Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2002-06-14
2004-05-18
Mai, Ngoclan T. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S038000, C075S351000, C075S370000, C075S371000
Reexamination Certificate
active
06737017
ABSTRACT:
This invention relates to the preparation of metallic-alloy articles, such as titanium-alloy articles, without melting of the metallic alloy.
BACKGROUND OF THE INVENTION
Metallic-alloy articles are fabricated by any of a number of techniques, as may be appropriate for the nature of the article. In one common approach, metal-containing ores are refined to produce a molten metal, which is thereafter cast. The ores of the metals are refined as necessary to remove or reduce the amounts of undesirable minor elements. The composition of the refined metal may also be modified by the addition of desirable alloying elements. These refining and alloying steps may be performed during the initial melting process or after solidification and remelting. After a metal of the desired composition is produced, it may be used in the as-cast form for some alloy compositions (i.e., cast alloys), or further worked to form the metal to the desired shape for other alloy compositions (i.e., wrought alloys). In either case, further processing such as heat treating, machining, surface coating, and the like may be utilized.
The production of metallic alloys may be complicated by the differences in the thermophysical properties of the metals being combined to produce the alloy. The interactions and reactions due to these thermophysical properties of the metals may cause undesired results. Titanium, a commercially important metal, in most cases must be melted in a vacuum because of its reactivity with the oxygen and nitrogen in the air. In the work leading to the present invention, the inventors have realized that the necessity to melt under a vacuum makes it difficult to utilize some desirable alloying elements due to their relative vapor pressures in a vacuum environment. The difference in the vapor pressures is one of the thermophysical properties that must be considered in alloying titanium. In other cases, the alloying elements may be thermophysically incompatible with the molten titanium because of other thermophysical characteristics such as melting points, densities, chemical reactivities, and tendency of strong beta stabilizers to segregate. Some of the incompatibilities may be overcome with the use of expensive master alloys, but this approach is not applicable in other cases.
There is therefore a need for an improved method to make alloys of titanium and other elements that present thermophysical melt incompatibilities. The present invention fulfills this need, and further provides related advantages.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for preparing an article made of an alloy of a metal such as titanium with a thermophysically melt-incompatible alloying element. The present approach circumvents problems which cannot be avoided in melting practice or are circumvented only with great difficulty and expense. The present approach permits a uniform alloy to be prepared without subjecting the constituents to the circumstance which leads to the incompatibility, specifically the melting process. Unintentional oxidation of the reactive metal and the alloying elements is also avoided. The present approach permits the preparation of articles with compositions that may not be otherwise readily prepared in commercial quantities. Master alloys are not used.
An article of a base metal alloyed with an alloying element is prepared by mixing a chemically reducible nonmetallic base-metal precursor compound of a base metal and a chemically reducible nonmetallic alloying-element precursor compound of an alloying element to form a compound mixture. The alloying element is preferably thermophysically melt incompatible with the base metal, but both thermophysically melt incompatible and thermophysically melt compatible alloying elements may be present. The method further includes chemically reducing the compound mixture to a metallic alloy, without melting the metallic alloy, and thereafter consolidating the metallic alloy to produce a consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article.
The nonmetallic precursor compounds may be solid, liquid, or gaseous. The chemical reduction is preferably performed by solid-phase reduction, such as fused salt electrolysis of the precursor compounds in a finely divided solid form such as an oxide of the element; or by vapor-phase reduction, such as contacting vapor-phase halides of the base metal and the alloying element(s) with a liquid alkali metal or a liquid alkaline earth metal. The final article preferably has more titanium than any other element. The present approach is not limited to titanium-base alloys, however. Other alloys of current interest include aluminum-base alloys, iron-base alloys, nickel-base alloys, and magnesium-base alloys, but the approach is operable with any alloys for which the nonmetallic precursor compounds are available that can be reduced to the metallic state.
In another embodiment, a method for preparing an article made of titanium alloyed with an alloying element comprises the steps of providing a chemically reducible nonmetallic base-metal precursor compound of titanium base metal, and providing a chemically reducible nonmetallic alloying-element precursor compound of an alloying element that is thermophysically melt incompatible with the titanium base metal, and thereafter mixing the base-metal precursor compound and the alloying-element precursor compound to form a compound mixture. The method further includes chemically reducing the compound mixture to produce a metallic alloy, without melting the metallic alloy, and thereafter consolidating the metallic alloy to produce a consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article. Other compatible features described herein may be used with this embodiment.
The thermophysical melt incompatibility of the alloying element with titanium or other base metal may be any of several types, and some examples follow. In the alloys, there may be one or more thermophysically melt incompatible elements, and one or more elements that are not thermophysically melt incompatible with the base metal.
One such thermophysical melt incompatibility is in the vapor pressure, as where the alloying element has an evaporation rate of greater than about 100 times that of titanium at a melt temperature, which is preferably a temperature just above the liquidus temperature of the alloy. Examples of such alloying elements include cadmium, zinc, bismuth, magnesium, and silver.
Another such thermophysical melt incompatibility occurs when the melting point of the alloying element is too high or too low to be compatible with that of titanium, as where the alloying element has a melting point different from (either greater than or less than) that of titanium of more than about 400° C. (720° F.). Examples of such alloying elements include tungsten, tantalum, molybdenum, magnesium, and tin. Some of these elements may be furnished in master alloys whose melting points are closer to that of titanium, but the master alloys are often expensive.
Another such thermophysical melt incompatibility occurs when the density of the alloying element is so different from that of titanium that the alloying element physically separates in the melt, as where the alloying element has a density difference with titanium of greater than about 0.5 gram per cubic centimeter. Examples of such alloying elements include tungsten, tantalum, molybdenum, niobium, and aluminum.
Another such thermophysical melt incompatibility is where the alloying element, or a chemical compound formed between the alloying element and titanium, chemically reacts with titanium in the liquid phase. Examples of such alloying elements include oxygen, nitrogen, manganese, nickel, and palladium.
Another such thermophysical melt incompatibility is where the alloying element exhibits a miscibility gap with titanium in the liquid phase. Examples of such alloying elements include the rare earths or rare-earth-like elements s
Ott Eric Allen
Shamblen Clifford Earl
Woodfield Andrew Philip
Ehresman Kurt L.
Garmong Gregory O.
General Electric Company
Mai Ngoclan T.
McNess Wallace & Nurick LLC
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