Frequently Asked Questions
There are several different capabilities of TIG welding power sources, and the selection of products ranges from a simple DC only power source, to highly advanced, multi-function devices, with a commensurate increase in cost.
In order to choose the right power source, a purchaser must consider the balance of price, availability and function. Many of the functions of a power source may not be necessary for a given application, so an overview is provided here of the different functions, to enable a purchaser to determine whether they are required.
AC vs. DC – An AC power source is typically needed for welding of metals that form persistent oxides, such as aluminium and magnesium. The AC cycle causes the welding circuit to switch between electrode positive and electrode negative, thus alternating between cathodic cleaning of the surface and high penetration. The ability to alter the portion of each cycle, the “balance”, will allow biasing towards cleaning or penetration as necessary. For welding of steel/stainless steel, only DC polarity is required.
More advanced power sources will also allow alteration of the shape of the current waveform. A typical sine wave AC cycle requires a high frequency start to re-ignite the arc. This is because the slow transition from electrode negative to electrode positive gives the arc sufficient time to extinguish. A square wave AC cycle eliminates this restart requirement, due to the rapid switching through the zero current point.
Pulsed current – Some power sources are capable of pulsing the welding current between a low “background” current, which primarily maintains the arc, and a high “peak” current which gives the required penetration. This has the advantage of reducing the average heat input while still giving good joint quality. The pulse frequency applied can vary between <10Hz (“thermal” pulsing) to several kHz (“high frequency” pulsing). High frequency pulsing (called “interpulse”, “superpulse” or “pulse on pulse”) is particularly suitable for maintaining a stable arc at low currents eg, <10A.
Duty cycle/Maximum current – There is a maximum current range achievable by a power source for a given length of time. The maximum current and the duty cycle are interrelated, such that a power source will typically operate at a lower current for a greater duty cycle. There will be different requirements in terms of duty cycle for a manual TIG welding set used occasionally for repair or light fabrication compared to a power source being used for heavy manufacturing on a mechanised/robotic system. The maximum current achievable by a welding power source will control the thickness of material it can weld and the speed with which it can weld a given thickness.
The maximum current also places requirements on the tungsten electrode size and the cooling system for the torch. Lower currents will use smaller diameter electrodes and air cooling, but higher currents will necessitate a larger torch to accommodate larger tungsten electrodes and water cooling.
Control of the welding current can be either via a switch on the welding torch, with 2T or 4T control, that initiates and interacts with a set program, or via foot pedal control, used to throttle the welding current reactively during manual welding.
Additional control possibilities may be needed if the welding system is to be used for mechanised/automated/robotic welding. It may be necessary for the welding power source to be integrated with a robotic or CNC-type system, and this will require certain power and control connections, and may necessitate separate computer systems to allow programming of the power source.
The final possibility is the use of filler wire. For manual TIG welding, this may simply be hand fed into the weld pool, but for automated processes, a wire feed system will be required. This may be part of the power source, or may be a stand-alone unit, which receives a control system from the power source. It may also be “hot” wire, using resistance heating to increase wire feed speed, particularly for high deposition applications such as cladding and overlaying.
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/)
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