Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1999-11-29
2001-05-08
Jenkins, Daniel J. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C102S501000
Reexamination Certificate
active
06228140
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to powder preform consolidation processes, and more particularly to such processes wherein consolidated tantalum powder parts are produced. The use of higher density metals such as tantalum for replacement of copper in the fabrication of explosively formed penetrators (EFP's) and shape charge liners (SCL's) is of considerable interest in the field of ballistic devices. However, certain metallurgical, fabrication and cost related issues currently limit the use of tantalum for task specific ballistic applications.
The conventional fabrication technique for sheet and plate is ingot metallurgy followed by standard thermo-mechanical metal working practices such as forging and rolling. These fabrication processes, however, produce highly undesirable textured microstructure which yield anisotropic static and dynamic properties over both low and high strain rate regimes. Machining of the tantalum plate or sheet stock to its final EFP or SCL geometry contributes not only to an additional loss of ductility through work hardening mechanisms, but also adds significant cost to the final product.
The role of texture on microstructure development and dynamic mechanical properties has been recognized by a number of investigators(
1-4
). Several common metal working practices such as extrusion, rolling and forging have undergone careful scrutiny as methods of producing ballistic grade tantalum. These studies have shown that the presence of a <lll> texture orientation improves formability (ductility) of the tantalum metal. However, these thermo-mechanically oriented processes also cause the tantalum to exhibit an anisotropic mechanical behavior due to the creation of a non-uniform texture. Through orientation distribution function (ODF) analysis forged and rolled tantalum is found to exhibit a pole density of 5×random. This non-uniform texture is known to have deleterious effects on the high-strain rate performance of the EFP which results in both an uneven collapse of the tantalum body upon impact, and the subsequent generation of unpredictable fin configurations.
SUMMARY OF THE INVENTION
It is a major object of the invention to provide a powder metallurgy (p/m) process overcoming the above problems associated with tantalum processing. The process of the invention is capable of producing a fine grain, virtually texture free, ballistic grade tantalum with significantly improved high strain rate properties, with the forged material exhibiting more uniform mechanical behavior under high strain rate regimes (4000 S
−1
) than its thermo-mechanically processed predecessor. Tantalum processed via the herein disclosed powder metallurgy approach provides a higher level of performance over conventionally processed ingot material even if the oxygen content of the powder processed tantalum is two or three times higher than the upper limit of 100 ppm currently established for ballistic application. Orientation distribution analysis of the forged powder metallurgy processed tantalum confirms a <lll> texture of only 2.8×random. Additionally, there is very little preferred orientation and no significant difference between the texture in directions perpendicular to a normal plane. The herein disclosed process provides for a reliable and reproducible manufacturing alternative for high quality, dynamically predicable, ballistic grade tantalum.
Basically, the process of consolidating tantalum metal powder includes the steps:
a) pressing said powder into a preform, and preheating the preform to elevated temperature,
b) providing a bed of flowable pressure transmitting particles,
c) positioning the preform in such relation to the bed that the particles encompass the preform,
d) and pressurizing the bed to compress said particles and cause pressure transmission via the particles to the preform, thereby to consolidate the preform in to a desired shape,
e) such pressurizing being carried out to effect a <lll> texture of less than about 3.0×random.
Another object of the invention includes effecting consolidation pressurization over a time interval of sufficient shortness that said <lll> texture is less than about 2.8×random. Such pressurization is typically effected at levels greater than 100,000 psi for a time interval of less than about 30 seconds.
Yet another object includes providing a sealed, evacuated, deformable metallic container in the bed, and locating the preform in the container with bed particles both inside the container and outside the container, prior to pressurization. Bed particles outside the container are typically pressurized to deform the container and transmit pressurization to bed particles in the container. In this way, oxygen access to the tantalum preform is virtually eliminated, to provide a more ductile material.
An additional object is to provide an improved tantalum product, produced by the method or methods of the invention, as referred to. Such a consolidated powder metal preform product is characterized by substantially completely random grain textural orientation. For example, the product consolidated preform typically has a <lll> texture of less than about 3.0×random.
These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more full understood from the following specification and drawings, in which:
REFERENCES:
patent: 4499048 (1985-02-01), Hanejko
patent: 4499049 (1985-02-01), Hanejko
patent: 4501718 (1985-02-01), Bradt
patent: 4539175 (1985-09-01), Lichti et al.
patent: 4640711 (1987-02-01), Lichti et al.
patent: 4766813 (1988-08-01), Winter et al.
patent: 5032352 (1991-07-01), Meeks et al.
patent: 5331895 (1994-07-01), Bourne et al.
Fleming Marc A.
Lansing Lucile
Meeks, III Henry S.
Ceracon, Inc.
Haefliger William W.
Jenkins Daniel J.
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