Multi pass question.

[Splatterer]

Hi,
Beginer question.
I know from reading threads on here that ARC can be used in layers to build up a thick weld on thick materials due to the parent metal being melted by the arc .
I also found out that MIG melts the parent metal by heat from the weld pool and therefore seems to need a single pass with a suitable sized machine due to heat dissapation on multi pass.

The question is, Is that entirely right? and,
Is it entirely due to the electrode size, ie would a MIG with 2.5mm wire then act as just like an ARC rod at 2.5mm and melt the parent metal with the arc and previous welds to form a strong bond.
Or are there more of those mysterious forces at work that seem to crop up when trying to learn welding.
Thanks.

 

[jonjoe]

first you would need a very big mig to run 2.5 wire it usely 1 mm and a 2.5 mm rod would be about 8mm

 

[jonjoe]

0.8mm

 

[pedrobedro]

All welding melts metal by heat of the weld pool. Some use two different processes like tig for the root to ensure penetration and arc or mig for capping to make the finish good. Multi pass made me think of the fifth element though.

 

[hotrodder]

^^

Splatterer, there's several reasons behind running multiple passes from simple ones like keeping the weldpool a 'managable' size to more complicated metallurgical* effects and a bunch inbetween too. MIG, MMA/stick and TIG... all get used to make multiple passes, as said often more than one process on a single joint (precision of say TIG for the root pass and then filling the joint up with something faster)


* each pass can 'heat treat' the preceding ones via grain refinement/ one big hot run puts more heat into things than smaller runs... more distortion, reduced corrosion resistance of stainless etc etc

 

[Splatterer]

Thank you,
So does that mean I could weld thicker plate with a v groove and make several passes with a single phase MIG.
Or would best practice be a root weld in a v then filling the remainder up with MMA?
Sorry if this is dumb.

 

[Rrunnach]

Thank you,
So does that mean I could weld thicker plate with a v groove and make several passes with a single phase MIG.
Or would best practice be a root weld in a v then filling the remainder up with MMA?
Sorry if this is dumb.

Not dumb at all, say you were welding 6mm carbon plate, Vee-Butt, you could go MIG or MMA all the way, be pointless to mix processes.

Assuming you have a mig set, use this, faster, with less problems such as slack traps......