[Olivero]

OK,

It's been a while but here's a question, as I teach people how to fix stuff I tend to get asked a lot of questions, most I can answer with utter confidence.

"Because that's how it works, now do it"

That seems to get me by and makes me look smart.

So here's a question I've had

In aluminum welding, the whole theory of why A/C is so efficient is because the constant polarity swithcaroo from + and - is bombarding the oxide layer with particles which breaks it down, or something like that, I read about it a long time ago.

So why is that? what makes it so it can eliminate that oxide layer despite it melting at a much higher temperature than the base metal itself?

Now, I could probably google it and find out but I like it when i can ask more questions and you guys probably know, so why not.

[LtBadd]

The positive side of the AC wave (flowing toward the tungsten) strips off the oxide layer, in simple terms

[Arno]

The positive side of the AC wave (flowing toward the tungsten) strips off the oxide layer, in simple terms

Yup..

When the current swaps direction because of the polarity change, the electron flow goes from workpiece to the electrode and this basically breaks apart the oxide layer on the surface as it does so. The argon in the meantime keeps oxygen out, so a new skin won't form quickly and when the polarity swaps again the base metal melts (now without the oxide layer) and you can access the puddle.

Don't want to keep it that way (eg. using DC) as it will also put all the heat into the electrode and not much into the workpiece, so AC is used.

By playing with the balance of the AC wave you can adjust how long it's in each phase and move the % of cleaning vs. melting/welding around.

More in-depth reading:

https://www.millerwelds.com/resources/a ... -frequency

BTW.. You can weld alu with plain DC TIG, it's just not very nice as you have to basically poke the filler through the oxide layer on top of the molten puddle and it looks pretty awful and tends to introduce contamination into the weld for that reason, but in a pinch (and not needing to fulfill code requirements) it's doable. Jody did at least 1 video where he shows this.

Bye, Arno.

[tweake]

OK,

In aluminum welding, the whole theory of why A/C is so efficient is because the constant polarity swithcaroo from + and - is bombarding the oxide layer with particles which breaks it down, or something like that, I read about it a long time ago.

AC is not efficient with tig as you waste a bit of heat due to the positive side of the wave.
if you want details on how it works look up cathodic cleaning. by mem i think its the ions bombarding the surface which breaks up the oxide.

mig is positive electrode which does the cleaning as well as the heating.

you can weld aluminum with dc tig but you either have to clean oxide off manually and still put up with some crap, or run positive and use a big electrode to handle the heat and weld thin aluminum. neither is very good for most people.

[kiwi2wheels]

DC negative TIG and helium is still the go to method for a sound weld and eliminating eliminating distortion in repairing aluminum engine blocks and other repairs and heavy fabrications.

https://weldingweb.com/showthread.php?6 ... ost7342151

http://forum.weldingtipsandtricks.com/v ... inum#p8357

It would be interesting to know how the latest inverter machines with hybrid wave forms ( DC neg, AC pos ) compare with the old method on castings / thick materials regarding distortion.

[Olivero]

Interesting,

So it's really the reversing of the current flow which breaks apart the oxide layer and opens it up to the base metal.

I read it was bombardment of the ions but that's pretty general, I wanted a more in depth idea of how it worked.

So the polarity reverses and that somehow rips the oxide layer off, why does it come off though? I can't imagine it's heat causing it, is there some reaction that occurs at that polarity switch?

[Poland308]

Yes it has to do with the direction of the electron flow. A slower version would be the electroplating process. Same principles just happening faster.

[Olivero]

Interesting, so how does that work?

What makes the oxide layer come off? It's obviously not magnetic so how come it just peels off when it requires so much heat to get rid of it otherwise.

[tweake]

Interesting, so how does that work?

What makes the oxide layer come off? It's obviously not magnetic so how come it just peels off when it requires so much heat to get rid of it otherwise.

it doesn't peel it off.
it actually breaks it up using the ions (ions go in the reverse direction to the electron flow)
for a full explanation your best to google up some of the papers on it. theres no way i can shorten it into a few sentences.
its very nerdy stuff.

[tweake]

DC negative TIG and helium is still the go to method for a sound weld and eliminating eliminating distortion in repairing aluminum engine blocks and other repairs and heavy fabrications.

i have some doubts on that.
distortion is about heat. AC reduces heat slightly. you can use helium on AC and using helium on thick parts helps a lot.
i doubt you have to you DCEN instead of AC. i suspect a lot of that came from a time when there was no AC balance and that limited he amount of heat it could put out without melting the torch.

[sschefer]

Cathodic erosion is caused by the turbulence created with the flow of the free electrons changes from one direction to the other. Electrons flow from negative to positive. In A/C tig welding your work clamp is positive and your torch is negative so electrons flow from the torch to the work piece when the current is flowing in that direction and from workpiece to torch when the current is flowing in the opposite direction. When you are in an EN condition then the workpiece heats and when you are in EP the tungsten heats.

[Oscar]

Interesting, so how does that work?

What makes the oxide layer come off? It's obviously not magnetic so how come it just peels off when it requires so much heat to get rid of it otherwise.

This my take on it in simple terms, from the few papers I've read on it, so don't take it as gospel.

As Tweake mentioned, it is cathodic erosion that is responsible, which is metal erosion due to cathode emission, which is basically emitting electrons. During this portion of the AC cycle, where the electrons are coming from the base metal toward the tungsten, they have to break loose/free from the base metal in order to do so. To break free [the electrons] from the base metal requires a certain break-down energy at the surface of the metal, related to what is known as emissivity. Aluminum and some other metals do not want to emit electrons from it's surface because it is a non-thermionic metal (they just don't have that "property"). But even a non-thermionic metal can become an electron emitter with enough temperature and a large enough electric field. When those two conditions are met, the oxide layer which is more of a dielectric material (like an insulator), ends up breaking down/apart (the "erosion" aspect) when it itself is has no choice but to permit electron emission. The aluminum oxide lattice structure is broken apart with so much electron emission, and it starts to float away leaving the clean molten aluminum underneath.

If you look at some of Jody's videos on his aluminum welding techniques where he sometimes "lets the cleaning action work" to help start off an aluminum weld, and slow it down some, you can see the oxide layer actually become the emitter when it just barely starts to "frost over".

[Poland308]

These are the reasons that the fumes from the welding process’s are often laden with heavy metal particles.

[cdxtrooper]

Hi everyone, I’m looking to buy a plasma welder. Any clue as to how much cfm is needed to cut 1/2 inch mild steel? Perhaps a miller 635 plasma cutter would be something I might be looking at


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[LtBadd]

Hi everyone, I’m looking to buy a plasma welder. Any clue as to how much cfm is needed to cut 1/2 inch mild steel? Perhaps a miller 635 plasma cutter would be something I might be looking at


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Hello cdx, welcome to the forum, you're posting this question in a thread about the oxide layer while TIG welding aluminum, please start your own thread so everyone can see it, and the answers will be in response to YOUR question.

[cdxtrooper]

Sorry, thanks for the info. I’ll try to figure out how to do what was suggested!


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[weldin mike 27]

viewforum.php?f=29 here's the spot for that.

[Olivero]

Wow.

I missed some of this stuff.

So essentially the oxide layer is being sucked up by the electron flow and it's almost being ripped apart on a molecular level?

[Poland308]

Yup

[Olivero]

Man that's cool.