Originally Posted by
sn0border88
This is the byproduct of me having way to much time on my hands, and noticing that about every week the same questions pop up. Hopefully this can shed some light about the mystery and general misconceptions about TIG welding of aluminum.
Polarity: AC current consists of a positive and negative portion, and alternates very rapidly between the two. This produces the "buzzing" sound that you hear, and what contributes to the nature of the AC arc. The AC balance control determines how much time is spend on either portion, a value higher on the negative side will giver better penetration and arc stability with decreased cleaning action, and vice-versa.
The positive portion of the wave (electrode positive) puts about 70% of the heat generated into the tungsten, this causes the tungsten to melt and beings to form a ball at the end. This portion of the wave is also what produces the cleaning action, in which ions bombard the surface of the material, removing some surface contaminates. DCEP can be used for welding of very thin aluminum but the tungsten will rapidly overheat.
The negative portion of the wave (electrode negative) puts about 70% of the heat generated into the workpiece, yielding much better penetration. This also enables the tungsten to better retain a point which makes the arc more stable and easier to control. This part does not offer any cleaning action. DCEN welding can be done on aluminum but it requires removal of all surface contaminates and generally done with helium shielding gas. (I dont know why, if any one can shed some light on this than im all ears)
Shielding Gas: 100% Argon is the most common, while mixtures of Ar and He can be used to achieve greater heat input. Argon is preferred because it is cheaper than He, very dense (better cleaning action) and produces a stable arc. Any mixture containing O2 or CO2 cannot be used because of the rapid formation of aluminum oxides, along with the rapid erosion of the tungsten.
Tungsten: The electrodes used on AC welding vary greatly with personal preference, but I do not know of any commonly found electrode that cannot be used. Some of the more popular ones include 2% ceriated, 2% thoriated, 1.5% lanthinated and pure tungsten. Pure tungsten is not required for use on any machine, in fact most inverter machines will not proper functionally with pure tungsten.
Forming a ball on the end of the electrode is not necessary, often it is preferred to weld with a slightly blunted point. If the ball grows too large and the arc seems unstable, stop welding and regrind the tungsten. If it continues, either decrease amperage or go to a larger tungsten diameter.
AC Controls: Older machines will likely have a simple balance control which was explained earlier. Newer inverter machines typically have an additional frequency control. This allows the user to adjust the amount of times (per second) that the arc switches from EP-EN. The standard is 60Hz, but increasing this ,typically >120Hz will stabilize the arc, making it "stiffer" and increasing penetration while reducing the width of the weld pool.
This is by no means the definitive guide, obviously I hardly even touched the surface. If you have something to add, do me a favor and just make it easy to read but feel free to add it in here. The goal is to make this a one-stop-shop for aluminum. If you disagree with something, PM me and we'll work it out.
That's all for now, more to come tomorrow.