Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-03-19
2003-04-22
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
Reexamination Certificate
active
06551377
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to substantially spherical powders of rhenium optionally alloyed with tungsten or molybdenum and the process by which such powders are produced.
Rhenium (mp 3,180° C.; D 21.04 g/cc) is a refractory metal that has no known ductile-to-brittle transition temperature and a high modulus of elasticity. Components formed from rhenium can withstand repeated heating and cooling cycles without incurring mechanical damage. For these and other reasons, rhenium is often used to manufacture thrust chambers and nozzles for rockets used on spacecraft and other critical components. An example of a thrust chamber having a body formed of rhenium is disclosed in Chazen et al., U.S. Pat. No. 5,720,451.
It is well known that rhenium can be alloyed with tungsten or molybdenum to impart improved ductility and other desirable properties to such materials. Alloys of rhenium and molybdenum typically containing 41-47.5% by weight rhenium are used in the electronics, aerospace, and nuclear industries. Alloys of rhenium and tungsten typically containing 3-5% or 26% by weight rhenium are used, for example, in the electronics industry as filaments and thermocouples.
Rhenium is derived primarily from the roasting of molybdenum concentrates generated in the copper mining industry. During the roasting of molybdenite, rhenium is oxidized and carried off in the flue gases. These gases are scrubbed to remove the rhenium, which is then recovered in solution using an ion exchange process. The rhenium solution is then treated and neutralized with ammonium hydroxide to precipitate ammonium perrhenate. Ammonium perrhenate can be reduced in a hydrogen atmosphere to form rhenium metal powder.
Rhenium metal powder derived in the manner thus described consists of discrete particles that have a random shape and an uneven surface texture. The particles, when viewed under high magnification, resemble flakes. For purposes of clarity, throughout the instant specification and in the appended claims such material shall be referred to as rhenium powder flakes.
Rhenium powder flakes exhibit very poor flow characteristics, have a relatively low density (typically only 15% of theoretical density), and contain approximately 1,000 ppm or more of oxygen. Due to these inherent properties and characteristics, it has heretofore been very difficult to manufacture rhenium components via conventional powder metallurgy techniques. In general, only relatively simple shapes such as rods, bars, plates, and sheets could be produced. To produce complex shapes, rhenium in the form of these simple shapes had to be machined to specified dimensions and tolerances. The machining of rhenium is also problematic and it results in the creation of a significant amount of scrap, which is extremely cost ineffective. Numerous attempts to produce components of complex shape using near-net-shape powder metallurgy techniques have met with very limited success over the years. Some of the problems associated with the fabrication of products using rhenium powder flakes are discussed in an article entitled Mill Products and Fabricated Components in Rhenium Metal and Rhenium Rich Alloys, by Jan-C. Carl én, Rhenium and Rhenium Alloys, B. D. Bryskin, Editor, The Minerals, Metals & Materials Society, 1997, p. 49-57, which is hereby incorporated by reference.
It would be highly desirable to be able to produce rhenium and rhenium alloy components having complex shapes using conventional near-net-shape powder metallurgy manufacturing techniques such as, for example, vacuum plasma spraying, direct-hot isostatic pressing, directed light fabrication, and metal injection molding. In order to use these techniques, special powders are needed. Such powders should exhibit good flow characteristics, have a higher density than rhenium powder flakes, and should preferably contain a minimum amount of oxygen.
SUMMARY OF INVENTION
The present invention provides powders comprising substantially spherical particles consisting essentially of at least about 10% by weight rhenium optionally alloyed with up to about 90% by weight tungsten or up to about 60% by weight molybdenum. Preferably, the spherical particles have an average diameter of less than about 150 microns, and more preferably, an average diameter within the range of from about 10 to about 50 microns. The powders can also have a bimodal or multi-modal particle size distribution. The powders according to the invention exhibit good flow characteristics and can be used to fabricate components of complex shape using conventional powder metallurgy techniques.
In one embodiment of the invention, the spherical particles consist essentially of rhenium. In another embodiment, the spherical particles consist essentially of an alloy of from about 15% to about 35% by weight rhenium with the balance being tungsten. In yet another embodiment, the spherical particles consist essentially of an alloy of from about 35% to about 60% by weight rhenium with the balance being molybdenum.
The spherical powders according to the invention exhibit excellent flow characteristics. In addition, the spherical powders according to the invention have a significantly greater density than powder flakes. Moreover, the spherical powders according to the invention have a reduced oxygen content as compared to powder flakes. Thus, the spherical powders according to the invention are particularly well-suited for use in conventional powder metallurgy techniques such as, for example, vacuum plasma spraying, direct-hot isostatic pressing, directed light fabrication, and metal injection molding.
The present invention also relates to a process for producing a powder comprising substantially spherical metal particles. The process comprises: providing flakes consisting essentially of at least about 10% by weight rhenium and optionally up to about 90% by weight tungsten or up to about 60% by weight molybdenum; entraining said flakes in a stream of gas for transport to an induction plasma torch; creating a plasma in said stream of gas to melt said flakes into droplets; permitting said droplets to cool so as to form discrete substantially spherical solid particles; and collecting said particles. In one embodiment, the process can be used to manufacture about 70 g. of powder per minute.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
DETAILED DESCRIPTION
As noted above, the flakes used in the process consist essentially of at least 10% by weight rhenium and optionally up to about 90% by weight tungsten or up to about 60% by weight molybdenum. Rhenium powder flakes can be obtained via the reduction of ammonium perrhenate in a hydrogen atmosphere as described in an article entitled Powder Processing and the Fabrication of Rhenium, by Boris D. Bryskin and Frank C. Danek, Journal of Materials, Jul. 19, 1991, pages 24-26, which is hereby incorporated by reference. Peters et al., U.S. Pat. No. 3,375,109, which is also hereby incorporated by reference, discloses methods of obtaining pre-alloyed powders of rhenium and tungsten or molybdenum.
Rhenium powder flakes can more conveniently be obtained from Rhenium Alloys, Inc. of Elyria, Ohio, which sells rhenium powder flakes in several grades and particle sizes. The −200 mesh powder metallurgical grade of rhenium powder flake possesses a purity of 99.99%, an average particle size of about 3.5 &mgr;m, an apparent density of about 1.84 g/cm
3
, and a tap density of about 3.03 g/cm
3
. As noted above, rhenium powder flakes of this type have a rough surface texture and thus exhibit poor flow characteristics.
An induction plasma torch must be used to prepare the powders according to the invention. The preferred induction plasma torches for use in the process of the present invention a
Mai Ngoclan
Rankin, Hill Porter & Clark LLP
Rhenium Alloys, Inc.
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