We can choose AC welding machine or DC welding machine when welding. When using a DC welding machine, there are positive and negative connections. We must consider the conditions of the welding rods, construction equipment, and welding quality. Let's take a look with the editor of Chengdu welding equipment manufacturer:
(1) Welding of ordinary structural steel
For ordinary structural steel electrodes and acid electrodes, AC and DC can be used. When welding thin plates with DC welding machines, it is better to use DC reverse connection. For thick plate welding, DC forward connection can be used to obtain a larger penetration. Of course, DC reverse connection is also possible, but for reverse welding of thick plates with grooves, DC reverse connection is still better. Alkaline electrodes generally use DC reverse connection, which can reduce porosity and spatter.
(2) Fused arc welding (MIG welding)
DC arc welding is generally used in fusion arc welding, which not only stabilizes the arc, but also removes the oxide film on the surface of the weldment when welding aluminum.
(3) TIG welding
Tungsten arc welding of steel parts, nickel and its alloys, copper and its alloys, and its alloys can only be connected by direct current, because if it is reversed by direct current, the tungsten electrode is connected to the positive electrode, the temperature of the positive electrode is high, and the heat is high. Extremely fast melting, unable to make the arc burn stably for a long time, and the molten tungsten falling into the molten pool will cause tungsten to be trapped and reduce the quality of the weld.
(4) CO2 gas shielded welding
In order to maintain stable arc, good weld formation and reduce spatter, CO2 gas shielded welding generally adopts DC reverse connection. However, when surfacing and repair welding cast iron, it is necessary to increase the metal deposition rate and reduce the heating of the workpiece. DC direct connection is mostly used.
(5) Stainless steel welding
The stainless steel electrode is better connected by DC. If you do not have a DC welding machine and the quality requirements are not too high, you can use an AC welding machine with a calcium carbonate electrode.
(6) Repair welding of cast iron
DC welding is generally used for repair welding of cast iron parts. The arc is stable, the spatter is small, and the penetration depth is shallow during welding, which just meets the requirements of low dilution rate for reducing the formation of cracks.
(7) Automatic submerged arc welding
Automatic submerged arc welding can use AC or DC power source welding. According to the product welding requirements and flux type selection, if nickel-manganese low-silicon flux is used, DC power source welding must be used to ensure the stability of the arc. DC reverse connection is used to reduce air holes. Large penetration.
Compared with AC power source, AC power source and DC power source can provide stable arc and smooth droplet transfer. -Once the arc is ignited, the DC arc can continue to burn; when welding with AC power, due to the change in the direction of current and voltage, and the arc is extinguished and re-ignitioned 120 times per second, the arc cannot continue to burn steadily. In the case of low welding current, DC arc has a good wetting effect on the molten weld metal and can regulate the bead size, so it is very suitable for welding thin parts. DC power is more suitable for overhead welding and vertical welding than AC power because the DC arc is shorter.
But sometimes the arc bias of DC power supply is a prominent problem. The solution is to change to AC power supply. For AC and DC welding electrodes designed for AC power or DC power welding, most welding applications under the condition of DC power are better. In manual arc welding, the AC welding machine and some of its additional devices are inexpensive and can avoid the harmful effects of arc blowing force as much as possible. But in addition to the lower cost of equipment, the effect of welding with AC power is not as good as DC power. The arc welding power supply (CC) with steep drop characteristics is most suitable for manual arc welding. The change in voltage corresponding to the change in current indicates that as the arc length increases, the current gradually decreases. This characteristic limits the maximum arc current even if the welder controls the size of the molten pool. When the welder moves the electrode along the weldment, it is inevitable that the arc length continuously changes, and the arc welding power source with steep drop characteristics ensures the stability of the arc during these changes.