On the other hand, with AC output, the welding current alternates from positive flow to negative flow and back again.
In North America, electricity alternates at a rate of sixty times per second or 60 hertz while most other global regions produce electricity at 50 Hz.
Figure 2 is a graph of AC output, often referred to as an AC sine wave graph. Note that times per second the welding output crosses the centerline, representing zero amperage or no output. To overcome this problem, some electrodes are designed specifically to operate on AC.
They have certain elements in their coating which help keep the arc ignited as the output goes through periods of low and no output loosely represented by the red zone on the figure 2 graph.
However, the resulting arc still tends to have more fluctuation or flutter than it does on DC polarity. Figure 3 lists the various types of coatings and currents, per the AWS A5. Note also that the polarities are listed alphabetically rather than by primary and secondary recommendation. In general and in at least North America , DC is the preferred polarity with all electrodes.
However, there are a few situations where AC polarity is used. The first, most common situation is when you have no choice. This is because you are using a power source with AC output only. Since AC is half positive and half negative, its welding properties are right in the middle of DC positive and DC negative polarity. Some welders choose AC if they want to avoid deep penetration, such as in repair work on rusty metals. Understanding welding currents and polarity is important for completing a welding job correctly.
You want to consider the type of metal, welding conditions, penetration levels, and deposition rate when choosing AC or DC currents and electrode-positive or electrode-negative polarity.
Knowing how these factors impact your weld will make your job easier. Subscribe to our blog via the form to the right. Apply Now Request Info.
Info for Parents. Out of State Students. Adult Students. Military Students. No two projects are exactly alike, and polarity must be adjusted for each material depending on the desired outcome. Keep reading to learn all about polarity in welding — what it is, how it works, the different types and more. When you turn on a welding machine, an electrical circuit is formed.
The circuit has either a positive or negative pole, and this property is referred to as polarity. Polarity is critical for welders to understand because it directly impacts the quality and strength of the weld. If a welder chooses the wrong polarity, problems can arise, like ineffective penetration, spattering and loss of control of the welding arc.
Welding machines and electrodes are typically labeled AC or DC, which is what indicates the polarity of the current in the machine. AC stands for alternating current, and DC stands for direct current. DC flows in one direction, which results in constant polarity. AC flows half the time in one direction and half the time in the other, changing its polarity times per second with a hertz current.
Typically, electrode-positive reversed polarity welding results in deeper penetration. Electrode-negative straight polarity welding results in faster melt-off of the electrode, and therefore a faster deposition rate. Deposition rate refers to the amount of filler metal melted into the weld joint. This method allows you to strike the arc easier and makes for a smooth, stable arc. It also reduces spatter and the chance for outages to occur, and can help you to avoid complications when overhead or vertical welding.
On the other hand, AC welding is often used with low-cost, entry-level machinery, making it a good choice for welding training. Many welders prefer it in conditions where the arc can blow side-to-side.
There are three main types of polarity: direct current straight polarity, direct current reverse polarity and alternating current polarity. Direct current straight polarity happens when the plates are positive and the electrode is negative. This causes the electrons to go from the electrode tip to the base plates.
As a result, the electrode melts down quickly and the metal deposition rate increases for consumable electrodes only. On the other hand, base plates tend to not fuse properly due to lack of sufficient heat. Therefore various defects arise, such as insufficient fusion, lack of penetration and high reinforcement.
Weld reinforcement is a term used to describe metal in excess of that needed to fill a joint. When the electrode is positive and the plates are negative, this results in direct current reverse polarity. The electrons switch directions and go from the base plates to the electrode.
Consequently, more heat generates at the base plate as compared with DC straight polarity. This type of welding is less likely to cause inclusion defects nonmetallic particles trapped in the weld metal or at the weld interface due to its arc cleaning action.
It makes for faster welding and performs better for welding thin pieces of material. The potential downside to this type of welding is that it has a shorter electrode life. While it works great for thinner materials, this method may be ineffective for joining thick plates with higher melting points.
If an AC current is supplied by the power source, reverse and straight polarity will take place one after the other. In half the cycle, the base plates will be positive and the electrode will be negative. In the other half, the electrode will be positive and the base plates will be negative. Depending on frequency of supply, this cycle repeats 50 to 60 times per second. Some power sources also provide provisions, which can alter frequency.
AC polarity has attributes of straight and reverse polarity, since both are occurring in the same cycle. It is effective to use with most electrode types and is suitable for many different plate thicknesses, making it a great all-around choice.
For example, magnesium and aluminum work better with reverse polarity, as their melting point is low. For stainless steel or titanium, alternating current polarity might be a better choice, since it will provide the benefits of straight and reverse polarity while keeping the heat-affected zone from becoming too large.
As welders learn to work with different materials, they will become more familiar with the types of polarity that should be used. This is all part of learning the craft! Are you interested in learning more about polarity and how to work with different types of welding equipment?
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