Preparation Of Graphite/AZ91D Semi-solid Mechanical Stirring And Thixoforming Die Forging

With the depletion of energy,lightweight has become the mainstream target of industry.Magnesium alloy is currently the lightest alloy commonly used in engineering,and it is widely used in industry.However,the magnesium alloy has low strength and poor plasticity.The corrosion resistance and wear resistance of magnesium alloy are worse than that of the aluminum alloy,which greatly limits the application of the magnesium alloy.It can effectively overcome these problems by adding a discontinuous reinforcement to the magnesium alloy.In this paper,graphite/AZ91D composites were prepared by semi-solid mechanical agitation casting process.In order to improve the mechanical stirring efficiency and optimize the equipment,the influence of the tilt angle of the stirring rod blade,the number of blades and the length and width of the blade on the mechanical stirring effect was explored by ANSYS（fluent）simulation.The function and principle of the stirring rib during the mixing process were analyzed.In this paper,the effects of mechanical stirring temperature,speed and time on the distribution of graphite in the matrix of AZ91D magnesium alloy were investigated,and the way of adding graphite was optimized.Different mass fractions of graphite（nickel-coated graphite）/AZ91D composites were prepared by mechanical agitation casting process.Observing the composite structure,it was found that there were a lot of defects such as pores,shrinkage and enhanced phase agglomeration inside the as-cast graphite（nickel-coated graphite）/AZ91D composite.The existence of these defects greatly reduced the mechanical properties of the composite and must be followed by thermal processing to eliminate these defects and improve the mechanical properties of composites.In order to eliminate the as-cast microstructure defects and explore the semi-solid molding process of magnesium-based composite materials,this paper adopts semi-solid die forging composite thin-walled three-way components,and explores the best semi-solid molding process.In the forged composite,the pores and shrinkage cavities are welded together.the internal voids of the graphite agglomerates are completely filled by the matrix material,the material density increases,and the theoretical density of the graphite/AZ91D composite material is reached,and the mechanical properties are also improved compared with the as-cast state.With the increase of the mass fraction of graphite and nickel-coated graphite in the composite,the mechanical properties of the composite first increase and then decrease,reaching the maximum at 0.5%,reaching 160MPa.The mechanical properties of the composite with the reinforcing phase of nickel-coated graphite are slightly better than the mechanical properties of the composite with the reinforcing phase of graphite,reaching 180MPa.The addition of graphite can greatly inhibit the precipitation of the brittle medium Mg₁₇Al₁₂,but there is a large amount of the second component which containing Al surrounded the graphite particles.The brittle medium Al₄C₃ may exist,which affects the material properties.The second item around the nickel-coated graphite is less than graphite.When the load is small,the wear rate of the graphite/AZ91D composite material is not significantly reduced compared with the matrix material,and the effect of the graphite reinforcing phase is not obvious.With the increase of load,graphite forms a black lubricating film between the friction surfaces during the friction and wear process.The larger the load,the larger the lubricating film,the friction coefficient of the composite material gradually decreases,and the wear amount is gradually reduced.The wear resistance of the material is significantly improved,improving 11%.