Study On The Mechanism Of Al-Sr-Mn-Ti-C Composite Refiner On Grain Refinement Of As-cast Mg-Al-Zn Alloys

Mg-Al-Zn(AZ series)alloy is a widely used cast magnesium alloy,but due to the low strength and plasticity,it can't be used as a structural part that can withstand large loads.Grain refinement can improve the mechanical properties of AZ-based alloys.Adding refiners in the liquid state to refine the grain structure of cast alloys is a cheap and practical method,which has attracted widespread attention and in-depth research.However,finding a grain refiner suitable for a specific alloy has become the focus of research.In this paper,a composite refiner Al-Sr-Mn-Ti-C containing a variety of alloying elements was smelted,prepared and added to liquid AZ31 and AZ91 alloys under casting conditions.The effects of the amount of composite refiner on the as-cast microstructure,grain size and mechanical properties of AZ31 and AZ91 alloys were studied firstly.Then based on the analysis of the micro structure of the refiner,the edge-to-edge matching model was used to explore the refinement mechanism of the refiner on AZ31 and AZ91 alloys.Besides,relative grain size model and interdependence theory were used to predict the grain size of as-cast AZ31 and AZ91 alloys.Refined by Al-Sr-Mn-Ti-C composite refiner,the secondary dendrites in the as-cast AZ31 and AZ91 alloys become developed,the grains are significantly refined,and the Mg17Al12 phase at the grain boundaries is intermittently distributed.The non-equilibrium solidification structures namely divorced eutectic and lamellar eutectic are found in metal-cast AZ31 and AZ91 alloys.But the number of lamellar eutectics and divorced eutectics in the AZ91 and AZ31 alloy respectively are reduced by the refinement treatment.The grain refining effect is the best when the amount of Al-Sr-Mn-Ti-C refiner is 0.5wt%,and the mechanical properties such as yield strength,tensile strength and compressibility of AZ31 and AZ91 alloys have been greatly improved because of grain refinement.The analysis results of XRD and EDS show that Al4C3,Al8Mn5,TiC and Al4Sr compounds with different morphologies are formed in the Al-Sr-Mn-Ti-C composite refiner.According to the calculation result of edge-to-edge matching model(E2EM),TiC and Al4C3 exhibit crystallographic orientation relationships with Al4C3 and Mg,respectively.Therefore,TiC can be a nucleat for heterogeneous nucleation of Al4C3 and Al4C3 is effective nucleant of ?-Mg.Crystallographic orientation relationship between Al4Sr and Mg17Al12 reveals that Al4Sr can be acted as a nucleating substrate for Mg17Al12 and promotes grain refinementThe relative grain size(RGS)of the AZ31 and AZ91 alloys was calculated using the relative grain size model,and the theoretical grain size of the AZ31 and AZ91 alloys was calculated using the interdependent theoretical model.The results show that the relative grain size model can qualitatively reflect the effect of solute Al content on the grain size,and the interdependence theoretical model can accurately predict the actual grain size of AZ-based alloys.