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Arc polarity for fusing wire?

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klugesmith

Sun Jan 14 2018, 03:05AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I've been forming balls on the end of 28 AWG (about 0.35 mm) copper wire by putting the wire in a torch flame.
Not unlike the method used on a much smaller scale in some wire bonders for IC's.
And in jewelry-making, with plenty of tutorials on the Internet.

Next step is to try fusing the end of the wire into a ball electrically, with a neon sign transformer on a variac. Goal is repeatability.

For a given current, I wonder if DC of some polarity would be better than AC, for making a well-behaved ball-shaped "puddle". Any advice, or guesses before I try it?
Thanks.

Sulaiman

Sun Jan 14 2018, 03:44AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Speculation:
Copper tends to form positive ions (as do most metals)
so vapourised copper ions would be attracted to the negative electrode, and repelled from the positive electrode,
so I'd try with both electrodes made of copper wire and see which forms the best shape.

Blackcurrant

Sun Jan 14 2018, 02:15PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

If using DC your negative electrode will get hotter but there will be more random things mixing with your metal. Also the melted metal ball will shoot back along the wire until too far for your power supply to keep the arc going. If you have a multicore wire fanned out the arc will melt one then jump to the next etc. Needless to say bigger wires will take a lot more current to melt.

johnf

Sun Jan 14 2018, 06:55PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

No No negative electrodes cool with emission anodes get hot just like in an x-ray tube

2Spoons

Sun Jan 14 2018, 08:01PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

For a while I was making thermocouples by fusing the ends using the arc from a neon transformer. Didn't bother rectifying - just used a fat counter-terminal opposite the thermocouple wire. Seemed to make reasonably consistent balls. One thing you might want to do though is use an inert gas blanket, such as argon.

radiotech

Mon Jan 15 2018, 06:28AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Just blueskying a couple of thoughts:

From work with fiber optic strand splicers , we got repeatable melt globs and splices
by carefully adjusting the arc in the splicer jig. (3-M portable fusion splicers- 40 years ago.)

From working with air blast circuit breakers, the arc was managed by 'blowout' coils,
i.e, magnetic fields proportional to the magnitude of current being interrupted. circa 1978.

As to globs forming at ends of fused wires: We found that when light bulbs were
burned out in groups after a big hit (because of our carelessness ) all the bulbs had
the same filament support wire blown open with a little glob of metal formed. (50 years ago )

klugesmith

Mon Jan 15 2018, 07:20AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I want to form balls about 2 or 2.5 times the wire diameter. Concentricity with the wire axis is more important than roundness. A first step toward making a few of what might pass for copper rivets, about 2 mm long.

First trials with NST and variac were successful this evening. I had read about GTAW polarities, so the rectifier experiment had a surprising outcome. Wire heated and balled _much_ faster when negative! I invite others to try it & see for yourself.

Wire workpiece is clamped vertically over the point of a steel thumbtack, which rests on a copper plate. The insulating "floor" is a scrap of plastic resting on top of a plain old NST.

Wire hot & beginning to ball, with variac at 44.

Wire not even glowing, with variac at 53, using what TIGgers call EP or DCRP.

Checking that the microwave oven diode (spec'd for 60 Hz, so it is NOT a microwave diode) isn't marked wrong.

Time to pick one (AC or EN) and get on with the show, as Sulaiman advised. It would be fun to understand the physics behind the large disparity.

klugesmith

Mon Jan 15 2018, 05:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Not so fast, Rich! The wire-negative balls are brittle & can't be beaten into pinheads.
Maybe the copper is oxidized, or contaminated with metal from the other electrode.

Back to wire-positive, I bypassed the thumbtack & let the arc go straight to the thick copper baseplate.
Ran the variac up to 140 (presumably for more than 30 mA, if not for the rectifier).
I think the wire tip never even got red hot. Hard to tell 'cause of arc luminance.

Wire-negative, with flat copper counter-electrode, the first couple of balls were brittle.
I'm going back to torch-flame balls, until there is time to hook up a shielding gas flow for the electric baller.
Most accessible are CO2 and propane. Any recommendations from you metallurgists?
For my long-unused argon bottle, I need to find the argon-to-oxygen nipple adapter & a regulator.

2Spoons

Mon Jan 15 2018, 09:05PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

I would suggest using whatever would be used for TIG welding copper - which is probably argon. I would think CO2 is likely to embed carbon in the metal - there will be plenty of C and CO ions in a hot arc like that. A little extra C is probably not an issue when welding steel - but I don't think you'd want it in copper.

Wire negative getting hotter is not so surprising - the heating will be due to positive ion impacts.

http://www.millerwelds.com/resources/communities/mboard/forum/welding-discussions/20568-tig-welding-copper here they suggest helium or helium/argon.

Re the brittleness: you may be forming copper nitride and oxide with an air arc.

klugesmith

Sat Jan 20 2018, 03:22AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

My end application was successful, starting with propane-flame balled wires. (I made about 20 and picked the best ones). Details to be reported later.

The Internet has taught me that in wire bonders, the balling process is called EFO (Electronic Flame Off).
As that industry moves from gold to copper wire, the main difference is the introduction of forming gas (95% N2, 5% H2) !

Here's a scholarly paper about forming gas details & its effect on things like FABs being round and concentric. A common defect is what they call golf clubbing.

Apparently the hydrogen helps to reduce oxides on the wire surface.

Some sources mention flammability as a reason not to use a higher percentage of H2. I would think there's a ton of safety margin, even though the explosive range for H2 in air is 4% to 75%. Because as this forming gas is progressivly diluted, the H2 concentration falls even as the O2 concentration rises:
]fg_dilution.jpg[/file]

Trillions of ball bonds are made each year. When the machine is running, it's reminiscent of a sewing machine zooming along in zigzag mode.

I just failed to re-find an 18th-century reference about industrial pinmaking, in which one step was done by a boy at the rate of 10,000 per day. Instead I found this early 19th century observation:

from Democracy in America, by Alexis de Tocqueville.

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