The concept and classification of gas metal arc welding (GMAW)
Based on the type of welding wire, welding can be classified into solid wire welding and flux-cored wire welding. Arc welding using solid wire with inert gas (Ar or He) shielding is called Metal Inert Gas Arc Welding, or MIG welding; arc welding using solid wire with argon-rich mixed gas shielding is called Metal Active Gas Arc Welding, or MAG welding. Arc welding using solid wire with CO2 gas shielding is called CO2 welding. When using flux-cored wire, arc welding with CO2 or a CO2+Ar mixed gas as the shielding gas is called flux-cored arc welding. It is also possible to weld without a shielding gas; this method is called self-shielded arc welding.
The difference between standard MIG/MAG welding and CO2 welding.
The characteristics of CO2 welding are its low cost and high production efficiency. However, it suffers from drawbacks such as excessive spatter and poor weld formation. Therefore, some welding processes utilize conventional MIG/MAG welding. Conventional MIG/MAG welding is an arc welding method using inert gas or argon-rich gas shielding, while CO2 welding has strong oxidizing properties, which determines the differences and characteristics between the two. Compared to CO2 welding, the main advantages of MIG/MAG welding are as follows:
1) Spatter is reduced by more than 50%. Under argon or argon-rich gas shielding, the welding arc is stable. Not only is the arc stable during droplet transfer and spray transfer, but also in the short-circuit transfer of low-current MAG welding, the arc's repulsive force on the molten droplets is smaller, thus ensuring a reduction in spatter by more than 50% during short-circuit transfer in MIG/MAG welding.
2) The weld bead is uniform and aesthetically pleasing. Due to the uniform, fine, and stable droplet transfer in MIG/MAG welding, the weld bead is uniform and aesthetically pleasing.
3) It can weld many reactive metals and their alloys. The arc atmosphere has very weak or even no oxidizing properties. MIG/MAG welding can weld not only carbon steel and high-alloy steel, but also many reactive metals and their alloys, such as aluminum and aluminum alloys, stainless steel and its alloys, magnesium and magnesium alloys, etc.
4) It greatly improves welding processability, welding quality, and production efficiency.
Differences Between Pulsed MIG/MAG Welding and Conventional MIG/MAG Welding
The main droplet transfer modes in conventional MIG/MAG welding are spray transfer at high currents and short-circuit transfer at low currents. Therefore, low-current welding still suffers from drawbacks such as high spatter and poor weld formation, especially for some reactive metals that cannot be welded at low currents, such as aluminum and its alloys, and stainless steel. This led to the development of pulsed MIG/MAG welding, whose droplet transfer characteristic is that one droplet is transferred with each current pulse; in essence, it is a spray droplet transfer.
Compared with conventional MIG/MAG welding, its main characteristics are as follows:
1) The optimal droplet transfer mode in pulsed MIG/MAG welding is one droplet per pulse. By adjusting the pulse frequency, the number of droplets transferred per unit time, and thus the welding wire melting speed, can be changed.
2) Due to the one-pulse-one-droplet spray transfer, the droplet diameter is approximately equal to the welding wire diameter, resulting in lower droplet arc heat, i.e., lower droplet temperature (compared to spray transfer and large droplet transfer). This improves the melting coefficient of the welding wire, thus improving the melting efficiency of the welding wire.
3) Due to the low droplet temperature, there is less welding fume. This reduces the burning loss of alloying elements and improves the working environment.
Compared with conventional MIG/MAG welding, its main advantages are as follows:
1) Low welding spatter, or even no spatter.
2) Good arc directivity, suitable for all-position welding.
3) Good weld formation, larger weld width, reduced finger-like penetration characteristics, and small weld reinforcement.
4) Perfect welding of reactive metals (such as aluminum and its alloys) at low currents. It expands the usable current range of MIG/MAG welding spray transfer. In pulsed welding, stable spray droplet transfer can be achieved in a wide current range from near the critical current of spray transfer to relatively high currents of tens of amperes.
From the above, the characteristics and advantages of pulsed MIG/MAG welding are clear, but nothing is perfect.
Compared with conventional MIG/MAG welding, its disadvantages are as follows:
1) The welding production efficiency is habitually perceived as slightly lower.
2) It requires higher skill levels from welders.
3) Currently, the welding equipment is more expensive. The selection of pulsed MIG/MAG welding is primarily determined by the welding process requirements.
Based on the above comparison, although pulsed MIG/MAG welding has many advantages that other welding methods cannot achieve or match, it also has drawbacks such as high equipment cost, slightly lower production efficiency, and difficulty for welders to master. Therefore, the selection of pulsed MIG/MAG welding is mainly determined by the welding process requirements.
According to current domestic welding process standards, the following welding applications basically require the use of pulsed MIG/MAG welding:
1) Carbon steel. Applications requiring high weld quality and appearance, mainly in the pressure vessel industry, such as boilers, chemical heat exchangers, central air conditioning heat exchangers, and turbine casings for hydropower turbines.
2) Stainless steel. Applications using low current (below 200A, referred to as low current below) and requiring high weld quality and appearance, such as locomotives and pressure vessels in the chemical industry.
3) Aluminum and its alloys. Applications using low current (below 200A, referred to as low current below) and requiring high weld quality and appearance, such as high-speed trains, high-voltage switches, and air separation equipment. Especially high-speed trains, including CSR Sifang Rolling Stock, Tangshan Rolling Stock Plant, and Changchun Railway Vehicles, as well as the small manufacturers that provide outsourced processing for them. According to industry sources, by 2015, all provincial capitals and cities with a population of over 500,000 in China will have high-speed rail connections, indicating the huge demand for high-speed trains and the corresponding demand for welding work and welding equipment.
4) Copper and its alloys. Based on current understanding, copper and its alloys basically use pulsed MIG/MAG welding (within the scope of gas metal arc welding).
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