Preparation And Properties Of B-containing Coatings On Pure Magnesium And AZ91D

Magnesium and magnesium alloys are widely used in transportation,aerospace,3C products,biomedicine,and armament technology due to their advantages of light weight,high specific strength,excellent damping and shock absorption,easy processing,and environmental protection.However,due to their low hardness and susceptibility to corrosion and other shortcomings,their application is limited.The cumulative stack rolling(ARB)Al/Mg/Al composite board can not only increase the hardness of the material,but also protect the surface of the Mg board from corrosion,but at the cross-section,galvanic corrosion between different materials is likely to damage the performance of the composite board.Among the many surface modification technologies,surface mechanical grinding(SMAT)and solid boronizing are widely used because they can obtain excellent properties on the surface of the material due to simple operations.Therefore,this paper uses the above two technologies to achieve the preparation of high-strength coatings through boron,in order to achieve the purpose of surface modification of the material or improve the performance of the material surface.Scheme 1:Adding AlB₂ powder to pure magnesium and AZ91D magnesium alloy for SMAT treatment and SMAT post-heat treatment respectively;Scheme 2:Treating the cross section of Mg/Al multilayer composite board with solid boronizing treatment at different temperatures and times.Through metallographic microscope,X-ray diffractometer(XRD),scanning electron microscope(SEM)and energy spectrum analyzer(EDS),micro hardness tester,electrochemical workstation,reciprocating friction and wear testing machine analysis and test methods.The surface modification layer's structure,phase composition,morphology,hardness,corrosion resistance and wear resistance were analyzed.In addition,the reason why the alloy layer is not formed in Scheme 1 and the effect of heat treatment on the surface modification layer were analyzed;the influence of different boronizing temperature and holding time in Scheme 2 on the infiltration layer was studied and analyzed.In summary,the following main research results and conclusions can be obtained:(1)Pure magnesium and AZ91D magnesium alloy added with AlB₂ powder for SMAT treatment and post-SMAT heat treatment can produce AlB₂ surface modification layers with different thicknesses and densities on their surfaces.No alloy layer is formed during this process.The thickness of the surface modification layer is inversely proportional to the density.Among all the samples prepared,the surface modification layer of the SMAT-400?-AZ91D sample is the densest,with an average thickness of about 12?m.Heat treatment is conducive to the diffusion of Mg element and the mutual adhesion between AlB₂ particles,making the surface modification layer more dense.(2)The corrosion resistance of the samples prepared by SMAT treatment and post-SMAT heat treatment is lower than the original samples,but among all the samples prepared by it,the corrosion resistance of the SMAT-400?-AZ91D sample is the most it is good.Heat treatment can eliminate the residual stress generated by some SMAT treatments and improve its corrosion resistance to a small extent.(3)Compared with the original sample,the hardness of the surface modified layer of the sample prepared by SMAT post-heat treatment has increased significantly,and with the increase of the depth,the hardness of the surface modified layer showed a gradual downward trend.The microhardness of the surface layer of the SMAT-400?-Mg sample is 212 HV,which is about 6.2times of the matrix,while the microhardness of the surface layer of the SMAT-400?-AZ91D sample is 228 HV,which is about 3 times that of the matrix.(4)The density,corrosion resistance and hardness of the surface modified layer obtained by using AZ91D magnesium alloy as the base are higher than that of using pure magnesium as the base.(5)The Mg/Al multilayer composite board is treated with solid boronizing at different boronizing temperatures and holding times,and all can have a continuous thickness of the infiltration layer on the surface of the substrate.The formation of the infiltration layer is closely related to the diffusion of elements The phase composition is mainly Mg AlBO₄,B₂O₃,AlB₁₂,KMg F₃,Mg F₂ and other substances,in which the AlB₁₂ hard phase is formed at a higher boronizing temperature.(6)Under different boronizing temperature and holding time,the hardness,corrosion resistance and abrasion resistance of the samples obtained after the solid boronizing experiment of the Mg/Al multilayer composite board are different to the matrix Increase,and the hardness of the infiltrated layer decreases gradually with increasing distance from the surface layer.(7)With the increase of the boronizing temperature and the increase of the holding time,the thickness of the infiltrated layer,the content of the phase composition,the hardness,the corrosion resistance and the wear resistance have been increased correspondingly.In this experiment,the effect of boronizing temperature on the structural characteristics and performance of the infiltrated layer is greater than that of the holding time.Among all the prepared samples,the infiltrated layer of the sample at 630?×10 h is the thickest,with an average thickness of about 20?m,the microhardness of the surface layer is the largest,up to 278 HV,and the corrosion resistance and wear resistance are also the best.