Concept and classification of gas metal arc welding
An arc welding method that uses a melting electrode, an external gas as the arc medium, and protects the metal droplets, welding the molten pool and high-temperature metals in the welding zone, called gas-shielded arc welding. Depending on the wire material and shielding gas, it can be divided into the following methods, as shown in the figure.
According to the classification of welding wire, it can be divided into solid wire welding and flux cored welding. The inert gas (Ar or He) shielded arc welding method with solid core wire is called molten electrode inert gas shielded welding, referred to as MIG welding (Metal Inert Gas Arc Welding); the argon-rich mixed gas shielded arc welding with solid core wire, referred to as MAG welding (Metal Active Gas Arc Welding). CO2 gas shielded welding with solid core wire, referred to as CO2 welding. When using flux-cored wire, arc welding that can use CO2 or CO2+Ar mixed gas as the shielding gas is called flux-cored wire gas shielded welding. It is also possible to add no shielding gas, this method is called self-shielded arc welding.
The difference between ordinary MIG/MAG welding and CO2 welding
The characteristics of CO2 welding are: low cost and high production efficiency. However, there are disadvantages of large amount of spatter and poor forming, so some welding processes use ordinary MIG/MAG welding. Ordinary MIG/MAG welding is an arc welding method protected by inert gas or argon-rich gas, while CO2 welding has strong oxidizing properties, which determines the difference and characteristics of the two. The main advantages of MIG/MAG welding compared with CO2 welding are as follows:
1) The amount of spatter is reduced by more than 50%. The welding arc is stable under the protection of argon or argon-rich gas, not only the arc is stable during droplet transfer and jet transfer, but also in the case of short-circuit transition of low-current MAG welding, the repulsion effect of the arc on the droplet is small, thus ensuring the MIG / The amount of spatter in the short-circuit transition of MAG welding is reduced by more than 50%.
2) The welding seam is uniform and beautiful. Due to the uniform, fine and stable droplet transfer in MIG/MAG welding, the welding seam is uniform and beautiful.
3) Many active metals and their alloys can be welded. The oxidizing property of the arc atmosphere is very weak or even non-oxidizing. MIG/MAG welding can not only weld carbon steel and high alloy steel, but also weld many active metals and their alloys, such as: aluminum and aluminum alloys, stainless steel and its alloys, Magnesium and magnesium alloys, etc.
4) Greatly improve the welding manufacturability, welding quality and production efficiency.
The difference between pulsed MIG/MAG welding and ordinary MIG/MAG welding
The main form of droplet transfer in ordinary MIG/MAG welding is jet transfer at high current and short-circuit transfer at low current. Therefore, small current still has the disadvantages of large amount of spatter and poor forming, especially some active metals cannot be used under low current. Welding such as aluminum and alloys, stainless steel, etc. Therefore, pulsed MIG/MAG welding has appeared. The characteristic of droplet transfer is that each current pulse transfers a droplet, which belongs to the droplet transfer in essence. Compared with ordinary MIG/MAG welding, its main features are as follows:
1) The optimal droplet transfer form for pulsed MIG/MAG welding is one droplet with one pulse. In this way, by adjusting the pulse frequency, the number of droplets transferred per unit time can be changed, that is, the melting speed of the welding wire.
2) Due to the droplet transfer of one pulse and one droplet, the diameter of the droplet is roughly equal to the diameter of the welding wire, and the arc heat of the droplet is lower, that is, the droplet temperature is low (compared with the jet transfer and the large droplet transfer). Therefore, the melting coefficient of the welding wire is improved, that is, the melting efficiency of the welding wire is improved.
3) Due to the low droplet temperature, there is less welding fumes. In this way, the burning loss of alloying elements is reduced on the one hand, and the construction environment is improved on the other hand.
Compared with ordinary MIG/MAG welding, its main advantages are as follows:
1) The welding spatter is small, or even no spatter.
2) Good arc directivity, suitable for all-position welding.
3) The weld seam is well formed, the fusion width is large, the finger-shaped penetration characteristics are weakened, and the residual height is small.
4) Small current perfectly welds active metals (such as aluminum and its alloys, etc.).
Expanded the current range of MIG/MAG welding jet transfer. During pulse welding, the welding current can achieve stable droplet transfer from near the critical current of jet transfer to a large current range of tens of amperes.
The characteristics and advantages of pulsed MIG/MAG can be seen from the above, but nothing can be perfect. Compared with ordinary MIG/MAG, its shortcomings are as follows:
1) The habitual feeling of welding production efficiency is slightly lower.
2) Higher quality requirements for welders.
3) At present, the price of welding equipment is relatively high.
The main process decision for the selection of pulse MIG/MAG welding
In view of the above comparison results, although pulse MIG/MAG welding has many advantages that other welding cannot achieve and compare, it also has the problems of high equipment price, slightly low production efficiency, and difficulty for welders to master. Therefore, the selection of pulse MIG/MAG welding is mainly determined by the welding process requirements. As far as the current domestic welding process standards are concerned, the following welding must basically use pulsed MIG/MAG welding.
1) Carbon steel. Occasions with high requirements on weld quality and appearance are mainly in the pressure vessel industry, such as boilers, chemical heat exchangers, central air-conditioning heat exchangers, and volutes for turbines in the hydropower industry.
2) Stainless steel. Use small current (below 200A is called small current here, the same below) and occasions with high requirements on weld quality and appearance, such as locomotives, pressure vessels in the chemical industry, etc.
3) Aluminum and its alloys. Use small current (below 200A is referred to as small current here, the same below) and occasions with high requirements on weld quality and appearance, such as motor trains, high-voltage switches, air separation and other industries. Especially the motor car, including the CSR Group Sifang Vehicle, Tangshan Vehicle Factory and Changke, and other small manufacturers who process outsourcing for them. According to industry news, by 2015, all provincial capitals and cities with a population of more than 500,000 will be able to use electric vehicles, which shows that the demand for electric vehicles is huge, and the demand for welding workload and welding equipment is huge.
4) Copper and its alloys. According to the current understanding, copper and its alloys are basically used pulse MIG/MAG welding (in the scope of MIG welding).
