Welding is a hands-on job, but welders still have to have a fair amount of technical knowledge. One of the terms you will often hear in classes and in shops is "welding current". You will see the welding machines and electrodes marked as AC or DC, which describe the polarity of the welding current. Why are current and polarity important in welding? Let's take a closer look.
The circuit created when you turn on the welding machines has a positive and a negative pole - a characteristic known as polarity. Polarity is very important in welding because choosing the correct polarity affects the strength and quality of the weld. Using the wrong polarity can lead to a lot of spatter, poor depth of melt and an uncontrolled welding arc.
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MAG Welding Machine
The terms "positive" and "reverse" polarity are commonly used for "electrode negative" and "electrode positive" polarities. Welding currents with positive (reverse) electrode polarity result in a deeper depth of melt, while welding currents with negative (straight) electrode polarity have a faster melting rate and a faster deposition rate. The different shielding gases may also further affect the weld.
AC stands for 'alternating current' and DC stands for 'direct current'. The former alternates its flow direction, while the latter flows in one direction only. Thus, plasma ARC welding machines and electrodes marked with "DC" have a constant polarity, whereas welding machines and electrodes marked with "AC" change polarity 120 times per second at a current of 60 hertz.
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Plasma ARC Welding Machine
In shielded metal arc welding (SMAW), DC is widely used for its many advantages. DC welding produces a smoother and more stable arc, you can start the arc more easily, there is less arc break and spatter, and vertical up and back welding is less complicated. However, AC is probably the preferred choice for beginners such as welding training students as it is often used with low cost entry level welders. AC is also common in boat building welding or in any conditions where the arc may blow from one side to the other.
Furthermore, the difference in welding is not only in the welding current, but also in whether the current has electrode positive or electrode negative polarity. dc positive allows high levels of penetration into the steel, while dc negative means lower penetration but higher deposition rates (e.g. for thin metal plates). As AC is half positive and half negative, its welding performance is somewhere between DC positive and DC negative. Some welders will choose AC if they want to avoid deep penetration, for example in repair work on rusty metal.
It is important to know the welding current and polarity to get the job done correctly. When choosing AC or DC current and electrode positive or electrode negative, you need to consider the type of metal, welding conditions, depth of fusion level and deposition rate. Understanding how these factors affect your welding will make your job easier.
