Studies On The Grain Refinement And Squeeze Infiltration Behavior During Pressurized Solidification Of AZ91D Alloy

Magnesium alloy has broad application prospects in the automobile andaerospace fields due to its advantages of high specific strength and specific stiffnessas well as low density. However, the wide freezing range, small heat capacity and lowlatent heat of crystallization of most magnesium alloys generate the occurence ofcoarse grain, poor feeding capacity during solidification, and various defectsincluding shrinkages and thermal cracking, et al. as for fabricated by conventionalcasting methods. As a consequence, the requirement of high performance structureunder extreme conditions cannot be met. As an effective method to modify the flowmorphology of liquid, pressurized solidification can effectively feed shrink in themushy zone via prompting the interdendritic flow capacity of the liquid. However, thedeveloped dendrite structure in most magnesium alloys reduces the melt flow capacityremarkably, even with the solid fraction of network structure is merely20%~30%. Forthis reason, the liquid feeding in the enclosed area of dendrite will be hindered.Nevertheless, grain refinement can not only improve the overall performanceincluding ductility and strength, but also can enhance the flow capacity in solid-liquidphase of the melt, which is beneficial to reduce the segregation of component andinhibit the formation of brittle phase with low melting point. Thus, for guiding thefabrication of high performance magnesium alloys, it is significant to research themicrostructure refinement of magnesium alloys and reveal the affected squeezeinfiltration behavior during the pressurized solidification.Vacuum induction melting coupled with protection of inert gas was performed onthis study. The variation of average grain size and phase structure of AZ91Dmagnesium alloys were investigated by changing the content of Ce and SiC inoculant,and the optimum refinement process were obtained. Moreover, the spheroidizingprocess of Mg2Si phase in the microstructure of magnesium alloys containing SiCparticles was investigated by solution heat treatment, which provides the theoreticalbasis for the microstructure modification of magnesium alloys. Besides, thequantitative description of the volume fraction variation of solid phase was realizedon the basis of differential thermal analysis technology. The effect of applied pressurewith different values on the microstructure and density of the refined AZ91D magnesium alloys were discussed, which reveals the corresponding squeeze infiltration behavior during the solidification, and thus providing a theoretical basis for improving the mechanical properties of magnesium alloys.Grain refinement results show that with the addition of0.75wt%Ce, the grain size of AZ91D magnesium alloys decreases from480μm for the unrefined AZ91D alloy to184μm, and the eutectic phase β appears smaller. The refinement can be attributed to the improvement of the undercooling in front of solid/liquid interface caused by the solute redistribution during solidification and the formation of Al11Ce3, which offers a pinning effect on grain boundary. In contrast, the addition of0.2wt%SiC enables the further reduction of average grain size to124μm. The optimum refinement is caused by the Al4C3and SiC phases with the similar lattice structure to a-Mg and act as the effective heterogeneous nucleation to promote the increase of nucleation number. In addition, the pinning effect on grain boundary migration by Mg2Si further inhibits the growth of grain. After solution heat treatment of400℃×16h,β phase disappears and the Mg2Si phase with Chinese script form presents evident spheroidizing, thus the fragmentation effect on the matrix structure is weakened.Pressurized solidification results show that the refinement of grain is beneficial to enhance the feeding capacity of liquid metal and improve the microstructure and density of the alloys. Moreover, gradual increase in grain number and further decrease in grain size are obtained with the enhancement of crystallization pressure. The tensile strength of AZ91D+0.2wt%SiC magnesium alloys reaches151.85MPa when applied with200kPa. After solution heat treatment of400℃×16h, the tensile strength is further improved to167MPa due to the spheroidizing of Mg2Si phase. In addition, a equation between the grain size D and the crystallization pressure Po was obtained according to the least square method:D=a0P3-a1P20-a2P0+a3. Combined with the feeding model during solidification, the mathematical relationship between the solidified feeding speed and grain size of AZ91D alloy fabricated by the vacuum pressurized solidification was established and shown as below:...