Effect Of Cooling Rates On Microstructure And Properties Of Aluminum Alloy

The research of Al-Ca alloy is not enough,and the related work of special casting process of Al-Ca alloy was not carried out adequately nowadays.The alloy prepared by the sub-rapid solidification technique can optimize microstructure and improve performance.The granulation treatment process can improve properties of the alloy by changing the morphology of second phase.In this paper,Al-8Ca?wt%?and Al-7Si alloys are prepared by iron and copper moulds,combined with granulation treatment process.The microstructure,phase composition and mechanical properties were analyzed by OM,XRD,SEM,EDS,universal tensile machine and other equipment.The effects of different solidification speeds and heat treatment processes on the structure property of Al-base alloy were investigated.The results show that the microstructure of Al-8Ca alloy prepared by difference cooling rates has small amount of dendritic primary?-Al,and lamellar eutectic structure composed of?-Al and Al₄Ca phases.As the cooling rate increases,the dendritic?-Al becomes finer,the crystalline grain is gradually refined,the lamellar spacing of the eutectic structure gradually decreases.The supersaturated solid solution is formed at high cooling rate.When the Al-8Ca alloy is treated by diffusion annealing,the eutectic structure is melted and show a granular shape,and the grain becomes coarse.As the cooling rate increases,the Al₄Ca phase becomes finer,the energy consumption of granular treatment reduces,and the granulated effect of the alloy solidified at a high cooling rate is better.The tensile strength of solidified Al-8Ca alloy at a cooling rate of 25K/s is 196MPa,and the elongation is 4.91%.As the cooling rate increases,the structure of the alloy is gradually refined,and the tensile strength and elongation are improved.When the cooling rate is1700K/s,the tensile strength is 392MPa,and the elongation is 8.74%.The granulation treatment improves plasticity of the alloy.The elongation of the Al-8Ca alloy with a cooling rate of 800K/s and 1700K/s after granulation treatment is 13.91%and 15.30%respectively,while the strength of the alloy slightly decreases due to grain coarsening.For the Al-7Si alloy,the primary?-Al is dendritic,and the?phase and Si phase in the eutectic structure grow in a branched form at low cooling rate.As the cooling rate increases,the microstructure of the alloy becomes finer,and the supersaturated solid solution is formed,which reduce component segregation.After diffusion annealing treatment,Si transforms into a granular shape,and the alloy solidified at a high cooling rate achieve better granulation transformation at lower energy consumption.As the cooling rate increases,the tensile properties of the alloy are improved,the tensile strength of the alloy is 248MPa and the elongation is 4.90%at a cooling rate of 1700K/s.After diffusion annealing,the elongation of the alloy is greatly improved and the strength is slightly decreased.