Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Sintering which includes a chemical reaction
Reexamination Certificate
2001-07-12
2004-07-27
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Sintering which includes a chemical reaction
C419S048000, C425S078000
Reexamination Certificate
active
06767505
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to the field of powder metallurgy and in particular to an improved and less expensive method and apparatus for generating controllable pressure pulses (both in the shock regime and in a rapid, but shock-free, regime in the same device) for the purpose of consolidating (compacting) powders to a contiguous rigid form, primarily for the purpose of producing material samples and manufactured parts. In this field, the emphasis is generally on producing higher quality and better performing parts, which generally means parts with higher density, strength and ductility, and doing so at lower cost.
2. Description of Related Art
Powder metallurgy (P/M) offers possibilities for the design of materials into near net shape which exhibit a wide range of unique and novel properties. In principle, there is no limitation on the production of a material with any desired composition by such methods. However, the high cost of powder metallurgy technology must compete with other low cost manufacturing methods.
Below, we first describe two specific market areas (soft and hard magnetic materials) of some commercial interest as an example of the need. Other markets for other materials are of equal interest. The dynamic compaction of conventional powders using the described invention as a low cost high tonnage compact press is expected to have high value in the part manufacturing industry. Then we describe present powder consolidation technology and dynamic powder consolidation techniques and their limitations.
Soft Magnetic Materials
The world-wide magnetic materials market is a multi-billion dollar industry, and soft magnetic materials comprise a significant fraction of this market. Soft magnetic materials play a key role in a number of applications, especially with respect to electric power applications. Some of these applications include electric motors, distribution transformers, and generators.
Soft magnetic materials find applications in electrical, electronics, and computer systems that characterize modern society. Soft magnetic materials play a key role in power distribution, make possible the conversion between electrical and mechanical energy, underlie microwave communication, and provide both the transducers and the active storage material for data storage in information systems. As the properties of these materials are continuously being improved, many new applications are likely to emerge.
The critical properties necessary in designing optimum soft magnetic materials include a high saturation magnetization, low coercivity, low hysteresis loss, high permeability, low magnetostriction, low eddy current losses, high Curie temperature, low temperature dependence of the magnetic properties, and cost. In practice, the available materials must compromise some of these properties in favor of others. For example, permalloys (Fe—Ni-based alloys) have a tremendously high permeability and very low coercivity, but the saturation magnetization is only approximately 60 percent of the value for &agr;-Fe.
Many of the requisite properties are intrinsic, such as a high saturation magnetization and magnetostriction. These properties are tailored through the specific design of the alloy. Other properties are influenced by extrinsic factors, most notably by microstructural features. The magnetic properties strongly influenced by the microstructure are those involving domain wall motion. For example, the eddy current losses arise because soft magnetic materials generally operate in alternating fields. The losses arise primarily because of difficulties in reversing the magnetization state of the material. This, in turn, is controlled by domain wall motion; if domain wall motion is inhibited, the losses are greater. Microstructural features such as precipitates and localized strain fields from dislocations and impurity atoms provide pinning sites for domain walls. The eddy current losses also increase as the size of the magnetic regions increases and as the resistivity decreases. Thus, the eddy current losses can be reduced by reducing the coercivity and the scale of the microstructure and by decreasing the electrical resistivity.
The coercivity is affected by both intrinsic and extrinsic factors. Intrinsically, the coercivity is controlled by the specific anisotropy of the crystal lattice (magnetocrystalline anisotropy). While cubic crystals possess the lowest anisotropy, there is variability between different cubic materials. For example, the anisotropy constant (the parameter that describes the degree of anisotropy) differs by almost an order of magnitude between Fe and Ni. As with other intrinsic properties, the anisotropy can be altered through alloy design.
The primary extrinsic influence on the coercivity is the microstructure. The microstructure, in fact, influences the entire shape of the hysteresis loop. Some examples of microstructural features that influence the coercivity by affecting domain wall motion include defect density, including dislocations and point defects (e.g., impurity atoms). Therefore, it is critical to control microstructural features in order to produce more efficient soft magnetic materials.
One microstructural feature that greatly impacts the coercivity is the structural correlation length (D). In crystalline materials, the structural correlation length is equal to the grain size, while in amorphous materials it is essentially the distances over which short range order exists. As grain sizes decrease from the millimeter size range to approximately 0.1 &mgr;m, there is a corresponding increase in coercivity proportional to 1/D. However, when the structural correlation length approaches the ferromagnetic exchange length, which is on the order of the domain wall width, the coercivity begins to decrease. With a continuing decrease in the structural correlation length, the coercivity was observed to decrease with a D
6
dependence. This dramatic decrease in coercivity has been attributed to the averaging of local anisotropies by the exchange interactions, with the net effect of eliminating (or significantly reducing) the influence of magnetocrystalline anisotropy on the magnetization process.
The strong dependence of coercivity on the structural correlation length has prompted a significant amount of research in the areas of nanocrystalline and amorphous alloys, where D ranges from 0.5 to 50 nm. Amorphous alloys typically consist of Fe- or Co-based alloys with additions of Si and B, which enhance the glass formability, and other alloying additions to control, for example, the magnetostriction. Currently, nanocrystalline microstructures are formed by the crystallization of specific amorphous alloys, with the resulting microstructure consisting of 10 to 15 nm crystallites surrounded by an amorphous matrix. The primary advantage of amorphous and nanocrystalline alloys is the reduction of the anisotropy and, in the case of nanocrystalline materials, magnetostriction. Amorphous metals have been used in place of grain-oriented Si steels in transformer applications, which provided a reduction of 75 percent in eddy current losses because of reduced coercivity and magnetostriction. However, the saturation magnetization is significantly reduced, with values ranging from 10 to 15 kG for current amorphous and nanocrystalline alloys (compared to 21.5 kG for &agr;-Fe). The lower saturation values result from the dilution of the Fe or Co alloys with elements that enhance glass formability or alter other intrinsic properties. Future advances in soft magnetic materials will be made by increasing the saturation magnetization while retaining the advantageous properties of amorphous and nanocrystalline microstructures.
Generally, nanocrystalline and amorphous materials are produced in particulate form by atomization or melt spinning techniques. Practically, the consolidation of nanocrystalline and amorphous particulate into useful engineering devices provides many unique challenges. Most densification techniques rely
Kruczynski David L.
Massey Dennis W.
Mozhi T. Arul
Ryan John M.
Witherspoon F. Douglas
Creighton Wray James
Jenkins Daniel J.
Narasimhan Meera P.
Utron Inc.
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