Cobalt tungsten carbide recast removal

we machine a lot of high cobalt tungsten carbide and i’m dealing with a layer of deposit that is about .0001-.00015 thick. the parts get assembled after this and we’re finding that this layer is screwing up our inspections leading to bad assemblies. it’s six parts per assembly so you can imagine that layer stacking up a bit.
does anyone know the composition of this layer?
thoughts on how to remove it efficiently? my research suggested extrude honing but that would a difficult sell in the current conditions. i find myself wondering if an acid of the right concentration might not be able to dissolve it without damaging the parts.
open to any suggestions.
our per piece cost is pretty low so i can spare a few on a wild goose chase if the theory is sound. free R&D, anyone?

Most likely, the recast layer consists of brass and zinc deposits. Honing or polishing will remove it but may not be practical. Additional skim cuts help to minimize the recast layer too. Some medical manufacturers use a chemical passivation process after Wire EDMing their parts but I do not know what the details are or how exactly that would work

excellent info. thanks for the reply.
i’ll do some research on the passivation process.

The amount of .00015 is relatively nothing…With all due respect,unless y’all are dealing with a Spaceship this should not be a issue with anything but Aerospace…Maybe ask for a deviation so you can open that tolerance a bit…Remember a hair is .003,so your customer needs to know it is fretting over a hair in lets say 20 to 30 pieces.My 2 cents.

We deal with removing and qualifying recast removal on many of our customers orders . The best
way to go , especially if there is no masking involved is to use a acid dip . I recommend giving this
Company a call . Precision Aerospace Corp. in Rancho Cucamonga Ca . Talk to Mike Archuleta and
he will help you out . I am sure there are other Company’s that do this but this is the one I use
most often . 909-980-8855.

Chris Marks
Wire Cut Company Inc.

Thanks, I’ll definitely give them a call.
I’ve tried acid dipping them in house with citric acid (which we use for lime scale removal) and AC500.
The AC500 is phosphoric acid that we use for general parts cleaning.
Both do a great job of removing brass recast, but not so great at removing the cobalt recast.
Maybe this company can help us get them both.

I should probably mention that I did finally get through to our EDM guru at our machine supplier who explained that part of the recast was deposits from the wire, brass, and that part would be the cobalt binder we use in our carbide.
The acids will react with the cobalt given enough time, but to remove the recast without damaging the surface layer of cobalt seems tricky. Also it turns the parts pink.
He actually suggested a combo of an acid dip in the AC500 and scrubbing with a soft bristle toothbrush. My boss is still hoping for the manual scrubbing/sanding free solution. I’m getting skeptical that that exists.

Your Welcome , its sound like your willing to do it yourself . He may even explain what Acid he uses
and how you can do it .
Good Luck !

Chris Marks
Wire Cut Company Inc.

Another method of removing the recast layer is via glass bead blasting (mechanical passivation). I suggest a number 7 or smaller glass impact bead . We have good results with this method. We too have found that AC500 mixed with DI water in an ultrasonic cleaner only yields mediocre results and stains the carbide.

You must use care when bead blasting to protect sharp edges by masking the non-wired sides and keeping the air pressure to an absolute minimum.

I have used in the past a micro sand blaster with a soda blast media like Arm and hammer.
Also might try some samples in citric acid passivation . Instead of the usual nitric acid they can use citric acid .

You can also use Cratex sticks . They are rubber sticks with different sizes of diamond dust .
All you do is rub you part with the sticks.
They are easy to buy at mscdirect.com or Amazon or just type "cratex sticks " in Google.

sand blasting with baking soda is a new one on me. sounds interesting.
i did try citric acid as well as the ac500 and didn’t really like the results.
i am a big fan of cratex in general. in the case of these parts, we can flat lap them pretty easily so if we go for one at a time mechanical that would be the route for us.
what we’ve been doing lately is a sort of pre assembly method. we push all six pieces through the assembly sleeve and all the way out. then wash them in the ac500 again. this seems to break up the layer pretty well on all the interfaces.
i really appreciate the ideas. might give the soda blasting a try if i can convince the boss.

Hello RSOOTER,

Here are some things to do to minimize.

-Your machine should have technology for 4 cuts. (I rough and 3 Skim cut) Please use this technology.

-Use a Zinc Coated Wire (Silver color)

-Bring your conductivity down to 5 micro or lower(3 would be best)

-This should bring your layer down to ,00005…For most machines.

Nick. C

Make sure that you use a micro sand blaster not a regular sand blaster that has a 80 to 90 PSI .
Micro blaster are low in PSI that way you don’t over do it specially on the sharp edges .
You can and should mask with painter’s tape so you concentrate only on areas where the EDM is

Check out this website
edmmax.com/hgh-micro-blasting-results.asp

Download the HGH MICRO BLASTER PRESENTATION
on page 7 of 19 you will find “the recast layer is removed with out damaging the surface or sharp edges .”
I didn’t use this particular machine back then.
We had a micro blaster from comcoinc.com/microblaster/
I recall doing several steps like an ultra fine blast media then arm and hammer and sometimes I also used micro glass beads to get like a lap finish.
I would also have some dummy parts to test out my procedure along with a good microscope.