Effect of alloying additions and high temperature t5-treatment on the microstructural behavior of al–si-based eutectic and hypo-eutectic alloys

Abstract

The present study has been undertaken to investigate the effect of alloying additions and high temperature T5-treatment consisting of high temperature direct aging after casting, on the microstructural characteristics of some Al-Si-based eutectic and near-eutectic alloys, typically used in the automotive industry. For comparison purposes and in order to estimate the effect of Mg, three sets of samples consisting of Al-11.9wt%Si (AS13, B413), Al-11.67wt%Si-0.22wt%Mg (AS10G) and Al-7.2wt%Si-0.44wt% Mg (AS7G06, A356) alloys have been first studied in the as-cast state. In order to examine the effect of heat treatments, three groups of alloys were investigated in two different states, namely: (i) after T6-treatment consisting of solutionizing at 540 °C for 10 hours, water quenching and then aging at 160 °C for 4 hours; and (ii) after high temperature T5-treatment consisting of annealing at 300 °C, 450 °C and 500 °C for different aging times in order to locate a possible peak hardness. Microstructural examination was carried out using optical microscopy, scanning electron microscopy, energy Dispersive Spectrometry (EDS), X-Ray Diffraction and Brinell-hardness measurements. Our as-cast alloys’ microstructure revealed that all the different intermetallic phases, such as the Q-Al5Cu2Mg8Si6 phase, the α-Al15(MnFe)3Si2, β-Al5-Fe-Si, π-Al8FeMg3Si6 and Al7Cu2Fe iron-rich compounds, as well as the Al2Cu and Mg2Si hardening precipitates, have been formed during solidification. Besides, X-rays diffraction patterns revealed that while some Al2Cu hardening precipitates have exhibited total dissolution, after solutionizing, the undesirable iron-rich phases have remained at least partially insoluble after the different heat treatments. A semi-quantitative X-Rays diffraction analysis, performed after the different heat treatments, has shed light on the interplay between the Fe-rich intermetallic compounds, denoting the transformation of the π-Al8FeMg3Si6 into the β-Al5FeSi phase on the one hand and between π-Al8FeMg3Si6 and Q-Al5Cu2Mg8Si6 phases in the precipitation and dissolution of Al2Cu and Mg2Si phases on the other hand. Both of the two processes, marking a certain interaction between the different intermetallic compounds, are observed to give rise to a significant release of Mg and Si solute atoms and to enhance the hardening precipitates volume fraction after heat treatments. It turns out that high temperature T5-treatment is more appropriate and even more efficient than the usual T6 one, in the sense that it yields a more significant amount of hardening precipitates giving rise to higher hardness peaks with just one step treatment.