Relationship Between Mechanical Properties And Morphologies Of ?-Si In Binary Al-Si Alloys

Due to excellent comprehensive properties(e.g.excellent castability,good mechanical properties,low thermal expansion coefficient and high thermal conductivity,etc.),near-eutectic and hypereutectic Al-Si series alloys are optimal light-weight materials which have been widely used in auto parts manufacture.However,the coarse primary ?-Si and flaky eutectic ?-Si(Si is short for ?-Si in following)have been identified as the main cause of the decrease of mechanical properties.Modification treatments are widely used in industry production to control Si phases morphologies of Si,and then tensile property can be improved.However,the systematical research of influence of Si phase(Si morphology,size and distribution)on mechanical properties of Al-Si alloys such as hardness and wear resistance is still needed.Therefore,in this paper,near-eutectic Al-13Si and hypereutectic Al-25Si alloys were selected as the main research subjects.Modification treatments by adding P or Sr element to Al-13 Si alloy were taken to form hypereutectic/hypoeutectic Al-Si microstructures.Al-P master alloy was added to refine Al-25Si alloy and obtain different primary Si size.With the same components,the effect rules between mechanical behaviors and Si morphologies of Al-Si alloys was investigated by macro hardness test,tensile test and wear test.The intrinsic reasons for the difference of sensitivity between them were analyzed.Meanwhile,the inhabiting effect of Ca impurity on P modification was studied.Based on the study,a new,effective and environmental process technique was proposed to remove the Ca impurity.The main results are as followings.(1)The relationship between Si morphologies and mechanical properties of near-eutectic Al-13Si alloys.It's found that under room temperature,when the chemical composition is certain,modifying of Si morphology can't change the hardness of Al-13Si distinctly.while the Si morphology can obviously affect the ultimate tensile strength(UTS)and wear resistance:The UTS can reach 205 MPa after Sr modification,which is 23%higher than that of P modification,the elongation increases from 1.03%to 3.48%;The weight loss is less than that of P modification as well,especially under high loading(120N,150N).However,under high temperature(350?),the uniform distribution of Si phase make alloy deform more coordinated.As a result,the UTS can research 72MPa,the elongation is 7.4%.friction coefficient curve becomes more smooth,and wear resistance is much better.The results above show that the tensile property and wear resistance of the alloys are sensitive to the morphologies and distribution of Si phase.(2)The relationship between primary Si size and mechanical properties of hypereutectic Al-25Si alloy.Under room temperature,when the size of primary Si is refined from over 200?m to 15 ?m with Al-P alloy,the UTS increases from 99MPa to 152MPa,improved by 53%,the elongation increases from 0.65%to 0.94%.The weight loss also decreases obviously under high loading,which presents a good wear resistance.However,the hardness keeps almost the same as that of without refinement.Data from some other hypereutectie Al-Si alloys reflects similar regularity,which indicates the hardness of Al-Si alloy is decided by Si volume fraction instead of the size,distribution and morphologies of Si.Furthermore,under high temperature,the UTS and wear resistance are improved due to the primary Si refinement:the UTS increases from 66 MPa to 69 MPa,the elongation increases from 0.4%to 2.7%;the frication coefficient curve also becomes more stable.(3)The inhibition effect of Ca impurity on P modification or refinement mechanism and new process of removing Ca.The interaction of Ca and P in Al-Si melt was systematically investigated.The microstructures and Si morphologies of Al-13Si alloy with different Ca/P ration was observed and analyzed.It was revealed that the main existence forms of Ca are Ca3P2 and Al2Si2Ca,which are irrelevant to the Si content and Ca/P ration.Based on the strong affinity of Ca and P,a new removal processo of impurity Ca is proposed:Adding Al-P mater alloy to react with Ca in Al alloy melt,and then removing the Ca-rich components.The process is simple,efficient,environmentally friendly,and can decrease Ca content to less than 10ppm.