Research Of Mg-7Zn-XAl Magnesium Alloy On Microstructure During Partial Remelting

Semi-solid metalslurry has good rheology and thixotropy,easy and conventional die casting molding method such as the combination of near-net shape.Semi-solid metal slurry can standing or moving when subject to the external small stress but also the stock showes freedom of movement as the viscosity of the liquid metal when subject to continuous large shear strain.Semi-solid non-dendritic structure have a direct impact on the molding process parameters and the final quality of the products,and the casting products can be done by heat treatment to improve product performance significantly.The microstructure evolution in the isothermal heat treatment process is analyzed,and its mechanism is discussed.The amount of Al alloy will affect the hardness and toughness to a certain extent.Test results show that the ZA72,ZA74 and ZA76 alloys selected are made semi-solid non-dendritic structure.The minimum equivalent diameter, shape fractor and the fraction solid phase of alloy also will change when the content is different.As can be seen from the test,when the mass fraction of Al in the alloy is 4% and constant 20min at 590℃,the alloy's solid spherical particles are relatively rounding,and the distribution is relatively uniform;Along with the increase of the quality of Al in the alloy,the rate of as-cast tissue to semi-solid organization evolution speeds up.When the mass fraction of Al in the alloy is at 4%,you can get the small relatively spherical particles.Vickers hardness of as-cast organizationthe alloy increases when the content of Al in the alloy increases,when the Al content exceeds 4%(wt%),the vickers hardness has occurred, when the Al(wt%)is at 4% in the alloy,the cast of the Vickers hardness is 101.52.Finally,on the basis of the above experiments,Cu alloy elements is added to improve the ductility of magnesium alloys and to strengthen the aging effect, improve the eutectic temperature of the alloy,the alloy can be carried out at high temperature for solid solution strengthening, and enhance post-treatment effects and improve the corrosion resistance of alloys.When ZA72+0.9Cu alloy is holding 10min at 610℃,the alloy of equivalent diameter is 43.09μm,shape coefficient of 1.82, and the solid fraction at 40%.Vickers hardness value of the cast ZA72 +0.9 Cu solid particles of is 94.26,between the liquid crystal,the Vickers hardness value is very large, up to 628.When ZA74+0.9Cu alloy is holding 10min at 600℃, the whiteα-phase is distribution of the gray matrix, the average grain size is 37.13μm, liquid rate was 74.43,shape coefficient of 1.62,alloy organization has evolved into spherical particles.The vickers hardness value of cast ZA74+0.9Cu magnesium alloy solid particles is 110.26, intergranular liquid coexistence vickers hardness is very large,up to 215.5.Cu alloy elements speeds up the rate of evolution from the cast structure to the non-dendritic structure,and elements added to increase the alloy Cu nucleation substances,making the alloy more small solid particles.Through this experiment,it enriched the theory of the preparation of non-dendritic structure by semi-solid isothermal heat treatment.Under energy and atomic,diffusion effect of isothermal the temperature is higher or the holding time is longer,the distribution of low melting point phase is more extensive and atomic diffusion faster,non-dendritic structure and finer particle size distribution will be made more easily in a relatively short period of time distribution.In the isothermal process,the initial curvature radius of cast different,the smaller radius curvature of dendrite,the lower the melting point of dendrite,the lower the dendrite near solute concentration is.The gradient is different between two dendritic morphology,in the heat treatment process,which will lead to high concentration of solute at the dendrite to the lower concentration of the diffusion,ultimately achieving balanced composition, organization of semi-solid uniform radius of curvature of the grain.Lattice distortion is larger between the two or more grain boundaries of particles, so there is grain boundary energy,grain growth and straightness of the interface can reduce the energy,the system tends to balance,this is one of the reasons that the syructure change from the cast alloy to the semi-solid non-dendritic microstructure evolution gradually under the system free energy reduced.