Study On The Effect Of Er Ratio On The Microstructure Evolution Of The Twin-roll Casting 6061 Aluminum Alloy Plate

6061 aluminum alloy has the advantages of light weight,high specific strength,corrosion resistance,and easy forming which is widely used in automobiles,ships,power electronics,construction packaging and other fields.Compared with the conventional forming method,the twin-roll casting and rolling process has the advantages of simple and centralized equipment,high efficiency and energy saving,which is a very promising preparation process.However,the produced plate has segregation along the thickness direction.At the same time the surface and internal structure are uneven and anisotropic.The produced plate is not suitable for deep forming.Based on this,This subject intends to use the microalloy element Er to improve the structure and comprehensive properties of the cast-rolled sheet.NF6-300 vertical twin-roll casting and rolling mill is used to prepare 6061cast-rolled plates with different Er content.Through metallographic observation,scanning electron microscopy and transmission electron microscopy and other tissue analysis techniques,as well as hardness,tensile strength and other performance testing methods,this subject studied the microstructure and properties of the experimental alloy in different states（cast-rolled state,homogenized state）,and analyzed the main existence form and mechanism of Er in the alloy in different states.The main conclusions obtained are as follows:（1）The addition of Er can refine the grain structure of 6061 aluminum alloy cast-rolled sheet,improve the morphology of the iron-rich phase,and improve the overall performance of the cast-rolled sheet.The addition of Er has an effect on the grain refinement of 6061 aluminum alloy cast-rolled sheets,mainly because the primary Al₃Er phase is the pre-precipitated phase during the casting-rolling process,which can become an excellent heterogeneous nucleation core ofα-Al and achieve grain refinement.In addition,Er element can interact with the Fe element in the cast-rolled sheet,so that the iron-rich phase in the 6061 aluminum alloy cast-rolled sheet is transformed from thick needles and bones to smaller short rods and blocks.At the same time,micron-sized spherical or spherical-like Er-containing iron-rich phases will appear in the matrix.However,when the Er addition amount is greater than0.4wt%,the number of rare earth compounds and impurity phases increases significantly,and they mainly precipitate along the grain boundaries,increasing the strengthening tendency of the second phase and reducing the shape of the material.When the Er addition amount reaches 0.4wt%,the comprehensive mechanical properties of the cast-rolled sheet are the best.At this time,the tensile strength,yield strength,and elongation are 160.14MPa,124.77MPa and 10.68%.（2）The addition of Er reduces the secondary dendrite spacing of the 6061aluminum alloy cast-rolled sheet,shortens the homogenization time of the cast-rolled sheet,and improves the overall performance of the cast-rolled sheet after the homogenization treatment.The 6061 cast-rolled plates with different Er content were subjected to homogenization annealing process for different time at 550^oC.When the6061 aluminum alloy cast-rolled sheet has been homogenized for 12h at 550^oC,the dendrites are basically dissolved and the homogenization process is completed.The best homogenization treatment process for cast-rolled plates with an Er content of0.4wt%is 550℃/6h.The best homogenization treatment process for cast-rolled plates with an Er content of 0.6wt%is 550℃/8h.The Er element added to the cast-rolled sheet will precipitate in the matrix the nano-level second phase Al₃Er which has a coherent relationship with the matrix,which improves the comprehensive performance of the 6061 cast-rolled sheet after homogenization.After homogenization treatment at 550℃/6h,the 6061 cast-rolled sheet with 0.4wt%Er content has the best comprehensive mechanical properties,with tensile strength,yield strength and elongation of 138.32MPa,107.76MPa and 21.77%.