TIG

TIG (Tungsten Inert Gas) welding is an arc welding process with an infusible electrode (tungsten), under the protection of inert gas, which can be performed with or without filler metal. TIG welding is one of the most widespread methods, it provides high quality joints, but requires highly specialized operators. The process is based on a torch in which the tungsten electrode is inserted, around which the shielding gas flows which, through a ceramic nozzle, is brought to the melting pool. The operator moves the torch along the joint to move the melting pool, while, if filler material is required, he simultaneously moves the rod of the material in such a way as to keep it constantly with the end within the arc and in any case under the protection of the gas. The equipment for carrying out TIG welding is therefore composed of:

• Current generator (Welding machine).
• Torch.
• Tungsten electrode.
• Shielding gas supply nozzle (Gas passage).
• Insulating sheath.
• Electrical conductor.
• Electrode holder.
• Shielding gas cylinder (Inert gas supply).
• Any filler metal rod.

One of the main advantages of this technology is that the material input into the weld pool is independent of the heat input during welding, unlike what happens in wire or consumable electrode welding. The TIG process is particularly suitable when small thicknesses of material need to be welded, starting from a few tenths of a mm, however it is not possible to weld thicknesses greater than a few mm (2-3 mm for steels) with a single pass (therefore, in general, it is not used to weld thicknesses greater than 5-6 mm), therefore, considering the low productivity, it is often used to make the first pass of a joint, while the filling is carried out subsequently with higher productivity processes. Given its characteristics, the process can be used in any position and can be used for continuous welding or for spot welding. It is not advisable to use this process in open places, since even a light wind can disperse the shielding gas.

The electrodes, having to be made of a material capable of resisting the temperatures of the electric arc, have been, for many years now, only made of tungsten or its alloys; at the beginning of this technology (the 1940s), graphite electrodes were also used. Tungsten, in addition to having better thermal and mechanical characteristics, is preferred for its high thermoelectric power (ability to emit electrons at high temperatures), which stabilizes the arc. To increase the thermoelectric power of the W, the electrodes are sometimes alloyed with small percentages (1-2%) of Th (thoriated electrodes).
The electrodes can be found on the market with different diameters from 0.25 to 6.4 mm. They are generally used in direct current (cc), direct polarity (pd), i.e. with the positive pole on the piece. The use of reverse polarity (pi), i.e. with the negative pole on the piece, is used for welding light metals (Al and Mg) or when the stability of the arc is important. However, since CCPI supplies less energy to the pool, and therefore requires higher arc currents, it is often preferable to replace it with alternating current (AC) welding, which can be symmetrical or asymmetrical.
Before being used in CCPD, the electrode must be sharpened so that the tip takes on a conical shape, with a cone height of about 1.5 times the diameter, in this way its ability to emit electrons is increased, thus obtaining good heating of the pool even with relatively low currents. Instead, in CCPI, the electrode must tend to take on a flat shape (and, of course, for these uses, thoriated electrodes are avoided), precisely to limit the emission of electrons, which would require a higher voltage for the same arc current.

Generally, the shielding gas is introduced on both faces of the joint (of course if this is accessible on both sides), while on the face where the bath is located (on the right side) the gas is brought directly from the torch, on the other face (on the reverse) it is blown in under controlled conditions, so as to ensure protection from oxidation even at the root of the weld. The most commonly used gases are Ar or He, used separately or in mixtures. In some special applications, mixtures of Ar with H2 are used. Generally pure Ar is preferred to other solutions, for the following advantages:

• It promotes arch stability.
• Surface cleaning on light metals (Al and Mg).
• Relatively low cost.
• Requires lower flow rates to provide the same shielding.
• Reduced penetration (particularly useful in manual welding on low thicknesses).

He is used for welding thick sheets (greater thermal conductivity, therefore greater penetration), it is used in a mixture with Ar to balance the characteristics of the two gases.
The use of H2 in a mixture with Ar is limited to austenitic steels and Ni-based alloys, due to the metallurgical damage that it could cause to ferritic steels (cold cracks). The presence of H2 in the shielding gas increases the energy transferred from the arc to the material to be welded, furthermore H2 acts as a reducing material, inhibiting the formation of oxides and therefore leaving very clean welding surfaces. For these reasons it is used (almost exclusively in automatic welding) for welding pipes for chemical or nuclear plants or tube plate pipes.
The shielding gas flow rates must be established by the welding technician, based above all on his own experience and on tests aimed at the particular job and the particular geometry.

MIG-MAG

MIG (Metal-arc Inert Gas) or MAG (Metal-arc Active Gas) welding (the only difference between the two is the gas used to protect the weld pool) is a welding process developed after the Second World War that has taken on an ever-increasing weight, in terms of product welded per year. One of the main reasons that have allowed this development has been the reduction in the cost of electronic products, for which semi-automatic welding machines have been developed at affordable costs even for medium-small sized companies.

The MIG/MAG welding process is a continuous wire process in which the protection of the welding pool is ensured by a covering gas, which flows from the torch onto the piece to be welded. The fact that it is a continuous wire process guarantees high productivity to the process itself, and at the same time the presence of gas allows for operation without slag (both of these characteristics increase the cost-effectiveness of the process compared to electrode welding). On the other hand, a MIG/MAG welding station is necessarily composed of the following components:

• Torch with a dual function: to strike the arc between the wire and the piece and to bring the shielding gas to the welding pool.
• Piece to be welded.
• Arc current generator (in modern machines the control of the arc characteristic is carried out electronically).
• Wire feed and control mechanism.
• Thread winding reel.
• Shielding gas cylinder.

MIG/MAG welding, like all continuous wire processes, is a process derived from the submerged arc, but, compared to the latter, it has the advantage that the operator can keep the arc under direct observation, therefore he can control the execution of the welding as in electrode processes (coated electrode and TIG). Other advantages compared to the submerged arc are the lack of slag formation and the possibility of welding even in non-flat positions.

The shielding gas has the function of preventing the contact of the weld pool with the atmosphere, therefore it must be brought to the weld pool directly from the torch.
The most used inert shielding gases are Ar and He, both are inert monoatomic gases, but, while Ar is heavier than air, therefore it stagnates on the weld pool, ensuring greater protection, He is lighter than air, therefore it provides less protection, however, having a thermal conductivity about 10 times that of Ar, it allows greater penetration of the weld. For this reason, the use of He is limited to joints of high thickness or to materials with high thermal conductivity (Cu or Al).
Active gases, on the other hand, are generally mixtures of Ar and CO2, with carbon dioxide, in extreme cases, replacing Ar (however, it is rarely used in a percentage higher than 25%). The presence of CO2 increases the stability of the arc positioning on ferromagnetic materials (carbon or low-alloy steels). Furthermore, the presence of active gas allows for greater penetration of the joint. On the other hand, the presence of CO2 causes an increase in the current required to have a spray transfer of metal between the wire and the weld pool, increases spatter and decreases the electrical stability of the arc. Therefore, in order to use active gases with spray transfer, a pulsed current is generally used, i.e. a current that has intensity peaks of pre-established duration and frequency, to have a continuous energy input, but the detachment of the metal droplet only during the high current intensity phase.

MIG/MAG welding is used when high productivity and sufficient flexibility of use are required. With this technology it is possible to weld both austenitic and ferritic steels, light metal alloys (Al and Mg), copper alloys, nickel alloys and titanium alloys. Since the protection of the weld pool is ensured by a flow of gas, this process is recommended only in the workshop, since, on the construction site, a moderate wind is enough to disperse the flow of shielding gas, with a consequent reduction in the quality of the welded joint. This technology can be used without difficulty to produce buttering (deposition of a layer of material on a different material) or for repairs.

In plasma welding, the torch used has a tungsten electrode at its center, which is infusible during the process. A plasmogenic gas reaches this electrode which, in the presence of the electric field present externally or internally to the torch, becomes plasma, that is, a highly ionized gas. The fundamental characteristic of plasma (which makes this welding technique very often used given its low costs and high performance) is that it can capture an enormous quantity of heat and transport it to the jet to be melted. A necessary condition for this to occur is that the plasma must arrive in a concentrated form on the piece, which is made possible by using an appropriate distance from the piece and a sufficiently high gas outlet speed. In these applications, the temperature can reach, in the most severe cases, orders of magnitude of 20,000 °C, which is unthinkable for other welding operations: it is precisely this high temperature that is responsible for the high precision and speed of carrying out these operations. To prevent blowholes in the welded part (which inevitably occurs with any type of welding, since according to Henry’s Law the solubility of a gas is a function of temperature) a system is used around the torch that allows the release of particular gases (usually mixtures of noble gases) which, having practically zero solubility even at high temperatures, do not allow atmospheric gases to penetrate into the molten metal bath.