TWI Frequently asked questions
As DC power with electrode negative polarity produces a concentrated arc with most of the heat (approximately two-thirds) in the workpiece, with one-third in the electrode, this type of power source isused for welding most types of metal. However, the arc with its cathode roots on the electrode ie, DC electrode negative polarity, will produce very little cleaning of the workpiece surface.
For materials, such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between electrode positive and electrode negative polarity at typically fifty times a second, the periods ofelectrode positive will be sufficient to remove the oxide and clean the surface, Fig.1.
Fig.1. Current and voltage waveforms for (sine wave) AC welding
Extracted from an article entitled 'Let's get technical - choosing an arc welding power source' by W Lucas and G Melton, published in Welding & Metal Fabrication, Vol.67, No.4, May 1999, pp.18-21. (Publishers - dmg BusinessMedia Ltd, Redhill, Surrey, UK - http://www.dmg.co.uk/dmgbm/ )
