Research On Electromagnetic Suspension Casting And Alloy Strengthening Technology Of Wrought Magnesium Alloy

According to the research status and insufficient of magnesium alloy,research works of this thesis aim at high performance wrought magnesium,and mainly focus on the effects of electromagnetic suspension casting(combines the virtues of the electromagnetic stirring and the suspension casting,ESC) and alloy strengthening on it.The formation of an efficient, pollution-free grain refinement of magnesium alloy melt processing techniques to improve the mechanical properties of the alloy,thereby contributing to magnesium alloy in the Industry.Electromagnetic Suspension Casting device is designed and set-up based on the principle of electromagnetic stirring and the suspension casting.The effects of suspension particles on typical wrought magnesium alloys,such as AZ31 and AZ61 alloy are investigated systematically for the relationship of their chemical composition,microstructures,mechanical properties and their applications.The results show that ESC technique has been efficient in refining the microstructure and intensify the tensile properties characterization of magnesium alloys.The increase of the relative density of alloys caused by the reduction of micro porosity and gas cavity by ESC has advantage in improvement magnesium alloy following workability.The results indicated that proper additions of CaC₂ particulates concentration were found to be efficient in refining the microstructure of magnesium materials.0.26wt.%CaC₂ addition reduced the mean grain size of pure magnesium 2350μm to 125μm。With the CaC₂ addition AZ31 and AZ61 alloy,the microstructures have significant improvement,the grain refinement and the ultimate tensile strength and relative elongation increase remarkable.The 0.15wt.%and 0.36wt.%CaC₂ addition refines the grains and improves tensile properties of AZ31 and AZ61 alloy most effective,respectively.The ultimate tensile strength is 195.3MPa, elongation is 12.9%of AZ31 alloy,which increased by 18.72%and 48.28%respectively compared to the die-casting AZ31 alloy.The ultimate tensile strength is 211.36MPa, elongation is 8.5%of AZ61 alloy,which increased by 24.2%and 46.6%respectively compared to the die-casting AZ61 alloy.Zr can be added to magnesium alloy not containing Al for grain refinement because of hypothesis relates to the lattice disregistry between Zr and Mg.Considering the Al element interferes with the grain refinement process by the Zr,the ESC technique has been used for casting.The Zr particles refined AZ31 and AZ61 magnesium alloy have been successfully fabricated by ESC technique.Appropriate amount of Zr are mainly dissolved inα-Mg or exist in the form of particles,and the size of Zr particles is less than 10μm.It is believed that Zr particles or elemental in solid solution may be a powerful nucleant of magnesium because of similarity in the lattice parameters,therefore providing efficient nucleation,improving mechnical properties.However,more amount of Zr can react with Al to Al3Zr,which weaken the strengthening effect of Al to magnesium alloy.Moreover,Al3Zr can not play the role of grain refinement due to no lattice match with magnesium.The grain of alloys coarsens with the more Zr addition.Consequently,it is allimportant of controlling the amount of Zr addition. The adding amount of Zr of about 0.07wt.%is appropriate.The effects of small amounts of rare-earth element Erbium on the microstructure and the mechanical properties of AZ31 and AZ61 magnesium alloy via the electromagnetic stirring(EMS) process have been studied.The results show that small amount of Er addition obviously refine the grain size,make alloy microstructure homogeneous,increase the rate of equiaxed grains,and improve the mechanical properties.More addition of Er obviously decrease mechanical properties of AZ31 based alloy because of the grain coarsening. Observed a small amount of rare earth Er in magnesium alloy to form solid solution atoms,an excessive amount of Er and alloys in the Al reaction generates Al₂Er phase,which weaken the strengthening effect of Al to magnesium alloy.Moreover,Al₂Er phase is face centered cubic structure,which is different with the structure of Mg(Hexagonal Close-Packed).It cannot be the heterogeneous nuclei and have no effect on the grain growth.During deformation,the constituents with Er and Al will fracture first and act as the microcrack sources due to the stress concentration,which would descent the mechanical properties.Research results show that the Er content 0.03—0.05wt.%is appropriateThe elecrtromagnetic casting(EMC) and electromagnetic Suspension Casting(ESC) magnesium alloy have been deformed by dynamic upsetting.The deformation limits and behavior of thermal deformation have been studied to provide the basis for the establishment of magnesium alloy hot-working forming crafts formulation.Due to the high strain rate, magnesium alloy deformation occur the dynamic recrystallization based on twinning during dynamic upsetting process.The dynamic recrystallization grains take the twin boundary as in the place to nucleation and growth.Nucleation of recrystallized grains of alloy increase in the volume with the upsetting deformation increases,twin organizations and large grains decrease, recrystallized grains increased gradually refined grain alloy.The results of research shows that the appropriate foring deformation condition of EMC AZ31 alloy is temperature of 380℃—430℃,maximum deformation of 40%.Extrusion process can extend the EMC AZ31 alloy forging temperature range to 350℃—450℃,which make alloy get a larger deformation at lower temperatures.The appropriate foring deformation condition of as-cast EMC AZ61 is temperature of 315℃—370℃,maximum deformation of 30%.Extrusion process extend the EMC AZ61 alloy forging maximum deformation to 40%,which make the alloys have a relatively large deformation while the forging temperature is essentially the same circumstances.The appropriate deformation condition of permanent mould casting AZ31 and AZ61 alloy is temperature of 380℃—410℃and 300℃C—350℃,respectively,maximum deformation of 40%and 30%,respectively. ESC technology can improve the plastic properties of alloys to enhance the maximum deformation of alloy,expand deformation temperature range.The maximum deformation of can be improved and the forging deformation temperatures range can be extend to 300℃—410℃with the Zr and Er addition.