What is Welding Polarity, Its types and their characteristics?
Welding Polarity is the electrical circuit which is created when we turn on the welding machine (DC type) having a negative and a positive pole. Polarity is important factor in the selection of the welding process for different materials. Choosing the right polarity affects the appearance, strength and quality of the weld. A wrong polarity selection will result into high spatters, in-sufficient or excessive penetration, and a lack of control of your welding arc.
Types of welding polarity/ Current in SMAW, GTAW & GMAW
The three different types of current or polarity used for welding are:
1. Alternating current (AC),
2. Direct-current electrode negative (DCEN), and 3. Direct current electrode positive (DCEP).
Direct-current electrode negative (DCEN)
DCEN or also known as straight polarity works with the electrode connected to the negative, and the work is connected to the positive terminal as shown in the below picture. The electrons are leaving the electrode and traveling across the arc to the surface of the metal being welded. This results in approximately one-third of the welding heat on the electrode and two-thirds on the metal being welded. DCEN welding current produces a high electrode melting rate. DCEN is used in GTAW/ TIG welding as well as some limited applications in shielded metal arc welding. DCEP heats the electrode much more than DCEN in TIG welding; thus, DCEP is rarely used in the TIG Welding process.
Direct-current electrode positive (DCEP)
DCEP is also called Reverse Polarity. In direct-current electrode positive, the electrode is positive, and the work is negative, as shown in the above figure. The electrons are leaving the surface of the metal being welded and traveling across the arc to the electrode. This results in approximately two-thirds of the welding heat on the electrode and one-third on the metal being welded. DCEP is used in SMAW, GMAW (MIG/MAG, FCAW), and SAW welding processes. Due to high heat at the electrode end, DCEP results in a higher melt-off of the filler wire or electrode thus yield in higher weld deposit comparing to DCEN.
Alternating current (AC)
In alternating current, the electrons change direction every 1/120 of a second so that the electrode and work alternate from anode to cathode, as shown in below figure. The positive side of an electrode arc is called the anode, and the negative side is called the cathode. The rapid reversal of the current flow causes the welding heat to be evenly distributed on both the work and the electrode—that is, half on the work and half on the electrode. The even heating gives the weld bead a balance between penetration and buildup.
Effect of Welding Polarity on penetration & Heat distribution
In GMAW, if we keep the electrode negative, the arc will become unstable, and the spatter is excessive as well as less heat on the welding wire, thus low melt-off rate. Similarly, in GTAW, if we keep the tungsten electrode positive, it will lead to the melting of the tungsten electrode fast. This can cause tungsten inclusion in the weld as well as other welding defects. Thus, a correct selection of the welding polarity is very important. The effect of welding polarity and heat distribution is shown in the below figure.