MIG welding polarity - why is it important?

[usenet]

I have been using my recently bought SIP 130 MIG welder over the past couple of weeks. At first I used it in'gasless' mode with cored wire but in the last few days I've been in a sheltered area so switched to working with gas and ordinary wire.

When I swapped over I forgot to switch polarity and I didn't notice this for quite a while. I did eventually realise and changed to the correct polaity. To be quite honest I couldn't really tell the difference, what effect does the polarity have in MIG welding?

[MrCheerful]

IIRC the main difference is in weld penetration, others include torch temperature.

MrCheerful

[Steve]

The metals being welded can also affect the required polarity. And aluminium needs AC....

Steve

[Andy Dingley]

Same as any process - the +ve electrode gets hotter

For MIG welding in spray-transfer mode, it's important to have the wire running hot and the torch +ve, or else you won't get a spray (which leads to globular transfer, poor penetration, and premature baldness)

If you're using dip transfer on thin sheet, then it doesn't matter which way you run it. Besides which, the voltage (and thus the power) is lower anyway. For cored wire, running the wire over-hot can disrupt the shielding effect of the generated gas - but even then, thin sheet isn't fussy.

MIG keeps the same polarity for all metals. TIG uses a -ve torch, to stop the tungsten melting. Welding aluminium (or magnesium) with TIG (but _not_ MIG) uses AC, to get a "scrubbing" action on the weld pool to break up the oxide film.

[usenet]

Thanks for the explanations, I'm working on fairly thin material so it would seem that polarity isn't all that critical.

Well my instructions definitely say:- +ve torch for using MIG with gas. -ve torch for using MIG gasless with cored wire. This is a SIP 130DP, specifically designed to be used for both gas and gasless MIG welding. It's also marked up on the welder itself.

[Richard Savage]

Chris,

Where do you buy your gasless wire, please?

Rgds Richard

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