Piston Materials. Aluminum Alloys

Pure aluminum is too soft and too susceptible to wear for use in pistons and for many other purposes. That is why alloys have been developed that are matched particularly to the requirements found in piston engineering. They combine, at low specific weight, good heat strength properties with a low tendency to wear, high thermal conductivity, and, in most cases, a low coefficient of thermal expansion as well.

Two groups of alloys have come into being, depending on the primary additive—silicon or copper:

Aluminum-Silicon Alloys:
- Eutectic alloys containing from 11 % to 13% silicon and smaller amounts of Cu, Mg, Ni, and the like. Included in this group of piston alloys, the ones used most frequently in engine construction, is MAHLE 124, which is also used for cylinders. For most applications they offer an ideal combination of mechanical, physical, and technological properties. The MAHLE 142 alloy, with a greater proportion of copper and nickel, was developed for use particularly at high temperatures. It is distinguished by better thermal stability and considerably improved strength when heated. A further step in this direction is the nearly eutectic MAHLE 174 alloy.

- Supereutectic alloys contain from 15% to 25% of silicon and use copper, magnesium, and nickel as additives to deal with high temperatures; examples include MAHLE 138 and MAHLE 145. They are used wherever a need for reduced thermal expansion and greater wear resistance is in the foreground. The MAHLE 147 (SILUMAL) alloy is used for cylinders and/or engine blocks without any special treatment for the running surfaces. Figures 7-23 and 7-24 show characteristic values for the materials.

Fig. 7-23. Physical properties of MAHLE aluminum piston alloys

Fig. 7-24. Mechanical properties of MAHLE aluminum piston alloys

Aluminum-Copper Alloys: To a lesser extent, alloys containing copper but almost no silicon and just a small amount of nickel as an additive are used for their good heat strength. In comparison with the Al-Si alloys, they exhibit greater thermal expansion and less wear resistance. While the Al-Si alloys can be both cast and reformed when warm, the Al-Cu alloys are more suitable for warm reforming.

Lightweight Alloy Bonded Materials. The introduction of bonded materials technology opened a number of different options for significantly increasing the load-bearing capacities of lightweight metal pistons. Here reinforcement elements such as ceramics, carbon fibers, or porous metallic materials are arranged in closely defined positions in regions of the piston that are subject to particularly high loading. The bonded material is manufactured by infiltrating the reinforcing elements with lightweight metals such as aluminum or magnesium using the squeeze casting process. High price and unfavorable creep properties are the primary reasons magnesium is not yet used in mass production.

Among the many options available, reinforcing aluminum pistons with short ceramic fibers made of aluminum oxide is the one most widely adopted for mass production. Following a washing process to remove components that are not fiber shaped, the fibers are processed to create mold components that can be cast (preforms) with fiber content of between 10% and 20% by volume. In this way considerable improvements in strength can be achieved at the edge of the recess in direct-injection diesel pistons, for instance.

A reinforcing element made of porous sintered steel with uniform porosity from 30% to 50% was developed for ring grooves. The Porostatik material offers favorable wear properties and a sure bond with the surrounding aluminum material. It is suitable, for example, for reinforcing ring grooves that are at an extremely high location, leaving hardly any room to cast around it on the side toward the piston head.