Study On Preparation And Thixoforming Of Semi-solid Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr Alloy

The semi-solid billet of this alloy was prepared by recrystallization and remelting?RAP?method and the experiments of partial melting and hot compression of Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr alloy were carried out on a multistage hot compression test machine.After these,Thixoforming experiments were carried out under different forming parameters?forming temperature,forming load?.The composition of alloy was analyzed by XRD.The microstructures of semi-solid slurry under secondary remelting parameters?heating temperature,heating rate,isothermal holding time?,hot-compressed samples and thixotropic cup parts were observed by SEM and OM.At the same time,the hardness of thixoformed specimens were tested and analyzed.Fracture morphology of thixoformed cup parts under different parameters?forming temperature,forming load?were also studied and the fracture structure was analyzed with OM.The interaction between mechanical properties of alloy parts,thixoforming conditions in semi-solid state and microstructure of alloy parts was studied.The results showed that affected by reheating temperature,heating rate and isothermal holding time,dissolution of eutectic compounds and partial melting occurred during reheating,dynamic arrangement of alloying elements and phase transformation took place during subsequent rapid cooling.With the increase of heating temperature,the compositions of the alloys changed from a-Mg,Mg₅?RE,Zn?and Mg₁₂?RE,Zn?eutectic compounds to a-Mg,Mg₅?RE,Zn?and?Mg,Zn?₃RE eutectic compounds.These microstructural behaviors not only changed the anisotropy in microstructural morphology of the extruded Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr alloy,but also affected the anisotropy in forming properties of it.Semisolid slurry with a homogeneous spherical microstructure was obtained when this alloy was heated to 580°C with a heating rate of 20°C/s and held isothermally for 20 s.The results showed that liquid segregation occured in Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr alloy due to the different forming behaviors of solid and liquid during thixoforming,resulting in uneven distribution of eutectic compounds in thixoformed cup samples.Due to the different volume fraction,morphology and distribution of eutectic compounds in Mg-8.20Gd-4.48Y-3.34Zn-0.36Zr alloy,the cup-shaped samples formed at different forming temperatures had different mechanical properties.The structure and mechanical properties of thixoformed specimens in the wall region were not affected by forming loads.However,It was found that the Vickers hardness at the bottom of thixoformed specimens increased sharply at higher forming loads,which was caused by the residual stresses involved in the formation of forming energy.