Aluminum alloy and article cast therefrom

Details Aluminum alloy and article cast therefrom Aluminum alloy and article cast therefrom

2002-07-11

2003-07-15

Wyszomierski, George (Department: 1742)

Metal treatment

Stock

Age or precipitation hardened or strengthened

C420S535000, C420S544000, C420S551000, C428S614000

Reexamination Certificate

active

06592687

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates generally to aluminum-silicon (Al—Si) alloys. It relates particularly to a high strength Al—Si based alloy suitable for high temperature applications for cast components such as pistons, cylinder heads, cylinder liners, connecting rods, turbo chargers, impellers, actuators, brake calipers and brake rotors.
2. Description of the Related Art
Al—Si alloys are most versatile materials, comprising 85% to 90% of the total aluminum cast parts produced for the automotive industry. Depending on the Si concentration in weight percent (wt. %), the Al—Si alloy systems fall into three major categories: hypoeutectic (<12% Si), eutectic (12-13% Si) and hypereutectic (14-25% Si). However, most prior alloys are not suitable for high temperature applications because their mechanical properties, such as tensile strength and fatigue strength, are not as high as desired in the temperature range of 500° F.-700° F. To date, many of the Al—Si cast alloys are intended for applications at temperatures of no higher than about 450° F. Above this temperature, the major alloy strengthening phases such as the &thgr;′ (Al
2
Cu) and S′ (Al
2
CuMg) phase will become unstable, rapidly coarsen and dissolve, resulting in an alloy having an undesirable microstructure for high temperature applications. Such an alloy has little or no practical application at elevated temperatures because, when the &thgr;′ and S′ become unstable, the alloy lacks the lattice coherency between the aluminum solid solution lattice and the strengthening particles lattice parameters. A large mismatch in lattice coherency contributes to an undesirable microstructure that can not maintain excellent mechanical properties at elevated temperatures.
One approach taken by the prior art is to use fiber or particulate reinforcements to increase the strength of Al—Si alloys. This approach is known as the aluminum Metal Matrix Composites (MMC) technology. For example, U.S. Pat. No. 5,620,791 relates to an MMC comprising an Al—Si based alloy with an embedded a ceramic filler material to form a brake rotor for high temperature applications. An attempt to improve the high temperature strengths of Al—Si alloys was also carried out by R. Bowles, who has used ceramic fibers to improve tensile strength of an Al—Si 332.0 alloy, in a paper entitled, “Metal Matrix Composites Aid Piston Manufacture,”
Manufacturing Engineering
, May 1987. Another attempt suggested by A. Shakesheff was to use ceramic particulate for reinforcing Al—Si alloy, as described in “Elevated Temperature Performance of Particulate Reinforced Aluminum Alloys,”
Materials Science Forum
, Vol. 217-222, pp. 1133-1138 (1996). Cast aluminum MMC for pistons has been described by P. Rohatgi in a paper entitled, “Cast Aluminum Matrix Composites for Automotive Applications,”
Journal of Metals
, April 1991. It is noted that the strength for most particulate reinforced MMC materials, manufactured from an Al—Si alloy, are still inferior for high temperature applications because the major &thgr;′ and S′ strengthening phases are unstable, rapidly coarsen and dissolve at high temperatures.
Another approach taken by the prior art is the use of the Ceramic Matrix Composites (CMC) technology. For example, W. Kowbel has described the use of non-metallic carbon-carbon material for making pistons to operate at high temperatures in a paper titled, “Application of Net-Shape Molded Carbon-Carbon Composites in IC engines,”
Journal of Advanced Materials
, July 1996. Unfortunately, manufacturing costs employing these MMC and CMC technologies are substantially higher than those using conventional Al—Si casting, which has hampered their ability to be priced competitively with Al—Si alloys in mass production for high temperature internal combustion engine parts and brake applications.
It is accordingly a primary object of the present invention to obviate the disadvantages of the prior art technologies.
SUMMARY OF THE INVENTION
According to the present invention, an Al—Si alloy containing dispersion of particles having L1
2
crystal structure in the aluminum matrix is presented. The alloy is processed using low cost casting techniques such as permanent mold, sand casting or die casting.
The alloy of the present invention maintains a much higher strength at elevated temperatures (500° F. and above) than other prior art alloys, due to a unique chemistry and microstructure formulation. The methods for strengthening the alloy in the present invention include: 1) Maximizing the formation of major strengthening &thgr;′ and S′ phase in the alloy, with chemical composition given as Al
2
Cu, Al
2
CuMg, respectively. 2) Stabilizing the strengthening phases at elevated temperatures by controlling the Cu/Mg ratio and by the simultaneous addition of Titanium (Ti), Vanadium (V) and Zirconium (Zr) elements. 3) Forming A1
3
X (X=Ti, V, Zr) compounds with L1
2
crystal structures for additional strengthening mechanisms at elevated temperatures.
In the present invention, key alloying elements of Ti, V and Zr are added to the Al—Si alloy to modify the lattice parameter of the aluminum matrix by forming compounds of the type Al
3
X having L1
2
crystal structures (X═Ti, V and Zr). In order to maintain high degrees of strength at high temperatures, both the aluminum solid solution matrix and the particles of Al
3
X compounds should have similar face-centered-cubic (FCC) crystal structures, and will be coherent because their respective lattice parameters and dimensions are closely matched. When the condition of substantial coherency for the lattice is obtained, these dispersion particles are highly stable, which results in high mechanical properties for the alloy during long exposures at elevated temperatures.
In addition to the alloy composition and microstructure, a unique heat treatment schedule is provided in order to optimize the performance for the alloy strengthening mechanisms and phases formation within the alloy. The advantages of the present invention will become apparent as the description thereof proceeds.


REFERENCES:
patent: 4648918 (1987-03-01), Asano et al.
patent: 04105787 (1992-04-01), None
patent: 1836476 (1993-08-01), None

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