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AC vs DC welding Science. Answered

AC vs DC welding.
I'm not contesting whether DC is better. Consensus is "yes" 

But what about the physics of _WHY_ it is better?
Theory: when a droplet of steel on the electrode is charged opposite of the workpiece, it is drawn toward the workpiece.  BUT in AC, there are moments when the charge drops to zero and reverses. At that time, might the droplets of steel, for just a moment, be electrostatically drawn back toward the electrode?
Y,N,M?

Discussions

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Downunder35m

3 years ago

I think it more chemical than pysical attraction or movement.
Taking aluminium as an example everyone will tell you that you need DC to weld it.
And if the old man back in the day explained it correctly than in this case:
The arc causes the aluminium to oxidize, the gas makes sure that is not too bad.
When the alu from the wire melts it can only form a galvanic connection if the polarity is correct, otherwise the material won't bond at all.

He made it very simple though but I accepted it ;)
For steel I guess it is in some way similar but with some other factors involved.
For static powers to be effective you need quite a low weight, these steel drops are quite heavy.
But take the normal elctrode melting of large steel amounts.
If they use AC the steel burns a lot, by using DC this is reduced quite a bit.
When welding the steel melts too and as we all know from thin meltal sheets it can vanish as well.
I assume that by using DC the burning of the stell is reduced allowing for a more controlled window to work with when the steel melts.
In high voltage arc welding you also have the ionisation to consider.
The protective gas acts quite differently on AC compared to DC.
With DC, in this case, you get a directed plasma beam that takes all energy towards the workpiece.
Using AC forms a plasma that is basically steady and provides even energy throughout with no direction towards the workpiece.
You can see the difference quite good using a very low feed on the wire.
With AC you can then burn off the weld to remove material - ugly but works.
With DC on the other hand you melt the welded area so the material flows together without burning away too much.


But I am sure we have some old welding guru here that can explain it all correctly and for everyone to understand...

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steveastroukDownunder35m

Reply 3 years ago

Taking Aluminium as an example you HAVE to use AC to weld it !

EP and EN have different heat transfer characteristics, and cleaning capabilities. The best kind of AC sets for AL allow a slight net EN if I remember correctly, so that the EP cycle breaks down the AlO2, but the EN supplies the heat.

Its very complicated...

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Downunder35msteveastrouk

Reply 3 years ago

Ahh, that makes sense, I now just wonder why I was told I would need DC for aluminium.
One day I will try it out and know for sure LOL
Guess that explains why a proper welder has so much more in terms of buttons, switches and knobs...
Glad I can braze alu and that at this stage I have no real need for high tech welds.
Have a simple stick welder and a gasless mig, enough for my hobby use, although I miss my big welder that brought all these all beetles back to life.
Only used CO2 with it as I had no argon bottle at hand but apart from the good finetuning for wire speed and amps it aso had some other nice features to make life easier.
One of the best was the high voltage arc starting, with that you could burn through some paint or rust and then start the wire feed to get going.
No more problems with bad contacts...
The voltage could also be adjusted, although I never really figured out what to use it for, so I kept it in the middle position LOL

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steveastroukDownunder35m

Reply 3 years ago

I use TIG mainly now, but only a DC set, so I can't weld aluminium. Welding aluminium is a bitch with any technique IMHO.

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Downunder35msteveastrouk

Reply 3 years ago

Have you tried reversing the polarity on your Tig (if it is an option on your system)?
You need thicker electrodes when working reversed but it works for aluminium.
Best to a use a helium rich argon mix though which makes it expensive, otherwise you have to find the exact right angle to work and be prepared for a bit more cleanup.
If you can set a pulsed mode try working in the 200 to 400Hz region, this worked best last time I had access to a proper welder.
A filler rod with some flux helps a lot to flush out the debris and get a cleaner result but I agree that welding aluminium is not a fun thing to do on a hobby level.

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Toga_DanDownunder35m

Reply 3 years ago

I'm just gonna reply to yer 1St statement for now. "Chemical attraction". How would that work in, say overhead welding? The metal droplets have to go _up_ from the electrode against gravity

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Toga_Dan

3 years ago

Replying from my phone. Cumbersome. Clarify. EP and EN? I may Google later, but can ya explain a bit more?

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steveastroukToga_Dan

Reply 3 years ago

EP = Electrode (stick, whatever) +ve with respect to the job

EN = Electrode -ve with respect to the job.

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bravoechonovember1

3 years ago

But you can weld EN or EP and if your theory is correct then if you weld EN the droplets would be pulled towards the rod. so to prove your theory you would need find a way to weld in a repeatable way and switch from EP to EN.