7056 Aluminum Alloy Heat Treatment Process And Organizational Performance Research

7056aluminum alloy,which was invented by French scientists in2004, is a very promising alloy.Compared to the7055aluminium,which is developed by Aloca company from the United States,it make a significant increase of the content of Zn as well as reduce the content of Mg and Cu,that is,the Zn/Mg ratio of the alloy is increased,at the same time,the purity of the alloy is very high,such as Fe、 Mn、 Si elements are refined strictly,those of which make the alloy has high strength, high fracture toughness and stress corrosion resistance and will be widely used in aerospace field in future.At present, the material of the fourth generation of fighters and military aircraft in our country are hoped to be equipped with the7xxx aluminium on the T77and T79states,especially with the development of the big aircraft and new millitary cargo plane,the need of this kind of adcanced aluminum alloy(such as the7056-T7951,7055-T77) is more urgent.However,the research of the7xxx alloy with the high Zn/Mg ratio in the domestic has not been started,so the study of7056aluminium is very valuable.The mircostructure and properties of the7056aluminium during the homogenization and multi-aging heat treatments are researched by using OM、 SEM、 TEM apparatus and mechanic perfomance testing、condu-ctivity testing、 SSRT testing methods.The characteristics of the alloy with different Zn/Mg ratio during single-aging are studied specialy. At the same time,the effect of various heat treatment in the micro-structure,mechanical and SCR properties of the alloy are researched also.All the conclusion are summarized as follows:(1)The crystal and area segregation are very serious in the micro-structure of as-cast7056aluminium.There are plenty of non-equilibrim eutectic phases as well as η (MgZn2)、T(Al2Mg3Zn3)、 S(Al2CuMg) pha-se and Al6(FeCu) phases between the metal compoundsand the grain boundaries.(2)The suitable single-homogenization of7056aluminium alloy is455℃/24h,in that case,most of the crystal and area segregation were eliminated,but there still had a small amount of residua bone eutectic compounds.The ieal heat treatment anneal of homogenization are455℃/24h+465℃/4h, which can dissolve more phases,and the microstructure of alloy are more homogenized.(3)The response speed of7056aluminum alloys is very rapid, its hardness can rise to188.3HV and206.2HV when aging only1h in105℃and150℃,which had reached87.4and97.2percent of its peak-aging hardness respectively.The suitable heat treatment of peak-aging (T6) states for the alloy is120℃×12h.The corresponding Vickers hard-ness,conductivity,tensile strength and yield strength were226.7HV,27.8%IACS,658.9MPa and571.3MPa respectively.(4)The increase of Zn/Mg ratios have no function of changing the seperation sequence of precipitates, but it is the reason for the accelerate of the kinetics of precipitation and the coarsening speed of them,which act more obviously during the over-aging stage especially the over-aging period of the higher temperature (135℃and150℃).Under the same condition,the higher the Zn/Mg ratio of the composition in the alloy is,the higher of its precipitation kinetics as well as the faster coarsening rate,and the hardness is lower with the higher conductivity.(5)The best traditional duplex aging treatment of7056alloy are as followes:120℃/4h+165℃/8h.The corresponding Vickers hardness, con-ductivity, tensile strength and SSRT strength-loss of7056were176.8HV34.8%IACS,559.3MPa and0.6%respectively.(6)The new double-aging treatment,in which the first temperature is higher,can improve the condctivity of the alloy on the basis of that maintein the strength of the alloy which is close to the T6peak-aging strength.155℃/2h+125℃/24h is the best aging treatment of it,in that case,the corresponding Vickers hardness, conductivity, tensile strength and SSRT strength-loss of7056were204.0HV,30.3%IACS,650.6MPa and12.7%respectively.(7)The retrogression temperature and retrogression time had a great effects on the properties of alloy, the higher retrogression temperature,the shorter time to achieve the bottom of hardness.With the increase of retrogression time, the hardness of alloy decreased more quickly at a higher retrogression temperature. (8)The ideal RRA treatments for7056aluminum alloy are as foll-owes: a.120℃/4h+180℃/40min+120℃/12h,The corresponding Vickers hard-ness conductivity, tensile strength and SSRT strength-loss of it were202.1HV,31.4%IACS,636.7MPa and8.5%respectively. b.120℃/12h+190℃/25min+120℃/12h,The corresponding Vickers hard-ness,conductivity, tensile strength and SSRT strength-loss of it were195.9HV,32.2%IACS,630.4MPa and6.7%respectively.(9)Compared to the T6peak-aging treatment,the bulky and inter-mittent grain boundaries which formed during the multy-aging treatments of7056aluminium alloy can improve the alloy stress corrosion resistance properties,and when the alloy is treated by120℃/4h+165℃/8h,it has the best SCR properties.