The gas-shielded arc-shielded arc welding method uses the short-circuit arc-ignition method for arc-ignition. Most of the tungsten-arc argon arc welding adopts the non-contact arc-ignition method, but the short-circuit arc-ignition method is also adopted. The principle of the short-circuit arc-ignition method is described below by taking the gas-shielded gas shielded welding as an example.
When the gas is arc-shielded, the arc is first fed into the arc and gradually approached to the base metal, as shown in Figure 1-2. Once in contact with the base metal, the power supply will provide a large short-circuit current, and the arc is triggered by the wire breaking near point A. If it breaks at point B, the arcing fails. Therefore, the explosion at point A is a necessary condition for successful arc ignition.
The explosion at point A or at point B is mainly due to the magnitude of the resistance heat generated by the wire near the point, that is, the magnitude of the contact resistance. The contact resistance of the two points A and B is shown in Figure 1-3. Point B is the contact point between the welding wire and the contact tip, and the contact resistance R B changes little with time and remains substantially unchanged. At point A, point A is the point of contact between the wire end and the base metal. R A is the contact resistance, and R A is infinite at the moment when the welding wire is in contact with the base material; as the short-circuit current increases, the point A softens rapidly, and the contact area increases, so that R A sharply decreases. It can be seen that in order to ensure the success of the arc ignition, it is desirable that the short-circuit current growth rate di S /dt is as large as possible, and the slower the R A decay rate, the better. That is, when R A is large, the short-circuit current i S is increased to a higher value, so that a burst occurs at point A.
The method to improve the success rate of arc ignition is as follows:
1) Improve the short-circuit current growth rate di S /dt, mainly to improve the working state of the power supply. For example, current transformers often use current inductance to adjust the dynamic characteristics of the welder in order to reduce spatter and improve forming, but reduce di S /dt and reduce arc ignition power. For this reason, the bypass inductor is often used to short-circuit the DC inductor during the arc-triggering, and the inductor is connected after the arc-triggering is successful. Here, when the inverter welder appears, the electronic reactor is fully utilized to adjust the dynamic characteristics of the power supply, and a small DC inductor is selected, so that it is not necessary to adopt the above method, and a very reliable arc-starting process can be obtained.
2) Reduce the decay speed of the contact resistance R A . When the arc is triggered, the wire feed speed is slower, so as to reduce the pressure increase speed of the welding wire and the base metal, and the R A decay speed is slowed down. It is also unfavorable that the wire feeding speed is too slow, and it is usually 1.5 to 3 m/min. After the arc is successfully turned on, it should be converted to the normal wire feeding speed immediately.
3) Use the shear effect to induce arcing. Under normal circumstances, the metal droplets remaining in the end of the wire are cut by pliers during welding to facilitate arc ignition. But this is cumbersome, so many gas shielded welding equipment now have increased ball removal, which is the shearing effect. At the end of the welding, the arc voltage and the wire feed speed are appropriately lowered to realize the automatic ball removing function.
4) When the contact tip wears a large amount, the contact resistance R B at point B will be increased, which is not conducive to arc ignition. To this end, the contact tip should be replaced in time.
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