Studies On The Preparation And Properties Of Fly Ash Cenosphere/magnesium Alloy Composites For Degradable Fracturing Ball

The degradable fracturing ball is the important component of the the multi-stage fracturing technique with ball and sliding sleeve,which is mainly dependent on importation.There exist some problems such as high cost and limited technology.Accordingly,it is urgent to develop degradable fracturing ball with independent intellectual property rights.In this paper,the optimal alloy proportion of fly ash cenosphere（FAC）/magnesium alloy degradable composites for degradable fracturing ball was obtained by orthogonal test and range analysis.The FAC/magnesium alloy degradable composites were prepared by stirring cast method with different contents of FAC and Al,respectively.The optical microscope（OM）,scanning electron microscope（SEM）,X-ray diffraction（XRD）,energy dispersive spectrometer（EDS）,mechanical testing machine,Brinell hardness tester and electrochemical workstation were used to investigate the microstructure,phase composition,compressive strength,degradable rate,degradation morphology,density,Brinell hardness and electrochemical properties of the composites.Finally,the compression mechanisms and degradation mechanisms of FAC/magnesium alloy degradable composites were discussed.Firstly,the optimal alloy proportion of FAC/magnesium alloy degradable composites was obtained by four factors and three levels orthogonal test.The results show that the orders of FAC/magnesium alloy degradable composites to degradable rate and compressive strength are Al>Zn>Ni>Cu and Al>Zn>Cu>Ni,respectively.The optimal alloy proportions of degradable rate and compressive strength are determined as the following:Al:15 wt.%,Zn:6wt.%,Cu:0.5 wt.%,Ni:1 wt.%and Al:20 wt.%,Zn:9 wt.%,Cu:1.5 wt.%,Ni:0.5 wt.%,respectively.The degradable rate of the composite is 0.34 g·h^-1,which made by the optimal alloy proportion of degradable rate,and the compressive strength is 352 MPa.The degradable rate and compressive strength of the composites are coincidental with the requirements of the fracturing ball materials.The effects of the content of FAC on the microstructure and properties of FAC/magnesium alloy degradable composites were studied.The results show that the interfacial reaction occurred between FAC and Mg alloy and generated Mg₂Si and MgO phase,while the grain size of Mg alloy matrix was refined.With the increase of the mass fraction of FAC,the density of composites has been increased slightly.The density of composites is 1.88-2.05g/cm³,which means the composites still maintain the characteristics of low density.The brinell hardness of composites increases gradually,and the compressive strength increases first and then decreases,the strain rate decreases.The maximum compressive strength is 375MPa when the content of FAC is 8 wt.%.The degradable rate of the composites becomes faster with the amount of FAC increases,the fastest degradable rate is 5.02 g·h^-1 at 80℃.The degradation of composites will get faster with the increasing temperature and Cl^-concentration.The content of Al also has important effects on the microstructure and properties of FAC/magnesium alloy degradable composites.The composites with different Al contents are mainly composed ofα-Mg,β-Mg₁₇Al₁₂,Mg₂Si and MgO phase.There are no changes on the phases of composites with the content of Al.The amount ofβphase gradually increases with the increasing Al content,the morphology of theβphase transforms from the shape of brand structure to a net-work shape.The compressive strength of composites first enhances and then weakens with the increase of the content of Al,and reaches peak when the Al content is 15wt.%.The compression fracture mechanism of the composites belongs to the brittle fracture of cleavage fracture.The trend of the degradable rate of the composites first speeds up and then slows down.The degradable rate decreases from 4.33 g·h^-1 to 3.28 g·h^-1 when the the content of Al increases from 15 wt.%to 18 wt.%.The degradation productions mainly containβ-Mg₁₇Al₁₂,Mg₂Si,Mg（OH）₂ and MgCl₂ phase.Through the analysis of the effects of FAC and Al content of composites on compressive strength and degradable rate,it can be found that the increase of the compressive strength mainly originate from fine grain strengthening,second phase strengthening and solid solution strengthening.And the increase of the degradable rate is due to the galvanic corrosion betweenα-Mg and the second phases such asβ-Mg₁₇Al₁₂ and Mg₂Si.The corrosion type of the FAC/magnesium alloy degradable composites transfered from local pitting corrosion to general corrosion.Among which,the FAC,β-Mg₁₇Al₁₂ and Mg₂Si will fall off as a whole in the composites.