Gas-electric welding of metals
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The essence of the method
The gas welding process (MIG) uses an electric arc burning between the consumable wire electrode and the workpiece. The process can be implemented as manual, semi-automatic or automatic, and the flexibility offered by the various process options is beneficial in many applications.
Gas-electric welding provides a significant increase in the deposition rate of the weld metal compared to conventional welding, and when used as a semi-automatic process, it usually does not require a high qualification of the welder. However, the equipment for such welding is more complex, less portable and requires more regular maintenance.
Gas-electric welding is the most common process for welding corrosion-resistant alloys and for welding thick sections.
The mechanism of gas welding
The mechanism of the welding process for this type of welding, by which molten metal at the end of the wire electrode is transferred to the workpiece, has a significant effect on the characteristics of the welded seam. There are three possible ways of transferring the metal: directly, by globular transport and jet. In addition, there is a variation in the spraying regime, called impulse spraying. The electric polarity for gas-electric welding of alloys is the reverse.
Since different power sources vary widely in design, operation and control systems, the parameters of gas-electric welding should be considered as an evaluation range to achieve the proper process characteristics using special welding equipment.
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The sequence of the
The speed of the gas-electric welding machine is usually from 150 to 250 mm / min. Excitation of the arc occurs in the lowest ranges of current and voltage, which leads to a low input power of the weld. It is usually used with a wire of smaller diameter and creates a relatively small and easily controlled bath with a melt, which is well suited for joining thin sections. However, a low heat balance makes the short-circuited transmission susceptible to incomplete joint defects, especially when welding thick sections or in multi-pass welds.
Globular transport occurs at higher current and voltage levels than the short-circuited, and is characterized by the presence of large, irregular drops of molten metal. The principle of globular transfer can be used for welding Ni / Co alloys, but is rarely used because it creates an inconsistent penetration and uneven contour of the weld, which contributes to the formation of defects.
Transmission by spraying occurs at the highest levels of current and voltage and is characterized by a directed flow of small metal droplets. This is a high heat transfer process with relatively high deposition rates, which is most effective for welding thick sections of material. However, it is generally useful only in a flat position, and its high heat input promotes the cracking of the metal and the formation of secondary phases in the microstructure that can degrade performance. The pulser is a completely controlled version of the spraying process, in which the welding current alternates between a high peak current when the jet is jetted and a lower background current. This leads to a stable process with an average welding current.
The optimum mixture of shielding gas depends on many factors, including the design / geometry of the weld, the welding position and the desired penetration profile. In most cases, a mixture of 75% argon and 25% helium is proposed; Good results are also obtained with a helium content of 15 to 30%.
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