Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1999-07-20
2001-06-19
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C419S032000, C419S062000, C419S066000
Reexamination Certificate
active
06248150
ABSTRACT:
BACKGROUND—FIELD OF INVENTION
This invention relates to tungsten-containing articles developed as alternatives to those traditionally made of lead and lead alloys.
BACKGROUND—DESCRIPTION OF PRIOR ART
Production of high-density, tungsten-containing materials by conventional powder metallurgical methods is a mature technology which is routinely used to produce a family of materials with relatively high densities. Of particular relevance to the present invention are a variety of materials developed to replace lead and its alloys. Most of these materials are produced by using a series of conventional powder metallurgical processes, for example, (1) selecting graded and controlled metal powders to be combined with graded and controlled tungsten powder to obtain a desired bulk composition, (2) blending the mixture (with or without the addition of lubricants or “binders”), (3) flowing the resulting mixture into a die cavity, (4) applying pressure to the mixture to obtain a mechanically agglomerated part (referred to as a “green compact”), (5) sintering the green compact in a furnace maintained at or near the melting temperature of one or more of the powder constituents to effect metallurgical bonding between adjacent particles, thereby increasing density and strength, and (6) finishing the sintered part by mechanical and/or chemical methods. Conventional tungsten powder metallurgy is at least as old as Colin J. Smithell's U.S. Pat. No. 2,183,359 which describes a family of alloys comprised of tungsten (W), copper (Cu) and nickel (Ni). Tungsten powder metallurgy has matured to include alloys such as W—Co—Cr, W—Ni, W—Fe, W—Ni—Fe et al. which are produced commercially by a large number of companies.
More recently, a variety of materials have been developed for the general purpose of offering alternatives to lead and its alloys. Lead has been outlawed in the U.S., Canada and some European countries for use in waterfowl hunting shot, due to its toxicity. In both civilian and military sectors, there is growing pressure for the outlawing or restriction of lead bullets. Similar pressures against the use of lead are gaining momentum in fishing (lures and sinkers), automotive wheel weights, and even in such household items as curtain weights and children's toys. Perhaps because of concerns pertaining to the health and safety of industrial workers, lead articles of virtually any sort are being viewed as undesirable. These and other social and political pressures have resulted in a spate of recent efforts to find acceptable alternatives to lead.
When one considers available and affordable materials which are denser than, for example, iron or steel, only a limited number of candidate elements come to mind. The choices (bearing in mind that iron and steels have densities of approximately 8 g/cc) include: copper (8.9), nickel (8.9), bismuth (9.8), molybdenum (10.2) and tungsten (19.3). Such metals as U (18.9), Ta (16.6), precious metals and certain “rare earth” elements are deemed too expensive to be economically feasible as lead alternatives. When one calculates the cost-per-density-gain (i.e., the cost/pound of a candidate material, divided by the gain in density over that of iron/steel), it is found that tungsten is the most attractive material available on a commodity basis. Furthermore, ferrotungsten is the most economical form of tungsten, being generally less than half the cost (per pound of contained tungsten) of pure tungsten powder. Many of the methods found in U.S. patents fail to recognize these economic factors. These will be individually addressed later in this section, following presentation of additional factors relevant to tungsten-based lead alternatives (WLA's).
All of the past and present WLA technologies are subject to structural and compositional limitations imposed on the various alloy systems by considerations of thermochemical equilibrium. For example, one may conclude by examining the phase diagram for the Ni—W alloy system that the Ni-rich phase (“alpha”) can dissolve only a certain maximum amount of W at a given temperature, and even this amount of W only under conditions of “thermal equilibrium” (i.e., when enough time is allowed at a specified temperature for the system to become stable). This type of limitation is referred to as “limited solid solubility.” In conventional WLA technologies, limited solid solubility restricts the amount of W which can be alloyed with another metal during melting or sintering, for example.
Another type of restriction which thermodynamic considerations may identify for certain alloy systems is referred to as “intermetallic compound formation.” An example of this may be found in the W-Fe system. If, for example, more tungsten than the amount which can be dissolved in ferritic iron is present in the bulk alloy composition, the “excess” W atoms chemically react with Fe atoms to form intermetallic compounds such as Fe
7
W
6
. Intermetallic compounds are generally harder and more brittle (i.e., less ductile/malleable) than solid solutions of the same metals. This is certainly true of Fe
7
W
6
, as alloys which contain significant amounts of this phase (e.g., “ferrotungsten”) are notoriously brittle and therefore difficult to fabricate into useful articles.
In addition to the difficulties associated with limited solid solubility and intermetallic compound formation, conventional WLA's suffer from yet another limitation inherent in conventional powder metallurgy. Because sintering generally involves temperatures above those necessary to cause grain growth, one must accept the fact that the “as-compacted” dimensions of constituent powder particles will be smaller than the dimensions of alloy grains observed in the final product, and that grain sizes will generally be larger at increased sintering times and temperatures. This “grain coarsening” is usually undesirable, as mechanical properties of such products are degraded in accordance with a principle of metallurgy known as the “Hall-Petch” effect.
Yet another problem associated with conventional WLA methods is the potential occurrence of a phenomenon encountered during sintering known as “gravity segregation.” If temperatures high enough to cause liquid to form during sintering are employed (referred to as “liquid-phase sintering”), the denser tungsten-rich phase particles will tend to settle out of the mushy mixture, resulting in an inhomogeneous product. In accordance with principles of physics such as Stokes' Law, which describes the settling rates of solid particles in fluids, “gravity segregation” effects will be exacerbated by coarser particles with higher densities.
The present invention offers the potential to significantly reduce problems in producing WLA's which are attributable to limited solid solubility, intermetallic compound formation, coarse grain structure and gravity segregation. Specifically, these improvements are effected by applying a relatively recent technology known as “mechanical alloying” (MA) to tungsten-containing products.
Mechanical alloying is one of several relatively new technologies by which novel materials may be synthesized under conditions described as “far from equilibrium.” Such processes are capable of producing metastable phases (i.e., phases not possible under conditions of thermal equilibrium), highly-refined structures and novel composites described as “intimate mechanical mixtures.” MA is essentially a highly specialized type of milling process in which material mixtures are subjected to extremely high-energy application rates and repetitive cycles of pressure-welding, deformation, fracturing and rewelding between adjacent particles. These cyclical mechanisms ultimately produce lamellar structures of highly-refined, intimately mixed substances. Localized pressures and temperatures may be instantaneously high enough to cause alloying (by interdiffusion between different constituents) and/or chemical reactions (“mechanochemical processing”). Because such repetitive, instantaneous events are relatively brief, the system is
Kolisch Hartwell Dickinson & McCormack & Heuser
Mai Ngoclan
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