Learn Me: Sintering furnaces, oxygen-free kilns, or vacuum kilns, whichever you want to call it.

I fully realize that this may be outside the bounds of anyone on GRM, but I'm hopeful.

So, I'm gearing up for printing metal impregnated filament in order to sinter it and have metal pieces.  (not necessarily going with this shop, just an easy to find place so I can show examples)

• 6061 Aluminum
• 316L Stainless

In the instructions for the sintering operation they recommend a sintering furnace for the low low price of "less than $30,000."  Obviously, this is way out of the realm of possibility.

So, I talked to a friend of mine who does ceramics.  Gave him the outlines of the problem and he came up with a couple potential options, but very much wanted to know what other options were out there and how the problem is normally handled.

I currently have an email in to the company shown here, but while I wait for that I figured I would check in and see if anyone has any experience with this type of situation.

Hmm. They only have instructions for copper and brass sintering. 

Vacuum oven wrong get hot enough, seems to want to much time and temperature accuracy to use a foundry. 

Would you be able to cast these pieces if you printed a mold? Lost PLA casting could work, depending on intricacy. 

Paul is building an electric heat treat oven/ kiln/ furnace. He is a member here.

https://www.youtube.com/results?search_query=pauls+garage

In reply to RevRico :

They've got an "all materials" section. (see below)

I've done lost PLA before, but this has the potential to be simpler.  I also don't have anything that can get hot enough to cast anything ferrous, so this would open up more options.

I've been in MIM (Metal Injection Molding) for 25 years. We do a lot of sintering. Sintering aluminum is tricky because it doesn't really have a forgiving solidus phase where it is melty/tacky but not liquid. Good luck with it. And most sintering furnaces I work with have inert shielding gas because oxygen is not your friend at most sintering temperatures - especially stainless alloys.

In reply to Mr_Asa :

Have you seen this stuff from BASF? A bit more expensive but has a processing network set up.  316L BASF Filament

I'd like to get my hands on some MIM feedstock and play around with a screw style extruder.

In reply to RacetruckRon :

I have seen that.  The announcement of it a year or so back actually got me back on track to actually complete the setup for these types of filaments. 

In reply to Sparkydog :

What gases do you guys use? Argon? Nitrogen? Anything as long as its inert?I'd love to pick your brain about this.

In reply to Sparkydog :

Any idea were to get MIM feedstock in quantities smaller than an entire gaylord or a metric ton?  Also very curious on your input in doing this in a GRM sorta way, would a flux coating on the outside of the model work well enough to keep the model free of oxygen?

In reply to RacetruckRon :

They recommend graphite powder to hold the printed part in place, already a fairly cheap method.

Modern kiln controllers should easily be able to handle the sintering programs on that website.  If you need an inert atmosphere, I'd seal up the kiln and spray argon or nitrogen in there like back purging stainless tubes for tig welding.  Sure it will take lots of gas, but beats a $30k oven.  The Orton standalone controller I use on my heat treat oven holds dead even on a temperature in my oven, no 20 degree differential, not even 5, it stays within +-1 degree F.  Very surprized how well it works.  You can retrofit an older cheap kiln with it, build your own, or just buy a normal digital modern kiln for a grand or 2, most of them can handle ramp rates and hold very well. If you need a vacuum in there I cant help you.  Thats beyond me.  And yeah I wouldnt try a foundry, they tend to be much too fast/uneven during heating.  They want metal molten and up to temp as fast as possible to prevent gas dissolving in the metal and oxidation losses

Edit: forgot to add my heat treat oven is about $800 in materials.  Mostly the controller, then the bricks, then the kanthal elements 

In reply to Rufledt :

The backpurging is roughly the outline of what my friend that does ceramics was thinking.   We've got the kilns taken care of.

Another idea beyond backpurging, your friend may know of something called saggar firing.  Basically seal stuff up in a ceramic case (called a saggar), fill it with combustibles to eat up the oxygen, creating a reducing atmosphere, and giving the ceramics some cool colors.  Maybe putting the part in a saggar full of charcoal or something would be enough to eliminate the free oxygen for the duration of the sintering. My other idea to cut the oxygen is a bit more dangerous and probable is best avoided at sintering temperatures...

Mr_Asa said:

In reply to Sparkydog :

What gases do you guys use? Argon? Nitrogen? Anything as long as its inert?I'd love to pick your brain about this.

3 actually. Argon & Nitrogen for the portion of the sintering cycle where we are keeping the bad Oxygen away and Hydrogen for reacting with the residual binder that needs to get scrubbed out of the part you are trying to sinter.

In reply to RacetruckRon :

Try either AMP (Advanced Metalworking Practices) https://www.ampmim.com/

or Ryer in Temecula California http://www.ryerinc.com/

It sounds like the OP wants to 3D print some powdered metal feedstock and then sinter it. If that is correct then I'm excited to see what kind of GRM ideas people come up with. But it won't be easy. Everybody wants to do that. Desktop metal has a system for about $750k (https://www.desktopmetal.com/products/studio) and the printer portion of their system is sort of the closest thing to what I want that anyone has come up with thus far. But they won't sell their printer by itself and if you screw around trying to use your own binder "crayon" they claim they will detect it and shut down the printer. So maybe someone can hack that for me.

For an intensive read on what kinds of obstacles you face in trying to get powder and binder to form a shape and then make the binder go away and the powder to fuse together, try reading most of this book:

https://www.amazon.com/Injection-Molding-Metals-Ceramics-Randall/dp/187895461X

The fundamentals of what you are up against are this:

1) You need the binder to be the glue to hold the shape of what you are trying to make.

2) You can't use too much binder or the metal particles won't be touching each other and therefore won't tack weld to each other as the binder is burned off. Less binder means less glue means hard to make filament and/or hard to squirt though a nozzle.

3) The binder has to have 2 components. One that dissolves/melts/burns/reacts off quickly so as to provide some swiss cheese passageways for the 2nd part of the binder to outgas as the powder starts to fuse together and shrink into itself. The 2nd part of the binder has to survive a high enough temp that the powder tack welds together before the binder disappears. In most cases (alloys) this requires a reactive scrub gas. 

4) Control of the time/temperature to accomplish #3 usually means you need a sophisticated vacuum furnace with computer controls. So far the industry has not been able to dumb down this stuff to make it DIY friendly. 

I should also mention that on those rare occasions when all the stuff I just wrote about works correctly, it's freakin awesome!

That all sounds like wonderful witchcraft and confirms to me that I should stick to casting.  As awesome as sintering is, its beyond me!