Studies On Preparation Technology Optimization And Microstructure Of Fly Ash Cenospheres/AZ91D Magnesium Alloy Composites

There still exist some problems in the present researches of magnesium alloy such as small hardness and low strength.In order to solve the problem existing in magnesium alloy,fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites were prepared by means of compo-casting technology.The possibility of reaction and reaction products of Si O2 which was the main component in fly ash cenospheres(FAC)and Mg were judged by Gibbs free energy calculated,and the kinetic equation between the reactants was established.The amount of reaction heat in the preparation of 6wt.% fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites process was calculated,and the thermal stability of fly ash cenospheres(FAC)-AZ91 D magnesium alloy system and reaction rate of fly ash cenospheres and AZ91 D melt were studied by DTA test.The optimal preparation process parameters of composites were obtained by orthogonal test,and the phase,alloy composition and microstructure of composites was studied by XRD,EDS analysis and microstructure analysis.The microstructure,Brinell hardness and compressive strength were studied after solution treatment of fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites.The main reaction equation and Gibbs free energy corresponded by calculated between Si O2 and Mg in fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites were:2Mg(l)+Si O2(s)=Si(s)+2Mg O(s)?GT=-76500+7.8T2Mg(l)+Si(s)=Mg2Si(s)?GT=-24000+9.4T4Mg(l)+Si O2(s)=2Mg O(s)+Mg2Si(s)?GT=-100500+17.2TThe amount of reaction heat between fly ash cenospheres and AZ91 D melt was 116.79J/g in preparation of composites containing 6wt.% fly ash cenospheres.The thermal stability of FAC-AZ91 D magnesium powder blend,fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites and AZ91 D magnesium alloy increased in turn.The thermal stability of fly ash cenospheres(FAC)/AZ91 D magnesium alloy composites containing 6wt.% fly ash cenospheres(FAC)was more and more stable with holding time increased in 720 ?.According kinetic model between fly ash cenospheres and AZ91 D magnesium alloy,the reaction product between fly ash cenospheres and melt of Mg O attached fly ash cenosphere wall surface forming a intermediate layer,while Mg2 Si was distributed in the matrix.The simple equation of fly ash cenospheres conversion rate in AZ91 D melt was:According to the equation,we can see that the reaction rate was related to fly ash cenospheres diameter,diffusion coefficient and intermediate layer thickness.Semi-solid stirring temperature range used in preparation of composites was 580?~600?,which was determined by DTA curve of AZ91 D tested by differential thermal analyzer.The optimal process parameters in preparation of composites containing 6wt.% fly ash cenospheres whice diameter were 145?m were stirring time 3min,stirring speed 675r/min,stirring temperature 590?.Which was obtained by three factors and three levels orthogonal test and made fly ash cenospheres most uniform distribution in composites.The density of the composites prepared by process parameters increased with the mass fraction of fly ash cenospheres increasing and decreased slightly with the diameter increasing.The variance of fly ash ceonspheres distribution increased with the mass fraction of the cenospheres increasing,and decreased with the diameter of cenospheres.There were reaction product like blocky or characters shaped Mg2 Si in microstructure observation.After the composites containing 6wt.% fly ash cenospheres was solution treated in 420? for 9h,18 h and 24 h respectively,?-Mg17Al12 decreased or even disappeared with the solution treatment time increased.And blocky Mg2 Si in composites turned to spherical,because angular gradually disappeared.After different composites containing different amount of cenospheres and different diameter of cenospheres were solution treated,grains in composites matrix became smaller with the mass fraction of cenospheres increasing,and grains in composites matrix became bigger with diameter of fly ash cenopheres increasing.Brinell hardness and compressive strength of composites both first increased and then increased with the amount of fly ash cenospheres,so did with diameter of fly ash cenospheres.Cleavage fracture was main fracture in fracture morphology,so the fracture category of composite was brittle fracture.There were pits formed after fly ash cenopheres shedding and fly ash cenopheres wall surface in fracture morphology.Because crack through the interface between fly ash cenopheres and matrix leaded to breaking.So interface was weak spot in the composites.