Original Date: 04/26/1999
Revision Date: 01/18/2007
Information : High Strength Aluminum Casting Alloy for High Temperature Applications
A hypereutectic alloy such as A390 is a conventional aluminum alloy used to make pistons in U.S. automotive engines. This alloy has adequate strength, is easily cast, and is low in cost. However, new congressional regulations require U.S. automobile manufacturers to continue to lower hydrocarbon emissions.
In a Conventional Piston Design, the large crevice volume between the piston and the cylinder at the top of the piston is a significant contributor to hydrocarbon emissions. A small volume of unburned gasoline/air mixture is trapped in this crevice volume during each firing stroke, and a portion of this unburned mixture is then expelled on the subsequent exhaust stroke of the piston. These unburned gases contribute directly to hydrocarbon emissions of the automobile. The Ford Motor Company in Dearborn, Michigan has developed and tested a Modified Piston Design. As expected, the thinner piston topland with its greatly reduced crevice volume significantly lowered hydrocarbon emissions. However, a typical alloy such as A390 aluminum alloy used in the design has inadequate tensile strength in the required 500° to 600° F temperature range. Figure 3-2 shows a comparison between the Conventional and Modified Piston Designs.
In 1995, Marshall Space Flight Center’s (MSFC’s) metallurgists began working with Ford under a Space Act Agreement to develop a new, castable aluminum alloy that had a 30% improvement in tensile strength during operation in the required temperature range. The result was an aluminum-silicon alloy that meets or exceeds all original automotive criteria. Figure 3-3 shows a 600° F, tensile strength comparison of available aluminum alloys against the NASA alloy. The projected cost of the aluminum-silicon alloy is 95¢ per pound versus 87¢ per pound for A390. Since less of the new alloy material is required due to its higher strength, the cost per piston is expected to be comparable. MSFC has currently filed a patent on the aluminum-silicon alloy, and negotiations are ongoing with Ford and other U.S. automobile manufacturers regarding its use.
Figure 3-2. Conventional versus Modified Piston Design
Figure 3-3. Aluminum Alloys Tensile Strength Comparison at 600° F
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