On 28 Aug 2003 20:12:13 GMT, wrote:

what effect does the polarity have in MIG welding?

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.