Research On Rheological Properties Of Hypereutectic Al-Si Alloy Melt Containing Iron

Semi-solid processing technology, being very rapidly developed in recent years, is anear-net forming, high-performance, energy-saving, environmentally friendly metalprocessing technology, and it is known as the21st century’s most promising near-net shapetechnology. It includes thixoforming and rheological forming processes. As researchersgot a better understanding of the formation mechanism of semi-solid alloy, there are somenew rheological forming methods and a breakthrough is being achieved. In the semi-solidrheological forming process, we must be strictly and precisely control the slurry rheologyto get qualified parts. Well, apparent viscosity is a very important parameter tocharacterize the rheological behavior of semi-solid metal. Iron is harmful impurity in Al-Sialloys, and it may decrease room-temperature mechanical properties of the alloys.However, with the addition of Fe, the hardness of aluminum alloys is improved to someextent, and the high temperature mechanical performance will be greatly improved. Andadding Manganese can change the morphology of iron phase in Al-Si alloy, and reduce theharmful effects of Fe at room temperature. Currently, there are few studies about apparentviscosity of Fe-rich hypereutectic Al-Si alloys at home and abroad. Reports about theeffect of Manganese content on the apparent viscosity of Fe-containing hypereutecticAl-Si alloys are even less. Therefore study on rheological properties of Fe-containingAl-Si alloys is very important.With a rotational viscometer, the rheological properties of semi-solid Al-17Si-xFe(x=2,3,4) were studied. This paper measured the viscosity of Al-17Si-2Fe in the constanttemperatures of100℃、50℃above the liquidus and10℃、40℃below the liquidusrespectively, and got the viscosity-temperature curve in the continuous cooling process.The results show the viscosities in the previous temperatures are0.7451、0.7878、0.9224、1.0829Pa s respectively. That is, as the temperature decreases, the viscosity increases. Asthe iron content increases from2%to4%, the viscosity of the alloys increases too. Butwhen the iron content is high, with the holding time increasing, the precipitation andsedimentation of the iron phases is obvious in the temperature of40℃below the liquidus. This phenomenon reduces the apparent viscosity of the melt to some extent. With theshear time increasing, the apparent viscosity of the alloys in the temperature of40℃below the liquidus decreases first and then increases. In the continuous cooling process,with the continuous decrease of the temperature, the solid fraction of the melt graduallyincreases, and the apparent viscosity of the melt increases, but the increase is slow. Whenthe temperature is reduced to some temperature near the solidus, the apparent viscosity ofAl-17Si-xFe alloys increases rapidly. In addition, at the same shear rate, to achieve thesame solid fraction, the bigger the cooling rate, the bigger the apparent viscosity of themelt.The rheological properties of semi-solid Al-17Si-2Fe-xMn (x=2,3,4) were studied,and its viscosity(apparent viscosity) at690℃,680℃,670℃……620℃,610℃,600℃is measured. The results show the viscosities of Al-17Si-2Fe-0.8Mn in the previoustemperatures are0.9088、0.8733、0.8170……0.8262、0.7779、0.8121Pa s. As theManganese content increases from0to0.8%, the viscosity increases. The apparentviscosity of Al-17Si-2Fe suddenly increases at640℃, and decreases at630℃. Basedon the microstructure quenched in water at640℃and630℃, iron-phase precipitates inthe melt of Al-17Si-2Fe alloys at640℃and some iron-phase deposits to the bottom of thecrucible at630℃. And iron-phase precipitates in the melt of Al-17Si-2Fe-0.4Mn alloys at650℃and some iron-phase deposits to the bottom of the crucible640℃. iron-phaseprecipitates in the melt of Al-17Si-2Fe-0.8Mn alloys at690℃and some iron-phasedeposits to the bottom of the crucible670℃.Adding Manganese to Al-17Si-2Fe alloys hassome effects to the morphology of iron-phase: When the Mn/Fe ratio is0, the iron phasein the melt is mainly needle-like-Al4FeSi2; When the Mn/Fe ratio is0.2, the iron phasein the melt is mainly short-strip-like-Al4(Fe, Mn)Si2and a little needle-like-Al4FeSi2;When the Mn/Fe ratio is0.4, the iron phase in the melt is mainly Irregular polygons type-Al15(Fe, Mn)3Si2and short-strip-like-Al4FeSi2, a little needle-like-Al4FeSi2. Theimpact of the melt viscosity:-Al4FeSi2>-Al4(Fe, Mn)Si2>-Al15(Fe, Mn)3Si2, that is,needle-like-Al4FeSi2increases the apparent viscosity of the melt the most.