The arc welding with infusible electrode and inert gas protection (commonly called more briefly TIG, from the English designation Tungsten Inert Gas) is an autogenous welding process in which heat is produced by an arc that strikes between an electrode that is not consumed (then said infusible) and the workpiece.
The electrode is made of tungsten or tungsten alloys, means a material at very high melting temperature, with excellent properties of thermionic emission that is used to facilitate the operation of the electric arc.
The welding is performed by bringing to melting the edges of the workpiece to be welded, creating the joint eventually also with chopsticks filler material.
The electrode, the solder bath, the bow, the filler material and adjacent areas of the piece are protected from atmospheric contamination by a flow inert gas that escaping from the torch.
This process has among its main features to use an infusible electrode; consequently, the welding can be performed for small thicknesses without filler material and, when this is used, always allows a good control of the solder bath due to the good visibility during the welding process and the absence of metal transfer phenomena in the arc.
The process is suitable for any working position and can also be applied on the laminations of a few tenths of a mm thick.
The TIG process is widely used for the realization of joints of high quality on materials sensitive to heating imposed by the welding.
Because of the limited productivity, the process is rarely used to perform the welding process of high thickness.
With the TIG welding is suitable for all types of carbon steels, low alloy steels, alloyed, stainless, nickel alloys, aluminum and its alloys, copper and its alloys, titanium, magnesium, and other non-ferrous alloys.
TIG welding is excellent to weld thicknesses of a few millimeters, because its heat source, intense and concentrated, allows discrete welding speed and then allows it melt the edges of the workpiece without excessive risk of breakthrough; also the possibility of using modulated current increases still these characteristics.
TIG welding of stainless steels
The TIG welding process is currently used for welding of austenitic stainless steels; performing techniques are similar to those used for welding of carbon steels and low alloy steels, with the modest differences described below:
– The weld is much more fluid, so it is necessary to suitably increase the welding speed when operating in different positions from the plane;
– The cleaning of the flaps is much more important, given the greater sensitivity of these steels to the formation of cracks (hot) in the molten zone;
– It is recommended the use of special filters the output of the torch and the cap to the reverse side (Figure 14) in order to reduce the effect of coloring of the welding bead (surface oxidation)
– At the end of welding it is very important to wait a few moments earlier to remove the torch, in order to prevent oxidation of the crater;
– For the cleaning and the machining of stainless steels it is always appropriate to use clean utensils and not contaminated on low-alloy steels.
Welding Paramenters and variables
The choice of the most suitable welding parameters is based on the diameter and type (pure or additivated) of the electrode, on the type of gas and on the power mode of the arc. The inclination of the torch follows the same techniques indicates for MIG/MAG welding.
1 Torch close to the surface – 2 Torch far from the surface – 3 Tilted Torch – 4 Perpendicular Torch
In Figure 1 it can be seen that with the torch close to the metal base and with the torch inclined, the welding beard appears more clean and tight, while it tends to increase the heat affected zone when the torch is distant and perpendicular to the workpiece.
In applications manual TIG is the preferred technique to push, with angles around 15 °, to the lower risk of operational defects, while in the automatic welding is instead typically used the technique with torch perpendicular to the workpiece, which guarantees intermediate results but it facilitates the management of the filler metal.
In TIG we have two methods of power management: DC and AC. Using the DC mode (Figure 2) with direct polarity, it obtains a melting bath very deep and narrow, a high feed rate and therefore the decrease of withdrawals and distortions, as well as minor consequences of the metallurgical nature of the base metal. In addition, because of the limited heating, the tungsten electrode is consumed very slowly and can withstand currents rather high even if it has modest diameter.
TIG welding in DC mode
TIG welding in DC mode
This mode is subject to fluctuations disordered causing alterations in the thermal regime of the arc.
Using current continues to reverse polarity, the end of the electrode tends to overheat until it melts, and takes on a rounded and it is easy that small drops of tungsten are placed in the bath, then quickly consuming the electrode and giving rise to defects often unacceptable (and splashes of tungsten inclusions) in the solder.
For these reasons, it cannot exceed 100 A.
This type of power supply provides to the considerable advantage of breaking the oxide film infusible coating some materials (aluminum) through the ion blasting.
The reverse polarity is used rarely, because of the impossibility of using high welding currents, the rapid consumption of the electrode, the solder bath wide and shallow, and the consequent lack of penetration.
When it is necessary to weld with currents greater than 100 A, materials that require the removal of the oxide film, it has to feed the torch with alternating current.
In this way, each half-period of the voltage wave in which the electrode is positive allows a good grinding of the oxide film, while the other half-period in which the electrode is negative, it serves to limit the heating of the its tip.
The drawback is the difficulty of re-ignition of the arc.
The weld obtained by TIG arc modulated, compared to that obtained with traditional TIG, has the following advantages:
– Greater penetration for the same heat input;
– Increase in the ratio depth / width of the welding bead: with correct values of the welding parameters can be obtained, for example, ratios of 2 to 1 in the welding of stainless steels;
– Significant reduction of the deformations and the extent of the heat affected zone, due to lower specific heat inputs that can be compared to traditional TIG;
– Sagging limited, since the high currents and short pulses allow the bath to cool quickly;
– Possibility to weld thickness very thin;
– Limited risk of having hot cracking, always thanks to the lower specific heat input, and the most beneficial form of the weld;
– Lower risk of gas inclusions, because the pulse arc shake the solder bath, facilitating the evolution of gas.
Among the main disadvantages, it is possible to consider the higher cost of the generator (which must be of type electronically controlled) and the difficulty that can be seen, in certain cases, in the regulation of the parameters of the pulsation.
In Figure 3 you can see a type of pulsed TIG welding without filler.
TIG welding without filler material
As for the effects found in the solder bath, a voltage variation results in a variation of the width of the bath, and can be obtained by lengthening the arc and by removing the torch from the bath.
As a side effect there is also a variation of the energy density which can lead, only in borderline cases, to variations of the penetration.
Finally, there is a maximum current value above which the arc tends to become unstable (this parameter is a function of the characteristics of the electrode and the gas employed).
The feed rate, in addition to influencing the heat input, also causes changes in the size of the welding bead.
For welding speed too low, the cord tends to swell too much and, if not properly maintained, it can be a source of breakthroughs; excessive speed may be due to lack of penetration and gluing (will consequently be necessary to act on voltage, current, and type of gas used).
Gas Protection backhand welding
Some materials are characterized by remarkable reactivity, that is, from the characteristic of easily react with oxygen, causing the formation of a film having surface characteristics generally much lower than those of the base material. In these cases, it is necessary to protect the reverse side by the oxygen of the weld at least until the heating caused by the welding heat is irrelevant to the wrong side of the weld itself. As can be seen in Figure 4, if the reverse side of the weld is not protected, you may get the formation of chromium carbides (number 1, Figure 4b) that deplete the steel of elemental chromium for a possible re-passivation after pickling.
Cordon TIG welded
Welded cordon with the protective gas to the other side of the weld
When the reactivity of the metal is extremely high (for example the case of titanium alloys), it also protects the coupling part located at the back of the torch by using suitable devices to prevent the presence oxygen until the coupling is not properly cooled. The shielding gas used in TIG welding can be classified as follows.
– Inert gases at high temperature. Are argon (Ar) and helium (He). Other inert gases (krypton, xenon, neon) are not used because, being very rare, are very expensive. Argon and helium are monatomic (that is, their molecules are composed of a single atom), are therefore not dissociable, do not react with any other element (vapors and metal droplets) present in the plasma of the electric arc; from this it derives its name.
– Gas protective. Nitrogen is a gas, in part dissociable but chemically inert, is used, in small percentages to achieve specific results. E ‘instead often it used for protecting the reverse side of the joints.
– Gas reducers. The hydrogen reducing gas for excellence. Of this gas is exploited the property to dissociate the temperature of the arc and to re-associate, with the development of thermal energy to the surface of the bath, improving the transfer of heat. In TIG welding, the hydrogen can be used, in admixture with the inert gas (mixture argon- hydrogen), for the protection of the solder bath, and with the protective gas (nitrogen-hydrogen mixture) for the protection in reverse. Rates typical hydrogen containing 1 to 8%; higher values of this gas may cause porosity and require very precise control of the welding parameters because of the instability of the arc.
Welding of aluminum alloys
The TIG welding process finds many application in the welding of aluminum and its alloys of aluminum, using, alternating current supply or with superposition of high frequency current modulated with square wave.
In the first case the alternating current allows the crushing of the surface oxide layer and the increase of the frequency of the power already includes a reduction of the times of arc off and it reduces the disposal of heat through the protective gas.
In the second case it includes an overcurrent trigger arc, which exerts a role of preheating the bathroom, useful especially in view of the high thermal conductivity of this type of alloys.
From the operational point of view, the aluminum alloys have certain special characteristics, including a high fluidity of the welding bath, which results in a risk of collapse of the junction, accompanied by a high thermal conductivity, which involves a certain risk of gluing the flaps.
E ‘finally should be remembered that the aluminum alloys are extremely sensitive to problems of cracking in the molten zone (hot) and porosity, it is therefore extremely important to the cleaning of the flaps and between passes, to be made with small cutters and chemically.
During the process of electrochemical pickling, the cleaning of the weld depends on the chemical elements present inside the electrode: an electrode to the silicon causes a bleaching of the welding bead during the pickling phase, an electrode to magnesium prevents whitening and results to be but causes more stable combustion processes.
At the end of the weld it can be noticed a halo very clear that circumscribes the weld bead (Figure 5a).
TIG welding aluminum
MIG welding aluminum
This halo is due to the ionic blasting process. The inert gas (argon), during welding, is ionized. The ions collide violently against the surface of the base metal by generating a process of erosion that removes a thin surface layer of the workpiece.
This effect is completely absent if you make a wire welding MIG (Figure 5b) because they change the electrical parameters used, but the welding bead is very little lying down and with the presence of inclusions and splashes.
During the pickling process, it is very difficult to remove or reduce the effect of ionic blasting which delimits the weld bead because the surface has undergone a heavy plastic deformation (erosion) which has totally changed the structure of the base metal.