Study On The Mechanical Property And Regulating Mechanism Of Fe-rich Phases In Al-Si Alloys With High Fe Content

Al-Si series alloys are widely used in automobiles,communications,electronic appliances and other fields due to their advantages of high specific strength and specific stiffness,excellent castability,corrosion resistance and good recyclability.In order to cope with global resource and environmental problems,more and more countries and regions have been using the recycled aluminum to replace the electrolytic aluminum.However,some impurity elements such like Fe,are inevitably introduced in the recycled aluminum alloys during the recycling process,which can deteriorate the mechanical properties of the alloy.In this work,we aim at one of the key problems during the recycling process:the high content Fe seriously deteriorates the mechanical properties of the recycled aluminum alloys,and therefore we studied the fundamental problems related to Fe removal and the regulation Fe-rich phase in the Al-Si alloys with high Fe content.In this work,the A356 alloy with a high Fe content was used as based alloy.The alloying elements such as Mn,B,and melt holding were used to regulate the Fe content and the morphology of the Fe-rich phases.The microstructure and mechanical properties of the A356alloys with a high Fe content under gravity casting were systematically studied using synchrotron radiation X-ray imaging,thermodynamic calculations,first-principles calculations,transmission electron microscopic（TEM）and other methods.We focused on the morphology evolution of Fe-rich phases during natural sedimentation and modification processes,and revealed the mechanisms of Fe removal and morphology regulation of the Fe-rich phase.The following conclusions are obtained:（1）The effect of the melt holding temperature on the microstructure and mechanical properties of the A356-1.0Fe-1.2Mn alloy was studied.When the melt holding temperature decreases from 680°C to 615°C,the area fraction of the Fe-rich phases and the remained Fe content in the alloys gradually decreased.The formation and sedimentation of the primary Fe-rich phase is the main reason for Fe removal.With the decrement of the holding temperature,the main factors of the Fe removal efficiency are morphology,size and density of the primary Fe-rich phases.（2）The effect of Mn/Fe mass ratio on the microstructure and mechanical properties of A356-1.2Fe alloy was studied.As the Mn/Fe ratio increases to 1.6,the Fe removal efficiency gradually increases to the maximum value of 64.5%.The in-situ synchrotron X-ray radiography results show that the time of the growth process for the primary Fe-rich phase during melt holding is about 36 minutes.With the increasing Mn/Fe ratio,the 3D morphology of the Fe-rich phases in the alloy was transformed from the network-interconnectedβ-Al₅Fe Si to the independently distributedα-Al₁₅（Fe Mn）₃Si₂ with various morphologies.It significantly reduces the volume fraction,equivalent diameter and interconnectivity ofα-Al₁₅（Fe Mn）₃Si₂,and greatly improves its sphericity.The synchrotron X-ray tomography results show that the3D morphology ofα-Al₁₅（Fe Mn）₃Si₂ can be summarized as massive and regular polyhedrons,hollow,regular polyhedrons,and multibranched polyhedrons.The volume fraction of the different 3D morphologies of Fe-rich phases is related to the Mn/Fe ratio.The excess Mn/Fe ratio increased the number and volume fraction of the large Fe-rich particles with a low sphericity,reducing the ductility of the alloy.（3）The effect of Mn/Fe mass ratio on the microstructure and mechanical properties of A356-2.4Fe alloy was studied.As the Mn/Fe ratio increases from 0 to 0.7,the Fe removal efficiency of the alloy increases gradually.When the Mn/Fe ratio exceeds 0.7,the Fe removal efficiency remains at 70%.Meanwhile,a peritectic structure forms showing irregular shape with double-layer,whose volume fraction and size gradually increase with increasing Mn/Fe ratio.The Mn/Fe ratio of the inner layer in the peritectic structure is more than 5.4 times higher than that of the outer layer and other Fe-rich phases.The inner layer consumed the Mn in the melt,leading to no more increase of Fe removal efficiency.The increase of the Mn/Fe ratio significantly reduced the volume fraction of the Fe-rich phases and modified their morphology,resulting in increasing the elongation by more than 6 times.（4）The effect of Mn on the microstructure and mechanical properties of the A356-1.2Fe alloy was studied.The Mn addition gradually changes the 2D morphology of the Fe-rich phase from needle-like to Chinese-script and star-like.The single Chinese-script Fe-rich phase is difficult to obtain by Mn modification.The synchrotron X-ray tomography results show a composite structure consisting of flake-likeβ-Al₅Fe Si and hollow polyhedronα-Al₁₅（Fe Mn）₃Si₂ in the 0.73%Mn contained alloy,and long rod-shaped hollow polyhedronα-Al₁₅（Fe Mn）₃Si₂ is found in the 1.09%Mn contained alloy.With the addition of Mn,the plasticity of the alloy is significantly improved,however,with too high Mn content the ductility will be reduced owing to the formation of the coarse Fe-rich phase.The thermodynamic calculation results show that the formation ofβ-Al₅Fe Si was completely eliminated when the Mn content is 1.06%.But the initial formation temperature of the Fe-rich phase is much higher than that ofα-Al,resulting into the formation of large-sized primary Fe-rich phases.（5）The effect of B on the microstructure of the A356-xFe alloys was studied.The addition of B significantly refined theα-Al grains,but has no obvious effect on the morphology of the Fe-rich phases.The Al-Al B₂ eutectic reaction provides a large number of nuclei forα-Al,significantly increasing their nucleation temperature and thereby transforming theα-Al dendrites to petal-like equiaxed crystals.After adding with B,the length of the primary Fe-rich phase in the A356 alloy with high Fe（2.4～3.0%）content is reduced by approximately 40%.But the existence of primary Fe-rich phase cannot be completely eliminated.The first-principles calculations and in-situ synchrotron X-ray radiography results show that the adsorption energy of B on the main sites ofα-Al₂O₃（0001）surface is significantly higher than that of Fe,and the B addition decreases the initial formation temperature of primary Fe-rich phase in the high Fe content alloys.In addition,α-Al and Al B₂formed in the Al-Al B₂ eutectic reaction hindered the growth of the primary Fe-rich phase.（6）The effect of the Mn/B synergistic modification on the microstructure and mechanical properties of A356-xFe alloy was studied.As the Mn/Fe ratio increases to 1.0,the morphology of the Fe-rich phase gradually changed from a single needle-like to a Chinese-script shape and a star-like under Mn/B synergistic modification.When the Mn/Fe ratio reaches 0.8-1.0,the uniform size and distribution of Chinese-script Fe-rich phases was obtained.After Mn/B synergistic modification,the elongation of the alloy gradually improved as the Mn/Fe ratio increases.However,it shows higher volume fraction of the Fe-rich phases at high Mn/Fe ratio,leading to the formation of shrinkage defects and resulting in decreasing mechanical properties of the alloy.With the increase of Fe content,the Mn/Fe ration needed to obtain a uniform Chinese-script Fe-rich phase gradually decreased,from 1.4 in 0.4%Fe alloy to 0.4 in 2.4%Fe alloy.（7）The Mn/B synergistic modification mechanisms of the Fe-rich phases were studied.The SEM and TEM results show that the B mainly existed as a fine Al B₂ phase with snowflake-like.It has no orientation relationship withα-Al₁₅（Fe Mn）₃Si₂ and therefore can not be used as effective nucleation sites.The first-principles calculation results show that the adsorption energy of B atoms on theα-Al₂O₃（0001）crystal plane is significantly higher than that of Mn and Fe atoms,which is owing the B atoms are preferentially adsorbed on the surface of the oxide film.Thereby the addition of B can suppress the nucleation of the Fe-rich phase at high temperatures.The in-situ synchrotron X-ray radiography results show that the B addition significantly increased the component undercooling of the Fe-rich phase and increased their nucleation rate.Furthermore,the growth of the Fe-rich phase competed with the Al B₂ andα-Al formed by the Al-Al B₂ eutectic reaction,promoting the transformation of the Fe-rich phase morphology to multi-branched hollow polyhedron.