Its all about oxidation...
Aluminum has a very thin coat of oxidation on its surface which melts at a higher temperature than the aluminum below it.
When I weld with oxygen and acetylene, I have to use aluminum flux and push thru this hard oxidized surface layer at the precise moment that I see a skin effect in the weld zone. This is when the aluminum is melting below the surface of the harder oxidized layer.
With MIG welding, we don't use flux for aluminum (we use Argon). This is because a flux cored aluminum filler material would be far too soft to push thru a MIG gun and it would bird nest all over the place and make life miserable.
We use DCEP (Direct Current Electrode Positive) to help break up this hard aluminum oxide surface layer. The electrons are jumping from the work piece towards the filler metal. (while at the same time breaking up the oxide layer.) The filler metal gets melted and sprayed back towards the work piece. The weld puddle is surrounded by an Argon shielding gas to prevent further oxidation until the weld has time to slightly cool down and solidify.
With TIG, we have the option of AC or DC welding. We choose AC (Alternating Current) because it allows us to have some extra balance control circuitry to adjust how long the cycle stays in either DCEP or DCEN modes. DCEN is fantastic for getting the heat to the work piece. While DCEP provides just enough time to help break up that hard aluminum oxide surface layer and get down to the business of welding something together.
https://www.youtube.com/watch?v=u-daLs9Q7ac
So, to answer your question, DCEN does have better penetration, but we need that DCEP to bust up that hard aluminum oxide surface layer that melts at a much higher temperature than the aluminum below it.
Its like trying to weld two chocolate bars together with the foil still on. You gotta get thru the foil before you can fuse the chocolate together. When you try to penetrate the foil with just straight heat, you make a mess of the chocolate bar before you ever melt thru the foil.
desmo wrote:Thanks for the links and the info. Can you please explain how the DCEP and DCEN work? I'm a bit confused, I though DCEN did the penetration. I guess the heat input from DCEP is enough to penetrate the work correctly in MIG? sorry if these questions are dumb. Thanks for the help
. From the little knowledge I know about metals, Aluminum needs AC to remove/clean the oxides that melt at a much higher temperature than Al in order to get a clean weld. Seeing that most MIG is done with DC, I don't understand how it gets cleaned (aside from brush and chemical) and what not. I'm aware of the equipment needed for MIG welding Al but not very familiar with the theory. If someone could please explain the theory, I would greatly appreciate it. Please correct me if I stated anything that is incorrect and if you can recommend any good books to read, please do so.
