Arc blow is the, usually unwanted, deflection of the arc during arc welding.
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There are two types of arc blow commonly known in the electric welding industry: magnetic and thermal.
Magnetic arc blow
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Eddy currents in an arc blow cross-section.Magnetic arc blow or "arc wander" is the deflection of welding filler material within an electric arc deposit by a buildup of magnetic force surrounding the weld pool. Magnetic arc blow can occur because of:
Arc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative).
Magnetic arc blow is popularly attributed to a change in the direction of current as it flows into and through the workpiece. Magnetic arc blow is known to begin at field densities as low as 10 gauss and becomes severe at densities of, equal to or greater than, 40 gauss; it is directional and can be classified as forward or backward moving along the joint, but can occasionally occur to the sides depending on the orientation of the poles to the workpiece.
Magnetic arc blow is more common in DC welding than in AC welding.
Thermal arc blow
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Thermal arc blow is widely attributed to variations in resistance within the base metal created by the weld pool as it is moved across the workpiece. Thermal arc blow can occur because of:
Thermal arc blow is not as severe as magnetic arc blow, but can still leave undesirable defects in the weld deposit.
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Mention AC/DC to most people, and they will immediately think of a popular Australian hard-rock band. On the other hand, welding professionals recognize it as polarity, referring to the negative and positive poles formed in the electrical circuit as soon as you turn on a welding machine.
What makes polarity so important in welding? Choosing the correct polarity can ensure the strength and quality of the weld, while the wrong one can cause poor penetration, excessive spatter, and less control of the welding arc.
Welding professionals often talk about the pros and cons of AC vs. DC welding. The primary difference is polarity: AC (alternating current) welding alternates rapidly between DC negative and DC positive, while DC (direct current) uses a straight polarity current to supply electricity to the electrode. Which of these you choose will depend on specific factors, so here's what you should know:
AC welding means you are using an alternating current in the welding process. The alternating current reverses its direction many times per second. For instance, a 60-hertz current will alter its polarity 120 times per second. Since the magnetic field and current reverse direction, it allows for a steadier arc on magnetic parts.
DC welding employs a direct current with constant polarity flow in one direction. The current can be positive or negative, with positive polarity flowing from the welder to the workpiece to the electrode and back to the welder. The current flows from the welder to the electrode to the workpiece and then back to the welder with negative polarity. Electrode positive welding offers deeper penetration, while electrode negative provides faster deposition rates.
Even though DC welding is preferred for most welding applications, AC welding works well in some specific applications. TIG welding aluminum, welding magnetized metals, downhand welding on heavy plates, with fast-fill electrodes, and welding where the only electrical outlets are 110 volts all lend themselves to AC welding. Unlike DC welding, arc blow is far less noticeable when welding with AC.
On the other hand, direct current maintains a constant and stable arc and is more reliable than AC. It typically produces a smoother weld with less spatter because of the continuous direction of the current. DC also welds thinner materials better than AC, working best with stick welding, stainless steel TIG welding, and vertical or overhead welding.
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As mentioned earlier, the DC current can be positive or negative. But how does that affect the welding process?
Sometimes called straight polarity, electrode-negative polarity is preferred for most welding jobs. It produces a hotter arc and faster electrode melting, allowing operators to be more productive and lay down a bead faster. Straight polarity DC will almost always be a welder's first choice unless a specific need for something else comes up.
Electrode-positive polarity, also referred to as reverse polarity, generates deeper penetration. Even though it is slower than straight polarity, it is usually the choice for welding thick material. The advantage of reverse polarity becomes evident when working with metal more than half an inch thick.
AC Welding
DC Welding
Polarity
Alternating polarity
A single polarity that could be positive or negative
Applications
Magnetized metals, aluminum, heavy plates
Stainless steel, overhead, vertical, and thinner plate welding (Most applications)
Weld strength
Weaker with more spatter
Stronger and smoother
Arc blow
Fixes arc blow issues
Can cause arc blow
Arc stability
Arc can be hard to work with
Stable arc
Welding machines
Less expensive, portable
More expensive and heavier
The answer is: it depends on your situation. While DC welding will probably be your first choice, each type of welding has some distinct advantages in specific circumstances. If you're starting your welding career, DC welding is easier to work with and results in cleaner welds. However, DC welding machines are typically more expensive than AC, prompting beginners to lean toward the more affordable welders.
Since each has its strengths and weaknesses, you might be better served with a welding machine that can do both!
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