Why doesn't the work piece melt when arc welding if a lot of amperes run through our electrode?

Question

I am recently learning the theory of Manual Arc Welding. So far I have seen that our electrodes are connected to a machine that outputs hundreds of amps. The work pieces is electrically connected through a clamp connected to the current source, and as we bring the electrode closer and make contact with the work piece then we get a current since the circuit would be complete. My question is: Wouldn't the work piece also get extremely hot due to the huge amounts of current passing through it, or at least wouldn't the area of the work piece connected to the cable that carries the current back to the source get heated up greatly? Does the current dissipate almost completely in the material?

I would think that a lot (most) of the energy of the current is deposited in melting the metal on the electrode and also melting the surface of the work piece to create the weld, and that is why the rest piece stays intact (apart from the welding zone). Is this correct?

Answer 1

I assume you're imagining something like ice and water where when it seems to go straight from solid to thin liquid, right? Metals (or at least alloys) don't generally behave that way.

You most easily experience this in soldering when you use a non-eutectic soldering alloy. Eutectic alloys are formulated to have a sharp transition between solid and liquid which makes it easy to solder. But non-eutectic alloys there is a gummy, viscous, plastic region between crystaline solid and liquid. It's basically whether all the parts of the alloy melt or solidify at the same temperature or not.

I think I heard a long time ago that this was why aluminum is difficult to weld: if you get it just a little to hot it flat out turns into a puddle.

Answer 2

In arc welding, the local spot where the arc intersects the workpiece is small, so the heat addition is very concentrated right at that spot. This makes it get much hotter than the surrounding metal.

That hotness of course drives heat conduction out of the arc spot and into the cooler surroundings, but the whole point of welding is to overwhelm the conduction process with heat addition that is very fast and very intense so the metal melts at the point of heat addition while leaving the rest of the workpiece unmelted.

Now, note that the intensity of heat generation at the arc spot depends on the current density flowing through it. Since the arc spot is small and the workpiece is large relative to it, the current density is a maximum right at the spot and then falls off quickly as you move away from the arc spot- and so does the power density. this means that the joule heating also falls off as you move away from the arc spot.

Answer 3

The piece is big enough to not heat up.

The rest of the piece is a lot larger than the weld point. The weld point is as thick as an electric arc. Heat (outside the arc) equals I²R. I is current (hundreds of amps typically). The resistance (R) of metal is proportional to the area the current flows through (R = ρL/A). Most of the pieces you are welding will have enough area to limit heating even at welding amps. Compare the area/thickness of the welded piece to the size of the welding cable, it's likely a lot larger. You will sometimes see the metal heat up a bit right where the ground clamp is, but it isn't usually a problem.

Welded items do get really hot, but it's from the molten metal, not from the amps. Some welded items require subsequent heat treatment.

If you tried welding a piece of metal the size of a welding rod you would have some problems. Welding small pieces is its own specialty, but generally you will need much smaller electrodes/wire and much less amps.