Hamid,

thanks for these additional descriptions.

Quote: "...and i think have enough information regarding TIG welding procedure..."

and

Quote: "...(b) isn't my answer its the answer of examiner. I don't know about its correctness..."

Hmmm... this again sounds a bit strange to me - forgive me the honesty - as it sounds a bit... contradictory?

Anyway, according to the nowadays valid theories the tungsten electrode must have positive polarity, i.e. act as the anode, to emit 'massive' ions. These again, to keep it very short, are accelerated across the column, impinge on the aluminium surface, or the Al-oxide surface layer respectively, to hereby enable a relatively 'stable' GTAW process behaviour.

Thus, in general and basically, DC+ would be thoroughly feasible to be used for GTAW of aluminium and its alloys. However, anode overheating effects are the result what's the reason again that this technique is just very rarely used. Amongst others, to avoid tungsten inclusions caused by molten off electrode material. Nonetheless, it is - from the theoretical and also practical standpoint definitely possible.

In this context by the way. No, I don't "...want to say that real answer is DC with straight polarity. (d)

Anyway, the 'regular' way of GTAW aluminium and its alloys is to use Alternating Current. Hereby, simply put, the electrode material is allowed to 'cool down' during the period of acting as the negative polarity cathode, and to be protected hereby from a deterioration due to superheating.

But... physically governing the GTAW process is as well-known the 'thermionic' emission. The stability of a thermionic emission again depends on the cathode temperature, i.e. in case of electrode polarity is negative, its temperature is determining the 'amount' of electrons emitted towards the plasma, simply put. Another crucial point besides the cathode temperature with GTAW, is the work function of the electrode material. Amongst others that's the reason for doping the pure tungsten with rare earth metal oxides like La2O3,...,.

For a general simplification, let us agree that the cathode temperature is very important for the arc plasma stability and thus, for the welding process stability itself. And let us just keep in mind, that the electrode work function is depending on the cathode temperature as well.

So far so good. What happens now when AC GTAW? Well, of course we are using variable polarity depending on a specific frequency. Once the tungsten electrode has positive polarity (anode) emitting ions to be impinged on the surface layer on the one hand. Being the anode however, it is bombarded by a high amount of low mass but highly accelerated electrons and thus being superheated. And once the electrode has negative polarity (cathode) emitting electrons to be accelerated across the column and to swap their kinetic energy into thermal energy - i.e. to melt the parent material, but to be otherwise allowed to 'cool down' during this period.

Coming slowly to the point...

As the polarity changes one can observe an inherent thermionic behaviour during a very particular period – which is as the current drops to 'cross' and rising again to 'leave' the normal current zero line. Here the thermionic behaviour of both tungsten electrode and base material does change as well. In other words, as the electrode is anode – i.e. having high temperature improving the emissivity for ions – the emissivity for electrons of the ‘colder’ aluminium weld pool does deteriorate. This again leads to a waveform imbalance which is referred to as ‘Inherent Rectification’.

There is, mentioned by the way, another phenomenon with AC GTAW of aluminium and its alloys, referred to as ‘Partial Rectification’. But due to this was not the issue of your original post, I don’t want to deal with this herein.

I am honest with you, I have never heard of the term ‘suppressor’ before. But however, to overcome the ‘Inherent Rectification’ phenomenon there are – to the best of my knowledge and as far as I was allowed to learn – different ways:

1.  Use of series resistance = inefficient means

2.  Parallel bias voltage = relatively costly and complicated

3.  Battery series capacitance = often used to overcome inherent rectification

4.  HF- superimposition to stabilise the ‘normal current zero line’ behaviour

5.  Use of pure rectangular wave form power supplies enabling the current to cross the normal current zero line rather ‘instantaneously’

There may be others, being unknown to me. Therefore it will be interesting to know what the real GTAW experts say –> Lawrence - Henry ?

I just can assume that a ‘suppressor’ might be a series capacitance suppressing the arc instabilities through ‘rectification’ phenomena by stabilising the arc behaviour in AC GTAW.

Hope this helps a bit.

Stephan