Mechanisms of Nanoparticles as Novel Grain Refiners for Thermo-mechanically Loaded Aluminum Cast Components

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Aluminum cast alloys are used in thermo-mechanically severely loaded areas, such as the cylinder head. The fatigue life of such components made of artificially aged alloys decreases drastically due to an extensive coarsening of the microstructure at elevated temperature (> 200 °C) in the most critical areas (e.g. the valve bridge). The mechanical properties, and thus, the lifetime, are then essentially determined by the grain size as solid solution hardening plays only a minor role in these systems.Given this scenario, the aim of the proposed project is to significantly increase the cyclic load-bearing capacity of cast aluminum alloys through the use of novel grain refiners. Specifically, nanoparticles will be used for this purpose. In-situ thermographic microscopy and microstructural analysis shall be used to study their influence on the solidification process and the corresponding grain structure. The alloy AlSi7Mg will be used in this project as it is commonly used in cylinder heads. In addition, binary alloy AlSi7 and pure aluminum are also investigated in order to obtain a fundamental understanding of the influence of the alloying components. Due to the successful use of boron and titanium in industrial grain refiners, nanoparticles based on these elements appear particularly promising. During solidification, nanoparticles can act as nucleation sites for the aluminum or they can interact with the crystallization front. Thus, it is necessary to obtain detailed knowledge of the transformation mechanisms of the nanoparticles during this process. Therefore, the objective of this investigation is to determine whether the particles used are effective nucleation seeds themselves, or if they interact with one of the alloying components and the occurring reaction products act as active ingredients. In this case, it is important to identify the effective reaction products as well as to investigate the necessary reaction conditions (temperature, time, and atmosphere). The microstructure and the solidification process analyses will be accompanied by mechanical testing in order to investigate the impact of the particles on the mechanical properties, particularly on the thermo-mechanical fatigue behavior. Thus, a better understanding of the circumstances under which nanoparticles exhibit a grain-refining effect in aluminum melts and the actual grain refinement mechanisms leading to the refined microstructure shall be gained.