| Al-7Si-0.35Mg alloy has the advantages of low density,high specific strength,good corrosion resistance and excellent casting performance.It is widely used in the manufacture of automobile hub,automobile engine cylinder block,cylinder head and other important parts,and is an important basic material for realizing automobile lightweight.However,the application and recycling of the alloy are faced with many problems and challenges.The primary problem is that the needle Fe-rich phase in the alloy will split the matrix and reduce the plasticity toughness.In the casting process,the lamellar Fe phase and the coarse α-Al grain easily lead to casting defects and significantly reduce the mechanical properties of castings.In this paper,the strength and toughness of Al-7Si-0.35Mg-xFe alloy are studied by means of microstructure regulation.Firstly,Al-TCB seed alloy(B-doped TiCx,Al-TCB for short)was used as a means to refine the α-Al grain in Al-7Si-0.35Mg-0.15Fe alloy to improve the mechanical properties of the alloy.Secondly,in order to further improve the toughness of the alloy,on the basis of α-Al grain refinement,Mn and Cr elements were used to improve the morphology of Fe-rich phase in the alloy,and the improvement effects of these two elements on the morphology of Fe-rich phase in the alloy were compared.Finally,to solve the problem of low elongation of Al-7Si0.35Mg-xFe(x=0.25,0.35,0.45)alloy,the morphology and size of Fe-rich phase in Al-7Si0.35Mg-XFe(x=0.25,0.35,0.45)alloy were regulated by refining α-Al grains in the alloy to improve the strength and toughness of the alloy,and Fe-rich phase morphology in the alloy was modified on the basis of fine grain strengthening.In order to further improve the plastic toughness of the alloy.The main work is as follows:(1)Grain refinement of Al-7Si-0.35Mg-0.15Fe alloyThe average grain size of Al-7Si-0.35Mg-0.15Fe was refined from 1135μm to 140μm,90μm and 91μm by adding 0.2%,0.5%and 1.0%Al-TCB seed alloy,respectively.The distribution of eutectic Si in Al-7Si-0.35Mg-0.15Fe alloy is more uniform due to the refinement of α-Al grains,and the cleavage planes of Al-7Si-0.35Mg-0.15Fe alloy are reduced and dimples are increased,which is conducive to the improvement of the alloy elongation.In addition to refining α-Al grains,the addition of Al-TCB also promoted the precipitation of aging phase and shortened the aging time.After grain refinement,the mechanical properties of the alloy are improved.With 0.2%,0.5%,1.0%Al-TCB,the tensile strength increases from 310 MPa to 325 MPa,335 MPa and 332 MPa,and the elongation increases from 6.2%to 7.6%,9.3%,9.6%,respectively.They increased by 22.3%,50.0%and 54.8%,respectively.(2)Regulation of Fe-rich phase in Al-7Si-0.35Mg-0.15Fe alloy after the grain is refinedIn order to further improve the toughness of Al-7Si-0.35Mg-0.15Fe alloy,the morphology and size of Fe-rich phase in Al-7Si-0.35Mg-0.15Fe alloy were improved by using Mn and Cr elements on the basis of α-Al grain refinement,and the improved alloy elongation increased from 9.3%to 11.4%.The results show that Mn transforms the needle sheet β-AlFeSi phase into Chinese character or long massive α-Al(Fe,Mn)Si phase.Cr transforms it into massive α-Al(Fe,Cr)Si phase,which reduces the cleavage of Fe-rich alloy matrix.Compared with α-Al(Fe,Mn)Si phase,α-Al(Fe,Cr)Si phase is smaller in size and more evenly distributed.Therefore,the mechanical properties of Al-7Si-0.35Mg-0.1 5Fe alloy can be improved more significantly with the addition of 0.1%Cr.TEM analysis showed that the two Fe-rich phases have the same lattice type and the gap between crystal faces is small,indicating that the metamorphic mechanism of Cr to β-AlFeSi phase is similar to that of Mn,which changes the crystal structure of β-AlFeSi phase by replacing Fe in the phase.In order to analyze the effect of Al-TCB on Ferich phase after modification,Al-7Si-0.35Mg-0.15Fe alloy with only modified elements was prepared.The results show that the addition of Al-TCB and Fe-rich phase modification are indispensable to obtain good comprehensive mechanical properties.(3)Strengthening and toughening of Al-7Si-0.35Mg-xFe alloyWith the increase of Fe content in Al-7Si-0.35Mg alloy,the temperature range of eutectic reaction L→α-Al+β-AlFeSi increases,and the β-AlFeSi phase increases.Compared with α-Al phase and eutectic Si phase,β-AlFeSi phase has higher hardness values.The experimental results also show that the Brinell hardness and yield strength of Al-7Si-0.35Mg-xFe alloy increase with the increase of β-AlFeSi phase,but the elongation of Al-7Si-0.35Mg-xFe alloy decreases significantly with the increase of β-AlFeSi phase quantity and size.In order to improve the elongation of Al-7Si-0.35Mg-xFe alloy,the average grain size of α-Al was refined from about 1340μm to about 90μm by adding 0.5%Al-TCB seed alloy to the alloy,and the strength and toughness was improved effectively.Even in Al-7Si-0.35Mg-0.45Fe alloy,its elongation can still reach 7.0%.After the α-Al grain refinement,the β-AlFeSi phase in the alloy is more evenly distributed and smaller in size,which not only plays the strengthening role of Fe-rich phase,but also weakens the cleavage effect on the matrix,thus effectively improving the strength and toughness of the alloy.When Al-TCB and Cr are added to the alloy at the same time,the refining ability of Al-TCB is weakened due to the large amount of Cr,resulting in the mechanical properties of the alloy are not significantly improved.This indicates that the mechanical properties of Al-7Si-0.35Mg-xFe alloys are improved more significantly by grain refinement than by Fe-rich phase modification of alloying elements. |
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