW process it may pay to research Welding Machines designed especially for GTAW.
The type of Welding current used for welding Carbon Steel and Stainless Steel is Direct Current Electrode Negative (DCEN) also known as Direct Current Straight Polarity (DCSP).
The type of Welding Current most often used for welding Aluminum is Alternating Current (AC).
A GTAW torch may be attached to any STICK machine using the remote receptacle on the machine or by attaching the Electrode Holder (Stinger) to the torch by the use of a special connector as shown below:
In this set up there is no foot pedal and the weld is initiated by touch start (similar to striking the arc in Stick). The gas is controlled by the valve on the torch. This setup may be readily assembled for welding on job sites.
MILLER ECONOTIG AND CONTROLS
USED AT DELTA
3 GAS SUPPLY
The two most commonly used types of gases used in the GTAW process are ARGON and HELIUM. A mixture of Argon and Helium may be used for some GTAW applications. Both Argon and Helium gases are considered INERT gases, meaning they are chemically inactive and do not react with the metal being welded.
Argon gas is the most widely used gas for manual welding with the GTAW process while Helium or a mixture may be used for heavier sections and automated welding.
HANDLEING GAS CYLINDERS
Because of the shape, contents and weight of gas cylinders used in the welding field it is important they be handled with extreme caution and in a safe manner. The following safety guidelines are suggested for safely handling and using gas cylinders.
Gas cylinders should never be moved by carrying, dragging or rolling them. They should only be moved by securing them in an approved cart. The cylinders must have the metal caps in place to protect the valve stem assembly of the cylinder before moving them.
If cylinders must be lifted they should be properly secured in an approved lifting cart or on a platform. Chains, slings, magnets, and other devices where there may be a possibility of the cylinder slipping and falling should never be used.
Cylinders that are transported over the road should be secured in the upright position with their protective steel caps in place.
Oil and grease should never come into contact with the oxygen cylinders, regulators or equipment, since a chemical reaction between the oil and the oxygen may cause a fire or explosion.
Hammers pry bars and similar tools must not be used to open a stuck cylinder valve or remove valve caps. If the cylinder valve is damaged the force of the escaping gas may cause high pressure cylinders to fly around uncontrolled and cause damage or serious injury.
If a cylinder is leaking gas it should be moved outdoors and the gas supplier should be notified immediately.
Cylinders must be chained or secured in the upright position secured on a cart or fastened into a welding machine cart. When not in use they should have the regulators removed and the caps in place.
Never weld on the cylinders, or allow the electrode to come into contact with them and establish an arc.
Use only the approved cylinder wrench to attach regulators and hoses to the cylinder. The use of loose fitting wrenches on the soft brass connections may cause excessive wear by slipping on the connections. Excessive wear may cause the edges of the connectors to become rounded and make it difficult to fit any wrench on them.
Use only the approved regulator and hoses for the type of gas being used and never use the same regulator and hoses for different gas contents.
The cylinder the cylinder valve must be cleaned before attaching the regulators. Standing behind the cylinder quickly open then close the cylinder valve to allowing the high pressure gas to blow out trapped dirt. This action is known as cracking the cylinder. Make sure no one is in front of the cylinder when you crack the cylinder.
Before attaching the regulator make sure the adjusting screw is backed out and loose. Since regulators are designed to blow out the front and back if they fail, stand to the side of the regulator when opening or pressurizing the regulator.
Fuel gas cylinders should only be opened about ¼ turn or until the gas registers on the regulator, so they can be shut down quickly in an emergency. The high pressure cylinders like the oxygen, argon and 75-25 have a double seating valve and should be opened all the way.
For most applications welders will be using 100 % Argon as the shielding gas with an approximate flow rate from 20-30 PSI.
If you are responsible for selecting the shielding gas for a given application, check with your gas and welding supply company for information about specific advantages of the available gases.
4 FOOT PEDAL
The GTAW Foot Pedal is an optional Pedal added to the welding circuit to remotely control the amperage and initiate the gas flow. When coupled with a Power Source that has a high frequency capability it allows arc starting without contaminating the tungsten electrode. Contaminating the electrode means the prepared point for welding becomes dull or has globules stuck to it causing the arc to wander and the weld bead to be wide. If the electrode gets contaminated the point will need to be re-sharpened for welding.
The use of a foot pedal is especially useful for welding aluminum where the heat needs to be lowered or increased during welding to avoid blowing away the sides of the joint, or melting through the joint.
The foot pedal works similar to the accelerator pedal in a car pressing down increases the amperage lifting the foot slightly lowers the amperage. When the foot pedal is fully depressed the amperage is at the setting on the machine. Lifting the foot lowers the amperage over a range until the arc is extinguished.
5 FILLER WIRE
Unlike Stick and Mig Welding, Tig uses a separate filler wire to add weld metal to the Joint to be welded in a similar manner to Oxy-Acetylene or Braze Welding. Some pieces may also be welded together by melting the edges of the joint called Fusion Welding.
Filler Wire is used to introduce more metal to the weld zone to fill the joint or increase the size of the weld. Filler wire is available in different diameters or sizes, some of the more widely used include 1/16th 3/32nds or 1/8th. The usual length is approximately 36 inches.
In general the filler metal or tig wire usually has the same composition as, or is compatible with the metal to be welded, so that Carbon Steel wire is used to weld Carbon Steel, Stainless Steel wire to weld Stainless Steel and Aluminum wire to weld Aluminum.
There is a selection of filler wire available for these and other metals using the GTAW process. Selecting a specific filler wire depends on many factors including strength requirements, joint design and condition of the joint surface.
When selecting an appropriate filler wire, consult filler wire guide books available at most welding supply stores and discuss job requirements and joint configurations with engineering personnel or experts at your local welding supply outlet.
Filler Wire used in the Gas Tungsten Arc Welding Process are classified by the American Welding Society using a letter and numbering system. The following are some examples:
Filler Wire for welding Carbon Steel
ER 70 S – 6
Filler wire for welding Stainless Steel
ER – 308 L
The Tensile strength of the weld metal is defined as the ability to resist forces acting to pull the weld apart, measured in thousands of pounds per square inch.
The Classification for GTAW filler wire is similar to that of the GMAW process.
GTAW is the most intricate of all the manual arc welding processes often requiring one hand to angle the torch, one hand to consistently feed the filler wire and a foot to control the amperage.
The metal is often thin and requires heat distortion control and must be kept clean. Welds beads are usually relatively small and flat with deep penetration.
Although GTAW produces less sparks with no slag, the intense arc produces ultraviolet and infrared rays that make the use of welding leathers and safety equipment mandatory. The same safety equipment used for SMAW should be used for GTAW except that the gauntlet gloves may be changed for tig gloves that provide adequate protection while allowing a better feel for manipulating the torch and filler wire