Passivation and alloying element retention in gas atomized powders
A method for gas atomization of a titanium alloy, nickel alloy, or other alumina (Al 2 O 3 )-forming alloy wherein the atomized particles are exposed as they solidify and cool in a very short time to multiple gaseous reactive agents for the in-situ formation of a passivation reaction film on the atomized particles wherein the reaction film retains a precursor halogen alloying element that is subsequently introduced into a microstructure formed by subsequent thermally processing of the atomized particles to improve oxidation resistance.
1. A method of atomizing an Al 2 O 3 -forming metal alloy, comprising gas atomizing a molten Al 2 O 3 -forming alloy to form a spray of atomized particles in an atomization chamber, first exposing the atomized particles in the atomization chamber downstream of an atomizing nozzle to a gaseous first reactive species to form a reaction product as a surface layer on the atomized particles and then exposing the surface layer on the atomized particles to a gaseous second reactive halogen species in the atomization chamber in a manner that an amount of the second reactive halogen species is retained in the surface layer on the atomized particles.
2. The method of claim 1 wherein the alloy is selected from the group consisting of a titanium alloy and a nickel alloy.
3. The method of claim 2 wherein the titanium alloy comprises titanium aluminide.
4. The method of claim 2 wherein the nickel alloy comprises a nickel base superalloy.
5. The method of claim 2 wherein the first reactive species is oxygen.
6. The method of claim 5 wherein the surface layer comprises an oxide product including titanium, aluminum, oxygen and a halogen element.
7. The method of claim 5 wherein the reaction product comprises an oxide including nickel, aluminum, oxygen, and the halogen element.
8. The method of claim 1 wherein the second reactive halogen species comprises fluorine.
9. The method of claim 1 including the further step of thermally processing the atomized particles in a manner to form a consolidated microstructure in which a halogen element is introduced from the surface layer into the microstructure to improve oxidation resistance.
10. The method of claim 9 including thermally processing the particles by one or more of hot isostatic pressing, pressing and sintering, spark plasma or other sintering, extrusion, metal particle injection molding, and additive manufacturing techniques including at least one of laser engineering net shaping, electron-beam engineering net shaping, and direct laser or electron-beam metal sintering.
11. The method of claim 1 wherein the molten alloy is atomized using an atomizing nozzle having inert gas jets.
12. The method of claim 1 wherein the gaseous first reactive species is injected into the atomization chamber by a first injection halo and the gaseous second reactive species is introduced into the atomization chamber by a second injection halo disposed downstream of the first injection halo.
13. The method of claim 1 wherein the atomization chamber is evacuated and backfilled with an inert gas before atomizing the molten alloy to promote an inert atmosphere to which the first reactive species and the second reactive species are introduced by injection halos flows.
14. The method of claim 1 including the further step of consolidating the particles.