Study On The Numerical Simulation Of Semi-continuous Casting And Homogenizing Annealing Process Of AZ80A Magnesium Alloy

Magnesium alloys are the lightest metal structure material in the practical application with low density,high specific strength and other advantages,which are widely used in the fields of automotive,aerospace and military.Large-sized cast rods of magnesium alloy are the necessary raw materials for processing large-sized extruded and forged parts,which have a broad prospect in armor,missile shell and vehicle wheels.However,the production of large-sized cast rods has many problems,the larger the size of cross section,the more serious the segregation and hot cracking sensibility,resulting in the consequence of low yield and poor processing ability.Aiming at above problems,this thesis selected large-sized AZ80 A magnesium alloy cast rods with the diameter of 600 mm as research object,so as to study the numerical simulation of semi-continuous casting and homogenizing annealing process.1.Numerical simulation of semi-continuous casting of AZ80 A magnesium alloy.Utilizing the casting simulation software Pro CAST to simulate the semi-continuous casting process,studying the effect of casting temperature and other casting process parameters on the distribution of temperature field and stress field.Simulation results show that: Depth of liquid phase zone increases with the increase of casting temperature and casting speed,and decreases with the increase of cooling strength;depth of mushy zone decreases with the increase of the casting temperature and cooling strength,and increases with the increase of casting speed;casting speed has the greatest influence on the depth of liquid phase zone and mushy zone.The edge of cast rod is subjected to compressive stress,and the central part is subjected to tensile stress,the value of effective stress increases with the increase of casting temperature,casting speed and cooling strength.In steady state,hot cracking susceptibility of the central part of cast rods is higher than that of edge;hot cracking susceptibility of cast rods decreases with the increase of casting temperature and cooling strength of secondary cooling zone,and increases with the increase of casting speed.The reasonable process parameters of semi-continuous casting of AZ80 A magnesium alloy with diameter of 600mm: casting temperature 680~700?,casting speed 20~25mm/min,first cooling coefficient 1000~1500W/m~2· K,senond cooling coefficient 4000~5000W/m~2·K.2.Homogenizing annealing process of AZ80 A magnesium alloy.Studying the diversity of microstructure and mechanical properties of as-cast alloys in different locations and homogenized alloys under different homogenizing annealing parameters,so as to determine the optimum parameters and establish the kinetic equations of homogenizing annealing.Experimental results show that: From the central part of cast rods to the edge,the distribution of ?-Mg17Al12 phase becomes intermittent gradually,the length of dendrite arm spacing and percentage of second phase decrease gradually,the microhard ess of ?-Mg matrix increases gradually;tensile strength and elongation increase gradually,tensile fracture of as-cast alloy shows the characteristic of intergranular brittle fracture.With the increase of temperature in the high temperature tensile test,tensile strength decreases gradually,elongation increases gradually,and the tensile fracture shows the characteristic of ductile fracture.The tensile strength and elongation of homogenized alloys are improved than as-cast alloys,the tensile fracture shows the cleavage fracture morphology with plasticity.For the central part and edge part of AZ80 A magnesium alloy cast rods,the optimum homogenizing annealing process parameters are held at 420? for 10 h and 400? for 15 h respectively,and selecting held at 400? for 20 h as the optimum homogenizing annealing process parameter of AZ80 A magnesium alloy cast rods with the diameter of 600 mm.Based on the dendritic distribution of element Al,the kinetic equation of homogenizing annealing of AZ80 A magnesium alloy is established as follows:(?),the calculated results are in good agreement with the experimental results.