Tungsten inert gas welding (TIG welding)

Tungsten inert gas welding is abbreviated to TIG in German. In English, however, the abbreviation TIG comes from the English word tungsten for tungsten. The process was introduced in the USA in 1936 as Argonarc welding. After the Second World War, it also became widespread in Germany.

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Advantages of the TIG process

TIG welding offers a number of advantages compared to other fusion welding processes. For example, it can be used universally. This means that as soon as a material can be welded at all, it can be welded using the TIG process.

The TIG process is also very clean. This means that there is hardly any spatter and very few harmful substances. In addition, the quality of the welded joints produced is particularly high.

In contrast to processes that use an electrode that melts away, the filler metal is supplied separately here. The current is also decoupled. This makes it possible to optimise both and thus match them precisely to the materials being welded. This makes it particularly suitable for root passages and constrained positions. With these advantages, the TIG process is particularly popular in the aerospace sector, as well as in nuclear technology and chemical plants and equipment.

The devices for TIG welding

The welding equipment used is a power source that supplies direct or alternating current, a welding torch and a hosepack for the connection to the welding torch. The hosepack contains the power cable, a supply line for the shielding gas, a control line and, in larger systems, an inlet and outlet for the cooling water. This means that no welding machine is used without gas.

There are two ways to ignite the arc. One is contact ignition and the other is high-frequency ignition.

The contact ignition

Contact ignition follows a process similar to gas-shielded arc welding with an electrode. A tungsten electrode is used for the TIG process, which is tipped onto the workpiece. This creates a short circuit. As soon as the electrode is lifted, an arc is created between it and the workpiece.

There is one disadvantage associated with this process. Each ignition causes a small amount of material from the workpiece to stick directly to the tungsten electrode and remain there. This results in an alloy of the material and the tungsten due to the high temperatures that prevail at the tip of the electrode. This slowly melts the tip of the electrode, which is initially designed as a sharp needle. Fine weld seams are therefore very difficult to create with this ignition.

High-frequency ignition for TIG welding

A high-frequency igniter is used for high-frequency ignition. This creates an extremely high voltage at the tungsten electrode. This voltage leads to the ionisation of the gas between the workpiece and the electrode. This in turn ignites the actual arc. Only safe currents are used for the high-frequency igniter.

Inert gases, including argon, helium, nitrogen or a mixture of these, are used as shielding gases for high-frequency welding. Hydrogen can also be added. The details look like this:

• Argon 4.6 - 99.996% Argon
• Argon 4.8 - 99.998% Argon
• Arcal10 - Hydrogen 10%, Argon 40%, Nitrogen 50%
• Helargon - Helium 10%, Argon 40%, Nitrogen 50%

The TIG process and the current

Both direct current and alternating current can be used for the TIG process. Direct current is mainly used to weld alloyed steels and non-ferrous metals or their alloys. The tungsten electrode forms the negative pole.

Alternating current welding is used for light metals. Alternatively, light metals can also be welded using direct current. In this case, the electrode forms the positive pole. In this case, the welding torches use a very thick tungsten electrode. Helium is used as the gas for TIG welding. The electrode must be the positive pole when welding light metals, as a passive coating has formed on it. This has a very high melting point on its surface. The workpiece is now the negative pole. This means that it now emits electrons. This leads to the passive layer being destroyed.

TIG pulse welding

TIG welding also gave rise to TIG pulse welding. Pulsed current is used here, which allows the heat input into the workpiece to be very finely dosed. This makes gap bridging very easy. It also enables good root welding and facilitates welding in constrained positions. Weld seam defects, which often occur at the beginning or end of a seam, are also avoided.

In this process, the welding current is pulsed. This means that it constantly alternates between a base current and a pulsed current. The frequencies, the basic heights and the pulse heights as well as their widths are varied. All these individual factors can be adjusted separately. This requires a special system that allows such a variable current flow.

TIG pulse welding can be carried out completely manually or partially mechanised. A filler material with a low thickness is used. The control of the welding current allows only the surface of thin sheets of light metals to be melted. This means that through-welding or melting can be prevented.

Especially when fillet welds are made, the corners are captured better than would be possible with a standard process. This means that even sheets with a thickness of just 0.6 mm can be butt-welded without any problems. The arc is so stable that the weld pool can be kept small and precisely defined.