Study On Forming Technology Of AZ91 Magnesium Alloy Sheet By Centrifugal Casting And Rolling

As the lightest metallic structural material,magnesium alloy has been widely used in the fields of automotive,aerospace and 3C.The magnesium alloy products are mainly used in the form of sheet.Therefore,the research and the development of magnesium alloy sheet with short process,large width and high performance are widely concerned.In this paper,a set of reasonable AZ91 magnesium alloy sheet forming process was determined by numerical simulation and casting of centrifugal casting,solution treatment,plate rolling and annealing treatment.The experimental process is as follows.(1)The flow field and the temperature field of the casting process of centrifugal casting AZ91 magnesium alloy pipe with length of 1500 mm,outer diameter of 400 mm and thickness of 20 mm are simulated by computer.The process shows as follows.During the centrifugal casting process,the alloy liquid flows from the gate into the mold and then it flows toward both ends which solidified by layer.The first layer of alloy liquid is filled with mold when the casting time is 8.4s and the pouring process ends at 20 s.During the pouring process,the temperature of the alloy liquid in the gate is the highest,and it is gradually decreased in the direction away from the gate.The ends of the pipe are completely solidified while the central part of the pipe is not solidified at the end of the pouring.(2)The AZ91 magnesium alloy tube with length of 1500 mm,outer diameter of400 mm and thickness of 20 mm is prepared by centrifugal casting which the parameters for the centrifugal speed of 900r/min,pouring temperature of 700°C and mold preheating temperature 200°C.The outer surface of the tube is smooth and the thickness of the pipe is uniform and the filling is complete.The mechanical properties of the pipe are obviously improved compared with the as-cast.The tensile strength is 158 MPa and the elongation is3.4%.The grain of the pipe is refined and distributed evenly.(3)The solution treatment of centrifugal casting AZ91 magnesium alloy pipe was carried out.The results show that the mechanical properties of the pipe are improved with the increase of the solution temperature and the mechanical properties are the best when the solution temperature is 415°C.Its tensile strength is 176 MPa and elongation is 6.7%.The tensile strength of the pipe first increases and then decreases with the increase of the solutiontime.The optimal solution time is 18 h and its tensile strength is 181 MPa.The elongation increases with the increase of the solution time.The optimal solution time is 24 hours and its elongation is 9.2%.Solid solubility of ?-Mg17Al12 increases with increasing solution time.(4)Finally,the centrifugal casting AZ91 magnesium alloy pipe was flattened and rolled by means of pack rolling.Through the comparison of plates prepared at different rolling temperature,it can be seen that the surface of the plate has no crack,edge crack and thickness uniform when the rolling temperature is 400°C.The tensile strength and the elongation are significantly improved compared with the as-cast.The elongation and the tensile strength of the plate are 7.2% and 291 MPa respectively.The microstructure of the sheet after rolling is obviously refined,and the dynamic recrystallization occurr during the rolling process.Through the comparison of rolled plates prepared at total reductions,the tensile strength and the elongation of large reduction of rolled sheet are significantly improved,and the tensile strength and the elongation are 317 MPa and 7.4% respectively.Annealing treatment of rolled plate at different holding time was carried out and the result shows that the plasticity of the plate are improved after annealing and the best holding time is 90 min where the elongation is8.6%.