Coiling Tungsten Wire

The coiling process is more like wrapping. The wire is dragged of the reel and wound onto a 1mm wire.

It is definitely the wire as I can move the suspect wire over to another machine which is running well and the breakage also occurs on that machine.

These processes can be tricky. For instance, when three strands are laid into a spiral rope, the three strands are fed from a rotating carriage of three bobbins. This would impose a twist on the outfeeding rope, but the individual bobbins are set up to counter-rotate in order to prevent this. There may be some tricks about slight overtwist or undertwist so that the final rope lies neutral, with minimal tendency to twist/untwist when a load is imposed.

Not knowing your total geometry, I can't tell if it is set up to avoid accumulative twist. The raw wire might be at fault, but I wouldn't jump to that conclusion just yet.

Ok, I do uderstand where you are going with this but I am 100% sure there is no twisting of any kind. I run a rope making buisiness (www.custombraid.net). But if you go to web site you can also see a bit of the wire coiling that I am talking about.

The core wires (steel) are pretwisted to relieve the tension you are talking about. But the tungsten is wrapped around the outside.

Ok, I do uderstand where you are going with this but I am 100% sure there is no twisting of any kind. I run a rope making buisiness (www.custombraid.net). But if you go to web site you can also see a bit of the wire coiling that I am talking about.

The core wires (steel) are pretwisted to relieve the tension you are talking about. But the tungsten is wrapped around the outside.

What you describe sounds to me like either

• Poor annealing that is localised in the feedstock
• Contaminant in the drawn wire giving brittle portions

spectroscopic analysis would eliminate the second alternative (which I believe is unlikely).

Annealing issues could relate to spool size and heat capacity of ovens if annealed "on spool". There may be portions (centre of drum, centre of spool) that don't achieve the necessary soak heat/duration and thus are not annealed.

Hope this helps.

Thanks. I will question my supplier abour his annealing setup to see if that may be the problem.

I still think Tornado has a good answer, but you might also consider the geometry of what you are doing.

When you coil a 0.3 mm wire around a radius of 0.75 mm the inner diameter of the coiled wire is about 4.7 mm while the outer diameter is 6.6 mm. This is a difference of nearly 2 mm, or about a 40% larger diameter for the outer edge of the wire. So you may simply be putting too much stress in the wire.

You might want to do a partial pre-bend to the wire, or heat the wire just before it gets wrapped. Maybe you could pass the wire through a small induction heater just before the wrap starts so it is slightly softened.

http://en.wikipedia.org/wiki/Induction_heating

Sorry I do not see where you get those numbers from? are you ref to Cir?

???

I got the numbers from your original post. You said the wire has a diameter of 0.3 mm. You said you were bending it to a radius of 0.75 mm.* The circumference for a 0.75 mm radius (which would be the inner radius of the coil after the wire is bent) is 4.7 mm ( = 2 x pi x 0.75 mm). The outer radius of the coil after the wire is bent is 0.75 mm + 0.3 mm = 1.05 mm, which yields a circumference of 6.6 mm (= 2 x pi x 1.05 mm).

The geometry of what you are doing suggests that the wire diameter is too large for the bend you are subjecting it to, when you coil it; or, that the bend radius of the coil is too tight for that diameter wire. My suggestion is that you ought to perform an operation on the wire to minimize the bend stress which might be the cause the wire breakage you're having.

(* I assume that this is the inner radius of the coil -- if this is the 'center' radius, or the outer radius, the geometry of the bend is even worse!)

More info on tungsten.

According to the website below, tungsten has poor ductility at room temperature:

Mechanical properties of tungsten
The mechanical properties of tungsten can be influenced by the purity of the material, the type and amount of alloying elements, heat treatment (annealing condition) and microstructure. Tungsten has a body-centred cubic lattice structure and exhibits very low ductility at room temperature.

From the website: http://www.plansee.com/tungsten.htm

Here is a diagram (as I understand it) of your coil:

I can't tell from the Website how the tungsten filament is fed onto the multi-strand steel core. If the tungsten spool and the core are parallel, and the core rotates; then the tungsten should feed without twisting. But if the tungsten is fed parallel to the core and laid down by some precessing finger scheme, then I'm pretty sure twisting would accumulate.

Try this one...

http://www.exair.com/en-US/Primary%20Navigation/Products/Cold%20Gun%20Aircoolant%20Systems/Pages/Cold%20Gun%20Aircoolant%20Systems.aspx

The likely properties of the wire are such that it will pick up ambient temperatures easy (because it is thin), and mechanical properties will be eased on the coiling with warmer ambient temperatures. I may suggest this (warmer ambient Temperature) could be part of the problems' solution to increase ambient temperature around the general area or localised at the point of coiling.

Hope this may have helped. This is assuming the client, has specified chemical content of the wire. If not, then addition of a malleable content could be introduced to the wire (although this will alter properties of the wire) may relieve some of the problem, although the above will still have potential. The altering of chemical content of the wire may not lend itself to its final application and will alter its attributes.

Regards

Hugh Reid (Former Chemist and Manufacturing Engineer)

There are some simple things both you and your supplier can do such as checking spooling and de-spooling equipment for constant tension operation, making sure that the de-spooling doesn't introduce twist in the wire, etc. You can consider a smaller wire(which will lower wire bending stress) or switching to a ribbon. If the problem persists, look for metallurgical inconsistencies. I would section wire at a break and perform micro hardness testing at the known bad part and a good part of the wire. Depending on how frequently the problem occurs, you mich perform some tensile testing.

Realizing that this is a tall order but:

1 Look to see if the fractures have a very pronounced cup and cone aspect to them.

In any type of drawn product (wire) it is possible iif due to build up on the die, the angles change and instead of 'drawing' the material gets pulled through the die, while an internal tear (rupture) is formed.

In the industry we called these internal ruptures "chevron cracks". (they look like seargeants stripes or chevrons...)

In addition to the transverse cup and cone I described, you will note that the internal texture across the section is different than that of the outemost concentric layer that held the wire together until your winding exceeded its strength...

I apologize I can't be of much more substantive help as I am travelling for the balance of the week.

But My guess is that you have intermittent internal chevroning due to final drwing passes exceeding the yield strength of the material. A look at the dies to see if there was a build up or wer ring which changed the angles and thus the flow is the likely cause.

I really doubt that it is anneal or chemistry, though it might be inclusion related( Inclusion failure will not be the geometry I described...)

Good luck with your investigation.

Milo

Are you cold coiling? Should you heat wire and mandrel when coiling? The wire was heated when it was being made, to prevent fracture.

Tungsten wire is one of the most difficult metals to fabricate with sufficient ductility to allow forming over small mandrels. I would confer with your wire supplier to make sure he understands your application. If necessary, I would consider alternate sources that may be able to fabricate a more ductile product.

I am aware of high melting point of tungsten, but is there some way to anneal the metal?