Segregation Behavior And Evolution Mechanism Of Iron-rich Phases In Molten Magnesium Alloys

The impurity element Fe is generally considered detrimental to aluminum alloys and iron removal with high efficiency and low-cost is difficult, which to a large extent restricts the recycling aluminum alloys and causes a great deal of waste. Current methods of iron removal, i.e. filters, electromagnetic and centrifugal and so on, usually have certain limitations in industrial practice. Given that, this work propounds a new method to iron removal and recycling of Al-Si alloys with high Fe content. First, after adding Al-Si-Fe alloys into pure Mg melts, Fe-rich phases with high density and high melting point will deposit to the bottom due to the natural subsidence principle. Then, it can be easy to separate Fe-rich deposit layer from Mg-Al-Si alloys with few iron content, by now iron removal implements in Mg melts.In this paper, the phase compositions, microstructure morphology and sedimentation regularity of Fe-rich phases in different technical conditions were studied, and precipitation behaviors and revolution mechanism were also revealed. The natural sedimentation model of Fe-rich particles based on kinetics calculation and phase quantitative-X-ray diffraction method was established, and the solidification path and related phase diagram of quaternary Mg-Al-Si-Fe alloys based on thermodynamic analysis of Pandat software were simulated.The main research contents in the present paper are as follows:(1) Al-Fe segregation law of Mg melts added Al-Si alloys with high Fe contentAfter 4 wt.% Al-14Si-4Fe alloys are added into Mg melts, in a proper condition, about 2.0% (volume fraction) Fe-rich deposit layer forms at the bottom of the melts, while the rest is Mg-Al-Si alloys with tiny Fe content. When adding 9 wt.% Al-14Si-4Fe alloys, the volume fraction of Fe-rich deposit layer depends on the combined effect of melt temperature and holding time. In order to obtain optimum deposit result, the Mg melts is supposed to hold for 45 minutes in 720℃, and the volume fraction is about 3.4%.(2) The microstructure morphology of Fe-rich phases in deposit layer with various Mg melts compositions, holding temperatures and cooling speedsWhen the primary Fe content of Mg melts was 0.36 wt.%(mean content, the same below), with the increasing of Si content, the Fe-rich phases turned from single blocky phase (Al5Fe2 phase) to two blocky phases (Al5Fe2 phase and Al0.7Fe3Si0.3 phase) in the deposit layer. When holding temperatures of Mg melts vary from 650℃ to 800℃, the Fe-rich phases are initially needle-like Al13Fe4 phase and blocky Al5Fe2 phase, then turn into single blocky Al5Fe2 phase, at last turn into blocky Al5Fe2 phase and Al0.7Fe3Si0.3 phase. Moreover, the forms of Fe-rich phases could also vary with different cooling rates of Mg melts. It is shown that Fe-rich phases are mainly Al5Fe2 phase and Alo.7Fe3Sio.3 phase when the cooling rate is faster, and are mainly phase and AlsFe2 phase when it is slower.(3) The natural sedimentation model of Fe-rich particles based on kinetics calculation and phase quantitative-X-ray diffraction method was establishedIn Mg-7.38Al-1.26Si-0.36Fe melts, when the holding temperature was higher than 720 ℃, the density of Fe-rich particles was larger (about 4.98g-cm-3), and the deposition rate was faster (about 1.57mm-min-1). After holding for 30 minutes, a concentrated deposit layer could be formed at the bottom, and the volume fraction was about 6%, respectively. When the holding temperature was lower than 700℃, the density of Fe-rich particles was smaller (about 3.96gcm-3), and the deposition rate was slower (about 1.10mm·min-1). After holding for 30 minutes as well, concentrated deposit layer could not be formed at the bottom. As a result, the volume fraction was about 34%.(4) The solidification path and related phase diagram of quaternary Mg-Al-Si-Fe alloys based on thermodynamic analysis of Pandat software were simulated.In Mg-7.38Al-1.26Si-0.36Fe melts, primary Al0.7Fe3Si0.3 phase began to form during insulation stage (at 750℃). Hence, with the decrease of temperature, this phase evolves into transitional Al2Fe phase and stable Al2Fe2 phase sequentially. In addition, a small number of Al13Fe4 phase could precipitate in the melt when the temperature was low enough before solidification. It was shown that in variable temperature longitudinal section calculation diagram of Mg1.26Si0.36Fe-xAl alloys, the critical temperature of phase transition of Alo.7Fe3Sio.3 phase appears linear increasing trend with the increase of Al content.