Aluminum Alloys for Pistons
United Engine currently uses gravity feed permanent molds to produce aluminum pistons. Aluminum, alloyed with copper,magnesium, nickel and silicon are common piston alloys in use today.
Silicon is the major alloying element added to the aluminum. It offers a number of benefits in the area of piston production and piston operation.
Machinability Corrosion Resistance
Improvement in Hardness and Strength
Improvement in Expansion Characteristics
Improvement in Wear and Scuff Resistance
Aluminum silicon alloys used in pistons fall into three major categories: eutectic, hypoeutectic and hypereutectic. Probably the easiest way to describe these categories is to use the analogy of sugar added to a glass of iced tea. When sugar is added and stirred into the iced tea it dissolves and becomes inseparable from the iced tea. If sugar is continuously added, the tea actually becomes saturated with sugar and no matter how much you stir, the excess sugar will not mix in and simply falls to the bottom of the glass in crystal form.
Silicon additions to aluminum are very similar to the sugar addition to the iced tea. Silicon can be added and dissolved into aluminum so it, too, becomes inseparable from the aluminum. If these additions continue, the aluminum will eventually become saturated with silicon. Silicon added above this saturation point will precipitate out in the form of hard, primary silicon particles similar to the excess sugar in the iced tea.
This point of saturation in aluminum is known as the eutectic and occurs when the silicon level reaches 12%. Aluminum with silicon levels below 12% are known as hypoeutectic (the silicon is dissolved into the aluminum matrix). Aluminum with silicon levels above 12% are known as hypereutectic (aluminum with 16% silicon has 12% dissolved silicon and 4% shows up as primary silicon crystals).
Pistons produced from these alloy categories each have their own characteristics. Hypoeutectic pistons usually have about 9% silicon. This alloy has been the industry standard for many years but is being phased out in favor of eutectic and hypereutectic versions. Most eutectic pistons range from 11% to 12% silicon.
Eutectic alloys exhibit good strength and are economical to produce. Hypereutectic pistons have a silicon content above 12%.
Silvolite’s KB Signature Series pistons use an alloy containing 16-18% silicon. In addition to greater strength, scuff and seizure resistance, the hypereutectic will improve groove wear and resist cracking in the crown area where operating temperatures are severe.
It is the primary silicon that gives the hypereutectic its thermal and wear characteristics. The primary silicon acts as small insulators keeping the heat in the combustion chamber and prevents heat transfer, thus allowing the rest of the piston to run cooler. Hypereutectic aluminum has 15% less thermal expansion than conventional piston alloys.