Effect Of Zn/Mg And Cu Contents On Microstructure And Properties Of Al-Zn-Mg-Cu Alloys

Al-Zn-Mg-Cu series alloys are high strength aluminum alloys that can be strengthened by heat treatment.It has high strength,good plasticity and corrosion resistance,so it is widely used in aerospace field.Throughout the development history of Al-Zn-Mg-Cu series alloy,it gradually developed from a single high strength to a good comprehensive performance with high strength,high toughness,high fatigue resistance and corrosion resistance,and the significant improvement of comprehensive performance was achieved by controlling the alloy composition The main ways are to adjust the content of main alloy elements,add micro-alloy elements appropriately,and strictly control and reduce the content of impurity elements.The means of adjusting the main alloy is to increase the content of Cu element appropriately and change the value of Zn/Mg appropriately,so that the alloy has good comprehensive mechanical properties and corrosion resistance.And increasing the cooling speed can refine grain,reduce segregation,increase mechanical properties of the alloy.Therefore,in this paper,Al-Zn-Mg-Cu alloy was prepared by water-cooled copper mold casting process with fast cooling speed,and the influence law of alloy component changes on microstructure and properties was explored to obtain the alloy component with the best comprehensive mechanical properties.The main research contents are as follows:(1)The as-cast microstructure of Al-Zn-Mg-Cu alloy prepared by water-cooled copper mold casting is typical equiaxial crystal,and a large number of non-equilibrium crystalline phases exist at the grain boundary.Moreover,with the increase of Cu element content and Zn/Mg ratio,the grain size gradually decreases and the number of non-equilibrium crystalline phases increases(2)After the alloys are homogenized at 470?/24 h,most of the dendrites divided into reticular structure are eliminated,and the non-equilibrium crystalline T phase(AlZnCuMg phase)at the grain boundary has redissolved.Only a small part of the insoluble phase,such as Al7Cu2Fe,is distributed at the grain boundary.(3)After hot rolling with a total reduction rate of 80%and cold rolling with a total reduction rate of 40%,the microstructure of the alloy is elongated and fibrous,and the ratio of grain's diameter and length decreased significantly with the decrease of Cu content and the increase of Zn/Mg ratio.Compared with hot rolling,the microstructure of cold rolled state is more slender and the grain boundary is clearly visible(4)After solid solution and quenching at 480?/2 h and aging treatment at 120?/24 h,incomplete recrystallization occurred in the alloy.Moreover,with the increase of Cu content and Zn/Mg value,the recrystallization degree and grain size gradually decrease,and the number of remaining long strip microstructure gradually increases.(5)Macroscopic hardness test and tensile property test results show that with the increase of Cu content,both tensile strength and elongation of the alloy increase;with the increase of Zn/Mg,the tensile strength of the alloy decreases gradually,while the elongation first increases and then decreases.With the increase of Cu content and Zn/Mg,the hardness first increases and then decreases slightly.Therefore,the alloy with the best comprehensive mechanical properties is no.4 alloy,namely Al-5.3Zn-2.4Mg-1.6Cu-0.12Zr alloy.