Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2000-08-30
2003-01-07
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S039000
Reexamination Certificate
active
06502623
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the priority of Austrian Patent Application, Serial No. 1619/99, filed Sep. 22, 1999, the subject matter of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a process of making a metal matrix composite (MMC) composite comprised of a preform having a ceramic, porous consistence with pores filled with matrix metal.
MMC is a material in which a preform and a metal are embedded within one another at different quantitative ratios. In the following description, the term “preform” will denote a porous structure made from a reinforcing material. The configuration of the preform material may be selected in a wide variety. For example, the preform may be comprised of single parts of random geometry (spherical, angular, of short fibers or long fibers, or in form of whiskers). It is also possible to provide the preform of one or more bodies of different sizes and porous structure.
Metal is poured to infiltrate the spaces between the individual parts and/or in the pores of a porous body. The so-called infiltration process is substantially a casting process by which metal is introduced into the very narrow pores of the preform. In case of a poor wetting behavior between the material of the preform and the metal in the pores, the metal must be forced into the voids of the pores. This is typically implemented through pressure application by which pressure, e.g. gas pressure or mechanical pressure by means of a die, is applied upon the liquid metal for pressing the metal into the pores of the preform. Another example of an infiltration process includes the decrease of surface tension of the liquid metal to such an extent that the metal can penetrate the pores of the preform even without application of a particular pressure. This type of infiltration process is commonly referred to as “spontaneous infiltration”. The decrease of the surface tension of the metal can be realized by reacting the liquid metal with respective agents that decrease the surface tension, for example by mixing these agents to the liquid metal or by introducing these agents into the ambient atmosphere of the liquid metal.
The solidification of the liquid metal is realized in the event of non-wetted preform-metal systems under pressure to prevent a segregation, i.e. to prevent metal introduced into the preform from oozing out from the pores again.
The purpose of the preform is to impart the finished composite and/or the used matrix material with particular or special mechanical, thermal and electrical properties, which are substantially determined by the arrangement and composition of the preform. For example, the volume portion together with the modulus of elasticity governs the expansion coefficient. A high modulus of elasticity and a high packing density result in a low expansion coefficient whereas a low modulus of elasticity and a same packing density leads to a comparably high expansion or requires a very high volume portion. Different compositions of the material influence, for example, the thermal conductivity. A composite of aluminum and Al
2
O
3
has a substantially lower conductivity than a composite of Al and SiC. Moreover, the type of infiltration metal impacts on the mechanical, electrical and thermal properties of the resultant MMC. Examples for typical infiltration metal includes aluminum, magnesium, copper as well as alloys comprised of one or more of these metals. Of course, other metals or metal-like elements may be usable as well.
The production of porous ceramic components that can be used as preform for such MMC components includes a variety of shaping processes, such as injection molding, hot casting, dry pressing, film casting, vacuum casting and slip casting. Each of the mentioned processes uses ceramic powder with a binder, lubricant etc., and the mixture is made flowable and shaped into the desired configuration. Slip casting may be carried out without addition of a binding agent.
By way of example only, the following description refers in more detail to the hot casting process. In this process, thermoplastics (up to 45% by volume), such as, e.g., paraffins or waxes, are used as binders and added to the ceramic powder. By raising the temperature, this mixture becomes liquid, and the hereby obtained flowing mass is poured into a mold. After solidification of the binder, the preform is ejected, the binder is expelled through heating, and the material is sintered. Depending on the duration of the sintering process and the selected sintering temperature, a porous, self-contained or dense structure is produced.
Preforms may also be produced by an injection molding process in which a powder mixture blended with binder (up to 40% binder fraction is required) is injected into a respective mold and allowed to cure. The binder is removed thereafter in a same manner as described with reference to the hot casting process.
A further possibility is the dry pressing process of ceramics, in which a flowable powder mixture is filled into a respective preform mold and pressed by a die into the desired shape. The required binder or also assisting pressing agents may hereby be provided of less complex configuration compared to the afore-mentioned hot casting process, and the quantity being used is also substantially smaller. Suitable binding agents include stearates and paraffins.
The use of a binder, e.g. stearic acid as oftentimes employed in conventional processes, has the drawback that the extraction of the binder requires a time-consuming process—typically heating of the ceramic particle mass to a temperature above the evaporation point of the binder, after removal of a liquid carrier—and entails the risks that the involved high temperatures result in a deformation of the molded product, and a reaction of gases of the ambient atmosphere, in particular oxygen with the ceramic particles, leading to undesired compounds that adversely affect the properties of the resultant preform and ultimately of the resultant MMC component.
In all these shaping process or preform processes, the required amounts of organic additives are removed again after obtaining the desired configuration, typically through thermal processes. As a consequence of the high fraction of organic substance, this process must be carried out at a slow speed, in particular when the formed body has an irregular geometry because, otherwise, the surface may crack, or deformations in the formed component may be experienced, rendering the use unsuitable. Depending on the particular addition of binder and lubricant, their removal is carried out at temperatures of 300° C. to 700° C. according to a time schedule that is selected to the specific material and product. Kilns with complicated temperature programs and temperature profiles, which must be maintained very accurately, are required hereby. The process may take several days, depending on the complexity of the component. Also, the subsequent sintering process requires a very slowly and careful heating profile to prevent internal tensions as best as possible. The same care is required for the subsequent cooling to room temperature.
Practice has shown that slip casting is a particular simple process to make ceramic components, in particular preforms as basis for MMC components. Slip casting is in particular suitable for the production of preforms comprised of SiC, although preforms made of different ceramic powders such as carbides, nitrides, borides, oxides or mixtures thereof such as, oxynitrides, can be made by slip casting as well. Concrete materials for the ceramic powder include, for example, SiC, TiC, B
4
C, AlN, Si
3
N
4
, BN, and Al
2
O
3
.
In the description, the term “slip” will denote a mixture of a powder with a particular amount of liquid carrier. In this context, the slip includes a ceramic powder to which, for example, water as liquid carrier has been added. When mixing liquid carrier to a ceramic powder, the flow limit of this dispersion is decreased to such an extent as t
Electrovac, Fabrikation elektrotechnischer Spezialartikel Gesell
Elve M. Alexandra
Feiereisen Henry M.
Tran Len
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