Research On Solidification Behavior And Application Of Magnesium Alloys By Sub-Rapid Solidification

Magnesium and its alloys,with a number of desirable features including low density,the highest strength-to-weight ratio of any of commonly used non-ferrous and ferrous metallic materials,better damping characteristics and shielding properties,well castability,and abundant resources,are thus very attractive for applications in the automotive,electronic and aeronautical industries.However,the magnesium alloys produced by conventional casting process exhibit the drawbacks of less than desirable strength,inferior formability,low thermal stability,inadequate creep resistance,poor oxidation resistance and inferior corrosion. Consequently,the extensive use of the magnesium-base alloys is restricted.In general,the grain size and microstructure of cast metal are directly influenced by the cooling rate. Different degrees of structural refinement and transformation can be obtained by the sub-rapid solidification technique,which presently provide high cooling rates up to 10³ K·s^-1.The microstructure changes,which involve microcrystalline structures,phases with large and non-equilibrium solid solubility,new metastable crystalline phases,can improve the mechanical properties,processability and corrosion resistance.Therefore,sub-rapid solidification process may be an effective technology to achieve better microstructures and obtain Mg alloy castings with the maximum combination of mechanical properties and corrosion resistance.In the present paper,sub-rapidly solidified Mg alloy castings were prepared by using several different methods with self-designed devices.The microstructure characteristics and formation mechanism of Mg alloy castings produced by different processes were studied.In addition,according to the investigation of the performances of the Mg alloy castings,it could be clearly seen that the final microstructures had a close relationship with mechanical properties and corrosion resistance.And then strengthening mechanisms and corrosion behavior of Mg alloy castings produced by different technique were investigated.Main research details and results are as follows:The small size AZ-series Mg alloys sheets were prepared by using sub-rapid solidification technique with the self-designed vacuum suction casting(VSC) setup.The microstructure characteristics of sub-rapidly solidified Mg alloy sheets,including grain size, phase composition and the microdistribution of alloying elements,were studied.For comparison,conventionally solidified castings were also produced and investigated.The results show that the typical micrographs of conventional castings consist of well-developed primaryα-Mg dendrites withβ-Mg₁₇Al₁₂(including divorced eutecticβphase and secondary precipitatedβ" phase) along theα-Mg grain boundaries in the form of network;In addition, grain size is quite large and its distribution is non-uniform.The alloying elements Al,Zn elements mainly distribute on grain boundaries and only a very small amount exist in grains (primaryα-Mg phase),and thus,the microsegregation of alloying elements is severe in the conventionally solidified microstructure.Whereas,the grains of sub-rapidly solidified sheets are obviously refined,and mean grain size is only a few micrometers.The higher cooling rate there is,the smaller grain size become.The elements segregation and enrichment inherent for conventional castings are suppressed to a great extent due to high cooling rates and thus,the microstructures of sub-rapidly solidified sheets dominantly consist of supersaturatedα-Mg solid solution,which may lead to the change of lattice constant of Mg.Therefore,it can be seen that a little migration of diffraction peaks in the XRD patterns of sub-rapidly solidified sheets.The cooling rate during sub-rapid solidification process was estimated in order of magnitude by measuring the secondary dendrite arm spacing.The results show that the cooling rate of the Mg alloys sheets produced by sub-rapid solidification technology is up to about 10²ï½ž10⁴ K·s^-1.Solute redistribution mechanism,microstmcture evolution and formation mechanism under different solidification conditions are discussed systematically.The analysis shows that in sub-rapid solidification process,the movement velocity of solid/liquid interface is very fast and the solidification time is very short due to the extremely high cooling rate. Consequently,there is no time for the solute atoms near interface to enrich and diffuse sufficiently to the far distance of the melt and thus,they are captured by the high-speed moving solid/liquid interface and solubilized in the matrixα-Mg phase.In addition,under sub-rapid solidification condition,the vast majority of nuclei have no time to grow up and thus,they generate large numbers of fine equiaxed grains.A portion of nuclei can grow up and generate the petal shaped microstructures,which are essentially the primary dendrite arms. Only a small quantity of nuclei eventually generates the dendrite structure with extremely fine secondary arms showing sixfold symmetry.The mechanical properties and corrosion resistance of AZ61A Mg alloy castings produced by different technique were studied in detail and subsequently,the influence of sub-rapid solidification microstructure on these properties was investigated.Moreover,the strengthening mechanisms and corrosion behavior were discussed.The results show that the mechanical properties and corrosion resistance of sub-rapidly solidified sheets are improved in varying degrees compared with those of the conventionally solidified Mg alloy castings. The improved mechanical properties of Mg alloy castings can be ascribed to the conjoint and mutually interactive influences of the fine-grain strengthening,solution strengthening and dislocations strengthening.The improvement of corrosion resistance is mainly attributed to the refining and homogeneous distribution of grains,the disappearance of the discontinuously distributedβphase on grain boundaries,and the supersaturated matrixα-Mg solid solution containing abundant Al element.A novel semi-solid processing technique,called vacuum suction casting—damper cooling tube method(VSC-DCT method),was used to manufacture high quality components of AZ91D Mg alloy directly from a liquid metal.The outstanding feature of the VSC-DCT process is attributed to the fact that it combines the semi-solid slurry making and component forming operation into one step and thus,the sub-rapid solidification of Mg alloy semi-solid metal(SSM) are achieved.The resulting apparent morphologies and microstructures are characterized in detail,and linked to the corresponding mold-filling behavior and subsequent solidification behavior.It is revealed that the SSM with higher viscosity can be caused to fill the mold with "solid-front fill",as compared with the liquid metal "spraying" in the conventional vacuum suction casting(CVSC) process,and the surface quality of the sheets fabricated by the VSC-DCT method is improved significantly.Solidification in the VSC-DCT process takes place in two distinctive stages;one is primary solidification in the shear-cooling system under high intensity of turbulence,and the other one is secondary solidification inside the mold cavity with high cooling rate.The final microstructures of the sheets via VSC-DCT method exhibit that the "preexisting" primary solid particles formed in the primary solidification,with the morphology of near-globules or rosettes,are surrounded by the eutectic mixture of fine secondaryα-Mg grains and fine precipitates ofβ-Mg₁₇Al₁₂ intermetallics,resulting from the rapid solidification of the remaining liquid of SSM in the secondary solidification step.The final primary solid size is within a range of 15μm～35μm as examined by quantitative metallography,and the average size of the fine secondaryα-Mg grains is about 6μm in diameter.On the basis of the experimental results and sub-rapid solidification principle presented above,a thin-walled AZ61A Mg alloy tube was fabricated by using centrifugal casting technique,and the sub-rapid solidification of large-sized Mg alloy casting was achieved.The microstructure and mechanical properties of the centrifugal casting tube were presented and discussed.The results show that the grain size of the tube is significantly refined.The eutectic transformation L→α-Mg+β-Mg₁₇Al₁₂ and microsegregation are suppressed to a great extent. As a consequence,the alloying elements Al,Zn show much higher solid solubility and the microstructure of the tube dominantly consists of supersaturatedα-Mg solid solution. Mechanical properties and corresponding fractography of the centrifugal tube are significantly improved compared with those of the conventionally solidified Mg alloy casting.The fracture surfaces of the centrifugal tube exhibit ductile fracture characteristics.