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closed impellor brazing?

I want to make small closed-type impellors for pumping cryogenic liquids (LOX, perhaps later LH2) under high stress, low weight conditions. This is for experimental purposes, at least at first. Suggestions for a production version are also welcome.

It's part of my ongoing effort to make turbopumps for bipropellant cryoliquid model rocket engines. :) I'll probably use mini-jet turbine parts for the turbines (if Wren ever answer my emails ...).

The impellor has complex interior surfaces, so I thought of making it up from computer generated plasma/laser (?) cut ~1 mm layers of 304 (?) stainless. I'd put removable tabs on the outside of each layer in order to get the registration right, and bulk braze them together to form a whole. Then heat-soak for a while, partly to distribute stresses and partly to allow the braze and the base metal to mingle.

Any suggestions for a brazing material?

I'd then like to coat all the interior surfaces in a lower melting braze, in order to smooth the interior surfaces - basically two-step brazing, except the second stage is just a covering/smoothing layer.

Again I'm looking for suggestions for a brazing material. I thought of using

449 and 456, any comments? Any sources for 449 in small quantities?

Any other suggestions, comments?

The finished impellors will have to pump LOX at cryogenic temperatures.

Thanks,

Reply to
Peter Fairbrother
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I don't think brasing ( or silver soldering which is the closest equivelent that work on stainless ) would be suitable.

You might be able to get the impelor " rapid prototyped " in wax and use that to investment cast the impeler, but it's not going to be cheap :-(

Reply to
Jonathan Barnes

Your best option is to get the stainless tig welded, you don't want anything that might be upset by the lox...brazing, dissimilar metals etc... With tig, you can fabricate the impellor. Contact me off group, as we have Rapid Pro facilities here, if you should want a wax prototype.

Joules

Reply to
Joules

Sorry, I can't help you, but it does all sound very intriguing. Do keep us informed of progress!

Reply to
Airy R. Bean

I don't think a wax could be made in one piece.

The impellor has top and bottom horizontal surfaces, with curved vertical vanes joining them. There is no way to get at the inside corners.

It could be through-laser welded perhaps, machine the bottom surface and the vanes from solid and weld on the top surface, with a bit of trial and error, although that would be expensive - but I don't think tig would do it.

I am not an expert though, brazing is about my limit of skill - I made my first weld only a few days ago - so I'd like to hear if I'm wrong.

Jonathan Barnes suggested lost wax casting - would that be expensive? I could supply the wax formers, and the castings would weigh about 75 grams, 3 ounces. One problem is likely to be the complex interior shape though, and another is that the interior surfaces have to be smooth. Could a foundry do that?

Anything other suggestions? Thanks,

Reply to
Peter Fairbrother

Perhaps you should let your keyboard do the research and check out lost wax casting?

Tom

Reply to
Tom

temperatures.

You can get a good finishon a casting by first bead blasting and then electro polishing.

If this is a high speed application getting the precision to minimise vibration could be a problem.

Many difficult shapes can be formed with very high precision and very good surface finish by spark erosion, but it's a costly process.

It might be worth your while contacting an investment casting company. S&T Precision castings ( tel 01536 265 351 ) quote +- 0.005 per inch as the linear tolerance for casting in an old flyer I have.

Reply to
Jonathan Barnes

temperatures.

You can get a good finishon a casting by first bead blasting and then electro polishing.

If this is a high speed application getting the precision to minimise vibration could be a problem.

Many difficult shapes can be formed with very high precision and very good surface finish by spark erosion, but it's a costly process.

It might be worth your while contacting an investment casting company. S&T Precision castings ( tel 01536 265 351 ) quote +- 0.005 per inch as the linear tolerance for casting in an old flyer I have.

Reply to
Jonathan Barnes

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