TIG Welding

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Important parts and accessories of TIG Welding Equipment

TIG welding equipment consists of mainly gas valve with flow regulator, welding torch, power cable, power source for welding, tungsten electrode, gas nozzle, collet to hold the nozzle, welding power source, high frequency unit, DC suppressor unit, inert gas cylinder, cooling water supply, filler wire and gas solenoid valves.

The welding torch may be air or water cooled if welding current is above 150 Amps.

The electrode material can be of tungsten or an alloy of tungsten which is generally thoriated tungsten or zirconiated tungsten which possess the capacity of holding higher current, high resistance to contamination and produce a stable arc when compared with pure tungsten electrodes. The tungsten alloy is used when copper is welded in nitrogen atmosphere nitrogen gets reacted with tungsten quickly but for aluminum welding with argon gas shield, pure tungsten electrode is found more suitable.

The other inert gases used in welding are helium, argon-helium mixtures, hydrogen and argon-hydrogen mixtures. Helium gas increase heat input when added thereby increasing weld speed and penetration. By adding hydrogen, the welds are cleaner and heat is increased too but at the same time may encourage porosity and cracking too. Metals like aluminum, copper, nickel and their alloys, mild steel and stainless should be welded using argon.

AC welding machines or DC with reverse polarity is used to weld aluminum, magnesium and their alloys because reverse polarity on these two metals removes oxide deposition film. DC welding machines are in practice to weld copper and its alloys, nickel and stainless steel. To keep the arc ignited and steady, a high frequency unit is engaged. Also, to avoid arc blow, erratic arc and overheating of welding transformer, a DC suppressor unit is integrated in the electric circuit. The DC suppressor balances the current wave and reduces the effect of current which comes into function due to the fact that arc voltage is higher when electrode is positive than when it is negative.

Filler rod is used normally when welding thicker metal and should be of same chemical composition that of the metal work piece. The rods come in either in the form of straight fixed lengths or coils.

To regulate the flow of gas and water, solenoid valves are installed in the system which start the water and gas before the arc ignites and stop the same after the welding is over, tungsten electrode has cooled and weld molten pool gets solidified.

Procedures of TIG Welding

The supply of water, inert gas and welding current is put on. The gas is fed through the welding torch for shielding the tungsten electrode and molten weld pool. The shielding is done because in this in this type of welding the molten weld pool on the metal may have contamination in the atmosphere or the quality of weld becomes poor. For this reason, inert gas like argon is used along with filler metal where required. The filler wire when used is added to the weld pool separately and not an integrated part of the TIG welding equipment.

Initially, arc is formed either by striking the electrode with a scrap metal or tungsten piece and broken by increasing arc length. This is a pre warming of tungsten electrode may be repeated for twice or thrice. Now the arc is made to strike between the work piece metal thoroughly cleaned. This procedure avoids breaking electrode tip, tungsten loss and contamination. There is another method wherein a high frequency current is applied on the welding current and the welding torch is carried close to the metal work piece to make the distance of 2 to 3 mm between electrode tip and the metal work piece. Arc is created due to a spark which shoots across the air gap between the electrode and the work piece. The welding puddle is built up and the welding torch is moved back followed by filler rod to move forward to be added to the weld puddle. Then the filler rod is taken out and further welding is continued in the same sequence.

Welding is started by moving the torch, striking the arc making it to impinge on the work piece metal and a molten weld pool is built. As the welding proceeds on the work piece and reaches the end, arc is made to break by increasing the arc length and the shielding gas is impinged on the solidifying weld pool for few seconds even after the arc is put off to avoid atmospheric contamination of the weld metal.

Advantages of TIG welding

Weld quality is good

Distortion of metal work piece is low

Spatter free

Precision welding

Flexibility of welding with or without filler rods

Welding up to 6 mm thickness of work piece possible

Butt, Lap, Corner, Edge and T joints- all used in TIG welding

Disadvantages of TIG welding

Deposition is low

Expensive for welding thick metal work piece