4D investigation of microstructure formation during solidification of Al-Si alloys
Near eutectic cast Al-Si alloys with main additions of Si, Cu, Ni, Mg and Fe are a frequent choice for piston applications in modern combustion engines due to their high strength-to-weight ratio, excellent castability, good wear resistance and low thermal expansion. Their microstructure typically contains highly interconnected 3D networks of primary and eutectic Si as well as various intermetallic phases which are embedded in a relatively soft -Al matrix. Besides the chemical composition of the alloy, the process of solidification during casting determines their 3D microstructure and thus, directly affects the thermo-mechanical behavior of these alloys. This is why, for the purpose of further optimization and design of novel Al-Si alloys with ever higher performance in their intended application, it is imperative to thoroughly investigate the sequence of phase formation and their evolution during solidification. Moreover, the applied cooling rate during solidification impacts the architecture of the phases.
In-situ solidification experiments during synchrotron micro-tomography have been conducted at PETRA III at the beamline P05 in a temperature range between 650°C – 350°C with cooling rates of 3K/min, 20K/min and 30K/min for several novel Al-Si alloys. This allowed us to investigate the kinetics of solidification three-dimensionally as a function of time and provided further understanding of the impact of variations in chemical composition and cooling rate on the development of the 3D inner architecture of the alloys during solidification based on the sequence of phase formation, growth, morphology and size distribution. It was found that the applied cooling rate strongly affects the resulting size distribution of solidified phases as well as their morphologies. Furthermore, variations in the content of intermetallic phase forming elements such as Ni and Fe can result in a change of the sequence in which specific phases form and also the temperature of their formation.