Gas Tungsten Arc Welding (GTAW)

Introduction Gas Tungsten Arc Welding (GTAW)also known as Tungsten Inert Gas welding (TIG) is an electric arc welding process that produces an arc between a non-consumable electrode (tungsten which does not melt due to its high melting point) and the work piece to be welded. The weld is shielded from the atmosphere by a shielding gas that forms an envelope around the weld area. However, a filler metal is usually used in the process.

GTAW set-up Below is a simple diagram illustrating the set-up of GTAW process...

Equipments use in gtaw Below are some of the tools used in GTAW process; Gas supply (cylinder) Electrical power source (AC/DC) Electrode holder, torch or gun Connection cables Hose (for gas supply) Tungsten electrode Coolant Filler rods NOTE: the above mentioned are not limited to the, however other special could be used for specific purposes...

Working principle An arc is established between the end of a tungsten electrode and the parent metal at the joint line. The electrode is not melted and the welder keeps the arc gap constant. The current is controlled by the power-supply unit. A filler metal, usually available in 1 m lengths of wire, can be added to the leading edge of the pool as required. The molten pool is shielded by an inert gas which replaces the air in the arc area. Argon and helium are the most commonly used shielding gases.

The process may use direct current electrode positive, direct current electrode negative or alternating current. The chart above indicates the operating characteristics of each of these current types. DCEN or “straight polarity” is used for welding most materials other than aluminium. The electrode tip geometry is generally a sharp point with a small blunted end since most of heat balance is on melting of the base material.

DCEP or “reverse polarity” is rarely used since it results in low penetration. Also the constant bombardment of the tungsten electrode by electrons in the DCEP mode degrades the electrode. Alternating current is used primarily to weld aluminium, which has a tenacious oxide surface layer. Although the diagram above states that there is a 50% cycle from DCEN to DCEP, it is possible through solid state electronics to vary the amount of time at each polarity and also the current at each polarity.

High-quality welds in metals such as aluminium, stainless steels, nimonic alloys and copper in chemical plants; sheet work in aircraft engines and structures; Mainly thin sheets.

Effects of Polarity

Advantages GTAW produces precise and clean, nearly spatter free welds on almost all metals with superior quality in comparison to the other arc welding processes. It has found use in the aerospace, food processing, and nuclear industries. It is particularly useful on smaller sectioned parts and on reactive metals such as titanium. It can be used with filler metal or without filler metal (autogenous). This process allows the heat source and filler metal additions to be controlled independently. It is easily automated and can produce welds in all positions, even with intricate geometries.

Superior quality welds, generally free from spatter, porosity, or other defects Precise control of arc and fusion characteristics Weld almost all metals Used with or without filler wire Easily automated Used in all positions Intricate geometries weldable

Disadvantages Deposition rates are lower with GTAW than any other arc welding process. In general, the process is limited to thicknesses of 3/8-inch or less since productivity makes the process cost prohibitive. Tungsten inclusions or contamination of the weld pool may occur if the electrode touches the weld pool or proper gas Higher operator skill Required Sensitive to drafts shielding is not maintained.

Manual GTAW requires more dexterity and welder coordination than with manual GMAW or SMAW. As with the other gas shielded processes, drafts can blow away the shielding gas, which limits the outdoor use of the process. Less economical than consumable electrode processes for sections thicker than 3/8 inch Lowest deposition rate of all arc processes Tungsten inclusions

Added notes; For TIG 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

Safety Precautions To prevent injury to personnel, extreme caution should be exercised when using any types of welding equipment. Injury can result from fire, explosions, electric shock, or harmful agents. Both the general and specific safety precautions listed below must be strictly observed by workers who weld or cut metals. Do not permit unauthorized persons to use welding or cutting equipment.

Remove all flammable material, such as cotton, oil, gasoline, etc Remove all flammable material, such as cotton, oil, gasoline, etc., from the vicinity of welding. Do not weld in a building with wooden floors, unless the floors are protected from hot metal by means of fire resistant fabric, sand, or other fireproof material. Be sure that hot sparks or hot metal will not fall on the operator or on any welding equipment components.

Mark all hot metal after welding operations are completed Mark all hot metal after welding operations are completed. Soapstone is commonly used for this purpose. Before welding or cutting, warn those in close proximity who are not protected to wear proper clothing or goggles. Remove any assembled parts from the component being welded that may become warped or otherwise damaged by the welding process.

Do not leave hot rejected electrode stubs, steel scrap, or tools on the floor or around the welding equipment. Accidents and/or fires may occur. Keep a suitable fire extinguisher nearby at all times. Ensure the fire extinguisher is in operable condition.