Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
1997-10-24
2001-11-06
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S026000, C419S030000, C419S038000, C419S049000
Reexamination Certificate
active
06312643
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the synthesis of nanoscale powders and the consolidation of these powders into bulk solid materials without inducing any significant grain growth into the nanoscale materials.
In conventional polycrystalline materials the majority of the atoms are in an ordered crystalline structure in the grains with a small percentage of the atoms being in the grain boundaries separating the grains. In conventional materials the volume fraction of grain and interface boundaries rarely exceeds more than a few percent, and therefore, the bulk properties are not affected significantly. Conventional powders can be prepared by methods such as gas condensation, rapid solidification, sputtering, mechanical alloying/milling, conventional vapor deposition, (either physical vapor deposition, PVD, or chemical vapor deposition, CVD), plasma assisted PVD/CVD, electrodeposition, plasma processing and sol-gel processes.
Of all the processes listed, only inert gas condensation, plasma processes and mechanical alloying are used widely to synthesize nanoscale materials. The inert gas condensation process is currently used only as a research technique because the production rate of nanoscale materials is only few grams per minute via this route. Plasma processes also are costly and difficult to scale-up for large scale production of nanoscale materials. Due to process flexibility and ease in scaling-up, mechanical milling/alloying has been widely used for producing nanoscale materials. The combination of increased strength, improved toughness and lower density of nanoscale materials makes them extremely attractive for space and propulsions applications.
Historically titanium alloys are the baseline material for aircraft and space applications. Although conventional coarse-grain aluminum alloys are lighter than titanium, they lack the sufficient specific strength needed for many applications as space and propulsion materials. However, aluminum nanoscale materials provide a unique opportunity for aircraft, space and armor applications as the properties of nanoscale aluminum alloy, such as tensile strength, hardness and toughness are vastly improved over those of traditional coarse-grain materials. In addition, the cost is less than that of titanium alloys.
Thus, there exists a need for a process for making a nanoscale material and products therefrom.
SUMMARY OF THE INVENTION
In this invention, a very simple and flexible powder synthesis and consolidation process is described which yields extremely high purity powders with small grain size and eliminates several cumbersome processing stages during consolidation of these powders. To mechanically mill and alloy the metal powders to form nanoscale alloy material, a commercially available attritor is used. The powder, which can have a narrow or a bimodal particle size distribution, is covered with a surfactant and blanketed with an ultra high purity inert gas. The advantage of surfactants is three-fold. First, it gets adsorbed around the particle surface and thus prevents oxidizing and nitriding of the particles. Secondly, it prevents cold-welding of the particles during attriting which eliminates particle size growth. Thirdly, during degassing at 100-250° C. and hot isostatic pressing at around 250-350° C. it desorbs completely leaving the virgin metallic surface which consolidates easily with full theoretical density even in a low temperature range such as 200-350° C. A continuous flow of ultra high purity inert gas is maintained over the top of the surfactant in order to prevent the escape of vapor surfactants and to prevent entrance of foreign gases. After the particles are attrited, the surfactant is removed, the powder is dried and degassed, and then the powder is consolidated at relatively low temperature.
Therefore, one object of the present invention is to provide a process for making nanoscale aluminum alloy powders and articles therefrom.
Another object of the present invention is to provide a process and articles therefrom having superior properties over titanium products.
Another object of the present invention is to provide products having improved properties for use in the aeronautics and astronautics area of technology.
These and many other objects and advantages of the present invention will be ready apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings.
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Hoffman Wesley P.
Upadhya Kamleshwar
Auton William G.
Jenkins Daniel J.
The United States of America as represented by the Secretary of
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