Rocket Alloys
GrCop-42 is a novel alloy composition developed by NASA for use in extreme environments, such as those found in rocket propulsion systems. This copper alloy was designed to withstand high temperatures, exhibit excellent mechanical properties, and offer improved oxidisation resistance, to ensure its suitability in rocket engines and other high-performance components.
Oxygen Content Too High
For high performance alloys such as GrCop-42, gas atomisation is a key process to produce the desired particle size, morphology and purity required for advanced manufacturing techniques such as DED and L-PBF. But what happens when these alloys fall out of specification?
Demonstrated oxygen reduction in GrCop-42 from 980ppm to 330ppm using the 235HT Flo-Bed
It is well established that due to the imperfect inert gas atmospheres in metal 3D printing machines powder not included in the built part becomes progressively oxidised until oxygen levels in the component exceed the
specified maximum.
MPP Ltd has produced tonnes of the alloy GrCop-42. This Cu-Cr-Nb alloy is difficult and expensive to manufacture due to the difficulty in incorporating the chromium and niobium alloying elements into the melt at the very precise levels required.
This leads rocket motor manufacturers such as SpaceX and Blue Origin to consider lower cost but lower performing alternatives. GrCop-42 is not only susceptible to oxygen pickup, not just in the printing process but also during even brief handling in air.
MPP now routinely recovers GrCop-42 powders in production-scale quantities in its in-house FBR, Flo-Bed model 235HT, for rocket nozzle manufacturers.
GrCop-42 powder produced on HERMIGA 120/250 V3I
Fluidising in reductant gas mixes typically lower oxygen levels from approx 900ppm to <400ppm, well inside the specification of newly made powder of one particularly rocket nozzle builder.
Considering that, in thermodynamic terms, oxygen is relatively loosely bound to copper, it is easy to imagine that oxide reduction takes place relatively easily. However, fine powders typically used in metal AM lie in the Geldart Category C range (See “From powder modification to rejuvenation: Fluidised Bed Reactors in metal powder production and Additive Manufacturing”) which means they are “cohesive” due to surface forces. Freshly reconditioned powders, due to the clean surfaces generated, are even more so. Therefore, reductant gas mixtures, and the flow and pressure regime within the FBR must be carefully controlled to produce a recovered powder of suitable flowability.
Oxide Reduction from 270ppm to 90ppm in Stainless Steel
In metal AM stainless steels are probably the most widely encountered powdered alloys. One might expect that the chromium oxide that confers such steels with their resistant property would be resistant to the lowering of oxygen in out-of-spec stainless steel powders.
Chromium oxide is thermo-dynamically stable due to the high negative heat formation of the oxide from the metal. Alloy 300M, a stainless grade with the addition of silicon, molybdenum, manganese and vanadium, has been the subject of an investigation at PSI to see if oxidised, out-of-spec powder could be recovered for re-use. Processing in the MPP 235HT FBR demonstrated reduction in oxygen levels from 270ppm to 90ppm.
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