The Investigation Of Al-Si-Fe-Mn(Y) Master Alloy Modification Of Al-Si Alloys

The lightweight of materials has become a new direction for the development of transportation,aerospace and other industries because of the increasingly serious issues of energy,environment and safety.Therefore,the cast Al-Si alloys have become the preferred material for the development of lightweight materials due to low density,high specific strength,wear resistance,corrosion resistance and other advantages.However,the well-developed?-Al dendrites and coarse Si phases are distributed in the matrix of the cast Al-Si alloys.The tip of the coarse irregular Si phase is easy to cause stress concentration and reduce the bonding strength between the Si phase and the matrix,which decreased the mechanical properties of the cast Al-Si alloys.Thence,in order to improve the mechanical properties of the cast Al-Si alloy,the chemical modification is an effective method to refine the?-Al and modify Si phases of cast Al-Si alloys.Therefore,the influences of alloying elements addition on the microstructure,mechanical properties and friction properties of the cast Al-Si alloys were investigated in this paper.It offers scientific basis and theoretical direction to achieve industrial manufacture of cast Al-Si alloys.In this study,the primary Si and eutectic Si phase in the hypereutectic Al-20Si alloy and the?-Al dendrites and eutectic Si structure in the hypoeutectic Al-7Si alloy were studied as the major objects.The influences of Al-Fe or Al-20Si-2.5Fe-2Mn master alloy additions on the morphology and size of the primary Si phase s and eutectic Si structure in the hypereutectic Al-20Si alloys and the effects of Al-20Si-2.5Fe-2Mn/Y additons on the?-Al dendrite and eutectic Si structure of the hypoeutectic Al-7Si alloy were investigated by the optical microscope,field emission scanning electron microscope,transmission electron microscope,X-ray diffractometer and differential scanning calorimeter.The relationship between the microstructure evolution and mechanical pr operties of the cast Al-Si alloy was clarified and the modification mechanism of Fe,Mn and Y elements on the primary and eutectic Si phases was researched.The effect of Fe addition on the primary Si in Al-20Si alloy had a significant modification.The morphology of primary Si changed f rom coarse irregular block and five-petal to fine regular block.When the Fe content was increased to 2.5%,the average size of primary Si decreased by 75.8%from 87?m to 21?m.The Fe can also effectively modify eutectic Si structure of Al-20Si alloy.The morphology of eutectic Si changed from coarse flake-like and needle-like to fine particles and short-rod.In addition,the tensile test illustrated that the ultimate tensile strength and elongation of Al-20Si alloy firstly increased with the increase of Fe content from 0 to 1.5%,and then decreased when the concentration of Fe improved from 2.0%to 2.5%.When the Fe content was 1.5%,the ultimate tensile strength and elongation were 175MPa and 1.84%,respectively.Moreover,the friction coefficient of Al-20Si alloy firstly decreased,and then increased with the improvement of Fe content.With the increase of Fe content,the wear rate of Al-20Si alloy reduced by 46.8%from 2.65×10^-5g/m to 1.41×10^-5g/m and the Brinell hardness value of Al-20Si alloy enhanced by 24.5%from 76.0HB to 94.6HB.The addition of Al-20Si-2.5Fe-2Mn master alloy clearly improved the size and morphology of primary Si phase and eutectic Si structure of Al-20Si alloy.With the increase of the Al-20Si-2.5Fe-2Mn master alloy contentration,the morphology of primary Si phase changed from coarse irregular block and five-petal to fine regular block and the average size of primary Si reduced by 61.1%from 90?m to 35?m.The morphology of the eutectic Si structure in Al-20Si alloy also changed from coarse flake-like and needle-like to fine fibrous and granular with the enhancement of the Al-20Si-2.5Fe-2Mn master alloy content.In addition,the ultimate tensile strength of Al-20Si alloy increased by 34.6%from 130MPa to 175MPa and the elongation enhanced by 45.8%from 0.85%to 1.24%when the content of Al-20Si-2.5Fe-2Mn master alloy increased to 6%.Moreover,the fracture mode gradually transformed from brittle fracture to ductile-brittle fracture.The complex addition of Al-20Si-2.5Fe-2Mn master alloy and rare earth Y can significantly refine the primary?-Al dendrites and modify eutectic Si structure in Al-7Si alloy.The average grain size of primary?-Al decreased by 89.5%from 483?m to 51?m and the secondary dendrite arm spacing of primary?-Al reduced by 48.1%from 27?m to 14?m.The morphology of eutectic Si changed from coarse needle-like and flake-like to fine fibrous and granular shape.In addition,compared with the unmodified Al-7Si alloy,the ultimate tensile strength of Al-7Si alloy with7%Al-20Si-2.5Fe-2Mn master alloy and 0.2%Y synergistic modification enhanced by 33.3%from 162MPa to 216MPa.The elongation increased by46.7%from 6.2%to 9.1%and the Q index value increased by 28%from280.9MPa to 359.9MPa.Moreover,the tensile fracture morphology of Al-7Si alloy was typical ductile fracture characteristics when the synergistic addition of the Al-20Si-2.5Fe-2Mn master alloy and rare earth Y.