AC and DC welding are forms of arc welding that use different currents to produce an electric arc. These types of welding involve the creation of an electric arc between an electrode and the metal being welded. The electric arc provides heat to fuse the metals together. A power supply is used to generate the arc, which can either use an alternating current (AC) or a direct current (DC). The choice of power source (AC or DC) decides the polarity of the electrical current that runs through the electrode. The power source used also impacts the weld produced.
Selecting the correct electrode polarity impacts the strength and quality of your weld. Commonly known as ‘straight’ or ‘reverse,’ the two types of current flow are also called ‘electrode negative’ and ‘electrode positive.’ The DC polarity is constant while the AC polarity flows in one direction for half of the time and half of the time in the other, reverse, direction.
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.
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
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
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.
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
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
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.
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