Effect Of Rare Earth Sc,Y And Heat Treatment Process On Microstructure And Properties Of 7055 Aluminum Alloy

7xxx aluminum alloy has been widely used in aerospace,automobile,military and other industries because of its advantages of high strength,low specific gravity,excellent processing performance and excellent welding performance.With the rapid development of science and technology,the public is more in pursuit of lightweight transportation,which puts forward higher requirements on the strength and toughness of aluminum alloy,and hopes to invent a new ultra-high strength aluminum alloy with low density,excellent processing performance and high cost performance.In this paper,trace Sc and Y were added into 7055 aluminum alloy by melting and casting process.After a series of processing and heat treatment processes,the electrical properties,mechanical properties and corrosion resistance of each sample were tested.The microstructure of the alloy was analyzed by OM,SEM,TEM and XRD.The effects of different contents of Sc and Y and different aging systems on the microstructure and properties of 7xxx aluminum alloy were studied in detail.The experimental results show that the grain size of 7xxx alloy can be refined effectively by trace Sc,and the dispersed and fine Al₃（Sc,Zr）phase can be formed in the process of heat treatment and deformation.These dispersed and fine Al₃（Sc,Zr）phase can hinder the dislocation,prevent the displacement of sub grain boundary,increase the recrystallization temperature and inhibit the recrystallization.The addition of Sc can improve the strength and elongation of7xxx aluminum alloy,delay the over aging and improve the stability of precipitates.The main conclusions are as follows:（1）The grain size of 7055 alloy can be refined by adding a small amount of Sc,when the Sc content is 0.25%,the grain size of 7055 alloy is between 30-40um,which is twice smaller than that of the alloy without Sc addition.Homogenization treatment can eliminate most of the segregation and residual stress,and the addition of trace Sc can form Al₃（Sc,Zr）phase,which can effectively inhibit the recrystallization behavior of aluminum alloy.（2）The results show that the hardness,conductivity,tensile strength,yield strength and elongation of 7055-0.25Sc alloy with 0.25%Sc are 212.6 HV,32.9%IACS,685.9 MPa,658.3 MPa and 18.3%,respectively,which are significantly higher than those of 7055 alloy without Sc.When Sc content increases to 0.45%,the self-etching current density is the lowest and the corrosion resistance of 7055 alloy reaches the best state.（3）The best comprehensive properties of 7055-0.25Sc aluminum alloy under three different aging systems are the aging system of 120℃/6 h+180℃/16 h,with conductivity of 42.9%IACS,hardness of 201.1 HV,tensile strength of 625.9MPa,yield strength of 596.5 MPa and elongation of 15.5%.Compared with the peak aging（T6）,the conductivity and corrosion resistance of the alloy are greatly improved.（4）The composite addition of Sc and Y can also significantly improve the properties of 7055 alloy,and the alloy with 0.25%Sc and Y has the best properties.The electrical conductivity of the alloy is 33.8%IACS,the hardness is 196.4 HV,the tensile strength and elongation are 398.6 MPa and 15.1%.