Study On The Effect Of CNTs And Rare Earth Pr On Properties Of AZ91 Magnesium Alloy

As a reinforcing phase,rare earth elements have been widely used in magnesium matrix composites.With the discovery of carbon nanomaterials,carbon nanotubes?CNTs?have received extensive attention due to their unique molecular structure,superior mechanical properties and physical properties,and have been gradually used as reinforcing phases for magnesium matrix composites.In this paper,the rare earth Pr reinforced AZ91 magnesium matrix composites were prepared by casting method.The composites were hot extruded and T6 heat treated.The microstructure,hardness and wear resistance of the composites that effected by hot extruded and T6 heat treatment was investigated.Then CNTs and Pr reinforced AZ91 magnesium matrix composites were prepared by powder metallurgy,vacuum hot pressing and hot extrusion.In order to improve the dispersibility of CNTs in the AZ91 magnesium matrix composite and enhance the interfacial bonding strength between the CNTs and the magnesium matrix,the CNTs were coated with TiO₂.The thermal simulation compression experiments of the prepared AZ91-Pr-TiO2@CNTs were carried out.The constitutive equation of the composite was constructed and the influence of thermoforming parameters on the microstructure of the composite was discussed.The results are as follows:1)Rare earth Pr refines the microstructure of AZ91 alloy and generates new reinforcing phase.When the Pr content is 1 wt.%,the refinement effect is best,and its hardness reaches a peak value of 76.2 HB.After the T6 heat treatment of AZ91-1.0%Pr alloy,the?phase precipitated out in grains and grain boundary again.The hardness of the alloy is further increased to 101.1 HB,which is a 32.7%increase in hardness compared to the AZ91 as-cast alloy.2)The addition of rare earth Pr helps to improve the wear resistance of the alloy.When the addition amount of Pr is 1.0 wt.%,the alloy has the best wear resistance.The T6 heat treated material exhibited the lowest wear rate and minimum friction coefficient.The result showed that the addition of Pr or T6 heat treatment is an effective method to improve the wear resistance of AZ91 alloy.3)The as-cast AZ91-Pr magnesium matrix composite material was hot extruded,and the T6 heat treatment followed.The microstructure,hardness and friction and wear properties of the composites of the three states were compared and analyzed.Extrusion and T6 heat treatment increased the hardness of the AZ91-Pr alloy by 18.8%and 58.4%,respectively,as compared to that of the as-cast alloy.The extrusion process not only refines the grain size,but also breaks the?-Mg₁₇Al₁₂ phase into a granular shape.Under the same conditions,the wear rates of the T6 and extrusion alloy are small relative to the as-cast alloy.The wear rate of the T6 heat treatment alloy is the smallest.4)The coating process can successfully coat TiO₂ on the surface of CNTs,and the coated TiO₂ is evenly distributed on the surface of CNTs.The coated TiO₂ can improve the dispersion uniformity of CNTs in the magnesium matrix composite and improve the interfacial bonding strength between the CNTs and the magnesium alloy matrix.5)The addition of TiO2@CNTs can hinder the dynamic recrystallization grain growth during the hot extrusion process and inhibit the movement of the grain boundaries,thereby refining the grain of the composite material.The addition of TiO2@CNTs increased the UTS,YS,and elongation of the alloy.When the addition amount was 1.0 wt.%,the maximum values of 194.19 MPa,389.67 MPa and 7%,respectively,were attained.Compared with the substrate,this showed increases of12.7%,49.1%,and 48.9%,respectively.The fracture modes of the alloys were all cleavage fractures.The carbon nanotubes carried tensile stress,but when the TiO2@CNTs concentration was too high,the CNT agglomerates easily formed cracks.6)The thermal simulation compression experiments of AZ91-Pr-1.0TiO₂@CNTs composites were carried out.The strain-stress curves of the materials were obtained at 300,350 and 400? and strain rates of 0.001,0.01,0.1 and 1S^-1.The composite material is sensitive to the forming temperature,and the flow stress is significantly reduced as the forming temperature increases.The material is also sensitive to the stain rate,and as the strain rate decreases,the flow stress value decreases.The stress-strain curve of the material under each forming condition appears as a steady-state flow pattern.7)According to the peak stress value of the stress-strain curve obtained by the thermal simulation compression experiment,the Arrhenius model constitutive equation of the material is constructed.The equation is as follows:???Using the Arrhenius model constitutive equation,the finite element numerical simulation of the thermal simulation compression process is carried out.The load results calculated by numerical simulation are compared with the experimental load.In most cases,the experimental load is close to the calculated load,and the degree of coincidence is high.However,for a strain rate of 0.001 S^-1,the accuracy of the constitutive equation is low,and the calculated forming load is significantly lower than the experimental load.8)Considering the strain rate,forming temperature and strain,a polynomial statistical equation of AZ91-Pr-1.0TiO2@CNTs composite was constructed.The SPSS software is used to regress the coefficients of the polynomial,and the statistical constitutive equation of the material is obtained as follows:???The stress value calculated by the constitutive equation of the polynomial statistical model is less than±15%error of the experimentally obtained stress value which was more than 95%.The statistical model has good calculation accuracy.9)When the forming temperature is 300?,the composite does not undergo dynamic recrystallization during the deformation process.When the forming temperature exceeds 350?,the composite material shows obvious dynamic recrystallization during the deformation process.As the strain rate decreases,the volume fraction of dynamic recrystallization in the composite increases.The CNTs contribute to the nucleation of the dynamic recrystallization of the composite material and inhibit the growth of the crystal grains.