Phase-field Simulation Of Microstructure Evolution During Solidification Of Magnesium Alloys

Microstructure is the key factors to determine the mechanical properties ofmaterials. The final material mechanics performance quality and service life depend onthe final solidification microstructure. so controlling the formation of themicrostructures effectively in solidification is meaningful in theory and practice. Withthe development of computer, numerical simulation of the solidification microstructurehas been an important method for the study of material. Phase field method as one ofthe most potential methods in the modeling of solidification, it has became a researchfocus in solidification field at home and abroad at present.Based on the KKS model and magnesium alloys with hcp structure which couplesthe temperature field and flow field, the phase-field model of single grains formagnesium alloys with sixfold rotation symmetry is established. Coupling a orientationfield which controls the preferential growth orientation of grains, a new phase-fieldmodel of random preferred growth direction of multiple grains of binary alloy isdeveloped based on the phase-field model of single grains for the sixfold rotationsymmetry of magnesium alloys and cellular automaton method. Based on the thephase-field model of multiple grains, coupling with a temperature field which wasdealed with temperature freezing method approximately, a new phase-field model whichis suitable for simulation of directional overgrowth behavior between multiple grainswith different growth orientation. The governing equations are discretized on uniformgrids using the Finite Difference method, the governing equation of temperature filed isdiscretized using an alternating direct implicit(ADI) method in order to avoid therestriction of time step. The simulation program is developed based on vc++6.0platform, Choosing Tecplot360and Origin to realize the visualization of the simulationresults.The growth process of the single and multi-dendrites of AZ91D alloys underwithout flow and forced flow are simulated using the the phase-field model of singlegrains. The effect of the model parameters on the dendrite growth under forced flowwas analyzed. The results show that the dendrites growed with sixfold rotation symme- try, there is a angle of60°between six adjacent primary dendrite arms and between theprimary dendrite arm and corresponding secondary dendrite arm, and form snow-likedendrite morphology in the finally under without flow; When dendrite grows underforced flow, the growth of three directions on the upstream side are much faster thanthose of three directions on the downstream side, and form asymmetric dendritemorphology in the finally; The larger disturbance strength, the more the side-branch, thethe ratios and velocity increase with the growth of the strength of anisotropy; As thevelocity of convection increase, the growth velocity of dendrite speeded up and thecurvature radius is getting smaller on the upstream side, the growth velocity of dendritespeeded down and the curvature radius is getting bigger on the downstream side; Thelower initial temperature, the quicker of the dendrite tip velocity, the smaller of thecurvature radius.Using the phase-field model of multiple grains, the growth process of themulti-dendrites of AZ91D alloys under without flow and forced flow are simulated. Theresults show that the grains nucleate and grow in a competitive way, the dendrite tip willbe bald and crooked, and form asymmetric dendrite morphology in the finally; Whendendrite grows under forced flow, because of the washing of supercooled solution, thegrowth velocity of the dendritic tip gets promoted and inhibited on the upstream anddownstream side respectively, and form asymmetric dendrite morphology in the finally.The simulation results agree well with the experimental results.Using the phase-field model of directional growth of dendrites, the directionalovergrowth behavior of AZ91D alloys between multiple grains with different growthorientation under without flow and forced flow are studied. The simulation results showthat when the dendrites with different crystal orientation grow in the directionalsolidification conditions, the dendrite which has a preferred orientation close to thedirection of temperature gradient will eliminates the one which greatly deviates from thedirection of temperature gradient. The simulation results are in good agreement with thetheoretical analysis of Walton-chalmers.