微量AlFeCuTi多元合金对Al-7Si合金微观组织及其力学性能的影响

Microstructure and Mechanical Properties of Al-7Si Alloy with Trace AlFeCuTi Alloy Addition

采用真空电弧熔炼炉制备了AlFeCuTi多元合金(原子比为1∶1∶1∶1),研究AlFeCuTi中间合金添加含量(0.3%和0.5%,质量分数)对Al-7Si合金显微组织和力学性能的影响。结果表明:当Al-7Si合金中添加0.3%的AlFeCuTi多元合金时,初生α-Al从粗大的树枝晶被细化为细小的等轴枝晶,其平均晶粒尺寸从218μm减小到53.6μm;二次枝晶臂间距(SDAS)从25μm减小到8.2μm;共晶Si组织从粗大的针片状被变质为细小的粒状和短片状,其平均长度从34.8μm减小到8.9μm,平均宽度从4μm减小到0.85μm。然而,进一步增加AlFeCuTi中间合金的添加量到0.5%时,α-Al和共晶Si组织没有进一步被细化,反而出现了粗化现象,但是比未变质组织要细小。拉伸测试结果表明:当Al-7Si合金中添加0.3%的AlFeCuTi中间合金时,抗拉强度(σ_b)和延伸率(δ)由未变质前的162 MPa和3%分别提高到190 MPa和6.5%。然而,当AlFeCuTi中间合金的添加含量增加到0.5%时,由于过变质现象的产生,导致合金力学性能下降。

The hypoeutectic Al-Si alloys have many advantages,such as low density,high strength-to-weight ratio,good corrosion and wear resistance,low thermal expansion coefficient and good castability.However,the coarse need-like eutectic Si phases seriously split continuity ofα-Al matrix.Moreover,the sharp edges and corners of Si phases easily give rise to the stress concentration leading to the decrease of mechanical properties of Al-Si alloys,especially ductility.In addition,the forming coarseα-Al dendrites in the solidification can result in many casting defects,such as pores,snowflake spot,shrinkage porosity and serious composition segregation,which obviously decrease the strength and elongation of Al-Si casting alloys.Therefore,it is necessary to refine the primaryα-Al,and change the morphology and distribution of eutectic Si structure through modification treatment to improve the mechanical properties of hypoeutectic Al-7Si alloy.Compared with these methods of refining and modifying the primaryα-Al and eutectic Si structure including rapid solidification,heat treatment,mechanical agitation,high energy ultrasonic treatment,and adding nucleating agent,chemical modification through adding modifier among these methods has been extensively applied due to their good refining efficiency and low cost.Fe element adding to hypereutectic Al-Si alloys can modify the primary Si from coarse five-folded shape to small blocky shape,and the eutectic Si from coarse acicular to short rods shape.Ti addition can refine the dendritic crystals into equiaxed grains in Al-Si alloys.In addition,Cu addition to Al-Si alloys can precipitate Al,Cu phases,and some Cu atoms solid solution to Al play an important role in strengthening.Hence,it is very necessary to investigate the effect of complex adding Fe,Cu and Ti elements on microstructure and mechanical properties of Al-7Si alloy.The influences of adding AIFeCuTi alloy(0.3%and 0.5%,weight percent)was prepared by vacuum arc melting to systematically investigate the effect of trace Fe,Cu and Ti on microstructure and mechanical properties of Al-7Si alloy.The microstructural evolution of Al-7Si alloy with trace AIFeCuTi alloy addition was observed by optical microscope(OM)and scanning electron microscope(SEM).The average grains size,secondary dendrites arm spacing(SDAS)of primaryα-Al and the mean length and width of eutectic Si phases were measured using image analysis software.The state of element distribution and existence form in modified Al-7Si alloy with introducing trace AIFeCuTi alloy were characterized by electron probe micro-analysis(EPMA)and energy dispersive X-ray spectrometry(EDS).The initial temperature of the reaction peak of Al-7Si alloy during the solidification process after addition trace AIFeCuTi alloy was analyzed by differential scanning calorimetry(DSC).The result indicated that the refinement and modification of primaryα-Al and eutectic Si were obvious when 0.3%AIFeCuTi master alloy was introduced to Al-7Si alloy.The coarseα-Al dendrites were refined into fine equiaxed grains,and the mean grains size was decreased by 75.4%from 218 to 53.6μm,and the second dendrites arm spacing(SDAS)was reduced by 67.2%from 25 to 8.2μm.In addition,the eutectic Si was significantly modified and the morphology was transferred from coarse need-like/plate shape to fine granular and short flake shape.The average length of eutectic Si was decreased by 74.4%from 34.8 to 8.9μm,and the width was reduced by 78.8%from 4 to 0.85μm.DSC analysis showed that the nucleation temperature ofα-Al improved 1.3°C from 602.5 to 603.8°C,and the nucleation temperature of eutectic Si raised 0.9°C from 572.2 to 573.1°C when 0.3%AIFeCuTi master alloy was introduced to Al-7Si alloy.Therefore,the nucleation rate of primaryα-Al and eutectic Si was improved refining microstructure.However,the solidification microstructure including primaryα-Al and eutectic Si structure occurred to coarsening phenomenon due to over modification when the 0.5%AIFeCuTi alloy was added to Al-7Si alloy melt.In addition,the microstructure and mapping analysis indicated that some fine Al,Cu,AISiFe and Al,Fe particles were precipitated and distributed in grain boundaries and eutectic matrix(α-Al+Si)in solidification from Al-7Si alloy with adding trace 0.3%and 0.5%AlFeCuTi alloy,and some elements occurred to part solid solution in Al matrix.According to the standard of GB/T228.1-2010,the tensile test samples were machined into standard tensile samples of 5 mm diameter and 28 mm gauge length,the tensile properties of the alloys were measured using a material testing machine at a speed of 1.0 mm'min'.The test results of mechanical properties showed that when the addition concentration of AlFeCuTi master alloy was O.3%,the ultimate tensile strength(UTS)was increased by 17.3%from 162 to 190 MPa,and the elongation(EI)was enhanced by 117.7%from 3.0%to 6.5%.The fracture mode of Al-7Si alloy also changed from ductile-brittle mixed fracture to ductile fracture when Al-7Si alloy was modified by adding 0.3%AlFeCuTi alloy.The strengthening mechanism could be contributed three facet factors:(1)fine grains strengthening through refining primaryα-Al dendrites and modification eutectic Si structure;(2)Fe,Cu and Ti elements solid solution strengthening;(3)precipitating strengthening of fine Al,Cu,AISiFe and Al,Fe particles.