copper backing of arc welds

[deltawhy]

Hello, I have a question regarding SMAW and MIG welds. People commonly use copper stock below, say, a but-weld to carry the pool along and create a better weld. I've tried to figure out physically why the copper does not melt or deform, with no luck. Does anyone have a chemical explanation for this phenomenon? I understand that the copper will carry the current far easier than the steel being welded, but the arc and molten pool seems to have no effect?

Thanks,
Daniel

 

[cloa]

Could it be that its good thermal conductivitity and that they use a big block of it keeps it cool.

 

[metengr]

Because relatively pure copper has an adherent surface oxide and is more efficient in conducting heat in comparison to steel. The suface oxide prevents the molten steel from fusing. You can locally melt copper if the arc is directly against the copper with no water backing

 

[Tmoose]

Copper's thermal conductivity still catches me by surprise sometimes when heating things up. Even when soldering small wires if I dawdle I will feel pain.

 

[moltenmetal]

The copper doesn't readily alloy with the iron, i.e. the copper is not rapidly soluble in the melt pool, especially if the copper itself keeps cool due to water backing and its native thermal conductivity. It also has a fairly high melting point, unlike aluminum.