Welding Aluminium with a DC TIG

All the books say to use an AC TIG for aluminium (good cleaning of the oxide etc), but someone on the Locost forum has produced a perfectly nice looking weld with a DC TIG: http://www.locostbuilders.co.uk/viewthread.php?tid=50458 (second post down).

It doesn't seem unreasonable that it should be possible to weld aluminium on DC - after all that's how it's done with MIG.

Is this a big no no, or could an occasional alu welder like myself save £600 and go for a DC TIC?

DC is fine for thick material, 10mm+
The trouble is you have to move very fast and its difficult to see a weld pool as such beceause you have to push the rod through all the surface crap.
For most jobs AC will produce a much cleaner and tidyer weld.

I would say that unless your into heavy alu fabrications AC is the optimum choice.

Be interested to hear others thoughts on it.

I don't understand the complete science of it myself! But DC won't penetrate the oxide layer of aluminium sufficiently. AC current alternates (as the name suggests!) rapidly every second & this gets through the oxide layer. On the more sophisicated AC Tig sets you can adjust the frequency & balance of the current fluctuations, to give a wider weld bead or deeper penetration.

Now Malcolm is gonna ask well how come MIG can weld aluminium & that is DC?!!
Well... I don't know!
The truth is MIG is not the ideal welding process for aluminium, certainly not with Hobby MIGs & when used in Industry it tends to be on thicker sections with sophisicated Pulsed, Synergic MIG systems & even AC MIG!
The car industry is now increasingly using Aluminium in their construction & thats only thin section - but again, Pulsed, Synergic MIG systems are used which I suppose is the next best thing to a TIG arc. (they can lay a beautiful weld down however).
All that said I've seen some pretty good results on Aluminium with home use MIGs on this site.
weldequip

Thats why its only suitable for heavy ally, you can put it sufficient amps to burn through the surface oxide.

Yep. Plus with a Pulsed MIG you get a similar effect to AC current from DC, with the arc pulsing & getting through the oxides so you can weld the thin section stuff aswell.
Technologies changing so fast though - difficult to keep up with all the latest gizmos!
You sussed out your £8k sets at work yet?! What are they, Kemppi??
weldequip

They are Hitachi, i havnt had a chance to realy play yet!
Getting a new Mosa genny welder soon too! TS300sc, silenced diesel has been choosen

You've been spoilt at work! Whats your home set (or do you do your 'foreigners' at work!)??
weldequip

My own set was some cheapo make, 200amp, sing ph, euro, £300
Fairly nice to use as far as single phase goes.
Cant remember the make, and its not at my house currently.

Although i rarely use it anymore due to not having room!

Hitachimad said:

Thats why its only suitable for heavy ally, you can put it sufficient amps to burn through the surface oxide.

Click to expand...

Makes sense - MIG is dip transfer so doesn't have to go through any outer oxide layer, while DC TIG does. There were some little pinholes in the weld in the forum link which could well have been related to oxide. Looks like the weld was in approx 2mm sheet though. I guess it's one of these things where the weld will work, but would be likely to have a few issues.

Think of Aluminium oxide as a surface diode blocking DC. The AC blasts the diode (oxide) to pieces so cleaning the surface to allow welding.

Something like that anyway.


Joules

Oxide problems

When trying to DC tig weld ally without a filler, the oxide layer on both sides of the weld acts as a membrane keeping the two molten halves behind the oxide apart. Soon as you introduce a filler rod into the pool the oxide membrane is broken and everything starts to flow.

I now have my MIG set up for Aluminium and regularly weld 5mm sheet while the DC tig is used exclusively for Stainless.

Just some thoughts which I hope will help

You can weld aluminium with a DC arc. In the early days Aluminium was done this way.

The point is that the electrode has to be DC Positive - not negative as for steel etc.

It is the DC Positive wave form that breaks up the oxide on the weld pool surface - when you weld with an AC arc it is the DC half cycle that does the cleaning and the negative cycle that allows a cooling down period for the tungsten.

The problem with using DC Positive arc is that as the heat distribution on tungsten is 2/3 on the positive side and 1/3 on the negative side it means that you have to use a larger tungsten electrode to take the additional heat - for instance if welding 3 mm Aluminium with an AC arc you use a 4mm electrode, to use a DC positive arc you would need an 8 mm electrode - this is the problem!

I thought the electrode was positive for steel, hold on, it is.

I get confused which is the electrode.

I think I get the idea though. If the torch is negative like with gasless welding the electrons go from the aluminium to the torch and blow the oxide off on the way. When you do it the other way you need to stir the weld pool a bit to break the oxide. That's why MIG works but why DC TIG could be considered suspect unless the polarity is reversed. Makes a lot of sense - thanks for that.

what about using a caustic flux as used with gas ally welding?