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

When considering the recycling of aluminum (Al) alloys, one of the most pervasive impurity elements is iron (Fe). In the aluminum-magnesium-silicon (Al-Mg-Si) system, the presence of Fe 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 Mg and Si 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 Fe impurity content on the melting and solidification behaviour of a model 6061 cast Al alloy (AA6061). Using differential scanning calorimetry, the melting of eutectics present in the as-cast microstructures was measured; and coupled with peak deconvolution, the influence of Fe on the presence of binary and ternary eutectics involving Si, Mg2Si, α-AlFeSi and Al3Fe was determined. During controlled cooling, the influence of Fe on temperatures corresponding to critical stages of solidification were determined and it was observed that the addition of 1 weight percent Fe to AA6061 caused a delay in the observed α-Al dendrite coherency, but that an earlier dendrite coherency point was established through the bridging of growing α-Al dendrites via the formation of Al3Fe intermetallic phase with α-Al.