MANUFACTURING TECHNOLOGY LAB

GAS TUNGSTEN ARC WELDING

SUBMITTED BY : SUYASH AGARWAL

ROLL NUMBER : 14135087

2ND YEAR MECHANICAL ENGINEERING,

GROUP 2

INTRODUCTIONWelding is a process of joining similar materials by application of heat with or without application of pressure and addition of filler material. The result is the continuity of homogenous material, of the composition and characteristics of two parts which are being joined together.

TYPES OF WELDING

Modern methods of welding may be classified under two broad headings

1. Plastic welding 2. Fusion welding

These are also called pressure welding and, non-pressure welding respectively.

PLASTIC WELDING

Unlike in fusion welding processes no liquid phase is present in the joint. The principle of solid-state welding is best demonstrated by following example. If two clean surfaces are brought into atomic contact with each other under sufficient pressure (and in absence of oxide films and other contaminants), they form bonds and produce a strong joint.

Applying external heat improves the bond by diffusion. Small interfacial movements on the contacting surfaces of the two pieces to be joined disturb the surfaces. Break up any oxide films, and generate new, clean surfaces; this mechanism improves the strength of the bond. Heat may be generated by friction, as well, an effect utilised in friction welding. Electric-resistance heating is utilized extensively in resistance-welding processes. In explosion welding, very high contact pressures are developed, to cause welding on the surface

FUSION WELDING PROCESSES

Fusion welding is defined as melting together and coalescing materials by means of heat. The thermal energy required for these welding operations is usually supplied by chemical or electrical means. Filler metals, which are metals added to weld area during welding, may or may not be used.

These processes include the oxy-fuel, arc, and high energy beam (electron-beam and laser-beam) welding processes that have important and unique application in modern manufacturing.

TIG WELDING

Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. TIG welding is a commonly used high quality welding process. TIG welding has become a popular choice of welding processes when high quality, precision welding is required.

In TIG welding an arc is formed between a non-consumable tungsten electrode and the metal being welded. Gas is fed through the torch to shield the electrode and molten weld pool. If filler wire is used, it is added to the weld pool separately.

In TIG welding the filler material is supplied from a filler wire. Because the tungsten electrode is not consumed in this operation, a constant and stable arc gap is maintained at a constant current level. The filler metals are similar to the metals to be welded, and flux is not used. The shielding gas is usually argon or helium. Welding with GTAW may be done without filler metals- for example, in the welding of close fit joints.

Depending on the metals to be welded, the power supply is either DC at 200 a, or AC at 500A. Maximum penetration is obtained with straight-polarity dc conditions. In general, AC is preferred for aluminium and magnesium, because the cleaning action of AC removes oxides and improves weld quality. Thorium or zirconium may be used in tungsten electrodes, to improve their electron emission characteristics. Power requirements range from 8 kW to 20 kW.

Contamination of tungsten electrode by the molten metal can be a significant problem, particularly in critical applications, because it can cause discontinuities in the weld.

The arc does not melt tungsten, which has a high melting point of over 3300 degree Celsius. The end of the welding gun where the arc is created is either made of high impact ceramic of is water cooled.

The TIG welding process lends itself ably to the fusion welding of aluminium and its alloys, stainless steel, magnesium alloy, nickel–base alloys, copper-base alloys, carbon steel and low-alloy steel. It can also be used for the combining of dissimilar metals, hard facing, and the surfacing of metals.

COMPONENTS USED IN TIG WELDING-Following components are required to perform TIG welding.

WELDING TORCH-TIG welding torch includes three main parts namely non-consumable tungsten electrode, collets and nozzle. A collets is primarily used to hold the tungsten electrodes. Nozzle helps to form a strong jet of inert gas around the arc, weld part and the tungsten electrode. The diameter of gas nozzle is selected

according to length of welded part. Current carrying capacity of welding torch directly affects the welding speed. Depending on current carrying capacity welding torch can be air cooled or water cooled. Air cooled welding torch is generally used for lower range of welding current. Whereas water cooled torch is used for higher range of welding temperature.

FILLER WIRE-Filler wire is generally not used for welding thin sheet by TIG welding. This is used to weld thick sheet by TIG welding to produce high quality weld. The filler wire can be fed manually or using some wire feed mechanism. For feeding small diameter filler wires, usually push type wire feed mechanism with speed control device is used. Selection of filler metal is very critical for successful welding. Filler material is used by taking some consideration as: mechanical property requirement, metallurgical compatibility, cracking tendency of base metal under welding conditions, fabrication conditions etc.

POWER SOURCE- The power supply used for TIG welding is a “Constant Current” power supply. In most cases it is the same power supply. TIG welding requires constant amperage to maintain the arc, while the voltage is what fluctuates depending on the arc length. The equipment for basic TIG welding is a DC power supply. In TIG welding current varies from 2-200A and welding voltage ranges from 10-35V.

SHIELDING GAS-Helium, Argon and their mixtures are commonly used as inert

shielding gas for protecting the weld pool depending upon the metal to be welded. Helium or hydrogen is sometimes added in argon for specific purposes such as increasing the arc voltage and arc stability which in turn helps to increase the heat of arc. The selection of inert gases to be used depends upon the type of metal to be welded. Argon and helium are the mostly commonly used shielding gases for developing high quality weld joints of reactive metals.

TUNGSTEN ELECTRODE-The tungsten electrode use here is non-consumable. Since tungsten has high melting point. Hence, tungsten electrode does not melt during the welding process.

ADVANTAGES OF TIG WELDING-

It is used to produce weld of high quality. It is use to weld thin work-piece. Does not use flux or leave any slag (shielding gas is used to protect the weld-

pool and tungsten electrode) Welded part is protected by inert gas during welding. TIG welding is highly resistance to corrosion. Concentrated arc permits pin point control of heat input to the work piece

resulting in a narrow heat affected zone. No spatter and fumes during TIG welding

DISADVANTAGES OF TIG WELDING-

It is a slow process. It is more expensive than MIG welding. Highly skilled labour is required. Welder has to work in high intensity of light.

APPLICATION OF TIG WELDING-

The TIG welding process is best suited for metal plate of thickness around 5- 6 mm. Thicker material plate can also be welded by TIG using multi passes which results in high heat inputs, and leading to distortion and reduction in mechanical properties of the base metal. In TIG welding high quality welds can be achieved due to high degree of control in heat input and filler additions separately. TIG welding can be performed in all positions and the process is useful for tube and pipe joint. The TIG welding is a highly controllable and clean process needs very little finishing or sometimes no finishing. This welding process can be used for both manual and automatic operations.

The TIG welding process is extensively used in the so-called high-tech industry applications such as

I. Nuclear industry II. Aircraft III. Food processing industry

IV. Maintenance and repair work V. Precision manufacturing industry VI. Automobile industry

Types of welding current used in TIG welding

1. DCSP (Direct Current Straight Polarity): In this type of TIG welding direct current is used. Tungsten electrode is connected to the negative terminal of power supply. This type of connection is the most common and widely used DC welding process. With the tungsten being connected to the negative terminal it will only receive 30% of the welding energy (heat). The resulting weld shows good penetration and a narrow profile.

2. DCRP (Direct Current Reverse Polarity): In this type of TIG welding setting tungsten electrode is connected to the positive terminal of power supply. This type of

connection is used very rarely because most heat is on the tungsten, thus the tungsten 5 can easily overheat and burn away. DCRP produces a shallow, wide profile and is mainly used on very light material at low Amp.

3. AC (Alternating Current): It is the preferred welding current for most white metals, e.g. aluminium and magnesium. The heat input to the tungsten is averaged out as the AC wave passes from one side of the wave to the other. On the half cycle, where the tungsten electrode is positive, electrons will flow from base material to the tungsten. This will result in the lifting of any oxide skin on the base material. This side of the wave form is called the cleaning half. As the wave moves to the point where the tungsten electrode becomes negative the electrons will flow from the welding tungsten electrode to the base material. This side of the cycle is called the penetration half of the AC wave forms.

4. Alternating Current with Square Wave: With the advent of modern electricity AC welding machines can now be produced with a wave form called Square Wave. The square wave has better control and each side of the wave can give a more cleaning half of the welding cycle and more penetration.