Metal matrix composite and piston using the same

Details Metal matrix composite and piston using the same Metal matrix composite and piston using the same

2001-04-05

2002-08-13

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Laterally noncoextensive components

C428S653000, C428S682000, C428S293100, C092S172000

Reexamination Certificate

active

06432557

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a metal matrix composite, including a light metal alloy, e.g., aluminum alloy, magnesium alloy, etc., as a matrix (base metal), and a piston using the same.
Conventionally, steel materials have been used as materials for mechanical element components. However, light metal alloys, such as Al (aluminum) alloy, Mg (magnesium) alloy, etc., are used for components that require reduction in weight. For some of components that require high-temperature strength, as well as reduction in weight, moreover, a metal matrix composite (abbrev. as MMC) may be used in the case where required characteristics cannot be obtained with use of a simple light metal alloy with a low melting point (i.e., with low high-temperature strength) or if the required characteristics cannot be obtained with use of a simple light metal alloy with poor wear resistance. The metal matrix composite is composed of a metallic matrix and reinforcements. Carbon fibers or ceramic fibers, such as SiC (silicon carbide), Al
2
O
3
(alumina), etc., are used for the reinforcements, for example.
Components such as automotive parts and aircraft parts of which the weight is closely associated with the fuel-efficiency eagerly require reduction in weight. Materials for the components of this type are being changed from the conventional steel over to light metal alloys, such as Al alloy, Mg alloy, etc. To meet this requirement, materials for internal-combustion engines that exposed to high temperature and their peripheral parts (engine parts such as pistons, cylinder heads, cylinder blocks, connecting rods, etc.) are being changed over to light metal alloys. With the progress of development of higher-output internal-combustion engines, however, high-temperature strength and wear resistance have ceased to be ensured with use of a simple light metal alloy with a low melting point (i.e., with low high-temperature strength) or a simple light metal alloy with poor wear resistance. The following is a description of a piston for a diesel engine of an automobile as an example.
Direct-injection engines have recently been becoming prevailing. The load on the side of the combustion chamber of the piston is expected to increase as the development of higher-output versions will advance hereafter. The combustion chamber for forming eddies of air called swirls is formed in an end face of the piston. Since the edge (lip portion) that requires machining for finishing is thin-walled, in particular, it is hard to secure satisfactory fatigue strength in a high-temperature zone (e.g., at 300° C. or thereabout) with use of conventional aluminum alloys (AC8A, etc.) for castings. The following is a description of the chemical ingredients of AC8A. In this specification, the chemical ingredients of the alloys are given by % by weight unless otherwise specified.
Cu: 0.8 to 1.3
Si: 11.0 to 13.0
Mg: 0.7 to 1.3
Zn: 0.15 or less
Fe: 0.8 or less
Mn: 0.15 or less
Ni: 0.8 to 1.5
Ti: 0.20 or less
Pb: 0.05 or less
Sn: 0.05 or less
Cr: 0.10 or less
Al: Remainder
Composites that use these Al alloys as their base metal (matrices) may possibly be subjected to surface treatment to improve their high-temperature fatigue strength. Since the effect of the strength improvement by the surface treatment is small, however, a metal matrix composite (MMC) is expected to be used.
Feasible reinforcements for the metal matrix composite include metallic fibers, carbon fibers, and ceramic fibers, and besides, porous structures and whiskers (crystal whiskers) formed of these materials, etc. Under the present conditions, fibers that are used as the reinforcements of the metal matrix composite are ceramic fibers, such as SiC, Al
2
O
3
, etc., and metallic fibers have not reached the level of practical use yet. This is so because no manufacturing technique has been established yet for metallic fibers of fiber diameters (several micrometers to tens of micrometers) that are required of reinforcements of a metal matrix composite, so that low-cost metallic fibers to serve for practical use cannot be obtained.
With the recent advance of performance that is required of various apparatuses, in particular, there is a growing tendency for higher fatigue strength or higher wear resistance to be required. Metallic fibers that can meet this high-level requirement are very hard and fragile, though not harder or more fragile than ceramics, so that they cannot be manufactured by the conventional wire drawing.
Usually, the casting method is used to compound a matrix and reinforcements. In the casting method, a preform (preformed piece previously molded to have a given shape and volume content) of fibers that serve as reinforcements is set in a mold. Thereafter, a molten matrix metal is poured into the mold. The preform is compulsorily impregnated with the matrix metal under a given pressure. A metal matrix composite is obtained by hardening the matrix metal.
In the case where carbon fibers or ceramic fibers are used for the reinforcements, they involve the following problems.
Carbon fibers and ceramic fibers have poor wettability with a light metal alloy that forms a base metal (matrix). Therefore, the light metal alloy of the matrix, e.g., Al alloy, fails to get well into spaces between the fibers, so that a large number of cavities (voids) are created inevitably. These defects lower the initial strength of the metal matrix composite and worsen the durability against corrosion or the like.
In order to improve the wettability with the matrix metal, therefore, the surface quality of the reinforcements that are formed of carbon fibers or ceramic fibers may be improved by plating or the like. However, the improvement of the surface quality requires many processes and much time, thus resulting in an increase in cost. Metallic fibers have a great advantage over carbon fibers and ceramic fibers with respect to the wettability with the matrix metal. As mentioned before, however, metallic fibers that are suited for reinforcements are expensive. It is hard for fibers of relatively low-priced stainless steel (SUS) to fulfill the high-level requirement for the high-temperature fatigue strength, wear resistance, etc.
Moreover, a composite that uses carbon fibers or ceramic fibers as its reinforcements must be preformed in order to prevent deformation of the reinforcements during casting operation. Preforming the carbon fibers or ceramic fibers requires a binder for use as an adhesive agent, and this binder causes the performance of the metal matrix composite to worsen.
A mold pressing method, extrusion molding method, and centrifugal molding method are known methods for manufacturing a preform with use of a binder. Any of these method requires many processes including a process for loosening fibers, process for applying the binder, temporary molding process, drying process, sintering process, etc.
There is also a problem that the composite using carbon fibers or ceramic fibers cannot be machined with ease. Primarily, carbon fibers and ceramic fibers are unworkable materials, so that it is natural that the metal matrix composite that uses them as its reinforcements cannot be worked with ease. Accordingly, there is a problem that the composite using carbon fibers or ceramic fibers entails prolonged working time or requires an expensive cutting tool. It is to be desired also in consideration of these circumstances that metallic fibers should be used for the reinforcements.
A metal matrix composite in which metallic fibers or ceramic fibers for use as reinforcements are mixed in a matrix metal of Al alloy has been developed as means for reducing the weight of and enhancing the strength of an engine piston. The casting method is adopted as a method for manufacturing the metal matrix composite of this type. Normally, heat treatment is carried out to enhance the mechanical strength of the composite after casting operation. There are close relations between conditions for the heat treatment and the chemical ingredients of the matrix (Al alloy). The Japanese Industr

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