TIG- Tungsten Inert Gas welding

                                   Principle of TIG                                 

TIG welding works on same principle of arc welding. In a TIG welding process, a high intense arc is produced between tungsten electrode and work piece. In this welding mostly work piece is connected to the positive terminal and electrode is connected to negative terminal. This arc produces heat energy which is further used to join metal plate by fusion welding. A shielding gas is also used which protect the weld surface from oxidization.

                         

                                     WORKING OF TIG

• First, a low voltage high current supply supplied by the power source to the welding electrode or tungsten electrode. Mostly, the
  electrode is connected to the negative terminal of power source and work piece to positive terminal.
• This current supplied form a spark between tungsten electrode and work piece. Tungsten is a non –consumable electrode, which give a highly intense arc. This arc produced heat which melts the base metals to form welding joint.
• The shielded gases like argon, helium is supplied through pressure valve and regulating valve to the welding torch. These gases form a shield which does not allow any oxygen and other reactive gases into the weld zone. These gases also create plasma which increases heat capacity of electric arc thus increases welding ability.
• For welding thin material no filler metal is required but for making thick joint some filler material used in form of rods which fed manually by the welder into welding zone

Advantages:

• TIG provides stronger joint compare to shield arc welding.
• The joint is more corrosion resistant and ductile.
• Wide verity of joint design can form.
• It doesn’t required flux.
• It can be easily automated.
• This welding is well suited for thin sheets.
• It provides good surface finish because negligible metal splatter or weld sparks that damage the surface.
• Flawless joint can be created due to non-consumable electrode.
• More control on welding parameter compare to other welding.
• Both AC and DC current can be used as power supply.

Disadvantages:

• Metal thickness to be weld is limited about 5 mm.
• It required high skill labor.
• Initial or setup cost is high compare to arc welding.
• It is a slow welding process.

Application:

• Mostly used to weld aluminum and aluminum alloys.
• It is used to weld stainless steel, carbon base alloy, copper base alloy, nickel base alloy etc.
• It is used to welding dissimilar metals.
• It is mostly used in aerospace industries.

Characteristics of welding process

WELDS can be characterized according to a number of criteria, including the welding process used, size, shape, mechanical properties, chemical composition, and a number of others. The appropriate methods of characterization depend on the weld's function and the particular set of properties required for the application. In some instances, the ability of a weld to function successfully can be addressed by characterizing the size or shape of the weld. An example of this is where factors related to the welding procedure, such as inadequate weld size, convexity of the bead, or lack of penetration, may cause a weld to fail. In other cases, it is important to characterize metallurgical factors such as weld metal composition and microstructure. Examples might include welds for which the goal is to avoid failures due to inadequate strength, ductility, toughness, or corrosion resistance. In general, the goals of weld characterization are to assess the ability of a weld to successfully perform its function, to thoroughly document a weld and welding procedure that have been demonstrated to be adequate, or to determine why a weld failed. In the first part of this article, characterization of welds will be treated as a sequence of procedures, each more involved than the last and concerned with a finer scale of detail. Initially, non-destructive characterization procedures will be the focus. The first level of characterization involves information that may be obtained by direct visual inspection and measurement of the weld. A discussion of nondestructive evaluation follows. This encompasses techniques used to characterize the locations and structure of internal and surface defects, including radiography, ultrasonic testing, and liquid penetrant inspection. The next group of characterization procedures discussed are destructive, requiring the removal of specimens from the weld. The first of these is macrostructural characterization of a sectioned weld, including features such as number of passes; weld bead size, shape, and homogeneity; and the orientation of beads in a multipass weld. Macroscopic characterization is followed by microstructural analysis, including microsegregation, grain size and structure, the phase makeup of the weld, and compositional analysis. The third component of weld characterization is the measurement of mechanical and corrosion properties. The goal of any weld is to create a structure that can meet all the demands of its service environment. In many cases, the best way of assessing the performance of a weld is to establish its mechanical properties. In addition to a number of standard material tests, many mechanical tests are directed specifically at determining a weld's capabilities. Examples of mechanical properties typically characterized for welds include yield and tensile strength, ductility, hardness, and impact or fracture toughness. Corrosion testing is often employed in situations where a welding operation is performed on a corrosion-resistant material, or in a structure exposed to a hostile environment. Although absolute corrosion performance is important, a major concern is to ensure that a weld and its heat-affected zone (HAZ) are cathodic to the surrounding metal. Following the discussion of the characterization procedures, the second part of this article will give examples of how two particular welds were characterized according to these procedures.

Advantages , disadvantages and application of Gas welding

A good flux should have following desirable properties:

1. It should have low melting temperature than the base metal.

2. It should easily and readily react with metallic oxides and form a low melting temperature fusible slag to float on the top of the weld.

3. It should be easily chipped-off after solidification.

4. It should also act as better cleaning agent.

5. It should not adversely affect the base metal.

6. It should not chemically react with the base metal.

7. It must not cause corrosion on the finished weld.

Applications of Gas Welding:

Oxy-acetylene gas welding is widely used in practical field.

Some important applications are:

1. For joining most ferrous and non-ferrous metals, carbon steels, alloy steels, cast iron, aluminum and its alloys, nickel, magnesium, copper and its alloys, etc.

2. For joining thin metals.

3. For joining metals in automotive and aircraft industries.

4. For joining metals in sheet metal fabricating plants.

5. For joining materials those requires relatively slow rate of heating and cooling, etc.

Advantages of Gas Welding:

The following are the advantages of gas welding:

1. Portable and Most Versatile Process:Gas welding is probably portable and most versatile process. The ranges of gas welding products are very wide. It can be applied to variety of manufacturing, maintenance and repair work.

2. Better Control over the Temperature:Gas welding provides better control over the temperature of the metal in the weld zone by controlling the gas flame.

3. Better Control over Filler-Metal Deposition Rate:In gas welding, the source of heat and filler metal are separate unlike arc welding. This provides better control over filler-metal deposition rate.

4. Suitable to Weld Dissimilar Metals:The gas welding can be suitable to weld the dissimilar metals with suitable filler and flux material.

5. Low Cost and Maintenance:The cost and maintenance of the gas welding equipment’s is low as compared to some other welding processes. The equipment is versatile, self-sufficient and portable.

Disadvantages of Gas Welding:

1. Not Suitable for Heavy Sections:Since the heat produced is not sufficient and hence heavy sections cannot be joined economically.

2. Less Working Temperature of Gas Flame:The flame temperature is less than the temperature of the arc.

3. Slow Rate of Heating:The rate of heating and cooling is relatively slow. In some cases this is advantageous.

4. Not Suitable for Refractory and Reactive Metals:Refractory metals like tungsten, molybdenum and reactive metals like titanium and zirconium cannot be welded by gas welding process.

5. Larger Heat affected Area:Gas welding results in a larger heat affected area due to prolonged heating of joint.

6. Flux Shielding is not so Effective:Flux-shielding in gas welding is not as effective as in case of TIG or MIG welding. The oxidation cannot be avoided completely.

7. Problem in Storage and Handling of Gases:More safety problems are associated with the storage and handling of explosive gases e.g., acetylene and oxygen.

Functions of Flux coating in Electrode

1. Improve the electric conductivity in the arc region to improve the arc ignition and stabilization of the arc.

2. Formation of slag, which;

(a) Influences size of droplet.
(b) Protects the droplet during transfer and molten weld pool from atmospheric gases.
(c) Protects solidified hot metal from atmospheric gases.
(d) Reduces the cooling rate of weld seam.

3. Formation of shielding gas to protect molten metal  (The coating give off inert gases as CO2 under arc heat,            which shields the molten metal pool and protects it from the atmosphere oxygen,Hydrogen and Nitrogen pick-          up,thus reducing contamination of the weld.)

4. Provide deoxidizers like Si and Mn in form of FeSi and FeMn.

5. Alloying with certain elements such as Cr, Ni, Mo to improve weld metal properties.

6. Improve deposition rate with addition of iron powder in coating since the thermal losses to the atmosphere             from the electro tip are reduced.

    One of the major concerns with the coated electrodes is the moisture pick-up by the coating. This moisture, when it enters the puddle, dissociates into oxygen and hydrogen with the hydrogen being absorbed by the liquid metal and subsequently relesed during solidification, causing porosity. Normally, some additives are added to the coating to reduce the moisture pick-up from the damp atmosphere. But they do not ensure complete safety from the moisture pick-up. The electrodes should be picked out of the oven only when welding is to be done. They should not be out of the oven for more than 30 min to 4 hours depending upon the moisture picking time of a particular coating