Experimental Study And Phase-field Modeling On The Microstructure Evolution In Pressurized Solidification For Applications In Multi-component Magnesium Alloys

Magnesium alloys are attractive materials for manufacturing lightweight structures.Squeeze casting is an advanced near-net-shape materials processing technology,in which molten metal solidifies under applied pressure,producing high-integrity castings with excellent mechanical properties.It is of great significance to investigate the microstructure evolution in pressurized solidification for application in multi-component magnesium alloys using phase-field modeling and experiments.A novel experimental apparatus for in-situ observation of dendritic growth under dynamic pressure and thermal conditions was built,based on the principles of direct squeeze-casting,and the influence of an increasing–holding–decreasing pressure,a periodic increasing-decreasing pressure and different pressure rates on dendritic morphology and growth kinetics was investigated using succinonitrile,a transparent model material.The relationship of the velocity of dendrite tip with undercooling and pressure was obtained through experiments and derivation,and the relationship between tip acceleration and pressure rate was also investigated experimentally and theoretically.A phase-field model for pressurized solidification of pure materials was built,where the effect of pressure on Gibbs free energy was taken into account via introducing a pressure term associated with pressure rate into the phase-field equation.The phase-field model was validated by the experimental results obtained by in situ observation,and was adopted to investigate the response of dendritic growth kinetics to dynamic pressure and pressure rates.Based on the phase-field model for pressurized solidification of pure materials,a phase field model for describing the microstructure evolution in pressurized solidification of a ternary magnesium alloy was developed.The model captured the essential physics associated with pressurized solidification of multi-component system,including pressureeffect-embedded thermodynamic modeling(PEE thermodynamic modeling)and interactive diffusion.The PEE thermodynamic modeling was established by calculating the Gibbs free energy under pressure according to the equation of state.The phase field modeling for pressurized solidification of ternary magnesium alloy was applied to squeeze casting Mg-Al-Sn alloy and Mg-Gd-Y alloy to investigate the influence of pressure on microstructure evolution and growth kinetics.The results showed that pressure increases the growth velocity of dendrites,promotes the growth of secondary arms and decreases the average grain size of the Mg-Al-Sn alloy.The microsegregation of solute Al aggravates with applied pressure considerably,while that of Sn is rarely influenced by pressure.The dendrites of Mg-Gd-Y alloy which were with fewer secondary arms were not as developed as those of the Mg-Al-Sn alloy.