SYNTHESIS AND CHARACTERIZATION OF AL-CE METAL MATRIX COMPOSITES FOR HIGH TEMPERATURE APPLICATIONS

Abstract

A primary advantage of Aluminum-Cerium alloys is their ability to retain their mechanical properties at elevated temperatures, i.e., in the range of 250 – 300 °C, compared to other aluminum alloys. This study focuses on the development of Al-Ce alloy matrix composites reinforced with SiC, Graphite, and Al2O3 to further improve the properties of Aluminum-Cerium alloys. Pressure infiltration and stir-casting were utilized for synthesizing Al-Ce-Graphite, Al-Ce-SiC, and Al-Ce-Al2O3 composites. The objective of this work was to study the solidification microstructure of the composites, including the effects of varying the spacing between reinforcement particles, variations in cooling rates, interactions between reinforcements and the melt, and to measure selected physical, mechanical, and tribological properties of the composites.The Al-Ce alloys studied in this work included Al-8wt%Ce (hypoeutectic), Al-12wt%Ce (eutectic), and Al-16wt%Ce (hypereutectic) compositions. The addition of 1 wt% Mg was made to the binary alloys to aid in the infiltration process and promote wettability between reinforcement particles and the melt in the stir-casting process. Nickel-coated graphite was stir-mixed with Aluminum-Cerium alloys, containing 1 wt% Mg for improved wettability, to form low-volume percent graphite containing Al-Ce composites. In a variation of the stir-mixing process, CeO2 was added to the melt of an Aluminum (Al), Nickel (Ni), Copper (Cu), and Magnesium (Mg) alloy to form Al-Ce melts via in-situ reduction of CeO2. In the pressure infiltration experiments, Silicon Carbide (SiC), Aluminum Oxide (Al2O3), and uncoated Graphite were used as reinforcement particles, and molten Al-Ce alloys were infiltrated into the loose beds of these particles. The SiC particles had a sphericity of 0.932 with two particle sizes, 75 µm and 15 µm. The Al2O3 particles had an angular morphology with two particle sizes, 90 µm and 50 µm. The uncoated graphite particles were spherical and had an average size of 121 µm. The stir-cast samples were synthesized at the UWM Center for Advanced Materials Manufacturing (6 lbs. melt size) and at Eck Industries (500 lbs. melt size). The first set of UWM samples cast in a water-cooled copper mold were to study the effect of high solidification rates on microstructure. The second set was synthesized by reducing CeO2 in the aluminum melt during stir-casting. The Eck Industries samples were made in a cast-in step-mold to study the effect of section size and cooling rate on the solidification of the composite melt. Select samples were heat-treated at 400 °C for 96 hours to study the impact of high-temperature exposure on the composite microstructure. Image analysis of the composite quantified area percentages of graphite, intermetallic phase, and Al and its secondary dendrite arm spacing, size of intermetallic, and eutectic lamellar spacing. The composition of different phases was identified and then morphologically analyzed using SEM-EDS and XRD. Hardness, density, tensile strength, and tribological properties of the composite were tested at room temperature and at 200 °C. SiC as the reinforcement led to a change in the melt composition due to the dissolution of Si in liquid Al, forming an Al-Ce-Si melt. The primary intermetallic observed in the samples was AlCeSi, preferentially forming between SiC particles, with no eutectic phase present, resulting in a featureless matrix. Al2O3 as a reinforcement did not alter the melt composition, leading to the formation of Al-Ce intermetallic particles. Increasing the Ce wt% concurrently led to larger intermetallic particles and a decrease in the interparticle spacing for SiC and Al2O3 particles required to form the intermetallic particles. The density and hardness of the composite were higher than those of the base alloys for SiC and Al2O3 reinforced composites. The density of graphite-reinforced composites was lower than that of the base alloys. The stir-cast rapidly solidified composites showed a decrease in the secondary dendrite arm spacing and eutectic lamellar spacing with increasing cooling rate. No measurable coarsening of the secondary dendrite arms and eutectic lamellae was observed after the heat-treatment of the samples. The density and hardness of the composite were measured before and after heat treatment, with the heat-treated samples showing a decrease in hardness. The coefficient of friction of the composite samples was 33% lower than the base alloys at room temperature and 32% lower at 200 °C. A similar methodology was used to analyze CeO2 reduced composites. The Al secondary dendrite arm spacing decreased, the intermetallic area percentage increased, and the hardness increased with increasing Ce wt% in the composite. The samples cast at Eck Industries were evaluated as a function of cooling rate for secondary dendrite arm spacing, eutectic lamellar spacing, hardness, and density. A decrease in the SDAS and lamellar spacing at higher cooling rates correlated with an increase in hardness. The properties of the composite samples were compared with those of the Al-Ce base alloy cast using the same casting technique, with the composite exhibiting higher hardness and lower density. The composites developed in this work are used in the automotive and aerospace sectors due to their ability to retain mechanical properties at elevated temperatures. Internal combustion engine components, such as pistons, cylinder liners, and turbocharger components, can be manufactured from these composites. The load-bearing structural components in aircraft, particularly near the jet turbine, can be manufactured using the presented composites. The low-cost casting methods and the availability of Ce in the U.S. help reduce costs for manufacturing industries, and the superior properties extend the life compared to conventional aluminum alloys.