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pulse welding wires instead of ferrules?

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coppercone2:
so this is kind of like the other thread about copper welding instead of solder,

I saw a video about a 'cold welder' that showed a copper wire being melted together with a pulse function on a TIG welder. I thought that if there was thermal collet to protect the insulation, then enough wire could be melted together to form a solid copper thing that can be put into a screw terminal, without a ferrule.

Do you think this could be done?


If the collet was good or maybe even water cooled, it might offer an avenue to braze wires to a distribution block after the strands are melted together to form a non mechanical connection that is theoretically all uniform material, which may have benefits in reliabiltiy in some conditions and thermal EMF, if welded, or otherwise stronger properties if brazed. I would be hesitant about brazing stranded wire but if it has a nub on it that is solid, maybe?

SeanB:
Problem will be that your transition to stranded will be very brittle, and will crack there. The ferrules will work though if you crimp them, and then spot weld the flat section, so that you get a flat surface for the cable clamp to use, or to attach the ferrule to a bus bar, or other conductive surface, permanently. The pulse weld will create a very sharp temperature gradient in the wire strands, so the ones right by the molten area will suffer a massive thermal shock, and probably a lot of oxidation, thinning out the material, while at the same time making the transition a region of varying ductility and brittleness.

There is a reason welded joins tend to fail right next to the weld, unless you anneal the part after welding, to relax all the material into a uniform state.

T3sl4co1l:
Oh interesting, there's a few papers on HAZ in copper -- in microscopic welds used in wirebonding!  They measure a significantly lower Young's modulus in the HAZ.  From the abstract, it's not clear if that's done in plain air or some kind of atmosphere; if so, oxidation might account for that?  (Heh, at a glance, that's spot on for Cu2O modulus? Yikes...)

Oxidation aside (and, I'd think it should be fairly insignificant, given the brief duration, and much larger cross-section of cable strands), the HAZ should be annealed more or less, which makes it more prone to flex, while the stronger (half or full hard drawn) cable has leverage against it.  It will strengthen rapidly with flex (work hardening), but that happens at the cost of lost cross section, so it will remain a weak point, until failure.

It shouldn't be at all brittle, at least any further than the oxidation has done, and again that should be fairly thin.  Ironic, I think it's a case of "right, for the wrong reasons" -- the above mechanism ultimately has the same effect, so it doesn't really matter, heh!

Now, if there's alloying elements around, that can cause embrittlement.  (And, yes, oxygen is a case of this, too: besides oxide formation, straight oxygen is soluble in molten copper.)  Probably good to avoid such a process, when using tin-plated wire.  Though that also creates the possibility of liquid-phase sintered or brazed joints, where the tin forms a surface layer of bronze; such a joint needs to be cooked long and hot enough that the concentration of tin is less than say 10%, thus avoiding the formation of large, brittle intermetallic grains.  It probably needs to be crimped anyways, to get enough contact area between strands for it to be usefully strong in the end.  And, mind this again can't be ensured all the way along the HAZ; and the process must go slower (it simply takes time, for diffusion to occur, keeping temp strictly under Cu melting point), meaning heatsinking is that much more critical; either that, or the HAZ is longer, plus other inconveniences related to melting/burning insulation.

So, if the cable can be restrained to the block after welding, at a point clear of the HAZ, it should be fine either way.

Really shows just how damn good crimped joints are.  They weaken the wire only slightly (when done with correct pressure!), at a point that's well constrained (i.e., the wire is compressed and stretched in the middle of the barrel, but clamped/contained by the rest of it), and often the wire is stronger than the tab itself so if there's enough movement and leverage, it breaks loose and you damn well know it!

Tim

T3sl4co1l:
On that note, interesting to ponder methods to deliver heat.  Is contact reliable enough to deliver a known peak temp on a fixed cycle?  Works fine for sheet metal (spot welding), but that's with a modest to large ratio of resistivity (modest as in for aluminum, which works well enough; steels are quite straightforward).  We don't have anything substantially better than copper to clamp the joint with; and also the heat loss through the cable and clamps is substantial, but equally poorly defined.

Is the ratio of electrical to thermal resistances reasonably constant?  Hmm, maybe.  Minus the default losses of course (conduction up the cable is independent of clamping).

We could use a constant-current source (a few hundred to some thousands of amperes, depending on wire/ferrule size), which accounts for the spread in electrical resistivity, but not thermal; it would likely deliver too much power into a high resistance.  Constant voltage would draw less power with poor contact, but perhaps the reduced heat dissipation in the same case, balances properly?  It seems like somewhere between these extremes should work, and that's quite convenient as it means a fixed impedance source should suffice; neat, shouldn't need any exotic controls.  And time and applied voltage/current/power can be more-or-less inverse, should be very simple.

I guess the best case would be, say of a crimp lug: clamp it such that the barrel is under axial compression, between two massive clamps, one holding the wire, the other the lug face.  This minimizes both electrical and thermal resistance outside of the barrel section, while leaving it as a point of least cross section, thus getting the most temp rise under load.  Question: should the barrel be under radial compression as well (say by ceramic swage jaws, or a tubular collar)?  It will release its internal tension as soon as it reaches annealing temps (sub red-hot), and it'll squish down like putty between the clamps.  Seems like it should.  And then, with tight clamping, internal oxidation should be at least greatly reduced, if not forming an entirely gas-tight seal in the process.

What of filler?  The crimp lug could perhaps be prepared with an excess of braze*, which then wicks into the cable material, without having to rely on platings or anything (likely, tin plated cable simply isn't enough anyways, it's a quite thin layer).

*Brazed barrel type lugs are made from a single piece of flat stock, curled around to form the barrel; in the same motion, a small sheet of braze filler is clamped in the joint.  These are sent into a reducing atmosphere furnace, where the braze melts into the gap, forming a solid, strong barrel (and also annealing the lug).  It would be interesting if simply "too much" were used, enabling a process like this.

I suppose that would work, without too much trouble.  Simple mechanical clamps could do, with periodic adjustment to account for wear / deformation.  Bonus points for pneumatics or hydraulics.  Regular cleaning of the contact faces would probably be needed, that's fine.

But yeah, still has the problem of no way to restrain the wire against fatigue.  You'd need a pretty stout piece of heat shrink to account for that.  Heatshrink or other wrapping is mandatory anyway, given the length of insulation that must be stripped to fit the clamp(s) -- but you'll need quite some thickness to account for both the missing insulation jacket AND the annealed metal joint.

The big question is, after such a process, can the metal in contact with the clamps, be held cool enough that it doesn't anneal appreciably?  Thus solving the fatigue problem.  If so, that'd be very interesting indeed.

Anyway, going to such efforts, when a simple cold (crimped) joint suffices, clearly isn't going to catch on.  Just interesting to think about.

Tim

Berni:
Well a ferrule is easier to apply while at the same time the plastic funnel part of the ferule additionally releaves stress from the joint and keeps the wire insulation a bit better in check.

Also for best results the part of the wire that is grabbed by the screw should be melted into solid copper, not just the very end of it. Otherwise the wire is still having the same properties as having no ferrule, it just wont fray apart at the end if you insert it multiple times. And for this task of keeping the strands in check i find its easier to just do the same thing with a soldering iron. You can solder just the last few milimeters of a stripped stranded wire. This keeps the strands from fraying, but does not put solder in the area gripped by the screw (solder is bad there because it slowly deforms and loosens). I use this tinning the end trick a lot when just prototyping stuff on the workbench to keep any stranded wires hanging off my project in check. Tinning the whole stripped wire is bad for screw terminals and tends to make the wire break off where the solder ends since all the bending is concentrated there.

If you are after welding (to not introduce other metals for example) these copper stranded wires id say spot welding should work well. You can have a wide bar that presses the strands tightly against the terminal then zap it with a huge current to fuse it on. Makes for lots of small welds on each strand so its less likely to break off and gets good low contact resistance. The terminal it is welded to also helps take away the heat after the pulse, so the insulation has a better chance of survival.

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