Influence of Cu on modifying the beta phase and enhancing the mechanical properties of recycled Al-Si-Fe cast alloys

C B Basak,N Hari Babu

doi:10.1038/s41598-017-05937-2

Abstract

High iron impurity affects the castability and the tensile properties of the recycled Al-Si alloys due to the presence of the Fe containing intermetallic β-Al9Fe2Si2 phase. To date only Mn addition is known to transform the β-Al9Fe2Si2 phase in the Al-Si-Fe system. However, for the first time, as reported here, it is shown that β-phase transforms to the ω-Al7Cu2Fe phase in the presence of Cu, after solutionization at 793 K. The ω-phase decomposes below 673 K resulting into the formation of θ-Al2Cu phase. However, the present thermodynamic description of the Al-Si-Fe-Cu system needs finer tuning to accurately predict the stability of the ω-phase in these alloys. In the present study, an attempt was made to enhance the strength of Al-6wt%Si-2wt%Fe model recycled cast alloy with different amount of Cu addition. Microstructural and XRD analysis were carried out in detail to show the influence of Cu and the stability range of the ω-phase. Tensile properties and micro-hardness values are also reported for both as-cast and solutionized alloys with different amount of Cu without and with ageing treatment at 473 K. The increase in strength due to addition of Cu, in Fe-rich Al-Si alloys is promising from the alloy recyclability point of view.

Highlights

In recycled Al-Si alloys iron impurity could be as high as 1.5 wt% or more, 2 wt% Fe was chosen as the upper ceiling for the level of iron impurity[2, 3]
A systematic investigation was carried out to study the evolution of phases and resulting microstructure during the heat treatments along with the thermodynamic calculation within the CALculation of PHAse Diagram (CALPHAD) framework
Chemical macro-analysis was carried out using glow discharge optical emission spectroscopy (GD-OES) and by micro-analysis using energy dispersive spectroscopy (EDS) attached to a field emission scanning electron microscope (FESEM)

Summary

Introduction

The energy requirement of primary aluminium production from bauxite ore is about 186 MJ per kg of metallic aluminium. This expenditure could be reduced to 10–20 MJ/kg by recycling the discarded aluminium products or scraps[1, 2]. Despite employing various physical separation techniques[7, 8], it is inevitable to have residual iron impurity in the scraps, which affects the properties of the recycled aluminium alloys because of two main reasons. It has been recently pointed out that properties of high Fe containing recycled Al-Si casting alloys can be improved by changing the morphology of the β-Al9Fe2Si2 phase[20, 21]. Microhardness and tensile testing was carried out and correlated with the microstructure to get a full picture about the influence of Cu in Al-Si-Fe alloy system

Methods

Results

Conclusion