A Study On Prediction Of Grain Size And Grain Refining Of Cast Mg-Al Alloys

The plastic deformation ablity is poor and plastic processing is relatively difficult of magnesium alloy at room temperature.Grain refinement can not only improve the plastic deformation ability,but also improve the comprehensive mechanical properties.Based on the dendrite growth dynamics model?i.e.,KGT model and LGK model?,and the effect of solute content and undercooling on the growth rate and the growth radius of dendrite was investigated.Using free growth theory,the grain size of AZ91 magnesium with addition of refiners was calculated.Meanwhile,a complex refiner Al-Sr-Mn-Ti-C was made,and the effect of refiner on grain size and mechanical properties of Mg-Al alloys was studied.The structures,morphology and size of the phases in the refiner was analyzed by SEM and XRD,and its refinement mechanism was discussed.The results show that the KGT model and LGK model have strong similarity under the Ivantson function.The higher content of solute in the alloys,the slower growth rate of dendrites,at the same supercooling degrees.However,with the increasing of undercooling,the growth rate increases rapidly,and the growth radius of dendrite tip decreases.The predicted value of average grain size of Mg-Al alloy is in good agreement with the tested one refind with the Al-Sr-Mn-Ti-C,on the condition that the size distribution of the particle is known.It is also found that Al-Sr-Mn-Ti-C refiner can effectively refine Mg-Al alloy.The results that?-Mg₁₇Al₁₂2 phase turn from coarse and continuous to discontinuous and uniform.The average grain size decreases,and the tensile strength is improved significantly.At the same time,the microscopic analysis results show that the Al-Sr-Mn-Ti-C refining agents contain TiC,Al₄Sr,Al₈Mn₅,Al₄C₃ and other compounds,these compounds show different forms,but the majority are rods,with diameter of megnitude of 10⁰ microns.And,they promote the grain refinement as the nucleus of heterogeneous nucleation.