Internal-combustion engines – Particular piston and enclosing cylinder construction – Piston
Reexamination Certificate
1999-09-23
2001-03-20
McMahon, Marquerite (Department: 3747)
Internal-combustion engines
Particular piston and enclosing cylinder construction
Piston
Reexamination Certificate
active
06202618
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to pistons for internal combustion engines.
BACKGROUND OF THE INVENTION
Cast aluminum alloy pistons are in widespread use in internal combustion engines. Such aluminum alloy pistons typically are strengthened after casting by a precipitation hardening heat treatment. To this end, the aluminum alloy includes alloying elements, such as Si, Mg, Cu, etc., that form intermetallic and other strengthening precipitates in the alloy microstructure as a result of the post-cast precipitation hardening heat treatment.
In service in an internal combustion engine, different regions of the piston operate at substantially different temperatures. For example, in a particular gasoline engine, the crown of the piston typically attains a temperature of approximately 300 degrees C., which is much hotter than the temperature of the piston boss and skirt (e.g. less than 200 degrees C.). The higher temperature at the crown of the piston subjects the microstructure thereof to what is called overaging whereby the strengthening precipitates present in the microstructure grow in size to an extent that the strength of the crown decreases more rapidly than that of the lower temperature piston boss and skirt. This is undesirable in that, in service, the crown of the piston should have high strength to withstand the combustion stresses at higher service temperatures involved.
It is well known in the art to improve one or more properties (e.g. strength, toughness, wear resistance, fatigue resistance, etc.) of metals and alloys by inclusion of reinforcing particles therein, either as dispersed particles or as one or more preformed inserts formed from particulates. The reinforcing particles can comprise elongated fibers, rounded particles, and other particle shapes. Such reinforced metals or alloys are referred to as metal matrix composites (MMC's) wherein the metal or alloy provides the matrix for the reinforcing particles. Metal matrix composites having various reinforcing particles and matrix metals/alloys are described in U.S. Pat. Nos. ,5679,041 and 5,588,477.
Aluminum based MMC's have been considered as candidate materials for use in selective reinforcement of pistons for internal combustion engines. For example, U.S. Pat. Nos. 4,920,864 and 5,505,171 describe pistons having a piston crown or head having an MMC structure for reinforcement purposes.
An object of the present invention is to provide a cast piston for an internal combustion engine, and method of making same, wherein by selective control of the piston microstructure the mechanical properties of the piston are selectively tailored to different service conditions experienced by different regions of the piston.
SUMMARY OF THE INVENTION
The present invention provides a cast piston for an internal combustion engine, and method of making same, wherein a crown region includes a microstructure, including a metallic matrix with reinforcing material and strengthening precipitates therein, providing strength properties, such as fatigue strength, suited to higher crown service temperatures by virtue of the precipitates having better resistance to averaging during engine service as compared to different strengthening precipitates formed in a microstructure of other regions of the piston. The different precipitates at the other regions of the piston provide strength properties, such as fatigue strength, suited to relatively lower temperatures experienced at those regions. The present invention thereby provides a piston having strength properties, such as fatigue strength, selectively tailored to different service temperatures encountered by different regions of the piston. In an illustrative embodiment of the present invention offered for purposes of illustration only and not limitation, a piston is cast by introducing molten matrix-forming alloy into a casting mold and selectively reacting a ceramic reinforcing material and the molten alloy at a crown-forming region of the casting mold to locally alter the alloy composition in the mold crown-forming region. The alloy composition is locally altered in a manner that a subsequent precipitation hardening heat treatment of the cast piston selectively forms strengthening precipitates (e.g. platelet precipitates) in the piston crown microstructure that impart superior fatigue strength to the piston crown region at engine service temperatures by virtue of reduced averaging at such service temperatures. At the same time, different strengthening precipitates (e.g. spheroidal precipitates) are formed in the microstructure of other regions of the piston to provide superior fatigue strength at lower service temperatures experienced by those regions.
The present invention provides a piston having mechanical properties, such as fatigue strength, tailored to accommodate operational temperature distribution of different regions of the piston in service in an internal combustion engine to improve performance of the piston.
The above and other objects and advantages of the present invention will become more readily apparent from the following detailed description taken in conjunction with the following drawings.
REFERENCES:
patent: 4708104 (1987-11-01), Day et al.
patent: 4735128 (1988-04-01), Mahrus et al.
patent: 4848291 (1989-07-01), Kawamura et al.
patent: 4939984 (1990-07-01), Fletcher-Jones
patent: 5074352 (1991-12-01), Suzuki
Baxter William J.
Mishra Raja K.
Sachdev Anil K.
General Motors Corporation
McMahon Marquerite
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