MIG Welding

The arc MIG/MAG welding with continuous wire is a set of welding processes in which heat is generated by an arc which strikes between a fuse wire and the workpiece. Consequently, the wire performs both the function of electrode, and the function of supplying material to the joint, since the passage of the current causes it to melt and it is fed continuously in the welding area by means of a torch. The protective atmosphere necessary to enable the operation of the electric arc and to avoid contamination of the bath by the atmosphere of air can be supplied from an influent gas from the torch (welding under gas protection) or directly from the cored wire, as it is the case also for coated electrodes (welding without gas protection). There are different versions of the wire welding, as shown in Table 1.

Types of welding wire

  • Welding with inerith gas protection (MIG – Metal Inert GAS);
  • Welding with active gas protection (MAG – Metal Active Gas);
  • Flux-cored arc welding with active gas protection;
  • Flux-cored arc welding with inert gas protection;
  • Flux-cored arc welding without gas protection.

Transfer pulsed arc
The welding generators with electronic control allow the use of modulated current for the management of the process of continuous wire welding. Consequently, it can use special waveforms that achieve a smooth transfer of the metal, regardless of the heat supplied to the bath, and consequently the contribution thermal. In its simpler forms, the welding with pulsed arc provides a current characterized by a base value, sufficient to maintain the arc on, and from a peak value, which determines the detachment of the drop. The heat input is instead assessed on the basis of the effective current, which is typically reported on the instrument, on the machine or from current clamp. Consequently, it obtains a penetration of the deposit directly linked to the peak current. The latter is associated to the contribution thermal, lower, calculated on the effective current. The waveform in Figure 1, represents the simplest form of the welding with pulsed arc transfer.

Figure 1 Waveform pulsed arc

However, they were developed in the time dedicated programs, which allow a more accurate management of the current (and sometimes of the voltage): pulsations are therefore often used in more peaks, with ramps of uphill and downhill slope different, or variable frequency. These programs therefore are intended to give the correct heat input during welding in such a way as to have the correct penetration of the filler material in the base metal. Changing the mode of pulsation (Figure 2), it can have a welding bead clean, clear, and with portions of the filler material which succeed with a rhythm narrower due to the increase of the pulsation of the welding.

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Figure 2 Mode pulse arc: 1 Pulse fast, Pulse Media 2, 3 Pulse slow

The pulsed arc welding is frequently used in the welding of thin metal sheets, especially in the case of particularly sensitive materials to thermal effects of welding (stainless steels, non-ferrous alloys) and in general is very frequent in the welding of light alloys, reducing the risk of inclusions due to the peak current, and overruns due to the reduced heat input. Finally, recent introduction of special programs for the execution of the first pass, for brazing, to contain the welding fumes and noise, to reduce the occurrence of porosity during welding aluminum.

WELDING PARAMETERS

Filler material
The choice of filler material determines 80% of a good weld. Over the years, the wire is becoming increasingly poor. This results in a very low quality of the weld. Visually the welding bead becomes very dark and contaminated by inclusions difficult to remove during the pickling process. The color of the wire used in the welding also controls the coloring of the welding bead. If the wire has a dark color, even the welding bead will become dark. For these reasons, the production process of the wire plays a fundamental role. A wire of excellent quality is obtained through a drawing process, the satin finish, and a double process of electrochemical pickling. The process of the satin finish is performed to lower the frictional resistance of the wire when it passes through the duct plastic power during welding. The glazing has replaced the old thread glossy white because it had a very high frictional resistance. A double pickling process of the wire has a cost of production by 30% more than a normal drawing process. Very often this last phase is removed in the production process for economic reasons, leaving the wire greasy residue of the grain and lubricating oils resulting from the drawing process. During welding, these residues determine a combustion process causing burns and a dark color along the welding bead. The welding process involves the formation of silicates along the cord, making very difficult the subsequent electrochemical pickling process.

Silicates that are created during the welding process are the result of the presence of silicon within the chemical composition of the filler material. The maximum amount of silicon permitted within the wire is 1%. The silicon is inserted between the alloying elements list because it has the ability to confer to the wire a lower stiffness, high workability and therefore an increase of the drawing speed. These silicates cannot be removed from the cord but can be reduced during welding. The reduction of the silicates occurs:

  • Using a wire with a low percentage of silicon
  • From the size of the welding bead
  • Using a low value of electric current

The wires that contain a high percentage of silicon results in an increase of the probability of obtaining the silicates on the welding bead. This probability increases when the size of the welding bead increases. There is an upper limit of the current that the concentration of silicate becomes very high, worsening the properties of the welded material. The increase in the current is very often linked to the idea of increasing business productivity, but only leads to lower quality of welding and the entire finished product.

Protective Gas
The amount of gas escaping from the nozzle is another important parameter during the welding process. A value of the gas flow too low does not protect adequately the smelting bath. A value of the gas flow is too high forms turbulence which allows to the oxygen in the air to react with the melting bath creating unwanted oxides. In Figure 3 it can be seen the welding bead obtained with a gas flow rate not optimal.

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Figure 3 welding bead obtained with not optimal flow rate of gas

Visually it can be noticed minute splinters grouped (splash, red circle) of filler material on the metal base with a welding bead not homogeneous and very prominent (Figure 3). The proper flow rate of gas is dictated by the inner diameter of the nozzle of leakage of the gas itself. If the diameter is 15 mm, the flow it will be set at 15 l/min. This setting allows a lowering of the number of splinters and the thickness of the welding bead. The nozzle diameter is chosen according to the size of the weld bead that it wants to accomplish: welding bead tight, narrow nozzle; welding bead wide, nozzle wide. The minimum flow rate of the gas to perform a weld is of 10 l/min.

Formerly using the mixture with 2% of oxygen. The welding result was a very dark cord. The advantage of using oxygen as active gas (MAG technology) was to keep down the level of carbon when were welded steels such as 304L and 316L, leaving welding bead blackened and difficult to pickle. Today this mode has been replaced by the use of a mixture based on carbon dioxide. Numerous studies have confirmed that a carbon dioxide content of less than 5% does not alter the chemical composition of steel welded. With these percentages of carbon dioxide, the carbon percentage in the steel remains unchanged. In addition, the active gas has a low temperature obtaining a welding with low heat input, clear and homogeneous. Special blends are made by adding hydrogen. During welding, the hydrogen binds to oxygen present in the welding area by preventing the oxidation of the bead. The maximum percentage of hydrogen is 2% to avoid problems of embrittlement and violent chemical reactions during welding. The effect of the hydrogen is to reduce the formation of silicates, to create a more stable melting bath and to remove the residues on the bead of the weld more easily. These ternary mixtures are suitable to machine parts that have a thickness of 4.5 mm, exceeded these values, it should use a binary mixture because the result becomes practically identical. The optimum thickness is 1-2 mm. The sheet thickness affects the welding process since sheets with large thicknesses have a silicon content higher than a sheet with thin thickness.

Electric Parameters
If the voltage is increased, the temperature of the weld pool increases and the chances of getting splinters is decreased. If the current is increased and the operator is sufficiently skilled to follow the weld quickly, the weld becomes impeccable because the cooling of the weld pool becomes faster. In this way, it lowers the H heat input during welding:

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Where V is the arc voltage (Volt), I is the arc current (Ampere), η is the efficiency of heat transfer between the arc and the welding bath and v the feed rate of the torch. So as it can see in the formula, increasing the speed of running, the heat input decreases. In practice (Figure 4) it can see how the welding bead with a welding speed higher has a more restricted section, a lighter color and with less inclusions.

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Figure 4 Effect of welding speed on the weld bead

If the welding speed is not enough to lower the heat input, it proceeds to lower the value of the current. Generally, to obtain a good weld, it is advisable to increase the voltage in order to avoid projections and splashing, lower the current in order to lower the heat input, the formation of silicates and burns as shown in Figure 5.

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Figure 5 MIG welding: 1 Value of high current, 2 low current value

From Figure 6 it can be seen that using the pulsed arc mode and all electrical parameters optimum, the welding bead becomes clearer homogeneous and well relaxed.

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Figure 6 optimal Weld Bead

To avoid the formation of the spatter are used very often sprays that cover the base metal but worsens the quality of the weld because we have a substance that causes a combustion reaction during welding, especially when using the pulsed arc mode. Using electrical parameters incorrect and not in accordance with a gas flow is obtained by a cordon dark, with many inclusions and splashes and little flattened (Figure 7).

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Figure 7 welding bead obtained with incorrect parameters

Comparing MIG standard welding with pulsed arc, the welding beard assumes a dark color, with many projections, oxide inclusions, and gets higher.

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Figure 8 Continuous wire welding: 1 Standard MIG, 2 MIG pulsed arc.

These problems are accentuated as the distance of the edges is reduced. For larger widths, the flaps are welded in spray arc mode (MIG high current) taking care that the feed rate is high so as to give the correct heat input and reduce the projections since this mode reaches a value of current of 220-230 amperes. Normally it is suitable for automatic systems to avoid the problems just described.

Angle torch
A welding performed in a non-optimal way may generate a cordon “glued”. In this case the welding beard partially melts the base metal and hovers above the flaps welding. The result is that if the part is subjected to a mechanical action, the welding beard can be destroyed causing the disjunction of the two edges. A weld bead between two plates placed perpendicularly optimum has, in section, a triangular shape with angles of 45 °.
In MIG / MAG / TIG, the angle of the torch with respect to the direction of travel has a significant influence on the shape of the welding bath and the penetration level that can be achieved (Figure 9).

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Figure 9 Different Angles of the Torch

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Torch tilted

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Torch perpendicular

Figure 10 Angle Torch: 1 Torch tilted, 2 Torch perpendicular to the workpiece

In Figure 10 it can be seen, at constant welding speed, tilting the torch, the welding bead becomes even more homogeneous and clear while, holding the torch perpendicular to the workpiece, the welding bead becomes darker with a wider heat affected zone. There are two ways of positioning of the torch (Figure 9). When the torch is positioned in the direction opposite to welding direction (technical pull), the energy of the arc is concentrated on the melting bath, and produces greater penetration, more stable arc and less spatter, although the visibility of the bath is made difficult; the maximum depth of the bath is usually achieved for values around 25° (flat welding).
In the case where the torch is oriented in the direction of advance (technique to push), it gets a melting bath more concave, with lower penetration and dilution; in this case the bath is well visible and more cold, therefore more controllable.

In manual applications it is therefore preferred technique to push, with angles between 5° and 15°, to the lower risk of operational defects; However, this cannot be used with cored wires that produce slag, due to the presence of the latter which, interposing between the electrode bath welds, would cause the extinguishing of the arc. The torch angle also allows to burn the residual impurity which owns the base metal before the molten bath reaches the affected area. In this way, the residues are not incorporated into the weld itself. The clear appearance of the weldind bead in the mode MIG / MAG only exists when, for a given correct parameters, the welding is at an internal angle. In this situation the protective gas is not dispersed but remains concentrated in the interested area by welding process.

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Photo credits:
Sparkalicious by Derek Gavey

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