Microstructure and Mechanical Properties of Al-7Si Alloy with Trace AlFeCuTi Alloy Addition
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 serious-ly split continuity of α-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 coarse α-Al dendrites in the so-lidification 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 primary α-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 primary α-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 al-loys.In addition,Cu addition to Al-Si alloys can precipitate Al2Cu 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 AlFeCuTi 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 AlFeCuTi alloy addition was observed by optical microscope(OM)and scanning electron microscope(SEM).The average grains size,secondary dendrites arm spacing(SDAS)of primary α-Al and the mean length and width of eutectic Si phases were measured using image analysis software.The state of element distribution and exis-tence form in modified Al-7Si alloy with introducing trace AlFeCuTi 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 AlFeCuTi alloy was analyzed by differential scanning calorimetry(DSC).The result indicated that the re-finement and modification of primary α-Al and eutectic Si were obvious when 0.3％AlFeCuTi master alloy was introduced to Al-7Si al-loy.The coarse α-Al dendrites were refined into fine equiaxed grains,and the mean grains size was decreased by 75.4％from 218 to 53.6 μm,and the second dendrites arm spacing(SDAS)was reduced by 67.2％from 25 to 8.2 μm.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 8.9 μm,and the width was reduced by 78.8％from 4 to 0.85 μm.DSC analysis showed that the nucleation temperature of α-Al improved 1.3 ℃ from 602.5 to 603.8 ℃,and the nucleation temperature of eutectic Si raised 0.9 ℃ from 572.2 to 573.1 ℃ when 0.3％AlFeCuTi master alloy was introduced to Al-7Si alloy.Therefore,the nu-cleation rate of primary α-Al and eutectic Si was improved refining microstructure.However,the solidification microstructure includ-ing primary α-Al and eutectic Si structure occurred to coarsening phenomenon due to over modification when the 0.5％AlFeCuTi alloy was added to Al-7Si alloy melt.In addition,the microstructure and mapping analysis indicated that some fine Al2Cu,AlSiFe and Al3Fe 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-1.The test results of me-chanical properties showed that when the addition concentration of AlFeCuTi master alloy was 0.3％,the ultimate tensile strength(UTS)was increased by 17.3％from 162 to 190 MPa,and the elongation(El)was enhanced by 117.7％from 3.0％to 6.5％.The frac-ture 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 re-fining primary α-Al dendrites and modification eutectic Si structure;(2)Fe,Cu and Ti elements solid solution strengthening;(3)precipitating strengthening of fine Al2Cu,AlSiFe and Al3Fe particles.
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