Tig welding Aluminium

[Old Dog]

Having only ever once tried Tig welding and that was on steel(joined but not nice to look at) I have a question about welding Aluminium. Every one says that you can't weld with a DC inverter and you need a AC welder. Having not tried it I'm just wondering if any one on here can explain why not(sorry it's one of those things that I just need to know, OCD moment ). To me it's one of those :- DO NOT PRESS THE RED BUTTON things that just have to be tried, the only thing stopping me trying is it's my welder that I might damage .

 

[Fazerruss]

The reason far as i'm aware is the alternating current is required to help bring the impurities in the aluminium out of the joint and to the top of the weld pool.but because of the AC more heat is put into the tungsten so you also need to use thicker ones than with welding via DC.

 

[pedrobedro]

AC cleans the oxide layer and lets the DC through to melt the weld pool.

 

[piman]

Hello Pedrobedro,

what if you clean the oxide layer as you do when gas welding aluminium?

Alec

 

[hotrodder]

DCEP moves the oxides (cathodic cleaning, to do with electron flows, proton bombardment and witchcraft), the white bands (etched zone) next to the weld are the result. DCEP is the 'normal' polarity with MIG/MAG hence migging ally = change of gas (so it really is MIG unlike with argon/CO2 which is an active gas), wire and torch liner followed by a tantrum due to the wire birdnesting if it's a small DIY type welder.

Approx 2/3 of the arcs heat occurs at the positive electrode, not a problem with MIG as you want the +ve electrode (the wire) to melt. More of a problem with TIG due to the tungsten that we don't want to add to the weld...

The origional patent for TIG was about welding magnesium using DCEP and helium (hence it being known as 'heliarc' in some circles). Using AC gave significantly better results and the various 'posh' versions of AC (squarewave etc) further improved things as the EN phase of AC puts heat into the work and lets the tungsten 'recover' from the higher temps during the EP phase used to move the oxides. Squarewave is more efficient than sinewave so the balance can biased further towards EN while maintaining enough cleaning action which = tungsten has an easier time and can weld faster as more of the heat is being put into the work

Flux is used to deal with the oxide layer when gas welding ally, doesn't matter how clean you get it with solvents/chemicals/wire brushes it ain't gonna work without flux as the oxide layer begins to form immediately and ally oxide has a melting temp about 3 times higher than ally

Ally can be welded with TIG using both DCEN and DCEP but ignoring automated/specialist stuff neither are practical 99.9% of the time. With DCEN there's no cathodic cleaning so you need to be properly ocd with the preweld cleaning and use helium or at least a helium rich argon mix. Helium is a LOT hotter than argon (higher ionisation potential and arc voltage) and very basically you just 'blast' the microscopically thin freshly formed oxide layer away. Helium makes it harder to strike an arc due the ionisation/voltage thing, welding grade helium is eyewateringly expensive and because it's lighter than air you need to use more of it than with argon (heavier than air). Because it's hotter it's not really suitable for thin stuff and the weldpool is poorly defined with DCEN. It won't work with argon, you get a wrinkly skin of oxide covering molten ally followed by a hole exactly like what happens with gas and no flux

With DCEP you get lots of 'cleaning' from the arc but the problem is stopping the tungsten melting... a 2.4mm tungsten can handle ~ 250A DCEN, anywhere between 160 - 200 with AC depending on balance settings (EN versus EP) and with DCEP about 30 or 40A. Need more amps for a given material thickness than with AC due to the heat being concentrated on the tungsten which compounds things and then there's the issue of striking an arc and lack of arc stability with an oversize tungsten plus limits of what can be fitted into the torch- 4mm max for aircooled torches and even if you could fit a 1/4" tungsten into the thing you don't want to know what they cost for a pack

 

[pedrobedro]

With heat and flux, how would that work with a tig ?

Hello Pedrobedro,

what if you clean the oxide layer as you do when gas welding aluminium?

Alec

 

[Old Dog]

Tig Aluminium

Thanks for the answer Hotrodder, so as I now understand it AC welding because of the sine wave is both DCEN and DCEP, therefore DCEN causes a high temperature and a good weld pool, the DCEP heats the Tungsten more than the metal which cleans the weld pool and surrounding area for the next cycle of DCEN. After such a good answer I'm now going to post a question in the MMA section.

 

[Drains]

AC cleans the oxide layer and lets the DC through to melt the weld pool.

Laugh? I nearly dropped my chips.

 

[hotrodder]

Ironically isn't it kinda, sorta, almost how your Lorch works using the MACS setting though A mixture of AC and DCEN?

 

[Shox Dr]

(Sorry Si)

 

[Drains]

Ironically isn't it kinda, sorta, almost how your Lorch works using the MACS setting though A mixture of AC and DCEN?

La la la can't hear you...

When I first saw Pedro's reply I had to think about it as MACS came to mind. It is a reasonable description of MACS, though I'm not certain it's quite right. I can't honestly say what the AC/DC combo does- obviously there's the cleaning component but why you would have an additional DC pulse I don't rightly know. I can't remember if its DCEN or DCEP. Either is possible if I recall correctly, though I reckon EP is the default. I'll have to check the manual.

The only time I've used it was on steel with ally bronze filler and what the switch from AC to DC did was wet the weld through really nicely, where only using AC or DC left a hard/ridged/cold or dirty edge to the weld.

Maybe Matt could give a decent explanation?


Shox, I won't blame you for gloating.

 

[Shox Dr]

Gloat I never gloat, a horrible trate to have. Just found it funny my good man.

 

[Drains]

Gloat I never gloat, a horrible trate to have. Just found it funny my good man.

 

[hotrodder]

The only thing i know about MACS is press release/sales spiel stuff like for example

The MACS procedure (= Modulated AC system) from LORCH is a selective mixture of alternating current and direct current welding. Depending if the direct current phase is applied at the minus or plus polarity, it results in different cleaning efficiency, electrode temperatures and controls the welding result: Broad welding seam, little penetration (with MACS+) or narrow welding seam, deep penetration (with MACS).

It was just too good an opportunity to pass on though Yeah, it'd be nice if Matt could expand on the above description