Investigation Of Effects Of Microstructure On The Mechanical And Corrosion Properties Of Mg-Al Series High-Pressure Die-Casting Magnesium Alloys

Magnesium alloys are the potential light weight engineering alloys in the automobile industry,and they have low density,high specific strength,excellent machinability,good cast ability and recyclability.High-pressure die-casting(HPDC)magnesium alloys are the most widely used form of all magnesium alloys,and steering wheel frames,seat frames,instrument board frames and other automobile parts have been researching and developing using HPDC Mg alloys by many automobile enterprises as present.However,the mechanical and corrosion properties of HPDC magnesium alloys are relatively poor,which restrict the development of this kind of alloy.Therefore,the research forced on the improvement of performance of HPDC magnesium alloys has become the key and difficult point in the field of light metals at home and abroad.The alloys used in present work are Mg-Al series HPDC AZ91 and HPDC AM50 magnesium alloys.By adopting multiple testing and observing methods including nondestructive testing,tensile testing,immersion corrosion,three dimensional microstructure reconstruction,metallographic observation,scanning electron microscopy observation,the microstructure,the characteristics of interior defects,the mechanical properties and corrosion properties of alloys were investigated systematically,and the relationship between them were discussed and simulated.Consequentl y,the effects of microstructure on the mechanical and corrosion properties of HPDC Mg alloys were acquired,which can provide some theoretical basis for the improvement of performance of the alloys.The concreted results prove that:1.The influence of microstructure on the mechanical properties of HPDC AZ91 magnesium alloy is analyzed.The microstructure of HPDC AZ91 is mainly consisted of ?-Mg phase,?-Mg17Al12 phase,and an amount of gas pores and shrinkages.The size and distribution of each phase and porosities in the space is inhomogeneous.On the direction from the skin region to the center region,the size of ?-Mg grains increases,the quantity of ?-Mg17Al12 phase decreases,and the size and number of porosities increases.According to the result of three dimensional microstructure reconstruction of the alloy,the size of gas pores is 4 to 505?m approximately,and most of them is about 100?m.Combining the results of tensile testing and finite element simulation,the size,distribution,the maximum value of gas pores areal fraction of serial cross section(MPAF)have significant influence on the ultimate tensile strength(UTS)and elongation of the alloy.With maximum value of gas pore size increases,the UTS and elongation of HPDC AZ91 Mg alloy decreases.The gas pores arranged perpendicular to the loading direction have more serious negative effect on the mechanical properties than those distributed parallel to the loading direction.The UTS and elongation decrease with the increase of the value of MPAF: when the MPAF increases 1%,the elongation decreases about 0.37% and the UTS decreases about 12.02 MPa.2.The effect of microstructure on the mechanical properties of HPDC AM50 magnesium alloy is investigated.The microstructure of HPDC AM50 Mg alloy is composed of ?-Mg phase,?-Mg17Al12 phase,Al Mn phase,and a number of micro defects.Compared with HPDC AZ91,the levels of quantity and size of porosities in HPDC AM50 are much lower.The size of most gas pores is below 100?m,except a few larger gas pores.And the mechanical properties are much better than HPDC AZ91.Integrating the tensile testing data with the results of finite element analysis,gas pores have adverse effects on the mechanical properties of the alloy.And the major influence factors are the size of gas pores and MPAF.With the increase of the size of gas pores in HPDC AM50 Mg alloy,both the UTS and elongation decrease.Meanwhile,with the MPAF increasing,the UTS and elongation decreases nearly linearly.When the MPAF rises 1%,the UTS reduces about 12.28 MPa and the elongation decreases about 1.96%.Micro defects like cold shuts and cracks have significant effects on the mechanical properties of HPDC AM50 alloy,which can lead to early fracture after yield stage.3.Different cooling velocities during the solidification can lead to various microstructure situations of HPDC Mg alloys.Due to different microstructure situations,the corrosion behaviors of HPDC AM50 specimens with different thickness are diversified.For the specimens with thickness ranges from 3.0mm to 4.5mm,the corrosion behavior decreases first,than increases.The specimen with the thickness of 3.5mm had the worst corrosion resistance of all.The corrosion rate depends on the size of ?-Mg grains,the distribution of ? phase,and the situation of porosities.The increase of size of ?-Mg grains,the weaker of the continuity of ? phase,and the increase of the content of porosities,all can lead to higher corrosion rate.In addition,when comes to specimen surfaces observation during corrosion using optical microscope,the dark field images(DFI)have more obvious advantage than the bright field images(BFI).Tiny changes can be easily observed usi ng DFI.4.There are three main ways that can improve the microstructure of HPDC magnesium alloys,which are the optimization of casting processing parameters,the application of new die cast method and the addition of alloying elements which can improve microstructure.For the purpose of improving the microstructure of HPDC AM50 magnesium alloy,ProCAST software was used in present work to testing and optimize three processing parameters.And the result shows the relatively good parameter set is: casting temperature at 670?,die temperature at 200?,and injection speed at 2.0m/s.5.For the practical problems during ultrasonic nondestructive testing in present work,two sets of automatic ultrasonic nondestructive devices were invented,which are used for rotatable specimens and irregular shaped specimens respectively.By the use of universal chucks,multiple direction motion mechanism and automatic spraying equipment,the problems including the inconvenience of sample clamping,the irregular orbit of probe operation,and the difficulty of ultrasonic coupling medium spraying,have been solved,and the automatic,three-dimensional,and highly efficient of ultrasonic nondestructive testing has been achieved.6.For the problem of the one-sidedness and randomness of metallographic capture of die-casting defects,three-dimensional microstructure reconstruction technique was applied to reconstruct the interior defects of alloys,which makes the size,shape,distribution of defects intuitive and accurate.