Research On Modification Refinement Of ZM-5 Magnesium Alloy

In this paper,the influences of different modificators additions on microstructures and mechanical properties of ZM-5 alloy are systemically studied by using measurements of Optical Microscope(OM),X-ray Diffraction(XRD),Scanning Electron Microscope (SEM)/ Energy Dispersive Spectrometer(EDS) and Erichsen test to ascertain the best range of every modificator.The influences of MgCO₃ on microstructures and mechanical properties of ZM-5 alloy are investigated.The results show that the microstructures of ZM-5 alloy are refined obviously by MgCO₃ addition.Its grain size reduces from 124μm which is the grain size without refining to 68μm.The comprehensive mechanical properties of ZM-5 alloy with the addition of about 0.3%MgCO₃ are the best,its increment of tensile strength is 17.3%, that of yield strength is 32.8%and that of elongation is 37.5%.The influences of La₂(CO₃)₃ on microstructures and mechanical properties of ZM-5 alloy are investigated.The results show that the microstructures of ZM-5 alloy with La₂(CO₃)₃ addition are consisted of matrixα-Mg supersaturating solid solution,β-Mg₁₇Al₁₂ phases,bacillary Al₁₁La₃ phases and massive Al₈LaMn₄ phases,and can be refined obviously by the addition of La₂(CO₃)₃.Its grain size reduces from 124μm which is the grain size without refining to 61μm.Most of La reacts with aluminum to form bacillary compounds of Al₁₁La₃ with high melting point and elevated temperature stability in the grain boundary.The mechanical properties of ZM-5 alloy with the addition of about 0.7%La₂(CO₃)₃ are maximizing,its increment of tensile strength is 19.5%,that of yield strength is 28.5%and that of elongation is 75%.The results of Al-Ti-C refinement show that:(1) the microstructure of ZM-5 alloy can be refined obviously by the addition of Al-8Ti-2C master alloy.Its grain size reduces from 124μm which is the grain size without refining to 89μm.When adding content exceeds 0.3%,refining effect is not obvious.Al-8Ti-2C refines not onlyα-Mg matrix,but also changes reticularβ-Mg₁₇Al₁₂ into granular and claviform phase.(2) The mechanism of the refinement of ZM-5 alloy using Al-8Ti-2C master alloy is formation single TiC phase which can serve as the grain nuclei so as to refine magnesium alloy effectively during the solidification process.(3) The comprehensive mechanical properties of ZM-5 alloy with the addition of about 0.3%Al-8Ti-2C are the best,its increment of tensile strength is 8.8%, that of yield strength is 24.2%,but that of elongation is not obvious.The results of Al-B-C modification show that:(1) the microstructure was refined remarkably by addition of 0.1%Al-0.5B-0.7C.The average grain size decreased from 124μm to 65μm.The course reticularβphases become scrappy and distribute dispersion. High melting point compounds(Al₄C₃) which have HCP crystal structure can serve as the nucleating sites ofα-Mg during solidification.Heterogeneous nucleation was the main cause of the refinement ofα-Mg grains.(2) The increment of tensile strength of ZM-5 alloy with the addition of about 0.1%Al-0.5B-0.7C is 17.3%,that of yield strength is 27.9%and that of elongation is 57.1%.