Hot Isostatic Pressing Primer? 1

[blueandwhiteg3]

Can anybody give me a quick primer on hot isostatic pressing?

I have an application where I would like to explore making a part using some kind of a metal-matrix composite / cermet. The objective is extreme hardness (for abrasion resistance) combined with a little more impact tolerance than most ceramics have, and as low a weight as reasonably possible. I do not have extreme tolerances here.

I have located a facility not too far from me that can handle the actual HIP part of the process. It's surprisingly inexpensive on a per-pound basis, particularly given that my part is unlikely to exceed 1 lb. Normal runs are 1680° F to 2180° F, and I believe "normal" pressure is 15 KSI.

I just need to supply them with the material prepared in such a way that it will go through their process without releasing anything (e.g. plastics, binders, aluminum, etc. are all issues.) If I have to go above the normal pressure or temperature, cost goes through the roof as I have to do custom runs.

The problem is that I simply don't know about this field. I would appreciate it if somebody could point me in the right direction on a few points:

1) The part is sheet-like, with a thickness of about 1 mm. Suggestions as to the kinds of materials I should be thinking about for this, given my needs?

2) Once powders are obtained and mixed, how are they formed to be 'ready' for HIP processing? Heat? Chemical binders? Pressure?

 

[CoryPad]

1) How about SiC, SiO2, or Al2O3 as the high hardness phase? A hard steel for the matrix? All are low cost.

2) Cold pressing, hot pressing

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

As I understand it, parts are sealed inside a chamber that probably used to be a gun barrel breech, then the chamber is filled with high pressure gas and the gas is heated by a calrod. I thought the most common gas was hydrogen, chosen for its thermal conductivity and price. Wikipedia says argon. That's the basics of the process.

The effect of the process is to collapse any internal voids, by virtue of the combination of high temperature and high pressure. E.g. a cube with a spherical cavity inside it will come out completely solid, and smaller... PROVIDED that the internal cavity has NO communication with the outside surface. So, if the cube is made from metal in two halves, the halves must be welded together. I think the collapsing cube was featured (and sectioned) in the ASME house rag some years ago.

I think porous/powder parts can be HIPed by first sealing their surfaces, e.g. by electroplating or electroless plating a metal, e.g. nickel, on their exterior.



Mike Halloran
Pembroke Pines, FL, USA

 

[blueandwhiteg3]

Thanks for the info. I am taking off some of your data and kind of asking questions... it's not to argue, just to glean understanding.

Also, I'm not very concerned about materials cost on a unit-basis, given that we're talking about fairly low total mass on the part, but I need to try and simplify the production as much as possible.

A) My tests with silicon carbide ceramics show they are rather brittle, I am learning towards silicon nitride which has proven more impact tolerant. Boron nitride is another possibility.

If we're talking about silicon dioxide, it isn't quite abrasion resistant enough, or at least more precisely, most glasses I have worked with aren't.

Alumina is markedly denser than most nitrides, unless it has some benefit I can't see, I'd tend to prefer a nitride.

B) For the metal matrix, I don't know if steel is the best choice. I'm concerned about rusting, and I don't know it has the best strength-weight ratio.

I am wondering about using titanium. I can't think of anything else that achieves a better strength/weight ratio at a relatively light weight.

Aluminum and similarly low-melting point metals are a no-go per the HIP company, throwing out a lot of the lightweight options.

I can't think of a ton else; nickel, molybdenum, etc. are sometimes used in cemented carbides as they have good wetting and even slight solubility with the carbide.

Tungsten is very dense, but also extraordinarily tough. However, I suspect the melting point precludes it from being useful.

C) I'm trying to roll the thing as simply as possible. Ideally, I'd like to actually be able to craft some kind of re-usable tooling by hand and/or based on a casting. I don't know what kind of pressures and temperatures are required for basic sintering suitable to prepare for HIP.

Steel tooling for pressing could be a challenge to prepare. Is it possible to build tooling out of something very easily workable? Aluminum would be nice, I could probably cast the tooling easily.

If not that, how viable is some variation on metal injection molding on a small scale?

 

[MikeHalloran]

I was thinking maybe you could draw a shallow cup out of something like the stainless foil that heat treaters use, pour in your mix, then apply a less shallow and slightly smaller cup of the same material, and crimp or EB weld the edges all around.

Warning; stainless foil is nasty to work by hand; you'll leave some blood behind.

Aluminum tooling is going to wear out instantly in contact with hard materials. Some urethanes do well in metal contact. You'll probably end up with steel tooling.



Mike Halloran
Pembroke Pines, FL, USA

 

[blueandwhiteg3]

Mike,

So you're suggesting that stainless foil might do the trick? Do you mean I would create a stainless steel foil mold almost, seal it off, then send off for HIP? Or that I would take this form and use it to heat sinter? With work (and sturdy gloves), I believe I could get a nice form together using stainless foil.

If I can sinter using heat alone (or with just a matter of a few to tens of pounds per square inch), I have a ton of good ideas. There's a lot of nice ceramics I have used in the past which tolerate mind-boggling high temperatures. However, I would need an idea of the kind of temperatures I need to sinter by heat alone. Anybody have ideas here?

Yes, I know aluminum tooling is not the best choice, but what about for a few prototypes? That's the primary goal here, producing just a handful of prototypes.

Another possibility that occurred to me was glass. Although this would not tolerate HIP, I suspect if I made thick tooling out of it (the design is not complex and I know people who work with glass extensively) it could be pressed at least somewhat and tolerate moderate temperatures.

Steel tooling is a non-issue if the prototypes come out well and I decide I want more than a few units. However, I don't think I can manage to do steel tooling myself.

I was not aware of urethane possibilities. I think it's similar to aluminum, it won't hold up for production, but might be good to prototype. Potentially, could I work with some kind of mixed-on-site urethane casting resin which sets at low temperatures?

Also, the discussion of glass makes me wonder if I could slip a little glass powder into the mixture to help "bind" everything together during sintering, without causing issues for the HIP processing. I know I could easily

 

[blueandwhiteg3]

If anybody could provide indications as to the kinds of temperatures and pressures involved in sintering common material combinations or your proposed arrangements, it would be useful.

One final note... thanks for the comments everybody. I really appreciate your willingness to share information. I know it's easy to say "just Google it" but the reality is that a few posts from intelligent forum members can be more useful than hours and hours of Googling around.

Once I get a handle on a new topic like this, I'll be a lot more able to work things out myself, but sometimes entirely new arenas take a little warm intro.

Thanks everybody!

 

[MikeHalloran]

Yeah, the stainless foil becomes a mold.. and the outer surface of the finished part you get back. No binder required, and no sintering step before HIP.



Mike Halloran
Pembroke Pines, FL, USA

 

[blueandwhiteg3]

Mike,

I just realized that I missed your first post, because I made a post at the same time. Yes, that is basically what I expected, I just need to keep the air spaces as small as possible, and everything will essentially contract within the same shape, and if the air spaces are very minimal, I'd end up with nearly none.

I'm working with a sheet-like part, so I think that the foil approach would be very simple and useful to test possible matrix compositions. The cost of HIP is like $4 a pound, so this is a dirt cheap way to see how the foil and various metal matrixes perform.

Is stainless the best foil to work with? Or would another foil be preferable? I was considering copper, because if I went with a lower temperature sinter, it wouldn't melt, and copper tends to be friendly to work with. Obviously, it would be a lot heavier than stainless, but this may not be too big a concern.

I am also liking the idea of a high temperature ceramic plaster type material, and doing a high-heat, minimal pressure sintering/fusing in a furnace myself, then just turn over a fused-but-not-solid unit. I could also even just put the whole thing in for HIP although the weight of the mold would increase the HIP cost significantly, I don't think it would be an issue for prototypes. I guess I would just have to be very, very sure that the plaster has no off-gassing and no air spaces!

I would think high temperature plaster type material would be a good way to make several identical prototypes, versus foil which is good for basic testing. I suspect it would also give me better "control" - meaning, I can use mild pressure and lots of heat to fuse things together and yield a pretty high density, so the shrinkage during the HIP is minimized somewhat.

However, all this discussion of the means through which to sinter or even avoid sintering entirely and go right to HIP leaves me wondering about materials.

Anybody up to warm me up a bit more on suggestions for lightweight, strong, very hard matrix composites?

I keep thinking about a titanium-boron nitride blend would be pretty optimal, at least on paper. Light weight, great strength, stiffness, impact resistance on the titanium, and extremely low weight and extreme hardness on the boron nitride. I'd tend to think about like a Ti6Al4V alloy, for its excellent strength and stiffness, but I'm no titanium expert.

The question becomes whether this is viable physically, and if so, what kind of temperature and pressure would be appropriate.

 

[CoryPad]

Your initial post provided no requirements for the final design, so I suggested low cost materials. If you have mass, corrosion, temperature, whatever requirements, it would be good to share those.

Titanium plus silicon nitride or boron nitride should be low mass and somewhat wear resistant.

When you say you are trying to roll the thing as simply as possible, you don't mean that you are trying to roll sheet metal by deformation processing, right? Your composite will have essentially zero formability.

Bodycote can do aluminium alloy HIP.

A typical HIP process for titanium would use a temperature of ~ 1000 [°]C and a pressure of ~ 100 MPa.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[blueandwhiteg3]

Cory,

Thanks for posting again. I cannot say why, but the Bodycote labs near me insists that aluminum is a no-go in their standard runs. The only way I could run aluminum would be a custom run, due to concerns around cross-contamination.

In any case, titanium still seems like a superior choice over aluminum, due to strength-weight ratios.

I appreciate your suggesting least-cost options first, and I'm sorry if I was unclear as to my needs and objectives. In my original post I wrote: "The objective is extreme hardness (for abrasion resistance) combined with a little more impact tolerance than most ceramics have, and as low a weight as reasonably possible."

I will add to this, as I am striving for surfaces around 1 mm thick on the part, I need to pack a lot of strength into a small areas, so it is more important to achieve high strength and hardness than low density. This is why I had given real consideration to something extremely dense, such as tungsten.

When I say I'm trying to "roll this as simply as possible" I am speaking metaphorically. I won't be modifying the shape of the finished part, I definitely know it will be quite unworkable.

Anyhow, given the parameters desired:
-High abrasion resistance (similar to boron nitride, silicon nitride, silicon carbide, boron carbide, etc.)
-Good strength, toughness to moderate impacts (greater than pure ceramics listed above)
-Cost is not important within reason ($100/kg or less would be nice for materials costs)

I would welcome suggestions, as to materials, ratios, and particle size.