Development and Casting of High Cerium Content Aluminum Alloys

Eutectic aluminum-cerium (Al-Ce) alloy systems have good mechanical properties at high temperatures and are very castable. Their castability is as good or better than the aluminum-silicon system with some deterioration as additional alloying elements are added. In alloy systems that use cerium in combination with common aluminum alloying elements such as silicon, magnesium and/or copper, the casting characteristics are generally better than the aluminum-copper system.

Additional alloying elements are used in aluminum casting, primarily to assist in the development of room-temperature mechanical properties. Cerium stabilizes those properties at high temperatures (392-752F [200-400C]). The primary intermetallic formed in the aluminum-rich region of the Al-Ce system is Al11Ce3.3

Microstructures typical of a eutectic aluminum-cerium alloy system are shown in Figures 1 and 2. The as-cast microstructures show a fine interconnected eutectic microstructure and the pure aluminum phase. The scale of the laths can be as small as 100nm and do not exhibit preferential direction at standard cooling rates. These structures are stable at higher temperatures. The intermetallics are trapped by the zero solubility of cerium in the aluminum matrix. This trapping prevents the system from minimizing surface energy through diffusion, which blocks the alloys from traditional coarsening interactions.

The idea of using cerium as a primary alloying element in aluminum casings at near eutectic compositions is not completely new. The element has been studied for the effect of its additions of up to 4 wt% on the solidification range, solidification volume change and cast microstructure in an Al-4.5Cu alloy. The microstructure and mechanical properties of Al-Ce-Ni alloys containing up to 16 wt% Ce and 8 wt% Ni have also been studied.
This study aimed to determine the suitability of using standard foundry processing parameters on metal quality and mechanical properties for the casting of an Al-10Mg-8Ce alloy. The study found production systems for melting, de-gassing and other processing of aluminum-silicon or aluminum-copper alloys can be used without modification for conventional casting of aluminum-cerium alloys. With the price of metallic cerium in the $4-5/lb. range during the study, the use of cerium as an alloying element is economically feasible for high-volume production.

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