How can DC mig weld Aluminium?

[abs_nz]

When using ac tig, the arc cleans the oxide layer allowing proper fusion. The oxide layer has a melting temp much much higher than the ali underneath so if trying to melt the oxide layer (in dc) to get to the ali, you will melt the ali and blow through before being able to penetrate the oxide layer and fuse the ali. I understand this completely, how the arc returning to the electrode from the parent metal lifts and breaks through the oxide layer allowing access to the ali. So how on earth is my dc based mig able to weld ali? Am i getting a proper fusion weld or is the sprayed ali just adhering to the oxide layer, meaning a week weld?

Also my tutor has told me that increasing the wire feed speed alters the voltage my mig is running at. The first mig i learned on had the voltage numbered around the dial, just like selecting the amps required for MMAW. So if i set the machine to 25 volts, doesnt it weld at 25 volts, irregardless of the wire feed speed being altered?
Cheers,
Andrew

 

[malcolm]

In AC TIG the cleaning cycle is when the torch is DC Positive. MIG is DC positive all the time. You can get a decent weld with MIG (especially fancy ones) but it's best using a big set on thick aluminium. Thin alu with MIG is a nightmare.

Normal TIG (DCEN) isn't any good for aluminium as you say, but you can TIG aluminium DCEP (torch positive) but you need an enormous tungsten to stop it melting, so AC TIG is the norm.

MIG is a constant volt process as oppose to TIG and MMA which are constant amps. With MIG the wire speed and diameter influences the amps rather than the volts.