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Arc welding involves the use of a power supply to create an electric arc between an electrode and the material intended to be welded to melt the metals at the welding point. The power supply can use either direct (DC) or alternating (AC) current. The type of current used can sometimes cause problems in welding.
What Do AC and DC Stand For And What They Mean In Welding
Any electrical current is either a direct current (DC) or an alternating current (AC). AC and DC are terms that refer to the polarity of the electrical current that the welding machine creates and runs through the electrode.
Direct current (DC) is electricity flowing in a constant direction and/or possessing a voltage with constant polarity, either positive or negative. Typical DC currents include the current in batteries and often used in low voltage devices such as cell phones and remote controls.
In welding, electrode negative (DC) or straight current provides faster deposition rates because there is quicker melt-off of the electrode. Electrode positive (DC) results in deeper penetration.
Alternating current (AC) is electricity that switches direction back and forth so the voltage also periodically reverses because the current changes direction. Typical AC currents are what you would expect to see from your electrical outlets in your home and often used in higher voltage devices such as household appliances. AC current changes its polarity 120 times per second with a 60-hertz current. Reversed polarity (AC) results in deeper penetration.
In Alternate Current (AC) welding, since the current and the magnetic field of the arc reverse direction many times a second, there is no net deflection of the arc.
Applications of AC Welding
AC welding can be used to weld magnetic metals. This cannot be done with DC welding. AC welding is ideal for the following types of welds:
- Downhand heavy plate
- Fast fill
- Aluminum TIG welding with high frequency
The primary advantage of using AC welding is that it allows the weld operator to weld on magnetized materials. In AC current, the current changes direction and is not affected by magnetism. The arc remains stable and is easier to control.
AC welding is also the preferred method for:
- TIG welding aluminum, because the current supports welding at a higher temperature.
- Making repairs on machinery because the machinery usually has a magnetized field and is older and may have rusty areas where there is concern about the higher heat penetration that can occur with DC welding.
- Seam welding in shipbuilding because the current settings can often be higher than those used in DC welding and a deeper penetration of plate metals can be obtained.
The biggest drawback to using AC welding is the quality of the weld. It is not as smooth as DC welding because of the continuous change in directional flow and there is likely to be more spatter.
Applications of DC Welding
Welding with DC is best used for:
- Hard facing
- Overhead or vertical welding
- Single carbon brazing
- Build-up of heavy deposits
- Stainless steel TIG welding
- Cutting tap
As a rule of thumb, DC is preferred for welding because:
- It produces a smoother weld and there is less spatter because of the constant linear direction of the current.
- It maintains a constant and stable arc and is thus is easier to handle and more reliable than AC current.
- Machinery that uses DC current is generally cheaper and easier to use.
- It welds thinner metals better than AC current.
The primary drawbacks to using of DC welding are:
- There’s greater potential for arc blow.
- DC currents are not supplied by electrical grids, so they need an internal transformer to change the current from AC to C for use. That makes this method more expensive to use.
- It’s not good for use in welding aluminum because it cannot produce the high-intensity heat necessary.
Arc Blow Issues
In welding, there can be a lot of reasons for a weld failure. One of the most frustrating is magnetic arc blow. This occurs when a magnetic field is present in the area where the weld is being prepared to join two metal components.
The quality of the weld can be affected by magnetic disturbance due to the interaction of the magnetic field of the welding current with the residual magnetic field present in the metal. The disturbance causes the welding arc to deflect away from the weld path and behave erratically.
The welding operator must fight arc blow by trying to keep the electrical current stable and the work usually suffers in quality. Arc blow occurs only in direct current (DC) welding. An effective fix for the problem is to switch from DC current to AC current.
Overall Strength of Welds
Overall the strength of the weld can be determined by many factors, such as:
- Proper electrode, welding apparatus and procedures;
- Properties of the materials being welded-magnetic vs. non-magnetic;
- Proper edge preparation-the cleaner the edge, the better the weld;
- Current settings – DC vs. AC;
- Speed of travel – the angle of the electrode needs to be maintained throughout the length of the joint as it is being welded.
A strong weld can be achieved in both AC welding and DC welding so long as you weld with the current and polarity appropriate for the material being welded. One doesn’t necessarily, always and everywhere, produce a better weld than the other. It’s a matter of choosing the right one for your job.
In order to achieve proper penetration, uniform beading and good welding results the correct current and polarity must be used when welding with any given metallic electrode. Using incorrect current and polarity results in poor penetration, irregular bead shapes, excessive splatter, overheating, lack of arc control and quick burning of the electrode. Be sure to research your materials and conditions so you can choose AC or DC for the task at hand!