Thermal analysis and characterization of cast 6061 aluminum alloy microstructures with elevated iron content

Abstract

When considering the recycling of aluminum alloys, one of the most pervasive impurity elements is iron. In the aluminum-magnesium-silicon system, the presence of iron can lead to the formation of a number of intermetallic phases in the microstructure, some of which can be quite deleterious due to their associated morphology. Since magnesium and silicon are responsible for the ageing response of 6xxx-series aluminum alloys, it is important to understand the phase formations that occur and consume these elements due to the addition of recycling impurities. Thermal analysis can be a beneficial tool in determining the influence of composition on phase formation, and the various stages of alloy solidification. The present study uses thermal analysis testing and microstructure characterization to assess the influence of iron impurity content on the melting and solidification behaviour of a model 6061 cast aluminum alloy. Using differential scanning calorimetry, the melting of eutectics present in the as-cast microstructures was measured; and coupled with peak deconvolution, the influence of iron on the presence of binary and ternary eutectics involving silicon, Mg₂Si, α-AlFeSi and Al₃Fe was determined. During controlled cooling, the influence of iron on temperatures corresponding to critical stages of solidification were determined and it was observed that the addition of 1 wt percent iron to the 6061 aluminum alloy caused a delay in the observed aluminum dendrite coherency, but that an earlier dendrite coherency point was established through the bridging of growing aluminum dendrites via the formation of Al₃Fe intermetallic phase with aluminum.