Study On Electromagnetic Twin-roll Casting Process And Theory For 7075 Aluminum Alloy

With the improvement of industrial production and living quality,the lightweight high strength metals are widely applied in transportation and aviation.Moreover,the preparation of the metal materials gradually developed to a direction of low pollution,low consumption and low cost.The 7075 aluminum alloy,one of the high strength construction materials,are widely used in fields of aviation,vehicle engineering and national defense.Hence,the 7075 alloy is one of material lightweight development in the further.However,traditional semi-continuous casting and hot rolling technology is considered as high energy consumption and cost process.The twin-roll cast(TRC)process has many advantages of low cast,good flexibility and high production capacity,recently,has been focused by researchers at home and aboard.That developing a new cast-rolling technology of 7075 aluminum alloy has notable economic and social benefits for aluminum alloy production technology.This paper studied the new cast-rolling process of 7075 alloy.The application of finite element was established to simulation of temperature and flow fields during the course of 7075 cast-rolling,and the technological parameters was determined.Later,the traditional TRC process was detailed studied.The applying of different external fields into cast-rolling areas,such as ultrasonic wave,magnetic field,electric pulse(EP)field and EP electromagnetic field and so on,which the influence on structure property of 7075 alloy strips was studied.At the beginning,the parameters of continuous TRC process was calculated by simulation that under roll diameter of 500mm and roll gap of 5 mm condition,the pouring temperature is in range of 670??680? and the rolling speed in range of 0.8 m·min-1?1.0 m·min-1.The extremes pouring temperature and rolling speed could not meet riser conditions because of easily producing "run soup" and "rolling block" phenomena.At the same time,some slabs defects are effectively controlled by studying their formation during TRC producing.The process parameters of implementation 7075 TRC production were determined,the pouring temperature is 675? the rolling speed is 0.9 m·min-1 and the roll gap height is 5 mm.Besides,the magnetic intensity in the range of cast-rolling areas was about 40 mT,which satisfies 7075 alloy electromagnetic TRC process.The influence of EP current on microstructure of 7075 ingots under solidification without and with magnetic field of 20 mT intensity was also observed in this paper.Under non-field condition,the finest structure of ingot was prepared under EP current of 20 Hz frequency,10%duty ratio and 300A intensity.Under magnetic field condition,the best optimum structure of 7075 alloy ingot was manufactured by EP current of 10%duty ratio.Moreover,combining the theories of classical pulse and melt nucleation,the refinement mechanisms of EP current on microstructure of aluminum alloy are careful studied and finally the introduction of EP current is optimized as frequency of 20 Hz,duty ratio of 10%and 300A intensity.During TRC manufacture,varies external fields were introduced into cast-rolling areas.Aluminum slabs without and with Al-Ti-B refiner were prepared for morphology and micro-segregation observation.The results show that under coactive with EP electric field and magnetic field condition,the microstructure of rolled slab is maximum refinement and uniformity.Under pulse TRC condition,the structure has good uniformity without segregation.Moreover,the structure of 7075 alloy slabs manufactured by TRC process respectively applied with magnetic field and ultrasonic field has also improved to a certain degree,while segregation zones still exist.The worst non-uniform microstructure of 7075 slab with severe segregation has obtained under traditional TRC process.Furthermore,the refining effect of Al-Ti-B is broadly apparent.The refining behavior of 7075 rolled slab with refiner is the same as that without refiner.At the same time,solidification theory model of 7075 TRC process under EP and EP electromagnetic conditions is established by introducing classical theory of ultrasonic and pulse electric fields.Finally after hot rolling and annealing,the 2 mm thickness strip with Al-Ti-B refiner was obtained for structure observation and mechanical testing.The result shows that the annealing structure has passed down the characteristics of cast-rolling morphology.Under strain rate of 8.3×10-4 s-1 condition,the tensile strength of 7075 strips after solid solution and aging treatment(T6)are all higher than 550 MPa,which the maximum strength could reach up to 630 MPa.The elongations of testing specimens are all about 9%,and fracture morphology presents ductile.Besides,after completely annealing in 410?,the highest tensile strength of 7075 strips is 260 MPa,and the elongations of tensile specimens is in a scope of 10%?20%.The fracture morphology presents the features of quasi-cleavage.Furthermore,the 7075 alloy manufacturing conditions which the tensile strength are sorted from high to low is listed,the EP electromagnetic field,the pulse electric field,the ultrasonic and magnetic fields and non-field.Above all,by combining with analogue simulation and experimental research,the 7075 electromagnetic TRC process is determined in this paper.At the same time,the 7075 TRC process theory is studied and the properties of rolled strips formed through different external fields are tested.Besides,the paper establishes a theoretical foundation for the electromagnetic TRC technology and application by means of producing some acceptable cast-rolled and subsequent hot rolled 7075 alloy strips.