Most recently, the TIG welding process has been facing greater and greater competition from the ever-perfected MIG/MAG process and its related processes. These processes drastically increase productivity without concessions to quality. Despite its slower welding speed and lower deposition rate, the TIG process has been and still is for many applications the best guarantee for the highest quality results. Last but not least, innovations in the power source sector ensure a sustained future for TIG welding. The following comments are meant as a more detailed discussion of the basics.
The core of a TIG welding torch is a non-consumable, temperature-resistant tungsten electrode. The arc that proceeds from it heats and melts the material. As required, a filler wire is fed in manually or with a wire-feed unit. In many cases, a narrow gap needs no filler material at all when being welded. Ignition of the electrode normally takes place without the tungsten electrode touching the workpiece. This requires a high-voltage source that temporarily switches on during ignition. For the majority of metals, welding itself takes place using direct current. Aluminium, however, is welded using alternate current.
The nozzle for shielding gas is fitted around the tungsten electrode. The gas that flows out protects the heated material from chemical reactions with the surrounding air, thereby ensuring the required strength and durability of the weld metal. Inert gases such as argon, helium or their compounds are used as shielding gases. Even hydrogen is used occasionally. All these gases are inactive, which is what the specialist term “inert”, taken from the Greek, refers to. The term used to describe the process, “tungsten inert gas” (TIG) welding, comes from the type of shielding gas and the electrode material used.
The most-used shielding gas for TIG welding is argon. It optimises the ignition properties, as well as the stability of the arc, and helps obtain a better cleaning zone than helium. This in turn ensures an especially wide and deep fusion penetration, thanks to its thermal conductivity, which is nine times higher than that of argon. Used in conjunction with aluminium, pore formation is less pronounced. Furthermore, hydrogen is also sometimes used for austenitic steels, the percentage often only 2 to 5 %, the rest consisting of argon. The heat conductivity of hydrogen is even eleven times greater than argon, leading to a very deep fusion penetration and extremely effective outgassing.
When welding corrosion-resistant materials, for example stainless steels, the heated edges oxidise because of contact with oxygen in the air, which cannot always be completely avoided. The so-called annealing colours appear. These can be removed by rework, which restores corrosion resistance. It is preferable however to prevent the annealing colours from forming in the first place. This happens by using so-called forming gases. Forming gases keep the air away from the edges of the weld seam and in some cases even influence the root formation of the seam. Forming gases are primarily compounds of hydrogen and nitrogen, but argon is also used.
TIG welding is a versatile process that can be used for all weldable materials and applications. The main application area is stainless steels, aluminium and nickel alloys. The concentrated, stable arc provides high weld metal quality and an even seam, with no spatter or slag. For applications with the highest demands on quality, for example pipelines in reactor construction, this process is the first choice. In addition, the use of filler metal is unnecessary. For sheet thicknesses of less than 4 mm, mechanised wire feeding produces economical welding speeds. Only the welding of thicker sheets means limited cost effectiveness, whereby only welding the root pass is recommended. Welding the filling runs is better with powerful processes such as MIG/MAG or submerged arc welding.
For many applications, a pulsed welding current is helpful for preventing overly intensive melting of the base metal and associated weld drop-through. For light-gauge sheets especially, the weld build-up is easier to achieve, as the base metal only melts in sections, and then solidifies again.
Wherever aluminium is exposed to the air, an oxide layer forms immediately on the surface. The layer has a melting point of 2015 °C. Aluminium itself however melts at 650 °C. If the oxide layer remains solid, the molten aluminium on the oxide layer would run off, and a weld joint would be impossible. The oxide layer must therefore be removed, by positive polarity of the electrode for example. One disadvantage however would be a deterioration of the welding properties, as the tungsten electrode must be negatively poled in TIG welding. The solution is to weld with alternate current. During the positive half-wave, the oxide layer breaks open. The negative half-wave increases the fusion penetration and generates the required welding power.
Regardless of the arc length, an ideal TIG power source possesses a virtually constant output current. Continuous current adjustment is also required for all sheet thicknesses, which is why conventional thyristor power sources feature a rectifier downstream from the welding transformer. A disadvantage of thyristor power sources is the low efficiency due to a very large, necessary output choke for smoothing the welding current.
Modern inverter power sources are free from such drawbacks, and offer the additional advantage of a faster reaction to changes in the welding process. A pulsed voltage with a very much higher frequency, rather than the mains voltage, arrives at the transformer. Due to the high frequency, this has a much lighter, compact and efficient design than the thyristor power sources. The low current ripple of the transformer output current means a substantially more compact design, or no need for the output choke. The rectifier simply consists of uncontrolled diodes.
For generating an alternate current (AC) for aluminium welding, AC-compatible power sources have an inverter downstream from the rectifier. Many power sources allow the user to set a sinusoidal or rectangular alternate current, as well as a combination of the two. A sinusoidal welding current has an unstable, if very soft arc. With a rectangular welding current, the current stabilises the arc. The audibly louder operating noise however requires the user to work with ear protection. A combination of sinusoidal and rectangular welding current is very stable and extremely soft at the same time.
TIG welding torches are available in gas-cooled and water-cooled versions. Gas-cooled welding torches are cooled by the shielding gas which flows through, while water-cooled welding torches also have effective liquid cooling using a pump and heat exchanger. There are also TIG welding torches with an integral device for mechanised wirefeeding.
The TIG process is definitely not the most economical welding process. Improvements in the power source sector, as well as mechanised and automated applications, however, qualify TIG welding for high volume production. In any case, the TIG process has been and still is the first choice for a whole range of applications that require high standards.