The shielding gas of carbon dioxide gas shielded arc welding (CO2 welding for short) is carbon dioxide (sometimes a mixed gas of CO2+Ar is used). Due to the special influence of the thermophysical properties of carbon dioxide gas, when using conventional welding power sources, the molten metal at the end of the welding wire cannot form a balanced axial free transition, and short circuit and droplet necking and blasting are usually required. Therefore, unlike MIG welding, free There is more splash than transition.
However, if a high-quality welding machine is used and the parameters are appropriately selected, a very stable welding process can be achieved and spatter can be reduced to a minimum. Due to the low price of the shielding gas used, the welding seam is well formed when short-circuit transition is used, and the use of welding wire containing deoxidizer can obtain high-quality welding joints without internal defects. Therefore, this welding method has become one of the most important welding methods for ferrous metal materials.
welding skills
1. Flat welding is divided into two types: right welding method and left welding method (the welding gun moves from right to left) according to the movement direction of the welding gun. In the right-hand welding method, the molten pool is well protected, the heat is fully utilized, and the weld shape is fuller; however, in the right-hand welding method, it is difficult to observe the welding direction and it is easy to bias the welding. Generally, the left welding method is commonly used.
2. In the left-hand welding method, the arc has a preheating effect on the base metal, the melting width increases, the weld is formed flatter, and the welding direction can be seen clearly, and it is not easy to deflect the welding. The inclination angle of the welding gun is about 10 to 20 degrees.
3. The welding wire, welding port and the surrounding 10~20mm range must be kept clean, and foreign matter such as rust, oil, water and paint that may affect the welding quality must not be allowed.
4. Arc ignition. Generally, direct short-circuit arc ignition is used. If the welding wire is in too close contact with the weldment or the contact is poor, it will cause the welding wire to explode in segments. Therefore, before arc striking, the distance between the welding wire end and the weldment is generally kept 2 to 3 mm, and attention should be paid to cutting off the spherical welding wire at the wire end. When igniting the arc, choose a good position and use the backward arc ignition method.
5. Close the arc. When closing the arc, the arc crater must be filled. The welding gun should stop for a moment at the closing arc and continue to provide gas protection. Then slowly lift the welding handle and make the first layer of weld overlap 20~50mm thick at the joint; it should not be lifted immediately. Welding gun, otherwise arc craters will easily form pores.
6. Welding current and arc voltage are key process parameters. In order to make the weld well formed, reduce spatter, and reduce welding defects, the arc voltage and welding current must match each other. During low current welding, if the arc voltage is too high, metal spatter will increase; if the arc voltage is too low, the welding wire will easily extend into the molten pool, making the arc unstable. During high-current welding, if the arc voltage is too high, metal spatter will increase and pores will easily occur; if the voltage is too low, the arc will be too short, resulting in poor weld formation.
7. Welding current is the main factor in determining penetration depth. As the current increases, the penetration depth and deposition speed increase, and the penetration width also increases slightly. The faster the wire feeding speed, the greater the welding current, which is basically a proportional relationship. When the welding current is too large, the molten pool will be too large and the weld shape will deteriorate.
8. As the arc voltage increases, the melting width increases significantly, while the weld reinforcement and penetration depth decrease slightly, and the mechanical properties of the weld are reduced. If the arc voltage is too high, weld pores and spatter will occur; if the arc voltage is too low, the welding wire will be inserted into the molten pool, causing the arc to be unstable and affecting the weld formation.
9. The increase in the extension length of the welding wire will reduce the welding current, reduce the penetration depth, and increase the width of the weld. When the extension length of the welding wire is too long, it is easy to form incomplete penetration and fusion, increase spatter, weaken protection, and form pores; when the extension length of the welding wire is too short, it will hinder the observation of the molten pool, and the nozzle is easily blocked by spatter, affecting protection. Form pores. In order to reduce spatter, try to keep the extension length of the welding wire as small as possible, but as the welding current increases, the extension length should be appropriately increased.
10. Gas flow directly affects the gas protection effect. If the gas flow rate is too small, defects such as pores will easily occur in the weld. When the gas flow rate is too large, not only is gas wasted, but the welding seam also forms oxide scale due to enhanced oxidation, reducing the quality of the welding seam.
11. If the welding speed is too fast, the gas protection effect will be destroyed, the weld will be poorly formed, and the weld will cool too quickly, resulting in reduced weld plasticity and toughness. If the welding speed is too slow, the weld seam will burn through and form a coarse weld seam structure.
12. It is forbidden for the electric fan to face the welding port, which will affect the protective gas.
13. When welding, the handle should be steady and the welding speed should be even. The main reason for the occurrence of welding nodules is uneven operation of the welding gun, which causes the temperature of the molten pool to be too high and the liquid metal to condense and fall slowly, thus forming metal nodules on the surface of the weld.
Advantages of carbon dioxide gas shielded welding
1. Low welding cost. Its cost is only 40~50% of that of submerged arc welding and manual arc welding.
2. High production efficiency. Its productivity is 1 to 4 times that of manual arc welding.
3. Easy to operate. Open arc, no limit to the thickness of the workpiece, can be welded in all positions and can be welded downward.
4. The weld has high crack resistance. Welds are low in hydrogen and contain less nitrogen.
5. The deformation after welding is small. The angular deformation is five thousandths and the unevenness is only three thousandths.
6. Welding spatter is small. When using ultra-low carbon alloy wire or flux-cored wire, or adding Ar to CO2, welding spatter can be reduced.