Effect Of Process Parameters And Zn Content On The Segregation And Cracks During The Twin-roll Casting Of Al-Zn-Mg-Cu Cast-rolled Strips

Al-Zn-Mg-Cu high-strength alloys have been extensively used as indispensable structural materials in the aeronautical industries because they have overall properties,such as high strength,low density and high resistance to fatigue.Al-Zn-Mg-Cu alloy sheet by twin-roll casting process has the advantages of short process,high efficiency and low cost.The 7xxx system aluminum alloy has many alloy elements and wide crystallization range,so there are some problems such as cracks and segregation in the cast rolling plate.In order to improve the structure and surface quality of cast rolling plate.Design process parameters and components,The casting and rolling plate with three technological parameters of pouring temperature,casting and rolling speed and casting and rolling thickness and five components of Zn content is taken as the object,By means of optical microscope,DSC thermal analysis,XRD diffraction analysis,SEM and EDS analysis,tensile test and hardness test at room temperature,The effects of process parameters and Zn content on the cracking and segregation of 7xxx aluminum alloy cast rolling plate were studied.In order to eliminate segregation and improve the properties of the alloy,the heat treatment of7 xxx system cast rolling plate was studied.(1)The influence of process parameters on the crack and segregation of 7xxx aluminum alloy cast rolling plate was studied.The results show that: When the pouring temperature is 670 ?,the phenomenon of "jamming" appears;when the pouring temperature is 710 ?,the plate cracking is serious;when the pouring temperature is 690 ?,the plate shape is complete.With the decrease of casting rolling thickness,the cracks of cast rolling plate decrease,and the degree of segregation is lighter.With the increase of rolling speed,the crack of cast rolling strip decreases.When the rolling speed is too high,the crack increases.Finally,the casting and rolling process parameters were determined.pouring temperature(690?),thickness of strip 4mm,roll casting speed14m/min.The alloy strips with different Zn content(3.5 wt pct,5 wt pct,6.5 wt pct,8wt pct,and 10 wt pct)were produced by twin roll casting.(2)The influence of Zn content on the crack and segregation of 7xxx aluminum alloy cast rolling plate was studied.The results show that: the dendrite gap becomes coarse,When the content of Zn is 3.5wt% and 5wt%,a small amount of eutectic phase is precipitated in the cast-rolled strip,and when the content of Zn is continuously increased from 6.5wt% to 10wt%,the coarse nonequilibrium eutectic phase is precipitated in the cast-rolled strip,and the size of coarse phase increases the width of the eutectic zone in the center of the cast-rolled strip.The solidification interval of Al-Zn-Mg-Cu cast-rolled strip narrows as the Zn content decreases.The solidificationinterval of the strip alloy is reduced by 30°C from 64°C to 93°C as the Zn content decreases from 10 wt pct to 5 wt pct.As the Zn content increases,the number of cracks in the Al-Zn-Mg-Cu cast-rolled strip increases.(3)The influence of Homogenization treatment on the crack and segregation of7 xxx aluminum alloy cast rolling plate was studied.The results show that: With the increase of homogenization time,dendrites gradually dissolved and non-equilibrium phase dissolved into the matrix.After homogenization,the segregation was basically eliminated.The second phase with high melting point in the alloy at lower solution temperature can not be completely dissolved into the matrix after solid solution treatment.Solution treatment at 490 C can cause Over burning of the alloy.Two-stage solid solution(460?,1h+480?,1h)can not only eliminate the insoluble phase in the alloy but also avoid over burning.After two-stage solid solution and peak aging,the strength,hardness and plasticity of the alloy were greatly improved.After peak aging,the tensile strength of 7000 series cast-rolling strip alloy increased from180MPa?240MPa to 360MPa?520MPa.With the increase of Zn content,the strength of the alloy increased continuously.The growth rate was obvious at 5%,6.5% and 8%,and the strength of the alloy increased slowly at 10%.