Introduction
MIG (or MAG) is the process of electric arc welding with a consumable electrode under gas shielding, which uses a solid wire as electrode and an inert gas (MIG) or an active gas (MAG) as a gas shield. Also known as Gas Metal Arc Welding (or GMAW).
How the process works
MIG/MAG welding uses the heat of an electric arc established between a continuously fed bare electrode and the base metal to fuse the electrode tip and the surface of the base metal at the joint being welded.The protection of the arc and molten weld pool comes entirely from an externally fed gas, which can be inert, active or a mixture of these. Therefore, depending on the gas, we can have the following processes:
• MIG process (METAL INERT GAS): injection of inert gas. The gas can be:
- argon
- helium
• MAG (METAL ACTIVE GAS) process: injection of active gas or a mixture of gases that lose their inert characteristics when part of the base metal is oxidized. The gases used are:
- 100% CO2
- CO2 + 5 to 10% of O2
- argon + 15 to 30% CO2
- argon + 5 to 15% O2
- argon + 25 to 30% N2
Slag formed in the processes of coated electrode welding and submerged arc welding, are not formed in the MIG/MAG welding process, because flux is not used in these processes. However, a glassy film (which looks like glass) of silica forms from high silicon electrodes, which must be treated as slag.
The figure below shows how the MIG/MAG welding process works.
- Higher deposition rate than coated electrode welding.
- Less gas and smoke from welding.
- High versatility.
- Large application capability.
- Welds a wide range of thicknesses and materials.
In the semi-automatic process, the electrode is fed automatically through a torch (or pistol). The welder controls the tilt and distance between the torch and the part, as well as the travel speed and handling of the arc.
The MIG/MAG welding process can also be used for surface coating application.
Welding Equipment
Basic MIG/MAG welding equipment consists of the following elements: a welding gun (better known as a torch), a welding power source, a shielding gas cylinder, and a wire drive system.The following figure shows the basic equipment needed for the MIG/MAG welding process.
The torch contains a contact tube to transmit the welding current to the electrode and a gas nozzle to direct the shielding gas to the vicinity of the arc and the weld pool. The wire feeder consists of a small direct current motor and a drive wheel.
The flow of the shielding gas is regulated by the flowmeter and the pressure reducer regulator. These allow a constant supply of gas to the nozzle of the gun at a preset flow rate.
Most MIG/MAG welding applications require direct current power of reverse polarity (DC+, electrode connected to the positive pole). In this situation you have a more stable arc, stable transfer, low spatter, and good weld bead characteristics.
Direct current in direct polarity is not often used, and alternating current was not used in this process until recently. Today there is already the possibility of welding aluminum with alternating current.
Types of filler metal transfer
In welding with consumable electrodes, such as in MIG/MAG welding, the molten metal at the wire tip must be transferred to the weld pool. The main influencing factors are:- Intensity and type of current.
- Arc voltage.
- Current density.
- Nature of the electrode wire.
- Stick-out electrode extension.
- Shielding gas.
- Power source characteristics.
Globular
It occurs with a low current in relation to the gauge (diameter) of the electrode. The metal transfers from the electrode to the workpiece as globules, each larger in diameter than the electrode. The globules transfer to the puddle without much direction and the appearance of splash is quite evident.By spray transfer
Occurs at high currents. The molten filler metal transfers through the arc as fine droplets. With spray transfer the deposition rate can reach up to 10 kg/h. However, this deposition rate restricts the method to position.By short circuit transfer
The fusion starts globular and the drop increases in size until it touches the molten pool, producing a short circuit and extinguishing the arc. Under the action of certain forces, the drop is transferred to the part. This process allows welding in all positions and is a relatively low energy process, which restricts its use for greater thicknesses.
By pulsating arc welding
It maintains a low current arc as a background element and injects high current pulses over this low current. The transfer of filler metal is by the droplet jet during these pulses. This characteristic of the welding current causes the welding energy to be lower, which makes welding in the vertical position possible by the use of large diameter wires.
The downside is that it requires a specific welding machine to control the pulses. Another disadvantage is making a root, as it is believed that low current levels lead to the lack of fusion defect.
Most spray MIG/MAG welding is done in the flat position. Pulsed arc and short circuit transfer MIG/MAG welds are suitable for welding in all positions. When welding in the overhead position, small diameter electrodes are used with the short circuit transfer method. Spray transfer can be used with pulsed direct current.
The short circuit mode has been widely used for its convenience but has a disadvantage due to the low heat input it produces. This little heat can generate a lack of fusion and for that reason it is being limited by some companies.
Types and functions of consumables – gases and electrodes
The main purpose of shielding gas in MIG/MAG welding is to protect the weld from atmospheric contamination. The shielding gas also influences the type of transfer, penetration depth, and bead shape.Argon and helium are shielding gases used to weld most ferrous metals. CO2 is widely used for welding low carbon steels (formerly called "mild" steels). When selecting a shielding gas, the most important factor to keep in mind is that the denser the gas, the more effective its arc protection.
The electrodes for MIG/MAG welding are similar or identical in composition to those of other welding processes that use bare electrodes, and, for the specific case of MAG welding, they contain deoxidizing elements such as silicon and manganese in certain percentages.
Just to be clear, the deoxidizing element is the one that takes the oxygen out of the molten pool or turns it into something less harmful. If you leave the oxygen in the puddle, it gets trapped in the weld after solidification in the form of pores (or porosity).
As a rule, the electrode and base metal compositions should be as similar as possible, and specifically for the MAG process, the addition of deoxidizing elements must be taken into account (because the joint cleaning is not as careful as in the MAG process).
Behavior of the active atmosphere in the MAG process
By active atmosphere is meant the injection of active shielding gas, that is, capable of oxidizing the metal during welding. To facilitate the reasoning about the phenomena involved, let's take as an example, the injection of carbon dioxide (CO2).
FeO + C -> Fe + CO
It may happen that there is no time for carbon monoxide (CO) to leave the weld pool, which will cause pores or porosity in the weld metal.
The problem is solved by adding deoxidizing elements such as manganese. Manganese reacts with iron oxide, giving rise to manganese oxide, which, not being a gas, goes to the slag (FeO + Mn -+ MnO).
Manganese, however, must be added in an amount compatible with the FeO formed. Excess Mn will cause part of it to be incorporated into the weld, resulting in greater hardness of the weld metal and, therefore, a greater probability of cracking. In summary, therefore, the following reactions occur:
• In the active atmosphere:
CO2 > CO + ½ O2
Fe + ½ O2 > FeO
• When liquid/solid transformation:
FeO + C > Fe + CO
• With the addition of deoxidizing elements:
FeO + Mn > Fe + MnO (MnO goes to the slag)
In theory GMAW does not generate slag but in practice it can form a glassy slag (as can be seen above). Another possibility is that MnO stays in the weld as an inclusion.
It is always convenient to pay attention to the following details in welding with active atmosphere (MAG process and all others with active atmosphere):
- As the speed of solidification increases, the likelihood of pores and porosities becomes greater;
- Oxidation can cause pores and porosity. Excessive deoxidation, by increasing the mechanical tensile strength of the weld, increases its hardenability (hardening by heat treatment). The risk of cracking will be greater.
Features and uses
The MIG/MAG welding process produces high quality welds with proper welding procedures.As a flux is not used, the possibility of inclusion of slag similar to the coated electrode or submerged arc process is minimal, and on the other hand, the inclusion of a glassy slag characteristic of the process may occur if the interpass cleaning is not done properly. Hydrogen in solder is practically non-existent.
MIG/MAG welding is an all position welding process depending on the electrode and the gas or gases used. It can weld most metals and can even be used for the deposition of surface coatings.
It is capable of welding thicknesses greater than 0.5 mm with short circuit transfer. The deposition rate can reach 15 kg/h depending on the electrode, transfer mode and gas used.
Process-induced discontinuities
In MIG/MAG welding, the following discontinuities may occur:Lack of Fusion
It can happen in MIG/MAG welding with short circuit transfer. It also occurs with spray transfer or axial spraying when using low current.Lack of Penetration
Its occurrence is more likely with short-circuit transfer (due to low heat input).Slag Inclusions
The oxygen contained in the base metal itself, or that captured during welding under deficient protection conditions, forms oxides in the weld pool. Most of the time these oxides float in the weld pool, but they can become trapped under the weld metal, giving rise to slag inclusion.Splinters, bends, double laminations and Interlamellar cracks
They can surface or appear in welds with a high degree of restriction.Undercuts (Resemble a bite)
When they do, it is due to the inability of the welder.Porosity
As we have already seen, pores and porosity are caused by gas trapped in the weld in MIG/MAG welding, the following mechanism is verified: the injected shielding gas without observing certain technical requirements can displace the atmosphere that surrounds it, which contains oxygen and nitrogen.Oxygen and nitrogen from the atmosphere can dissolve in the weld pool, giving rise to pores and porosity in the weld metal.
Overlap
It can happen with short-circuit transfer.Cracks
Cracks may occur in welding with poor technique, such as the use of inappropriate filler metal. By inappropriate I mean choice or specification of consumable (engineer responsibility)Conditions for personal protection
In MIG/MAG welding, the emission of ultraviolet radiation is high. There is also the problem of metallic projections. The welder must wear conventional safety equipment such as gloves, coveralls, eye protection goggles, etc.When welding in confined areas, we cannot forget the need for forced ventilation, as well as removing from the area containers containing solvents that can decompose into toxic gases by the action of ultraviolet rays.
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