For Aluminum Tig Welding, AC balance is not absolutely necessary.
But it is sure a nice feature.
Because not all aluminum tig welding jobs are created equal.
An aluminum handrail by the ocean will be much more heavily oxidized than a brand new piece of 6061 t6 tubing that just came off the shelf....and so its nice to be able to adjust the AC arc for different situations.
I have welded with lots of machines that were not equipped with AC balance. But when given a choice, I prefer to have it.
So... what is AC balance? how does it work? and why do I care?
Think of AC balance as the handle in a shower that mixes hot water and cold.
Straight hot water is too hot. and straight cold water is too cold.
But with a simple piece of technology,...a valve... you can mix hot and cold water to get something more useful and pleasing.
AC balance is kind of like that.
Except it mixes negative and positive cycles in proportions that are more useful than 50/50.
To understand AC balance, it helps to talk about direct current electrode negative first.
Steel is tig welded on electrode negative because its a smooth arc, and current travels from the electrode to the metal.. When current is moving in that direction it allows you to pinpoint heat.
But direct current electrode negative does nothing to help break thru the oxide layer on aluminum. Did you know that the oxide layer melts at 3600 F? while the base metal melts at around 1100-1200F?
We need something to break up the oxide for aluminum to weld clean.
Electrode positive .
When current is electrode positive, electrons dance around in the arc plume like little fairies and break up oxides . That is a good thing but the drawback is that when current is traveling up thru the electrode as it does on electrode positive, the electrode gets really hot really quickly.
Thats why aluminum is welded using alternating current.
Alternating current provides some of the properties of negative, and some of positive , but not all the drawbacks.
penetration and heat input come from the negative cycles, and cleaning comes from the positive.
So what AC balance does is let you adjust and tweak the penetration vs cleaning.
for nice clean metal, a higher setting that dials in more electrode negative can be used.
for more heavily oxidised aluminum, a lower setting that uses more positive current helps.
Personally, I find that a setting of 65-70 on a Miller Dynasty is a very good setting for general applications. But occasionally, there is the weld that is right next to something that you dont want to melt.
A balled electrode might cause the arc to wander and cause problems. It could be a polished area, a cooling fin, or a pressed in brass bushing. or...it could be that you just need to add a drop or two of filler metal to fill a ding in a polished injection mold. Maybe the extra pitting caused by excess cleaning action just means more work whereas if you could really pinpoint the arc , melt , add filler, and get out, you could save a lot of work.