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.