Welders must have a certain amount of technical knowledge to do their job correctly. One of the technical terms heard in any welding shop or when taking a welding class is polarity. Polarity is one of the factors that determine the quality and strength of a weld. Since no two welding projects are identical, welders must adjust for each material depending on the desired results.
Another term every welder comes across is welding current. Many welding machines are labeled either AC or DC, describing the polarity of the machine’s current. Here’s what you should know:
AC and DC welding
AC stands for alternating current and DC for direct current. While DC flows in one direction, resulting in constant polarity, AC flows in one direction for half the time and reverses for the other half.
What is polarity in welding?
As you turn on a welding machine, it forms an electrical circuit, having either a positive or negative pole. This property is called polarity. Because the correct polarity results in strong, high-quality welds, choosing the right one is critical.
Welding with the wrong polarity can cause multiple problems, including ineffective penetration, excessive spatter, and loss of control of the welding arc.
What are the different types of polarity in welding?
The three primary types of polarity in welding are direct current straight polarity, direct current reverse polarity, and alternating current polarity.
Direct current straight polarity occurs when the plates are positive and the electrode is negative, causing the electrons to go from the electrode tip to the base plates. In most cases, about two-thirds of the arc heat is generated at the electrode, while the other one-third is at the base plate.
Because of this, the electrode melts down quickly, and the metal deposition rate increases.
On the downside, plates tend to not fuse properly because of the lower proportion of heat. Insufficient fusion, lack of proper penetration, and high reinforcement are some of the quite common defects.
Direct current reverse polarity happens when the electrode is positive, and the plates are negative. The electrons reverse direction and travel from the base plate to the electrode, generating more heat at the plate when compared to DC straight polarity. DC reverse polarity welding typically has fewer inclusion defects and provides a faster welding process. It works well on thin materials and metals with a low melting point, such as copper.
Potential problems with DC reverse polarity include a shorter electrode life and a higher level of reinforcement required if the speed is not set correctly. While this method is excellent on thinner materials, it often is ineffective for joining thick plates with higher melting points.
If the power source supplies alternating current polarity, reverse and straight polarity will alternate with the base plate being positive and the electrode being negative half the time. In contrast, the electrode will be positive and the base plate negative the other half.
AC polarity has attributes of straight and reverse polarity, and both occur during the same cycle. It is effective with most electrode types and different plate thicknesses, making it a popular all-around choice.
Selecting from the various types of polarity in welding
When deciding on polarity, you will need to consider a few factors, including the material’s melting point and its thickness. As an example, aluminum and magnesium work best with reverse polarity because of their low melting point. On the other hand, stainless steel and titanium respond better to alternating current polarity. It provides the advantages of straight and reverse polarity and keeps the heat-affected zone from expanding too much.
Every welder will work with various materials, making it essential to become familiar with the types of polarity to use under different circumstances.
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