Microstructure And Mechanical Properties Of Direct-Chill Casting Al-Zn-Mg-Cu Alloy Assisted By Physical Field

Al-Zn-Mg-Cu alloy has been widely used in aerospace and aerospace field load-bearing structural parts because of its excellent mechanical properties.Besides,it has a wide range of applications in the civil field.A large number of studies have shown that during the preparation of Al-Zn-Mg-Cu alloy ingots,problems such as poor structure uniformity and unstable mechanical properties of the alloy have become industry problems.The main reason for these problems is the high degree of alloying and the wide range of crystallization,which results in the poor uniformity of cross-section crystallization of ingots.However,the more advanced physical field-assisted direct chill(DC)casting technology is superior in solving the above problems.In this paper,Al-Zn-Mg-Cu alloy was prepared by an electromagnetic field and ultrasonic-assisted DC casting,respectively.The microstructure evolution and mechanical properties of as-cast alloy with the assistance of different physical external fields was studied in order to prepare uniform microstructures and mechanical stable alloy material.At the same time,the law of alloy refinement and the mechanism of improving solidification by electromagnetic field and ultrasonic wave was explored,which provides theoretical basis for industrial application.With the help of electromagnetic field-assisted DC casting,the microstructure of ingot is refined and the homogeneity is improved obviously.With the increase of electromagnetic frequency,the refinement degree of the as-cast ingot center structure is increased,and the homogeneity of the microstructure at different positions is also gradually improved.At 45 Hz,the microstructure is the most uniform,and the center and edge grain sizes of the ingot are 76.6?m and 70.3?m,respectively.The grains are the finest,and the overall difference in grain size is the smallest.Compared with the absence of an electromagnetic field,the second phase size is significantly smaller.When the electromagnetic frequency is 45 Hz,the morphology of the second phase changes from net and rod shape to spherical and needle shape when no electromagnetic field is applied,and the second phase area fraction is also greatly reduced.The center and the edge are reduced from 1.87% and 1.79% when no magnetic field is applied to 0.78% and 0.70%,respectively.In addition,the tensile strength,yield strength and elongation of the center ingot are increased from 280 MPa,175MPa and 6.0% without the magnetic field to 308 MPa,212MPa and 8.5%,respectively.The edges increased from 309 MPa,215MPa and 11.5% to 322 MPa,221MPa and 12.0%,respectively.Under ultrasonic-assisted DC casting,when the ultrasonic power is 900 W,the microstructure of ingot at different positions is significantly refined.At 1500 W,the grain is the finest,and the microstructure uniformity and mechanical properties are the best.The grain size of the center and R/2 positions are 60.2?m and 66.3?m,respectively.The grain size at the different positions is the smallest,which is 42.3% and 12.1% smaller than that in the absence of ultrasound.The second phase is greatly refined,and the morphology of the second phase changes from the reticular shape in the absence of ultrasound to the spherical shape and small piece.The second phase area fraction of the center decreases from 3.04% to 0.42% when no ultrasound is applied.After the ultrasound is applied,the tensile strength,yield strength and elongation of the center increase from 280 MPa,161MPa and 9.5% to 326 MPa,218MPa and 14.5%,respectively.The R/2 position is increased from 304 MPa,191MPa and 13.5% to 318 MPa,205MPa and 14.0%,respectively.