Development and Casting of High Cerium Content Aluminum Alloys

David Weiss
Click here to see this story as it appears in the November/December 2017 issue of Metal Casting Design & Purchasing

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.

Casting Trials
Initial analysis performed on an Al-Ce system produced via powder metallurgy followed by hot forging showed promising strengths at temperatures up to 650F (343C). However, the casting characteristics of these alloys were unknown. A review of the phase diagram showed a promising eutectic composition at approximately 10 wt% cerium that suggested the alloy could be cast. Figure 3 depicts the calculated binary phase diagram of Al-Ce system.

To test the casting of the Al-Ce alloys, preliminary trials were performed using a permanent mold containing the standard ASTM B108 test bar geometry. The casting alloys were prepared in 55-lb. (25-kg) batches using P1520 ingot with the composition shown in Table 1. The alloy was not degassed and was poured into the mold heated to 752F (400C) using a casting temperature of 1,382F (750C).

When casting Al-Si alloys such as 356 or 355 containing 5% silicon or more at a mold temperature of 752F and a metal temperature of 1,382F, the test bar set easily fills and good test bars are produced. To fill consistently, alloys containing less silicon require additional superheat to either the mold or the metal.

In the casting trial, compositions of up to 10% cerium filled the mold completely and the production of sound bars was consistent with those produced with alloys containing 5% Si. At 12% cerium, mold-filling capability declined and the metal temperature was adjusted upwards to 1,427F (775C) to achieve complete fill.

Figure 4 shows that at 16% cerium, the mold did not fill completely at a mold temperature of 797F (425C) and a casting temperature of 1,427F. The arrow shows the extent of the fill on the riser which is usually filled to the top of the mold, in-line with the top of the sprue. This is a result of the rapidly increasing melting temperature above the eutectic point for the alloy. None of the test bars showed evidence of hot tearing.

A second trial was conducted using the same materials and processing parameters but using a step plate mold and a hot tearing mold to estimate feeding characteristics and susceptibility to hot tearing (Figs. 5-6). Overall castability of the studied compositions appears to be in line with currently available commercial alloys. As a comparison, hot-tear molds and step plates also were cast from A206. In comparing the A206 castings versus the identical Al-Ce castings, the A206 castings appeared to have larger and more pronounced macroscopic defects present than did the castings of Al-Ce alloys.

Since the casting characteristics of the binary system were acceptable, a complicated cylinder head was cast using an 8% Ce binary alloy (Fig. 7). The casting was poured successfully and inspected for hot tears or other defects. Besides a few minor misruns attributable to a lower melt superheat than currently is used to produce this head, the casting passed all inspection criteria.

In general, Al-Ce alloys near the eutectic composition exhibited good to excellent casting characteristics. However, the room temperature mechanical properties were not high enough for many commercial applications nor did the alloys have a positive response to heat treatment. Generally, the tensile strength increases with increasing Ce content up to the eutectic composition. The yield strength increased with increasing cerium content in all compositions studied. The extensometer slipped when measuring the 8% and 10% compositions, giving erroneous measurements (Table 2).

An additional 20 alloys were produced using Al-8Ce as a base composition with additives of silicon, magnesium, copper, zinc, nickel, titanium, manganese or iron. The 8Ce base composition was chosen for economic reasons, even though higher levels of Ce develop better mechanical properties. Except for magnesium, the addition of these alloying elements in excess of 1% reduced die filling capability even though many of the alloys had improved mechanical properties. For ternary Al-Ce-Mg alloys, yield strength increased with increasing magnesium levels without a noticeable reduction in castability up to the tested level of 10% magnesium. Mechanical properties for three of these alloys are shown in Table 3. The 500F (260C) properties were measured after stabilizing the samples at that temperature for 30 minutes. The data shown in Tables 2 and 3 is the result of the average of six test bars. The maximum recorded standard deviation for tensile and yield strength was 4.4 MPa. The elongation values were rounded to the nearest percent but no values were outside of +/-.4% of the reported data.

Preliminary work has been completed to develop mechanical properties after long-term high temperature exposure. After exposure at 500F (260C) for 336 hours and measured at room temperature, the Al-8Ce-10Mg-F alloy had a yield strength of 144 MPa, 33% higher than 354.0-T61 after 100 hours of exposure. This yield strength is higher than shown with 30-minute exposure in Table 3, indicating some positive effect from long-term thermal exposure.

Pilot Production
Since the Al-Ce-Mg alloys have both good castability and good mechanical properties, a pilot study was made using a 705-lb. (320-kg) heat of an Al-10Mg-8Ce alloy. Aluminum 535 was used as the base material with a chemistry as shown in Table 4.

Many commercial castings were poured using patterns and permanent molds that were gated for 200 and 300 series alloys (Figures 9 and 10). The gating was not modified to pour the Al-Ce. The casting quality was acceptable and equivalent to that produced in the production alloys. Test bars were produced from the production batch of material and tested to determine if the properties met those of the smaller batches of experimental material. The tensile strength of the material was 3.5% higher than the early experimental heats which were melted under argon and cover flux but had no degassing. Optical examination of test bars at 50X revealed that they had lower oxide levels than the early experimental heats. A total of 20 tensile bars from the pilot production lot have been tested at room temperature with average properties of 235 MPa tensile, 192 MPa yield and 1% elongation.

Next, 551 lbs. (250 kg) of the alloy was held for 17 hours at 1,384F (750C). The magnesium chemistry was checked at 9.78% or a 3.1% loss over that interval. This is a smaller than expected magnesium loss given the holding time and the lack of a protective atmosphere. The reasons for this unusual magnesium stability is being investigated.

Economic Considerations
Cerium is the most abundant rare earth element. With the price of metallic cerium in the $4-5 per lb. range at the time of the study, the use of cerium as an alloying element is economically feasible for high-volume production. The as-alloyed cost of Al-Ce material is competitive with other high-performance aluminum alloy systems. A graphical comparison is given in Figure 11.

Several test pieces and complicated castings have been produced in the Al-Ce alloy systems. The data and experience to date indicate Al-Ce or Al-Ce-Mg have castability equivalent to 300 series alloys. Other alloy additions have generally diminished castability but show promise with additional work. The use of production processing equipment resulted in better mechanical properties than the earlier development heats because of the more effective removal of oxides and hydrogen. Unexpected results that require further study include the apparent reduced solubility of hydrogen in alloys containing cerium and the role of cerium on the stabilization of magnesium in Al-Mg-Ce alloys. 

This article is based on a paper (17-013) originally presented at the 2017 Metalcasting Congress.