Arc blow - Wikipedia
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:
- Workpiece connection
- Joint design
- Poor fit-up
- Improper settings
- Atmospheric conditions
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:
- Improper surface preparation
- Improper travel speed
Thermal arc blow is not as severe as magnetic arc blow, but can still leave undesirable defects in the weld deposit.
References
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What is the Difference Between AC and DC Welding? - TWI
What is DC Welding?
Direct current is an electric current that has a constant polarity flow in a single direction. This current can be positive or negative. With DC welding, since the magnetic field and current of the arc are constant, stable arcs are produced.
Advantages
The advantages of DC welding are:
- A smoother welding output than with AC
- A more stable arc
- Less spatter
- DC negative offers faster deposition rates when welding thin sheet metals
- DC positive provides greater penetration into the weld metal
Disadvantages
The disadvantages of DC welding are:
- DC welding is unable to fix arc blow problems
- Equipment is more expensive as DC currents require an internal transformer to switch the current
Applications
DC welding is ideal for joining thinner metals as well as being used in most stick welding applications, including TIG welding of - steels. This form of welding is also good for overhead and vertical applications.
What is AC Welding?
An alternating current is an electric current that reverses its direction many times per second. A 60-hertz current will change its polarity 120 times per second. With AC welding, because the magnetic field and current rapidly reverse -direction, there is no net deflection of the arc.
Advantages
The advantages of AC welding are:
- The alternating current between positive polarity and negative polarity allows for a steadier arc for welding magnetic parts
- Fixes problems with arc blow
- Enables effective aluminium welding
- AC welding machines are cheaper than DC equipment
Disadvantages
The disadvantages of AC welding are:
- More spatter
- Weld quality is not as smooth as with DC welding
- Less reliable and therefore more difficult to handle than DC welding
Applications
When switched to AC positive, it also helps remove oxide from the metal surface - hence it is suitable for welding aluminium.
AC welding is also widely used in shipbuilding, particularly for seam welds, as it has the ability to set the current higher than with DC. AC welding also offers fast fills and is used for down hand heavy plate welds.
One of the main uses of AC welding is with materials that are magnetised. This makes it useful for repairing machinery.
How can TWI Help?
TWI has been at the forefront of developing arc welding processes and, as such, offers a number of associated services. Achievements include the invention of the twin wire MIG welding process (used to increase weld speed and metal deposition rates or to shape the weld bead) and transistor control technology, which paved the way for TWI to develop pulsed TIG, short-circuiting MIG and pulsed MIG processes.
Our team of over 20 welding professionals, including highly qualified International welding engineers, are able to provide expert guidance on any issue related to the joining of materials.
Please email contactus@twi.co.uk to learn more.
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