Study On Microstructure And Mechanical Properties Of Vacuum-Assisted Die Cast AE44 Magnesium Alloy

As a metal structural material with low density,high specific strength and easy recycling,magnesium alloy plays an important role in the field of lightweight.Die casting is an efficient near-net-forming process for light metals,which is widely used in automotive,communications,electronics and other industries.As the typical microstructure characteristics of die-casting magnesium alloys,ESCs（externally solidified crystals）,porosity,defect bands have important effects on the castings’mechanical properties.Therefore,carrying out a systematic study on the solidification microstructure and defects of magnesium alloy in combination with the die casting process,and establishing the internal relationship with the mechanical properties of die castings,are of great significance for optimizing the die casting process and improving the mechanical properties of die-cast magnesium alloys.In this thesis,AE44 magnesium alloy containing rare earth was selected as the research object,and a series of vacuum-assisted die casting experiments and microstructure characterization of die castings were carried out.Results showed that the microstructure of AE44 mainly containedα-Mg,ESCs,Al₁₁RE₃,Al₂RE,defect bands,and porosity.ESCs tended to accumulate towards the center,and their morphology varied.Double defect bands were prevalent,divided into an inner defect band surrounding the core,and an outer defect band near the surface,both of which were characterized by solute enrichment,but without the aggregation of porosity.The inner defect band was significantly wider than the outer one.Under various process conditions,differences existed in the changing trends of the inner and outer defect band,where the morphology and distribution of the inner one were greatly affected by ESCs,while the outer defect band was unrelated with ESCs and usually followed the contour of the casting,but its width and distance fluctuated.On the basis of microstructure characterization,combined with the simulation results of die casting process and theoretical analysis,the formation mechanism of double defect bands was studied.The formation of the outer defect band occurred in the fast filling stage.The semi-solid region near the chilling layer on the surface of the casting had a flow velocity difference with the surroundings,resulting in shear stress acting on this semi-solid region,causing the collapse of the grain network and the feeding of liquid,and eventually a solute-rich outer defect band was formed.The inner defect band was also developed from the shear deformation,collapse and liquid feeding of the semi-solid region,which existed near the core of the die casting where the ESCs gathered.The generation of shear stress was ascribed to the intensified casting pressure acting upon the whole melt,which was transmitted to the semi-solid region near the casting center.The solution and aging treatment of vacuum-assisted die castings were carried out.Results showed that the morphology of Al₁₁RE₃ phase transformed from lamellar to short strip and spherical after solution treatment.Its hindering effect on dislocations was weakened,but the deformation of adjacent grains could be better coordinated,so the strength of the alloy decreased slightly while the elongation increased significantly.The study of failure mechanism revealed that the defect band was not the main source for crack initiation,and the crack would preferentially originate from the pores in the core region of the sample or the defects near the surface during the tensile process.After the crack was initiated,it would propagate in a combination of inter-granular and trans-granular mode,and connect and merge with the surrounding pores and cracks,eventually leading to the fracture of the specimen.