Investigation On Single And Complex Modification Of Eutectic Al-Si Alloy

Due to low melting point,low density,low thermal expansion coefficient,high wear resistance,high specific strength,good casting properties and good weldability,eutectic Al-Si alloys are widely used in automobile and aerospace,especially in engine cylinder and piston.The mechanical properties of eutectic Al-Si alloys will be significantly deteriorated due to the splitting effect of thick needle-like eutectic Si on the matrix.Therefore,Na,Sr or RE modification of eutectic Al-Si alloys are usually conducted to improve the morphology of eutectic Si for alleviating its adverse effects on the mechanical properties of the alloys.At present,the research on modification of Al-Si alloys mainly focuses on the refinement of microstructure and improvement of mechanical properties.Little study on the change of area fraction ofα-Al and primary Si and its effect on mechanical properties of Al-Si alloys.Single modification of eutectic Al-Si alloys with Al-5Ti and Al-3P respectively as well as complex modification with Al-3P and Al-3B of that were carried out.The change of area fraction ofα-Al and primary Si in eutectic Al-Si alloy after modification and its effect on mechanical properties of alloys were systematically studied.Al-B-P ternary system related to complex modification was determined to achieve the purpose of designing complex modifier and interpreting the mechanism of complex modification.The results could be regarded as an excellent reference and guidance for further analyzing the microstructure of eutectic Al-Si alloys and broadening the application of it.Modification of eutectic Al-Si alloys with Al-5Ti master alloy was studied.The results show that when the modification temperature increased from 650oC to 700oC,the fraction of primaryα-Al in eutectic Al-Si increased sharply.Then,the fraction of primaryα-Al increased slowly with increasing temperature from 700oC to 750oC.The primaryα-Al and eutectic Si in eutectic Al-Si were finest,when the alloy was modified at 700oC.The fraction of primaryα-Al in eutectic Al-Si first increased and then decreased with increasing the added amount of Al-5Ti.The fraction of primaryα-Al in eutectic Al-Si was largest when the added amount of modifier was 0.2 wt.%.Compared to the unmodified eutectic Al-Si alloy,the tensile strength and elongation of the alloy modified with 0.2 wt.%Al-5Ti at 700°C increased by17%and 42%,respectively.Modification of eutectic Al-Si alloys with Al-3P master alloy was studied.The results show that the fraction of primary Si andα-Al in eutectic Al-Si alloy were largest after modification at 740°C when a certain amount of Al-3P was added.The fraction of primary Si andα-Al in Al-Si eutectic alloy first increased and then decreased with increasing amount of Al-3P,and reached maximum when the added amount of Al-3P was 0.4 wt.%.The size of the primary Si decreased continuously.Compared to the unmodified Al-Si eutectic alloy,the tensile strength and elongation of the alloy modified with0.4 wt.%Al-3P at 740°C increased by 7%and 74%,respectively.Complex modification of eutectic Al-Si alloys with Al-3P and Al-3B was studied.The results show that the fraction ofα-Al in eutectic Al-Si alloy modified with 0.4 wt.%Al-3P first increased and then decreased with increasing amounts of Al-3B,while the fraction and size of primary Si decreased rapidly first and then stabilized.The morphology of eutectic Si transformed from needle-like into fine short rods or granules after complex modification with Al-3P and Al-3B.The comprehensive mechanical properties of eutectic Al-Si alloy modified with 0.4 wt.%Al-3P and 0.2 wt.%Al-3B were improved compared with that of the alloy modified with Al-3P.The 700°C isothermal section of Al-B-P ternary system was studied by equilibrium alloy method.The eutectic Al-Si alloy was modified with the Al-1.5B-3P master alloy designed basing on this phase diagram.The results show that there exist the following five three-phase regions in the 700°C isothermal section of Al-B-P ternary system:Liq.+AlB₂+AlP,AlB₂+AlP+BP,AlB₁₂+AlB₂+BP,AlB₁₂+BP+B and AlP+BP+P_gas.The phase composition of Al-1.5B-3P alloy are Al,AlB₂ and AlP.After complex modification of Al-1.5B-3P alloy,a large number of fine-sizedα-Al dendrites and uniformly distributed fine primary Si appear in the eutectic Al-Si alloy,and the morphology of eutectic Si changes from lamellar to granular or short rod.Compared to the unmodified eutectic Al-Si alloy,the tensile strength and elongation of the alloy modified with 0.4 wt.%Al-1.5B-3P increased by 20%and 24%,respectively.