The Microstructure And Corrosion Resistance Of AZ91D Alloy Prepared By Spark Plasma Sintering(SPS)

Magnesium alloy has been widely used in automobile,aerospace,ship and other fields due to its characteristics of low density,high specific strength,good damping and good shock absorption.However,poor corrosion resistance is the bottleneck restricting the wide application of magnesium alloys.In this study,the application of spark plasma sintering technology in the preparation of AZ91D magnesium alloy was discussed,and its effects on microstructure,corrosion properties and mechanical properties were emphatically studied.A series of AZ91D alloy samples were prepared under different manufacturing conditions by adjusting the parameters of spark plasma sintering and particle size and adding different 2D carbon materials.The microstructure,crystal structure and corrosion properties of the samples were characterized by scanning electron microscopy(SEM),X-ray diffractometer(XRD)and electrochemical corrosion test.These results are expected to provide strong support for the development of high performance magnesium alloys and their industrial applications.The main contents are as follows:(1)The effects of different sintering temperatures on the microstructure and corrosion properties of AZ91D alloy prepared by spark plasma sintering were studied.By controlling the sintering temperature,the size and distribution of the second phase can be effectively adjusted.At higher sintering temperature,the second phase gradually dissolves,and the effect on grain fixation is weakened,which is conducive to the growth of grains.Grain refinement or coarsening can not improve the corrosion resistance of AZ91D,instead,the key is to obtain the best grain size and control the size and distribution of precipitated phase.Alloys sintered at 440℃ exhibit excellent corrosion resistance due to the dense distribution of a small residual second phase,which effectively acts as a barrier and prevents the corrosion solution from penetrating into the Mg matrix.The alloy sintered at 440℃ formed a relatively stable corrosion product film,while the alloy sintered at 320℃ and 520℃ had severe local corrosion and failed to form a uniform oxide film.(2)The effects of sintering temperature on the grain growth,texture evolution and second phase precipitation of AZ91D alloy during static recrystallization were investigated.Sintering temperature significantly affects the grain growth and texture evolution of AZ91D alloy.By controlling sintering temperature,grain growth,texture evolution and hardness response can be effectively regulated.At high sintering temperature,the grain boundary movement and grain growth are more obvious,resulting in a decrease in grain boundary area,which reduces the dislocation and strengthening effect of the material.High sintering temperature adversely affects the hardness and strength response of the material.At higher sintering temperature,the grain size increases and the preferred orientation of the grain boundary increases,resulting in a decrease in hardness and strength of the material.The alloy has high hardness and strength at 320℃.This is because the second phase has the largest number,wide distribution,high dislocation density,grain refinement and other factors that work together to make it have high hardness and strength.The process of nanocrystalline coarsening is studied by means of crystal phase-field method.The phasefield model of non-isothermal sintering reveals the evolution of the particles with time during sintering.(3)The effects of different particle size on microstructure evolution and corrosion behavior of AZ91D alloy were investigated.The particle size and distribution of the second phase can be effectively adjusted by controlling the powder size.Increasing the size of powder will reduce the size of the second phase,weaken the nailing effect on grain boundaries,and promote the growth of grain.Grain coarsening is helpful to improve the corrosion resistance of AZ91D.The sintered alloy with a powder particle size of 75-154 μm exhibits excellent corrosion resistance,primarily due to its lower and uniformly distributed second-phase content,which mitigates the occurrence of microgalvanic corrosion.A relatively stable corrosion product film was formed on the surface of the sintered alloy with the powder size of 75-154 μm,while severe local corrosion was found on the sintered alloy with the powder size of 48-75 μm and 1-48 μm,and no uniform oxide film was formed.The dissolution process of a phase with time is better understood by the method of phase-field simulation.(4)The effects of different 2D carbon materials on the properties of AZ91D alloy were discussed.By analyzing the microstructure and crystal structure,it was found that the distribution and interaction mechanism of different 2D carbon materials in the alloy,especially in terms of grain size and second phase content,had a significant impact on the corrosion resistance of the alloy.In terms of corrosion properties,the addition of graphene(G)showed the best performance,with lower corrosion rates and better electrochemical corrosion resistance.In addition,the mechanical properties of the materials,including hardness,compressive strength and elongation,were also studied.The addition of different two-dimensional carbon materials also influences these properties,with carboxyl graphene(GC)showing the highest hardness and graphene oxide(GO)showing the highest compressive strength.