Numerical Simulation Of Macrosegregation And Stress Field In Large Aluminum Alloy Ingots During Direct-Chill Casting

Semi-continuous casting is the main productive process of the high strength aluminum alloy ingots.Solidification is the controlling process for producing high quality ingots,which are the basis of subsequent processes.Defects,such as macrosegregation,tear and shrinkage porosity are the main problems to be overcome in semi-continuous casting.Therefore,it is of great significance to investigate the mechanism of macrosegregation and stress formation in semi-continuous aluminum alloy ingots.The numerical simulation technology has the advantage over experiments for its low cost and high efficiency to predict the casting defects,which is important for optimizing casting technology.The main research works of this paper include: numerical calculation of the macrosegregation based on the extended continuum model,and the verification of macrosegregation mathematical model;numerical simulation of the macrosegregation in 2024 aluminum alloy ingot during semi-continuous casting process;the investigation on the influence of casting parameters on macrosegregation;the macrosegregation calculation of Al-4wt%Cu ingot during semi-continuous casting based on three phase model;and numerical simulation of the temperature field and stress field in 7050 aluminum alloy ingot.The detail research results are as follows.The extended model(Vreeman-Krane-Incropera)based on the continuum model is used to calculation marosegregation.In the model,the macroscopic continuous equation,momentum equation,energy equation and solute transport equation are coupled with microscopic Gulliver-Scheil solidification equation.The approximate phase diagram method is adopted to deal with the solidification path,which can ensure the accuracy of the case and greatly improve the computational efficiency.Combined with the finite volume method of large fluid dynamics computing software,FLUENT,the mathematical model is solved numerically,and the source terms and boundaries are loaded by the user defined function of the open interface.The validity of the numerical method is verified by solving the H-H experimental benchmark and the Bellet numerical benchmark.The macrosegregation based on the continuum model was predicted during semi-continuous casting of 2024 aluminum alloy ingot with the diameter of 400 mm.The results show that the surface and along the radial direction from 100 mm to 175 mm of the ingot show positive segregation,while the center and the subsurface of ingot exhibit negative segregation.The formation of this segregation is caused by the movement of dendrites,thermosolutal convection and solidification contraction.The simulation results are in good agreement with the measured results in the literature.The influences of the ingot size,casting speed,pouring temperature and cooling intensity of water cooling zone on the macrosegregation of ingot are investigated.The casting parameters mainly influence the macrosegregation by changing the shape and depth of the sump.The casting speed has the greatest influence on the sump.With the increase of the ingot size,the cooling rate of ingot decreases significantly and the sump becomes deeper.The effect of the pouring temperature on the depth of the liquid pool is less significant.The cooling intensity in the water cooling zone is controlled by the water flow rate.The greater the flow rate is,the more shallow the depth of the sump will be.The solidification process of Al-4wt% Cu ingot is simulated by the three-phase model during semi-continuous casting.The effects of thermosolutal convection,equiaxed dendrite movement and CET(Columnar to equiaxed transition)transformation on macrosegregation are studied.In the model,the capture of the equiaxed dendrites by growing columnar tree trunks,the interaction and impingement between columnar and equiaxed dendrites have been under consideration.The results show that the cone zone of negative segregation is obviously observed at the bottom of ingot and W-type segregation in the overall scale of ingot.The prediction results are in agreement with the results of the reported experiment.The temperature and stress fields of 7050 aluminum alloy ingot are calculated using MAGMA Soft software.By comparing the three cases,it is found that the shape of the dummy bar is the cylindrical convex chassis and the shorter crystallizer can reduce the depth of the sump.The greater the drawing speed is,the deeper the sump will be.The maximum temperature gradient appears at the junction of the mo uld and the second water cooling zone,and the temperature gradient is gradually decreased from the ingot surface to the center of the ingot.With the increase of the mould height,the temperature gradient increases sequentially,while the temperature gradient increases significantly with the increase of the casting speed.Meanwhile,the maximum value of the temperature gradient at the junction is almost doubled in case where the speed increases.The results show that the tensile stress forms in the center of the ingot,while compressive stress forms near the surface.The maximum tensile stresses occur at the center and junction where the hot tears are most likely to generate.When the shape of the chassis is convex,flat and concave,the corresponding stress in the ingot increases successively,but the increase is small.The stress increases with the increase of the height of the mould,and the distribution of the central tensile stress is near the bottom.With the increase of casting speed,the stress of the whole ingot is significantly increased.All in all,the casting speed has the greatest influence on macrosegregation.