| With the continuous development of China's automotive industry,high speed rail and aerospace and other industrial fields,the demand for aluminum alloy materials is increasing.As early as in the 20 s of last century,high strength aluminum alloys were developed and applied in industry.With the development of technology,the toughness,strength and corrosion resistance of aluminum alloys are higher and higher.The proportion of impurities and trace elements is more and more reasonable.The 7 series aluminum alloy started in the 60 s of last century in China.After more than 50 years of development,the main technical indicators such as corrosion resistance and elongation have been significantly improved,but there is still a certain gap with other countries.7020 aluminum alloy is one of the high-strength aluminum alloy in the 7 series aluminum alloy of Al-Zn-Mg-Cu.It has the advantages of easy forming,high strength,light weight,corrosion resistance,good weldability and good thermal conductivity.It can be used as an ideal lightweight component in rail transit.Casting rolling is a short process,low investment,low energy consumption and high efficiency production process.However,there are cracks in the 7020 aluminum alloy plate after casting and rolling,which greatly reduces the mechanical properties of the sheet.In order to eliminate the crack,a warm rolling process is used in this study.The temperature rolling can not only avoid the oxidation caused by hot rolling,but also avoid the working hardening caused by cold rolling.In addition,the heat treatment process can further eliminate some defects caused by rolling and rolling,and then improve the comprehensive performance.By means of OM,SEM,tensile test and hardness test,the effects of 325?,350?,375?,400?,and 25%,50%,70% and 80% reduction on the crack evolution and microstructure and properties of cast aluminum alloy 7020 were studied.The results showed that:(1)In the warm rolling process,when the casting and rolling 7020 aluminum alloy sheets at the same temperature,the grain evolves from the approximate circle to the long strip with the increase of the reduction temperature,and the grain interacts with each other to make the spacing between grains smaller.Under the same reduction,the grain grows gradually with the increase of temperature.With the temperature approaching to the recrystallization temperature,the grain coarsening is serious,the grain boundary becomes longer and the grains are hard to intergrate.(2)When casting and rolling 7020 aluminum alloy sheets at the same temperature,the crack healing is better with the increase of the rolling load.When the reduction is 80%,the crack healing and mechanical properties reach the peak value.Under the same compressive load,with the increase of the temperature,the degree of crack healing is parabolic.It first increases and then decreases.When the temperature is 350?,the degree of crack healing and mechanical properties reach the peak value.(3)Both the degree of crack healing and the mechanical properties are at the peak at the temperature of 350? and the reduction of 80%.(4)after heat treatment,the mechanical properties of warm rolled 7020 aluminum alloy sheets increased in varying degrees.However,the warm rolling temperature of 350?,the reduction of 80% is still the best warm rolling process.Comprehensive analysis shows that the temperature rolling temperature is 350? and the pressing amount is 80%.It is the best rolling process. |
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