Aluminum-Boron Composite?

One of my clients is working on a new mechanism that's going to require a high performance control system.
Whenever this happens I start suggesting exotic materials to the
mechanical engineering team. This isn't so much because I think that the product will fail without them, or that I think they're a good idea (they're 'exotic' because people are generally smart enough not to use them, after all). Rather, it's because if the ME team is thinking about how to shut up the crazy systems engineer and his ideas of exotic materials, one of the things they do is think of how to keep the resonances up with _ordinary_ materials.
(RCM-ers: basically once you've gotten all the other limiting factors out of the way, the performance of a positioning system is limited by the resonances of the hunk of stuff you're trying to shove around, so raising the resonances as high as possible, and to some extent deadening them, makes for a higher achievable bandwidth).
We've already dismissed Magnesium (goes "thunk" when you hit it, instead of "piiiiiiiiing" like Aluminum, but its modulus is low and their favorite machine shop is scared of flammable metal), Beryllium-Aluminum alloys (stiff stiff stiff, but Marketing and that danged machine shop whine about toxicity, of all things!), Stainless brace members (stiff, but too heavy, too hard to figure out how to do Al on the inside and SS on the outside), and my latest piece de resistance, Silicon Carbide (stiffer than Be-Al, but totally wacko -- you may as well build the whole thing out of glass -- but if I can make it seem to be even remotely sane then other things start looking reasonable, no?).
I recall about ten years ago a lot of press on boron fibers in an aluminum matrix making for a very stiff material. Since then I haven't heard about it, or if I have it's been one of those things that's always 10 years down the road.
Have any of you seen this used anywhere? Seen any ink on it? Have any reason to believe it's any less wacko than building a heavy machine out of silicon carbide and hoping that no one drops it off a forklift?
Thanks in advance.
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Tim
I saw a program use some kind of aluminum fiber composite about 8-12 years ago for several heat sinks for electrical PWBs. Lighter and stiffer than straight aluminum. I don't remember the supplies or what the fiber material was.
CarlBoyd
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We looked at the Boron/aluminum composite for a piezo actuator for DARPA, ended up going with 4140 steel. It cost too much and was hard to machine.
Idea: Use carbon fiber composite. Carbon composite structures are about 80% as stiff as steel but weighs 40%
How about controlling the resonance with a piezo actuator?
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It's not easy keeping up with the favored ceramic-reinforcement-of-the-month for metal-matrix composites. One that seems to have lasted is 3M's Nextel aluminum-matrix composite; it's reinforced with fibers of either zirconia or alumina, I forget which.
This stuff is pretty miserable to machine, as one would expect, but the materials suppliers offer recommendations. The last I wrote about metal-matrix composites was maybe 7 or 8 years ago, and it was all diamond tooling at that time.
Mass is low, stiffness is very high, damping...I don't know. Since it's light and stiff, I would suspect very high resonant frequencies. Call 3M.
Silicon carbide, alpha or beta, is a nightmare to work with; special ultra-high temperature sintering and diamond grinding, all the way.
Your supplier shop is wise to stay clear of beryllium-aluminum. That's for people who feel comfortable machining plutonium pits for H-bombs. <g> It is dangerous and is only justified for some aerospace applications. They used it for Formula One engine blocks for a while, but they realized quickly they had an intolerable liability on their hands and outlawed it.
-- Ed Huntress
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I do not know where to find data, but fishing rod manufacturers were using boron (or maybe it was boron carbide )with epoxy. It did not have any advantages over carbon epoxy and was a pain to work with. The boron fibers made hellacious splinters.
I believe Toyota was using kaowool or fibrafrax as a reinforcement for aluminum pistons. As I remember they used a pressure system to force the melted aluminum into a mold where the reinforcing material had been previously placed.
Dan
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wrote:

Well, it did have one advantage: Those rods were solid, not hollow, and the shafts were very skinny. They didn't have to be made over a mandrel, but the fibers did have to be cut to specific lengths in a progression. It was tricky and nobody wanted to handle the damned stuff, because those fibers could poke an inch or more into your skin. They were downright dangerous.
I tried a boron fiber fly rod at a fishing show around 15 years ago. It was very nice, but it was too quick for me. Just like getting used to graphite flyrods, they required a big change in the timing of your casts.

Yeah, there are laminations that are sintered, and forced injection/infiltration that works somewhat like reaction-injection molding (RIM) of reinforced plastics, and a powder metallurgy process. There may be other ways by now. Those methods were all in use 20 years ago.
-- Ed Huntress
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Tim Wescott wrote:

Tim: Check out M Cubed Technology. They offer an aluminum entrained silicon carbide material that is really cool stuff. Most common materials have a Youngs/mass ratio all within spitting distance of each other. Beryllium is the exception as is silicon carbide. The aluminum entrainment toughens it considerably. It is offered on a few configurations, one takes a porous sintered preform and uses a vacuum to draw the aluminum into the matrix, another is a high percentage if SiC nodules in aluminum for casting. Both must be fabricated to near net shape while green and ground to final dimension. I used it in the past as a spar for an air bearing gantry system. nicely dead, fairly tough, and incredibly stiff. Oh, BTW, it ain't cheap.....
Jeff
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On Fri, 13 Mar 2009 20:57:33 -0400, jeff wrote:

It's OK if it isn't cheap. As a systems engineer my job isn't to do the other engineer's work for them, it's to get the system as a whole to work well.
Generally I find that the best way to do motivate the design engineers is _not_ to say "just make it this good because I am a systems engineer and I am therefore closer to God than you*". Rather, I try to find _a_ solution that will take things in the direction that I want them to go. Most good design engineers would rather die than adopt, unchanged, a design from some egghead systems engineer, so by suggesting a design I establish a bar that they are very motivated to jump over. Then they go out and find _the_ solution that really solves the problem.
It's kind of a Brer Rabbit thing. Or maybe Tom Sawyer. Something like that.
* I am, of course, but it doesn't do to rub their noses in it :-).
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That's a gem, Tim.

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Just as I was going to give up on RCM because of the off topic bulltish, a topic line this comes up, and I can learn something.
Very interesting.
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Tim Wescott wrote:

If I suggest using exotic materials in my place they will definitely consider it and probably end up using it. Then they are like that. For one job we had a copper and stainless steel composite piece where the two materials were friction welded together before being machined to the final shape. The copper was used because of its thermal behaviour and the stainless steel was used for its rigidity and strength. All of this had to end up with minimal inclusions (empty pockets in the material for those that are not familiar with the term) too.
However I know where you are coming from Tim.
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<snip>

Just out of curiosity, how do you get on with warping due to temperature change with what's essentially a bimetallic strip?
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Bruce Varley wrote:

At cryogenic temperatures (4.2K) this is hardly a problem.
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Tim Wescott wrote:

Oh, you sly fox, you!

You need also to raise the resonance of the rails the moving stuff rides on.

How about giant germanium crystals? (Laminated aluminum sheet has good damping.)
Jerry
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