Unit 215 – Welding by TIG Process TIG Welding Process

Process An arc is produced between a non-consumable tungsten electrode and the work piece. The arc burns within a shield of gas to protect the molten weld pool and electrode from atmospheric attack. Additional metal needs to be added to produce the weld reinforcement. Welding without additional weld metal is possible on close fitting joints and is termed autogenous.

Process Depending on the shielding gas being used this welding process can be: TIG – Tungsten Inert Gas TAG – Tungsten Active Gas Principles are the same, the only difference is the shielding gas being used.

Process

Electrode Polarity Direct current (DC) is used for the welding of the majority of metals Alternating current (AC) is used for welding aluminium. Electrical polarity has a great influence on the heat distribution of the arc. The polarity of the electrode can be changed when using DC

Electrode Polarity

Electrode Polarity Electrode negative will produce a deep penetration weld as 66% of the heat is in the plate. Electrode positive will produce a shallow penetration weld as 66% of the heat is in the electrode. The electrode will melt away.

AC Welding Aluminium and its alloys have an oxide layer that has a higher melting point than the metal itself. This oxide layer must be removed before welding can take place. The AC current removes the oxide during the positive half cycle and welding during the negative cycle. Damage to the tungsten is prevented as it is cooling during the negative half cycle. Oxide removal Welding and electrode cooling

Electrical Characteristics The distance from the electrode to the plate is known as the arc length. As the arc length increases more volts are required to maintain the arc. If the voltage increases, amperage will drop (Ohm’s law) If amperages drop too much due to arc length, poor welds will result. TIG machines use a drooping characteristic electrical form to reduce the effect of variable arc length.

Electrical Characteristics

Shielding Gases In addition to providing a protective environment, the shielding gas and flow rate also have a pronounced effect on the following: arc striking arc stability penetration and weld bead profile speed of welding cleaning action

Gas Flow Gas flow is measured in litres per minute (Lp/m) Low gas flow will not protect the molten pool from the atmosphere. Too high a gas flow will cause turbulence and drag the surrounding air into the weld, again causing defects. Gas flow rates can vary depending on the gas, material thickness and joint type. Gas flow is regulated by a flow meter. As a general guide flow rates should be: 1 - 3mm thick - 4 -9 lp/m 4mm and above - 10 -20 lp/m

Shielding Gases Argon and helium are both inert gases. Type Uses Pure Argon LCS, Stainless steel, aluminium Argon/Hydrogen Austenitic stainless steel Argon/Helium Aluminium Helium Argon and helium are both inert gases. Hydrogen is an active gas. Hydrogen and helium increase the heat input, producing a hotter weld with deeper penetration. Helium is a light gas, so flow rates will need to be increased.