Microstructure formation during solidification of Al-Si alloys with increased Fe-content

Abstract

Aluminum is an attractive sustainable material due to its use in several key sectors and can be recycled over and over again while retaining its properties, i.e. lightweight, conductivity, formability, durability, permeability and multiple recyclability. These characteristics make it a candidate with great potential to meet the climate neutrality and circular economy requirements of the European Green Deal. Compared to primary Al production, recycling requires only about 5% of the energy. Recycling is often associated with the introduction of impurities, e.g. increased Fe content from tooling or contamination from oil and dirt, which can affect the material performance. Since increasing the purity requires additional time, cost and energy, it is imperative to investigate effect of an increase in Fe content in Al-Si alloys and ways to handle the increase without the need to remove the impurities. The microstructure of Al-Si alloys typically contains highly interconnected 3D networks of primary and eutectic Si and various intermetallic phases which are embedded in a relatively soft -Al matrix. In addition to the chemical composition of the alloy, the solidification process during casting determines the 3D microstructure and thus directly influences the thermo-mechanical behavior of these alloys. Therefore, in order to further optimize and design suitable recyclable Al-Si alloys with satisfactory performance in their intended application, it is imperative to thoroughly investigate the sequence of phase formation and its evolution during solidification. In addition, the cooling rate applied during solidification has an influence on the phase architecture. In-situ solidification experiments during synchrotron micro-tomography were performed at PETRA III at beamline P05 in the temperature range 650°C - 350°C with cooling rates of 3K/min, 20K/min and 30K/min for Al-Si alloys with increased Fe content. This allowed us to study the solidification kinetics in three dimensions as a function of time and to further understand the effect of variations in chemical composition and cooling rate on the development of the internal 3D 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 influences the resulting size distribution of the 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 in the sequence in which certain phases form and also the temperature at which they form.