| Grain refinement has been proven to be one of the most effective approaches for enhancing the strength of Mg alloys.First,the traditional second-phase strengthening,solution strengthening,dispersion strengthening and other strengthening method are ineffective for Mg alloys because of lack of phase transformations below the solidification temperature to the room temperature,the low solid solubility of the other elements in the Mg matrix and lack of effective strengthening phases.Moreover,according to the Hall-Petch relationship,the mechanical properties of metals are violently influenced by their Hall-Petch slope constant,so the mechanical properties of Mg alloys are remarkably affected by the grain size due to their higher Hall-Petch slope constant(280 MPam1/2).In order to furtherly refine the grains of AZ31 alloy and improve their mechanical properties,this thesis aims to acquire ultra-fined bulk Mg alloys through increasing the driving force and nucleation rate of the static recrystallization of the rolled Mg alloys pretreated using equal-channel angular pressing?ECAP?by high-current-density electropulsing treatment?EPT?,which is based on domestic and overseas latest research results and the research foundation of our team.Accordingly,the relationships among EPT parameters,recrystallization microstructure evolution and mechanical properties of AZ31 alloy were investigated.In this study,a series of experiments were designed to investigate the effect of EPT and ECAP on the microstructure evolution and the mechanical properties of AZ31 Mg alloys.To begin with,the homogenized AZ31 Mg alloy is ECAPed by 8 passes at 300??250??220?,respectively,by which the grains of Mg alloy are refined and the elongation is increased significantly.Second,the ECAPed AZ31 Mg alloy is rolled by multiple passes at room temperature to accumulate a higher vacancies and dislocation densities inside the sample.Finally,the rolled AZ31 Mg alloy is processed by EPT to further refine the grain and optimize the mechanical properties.The Gibbs free energy change in the alloy system and the effect of EPT during this process are analyzed to reveal the recrystallization principle under EPT.With the extrusion temperature of ECAP decreasing,microstructures came to be more homogeneous and finer.Microstructure with double peaks is obtained at 300°C and the average grain size is 8?m while the large grain size is 17-18?m and the small grain size of2-3?m.When the extrusion temperature is reduced to 250°C,the grain size of sample is uniform obviously,and the average grain size is about 3.5?m.When the extrusion temperature of sample is reduced to 220°C,the grains are furtherly refined with an average size of 1.7?m.There is no obvious deformed structure in the rolled sample with rolling reduction of10%.When the rolling reduction was increased to 20%,the shear bands occurred along 45+5°to RD?ND?direction.In this experiment,effect of the output duration and processing time of the EPT on the microstructure evolution and mechanical properties was investigated so as to find the optimum parameters of EPT and analyze the mechanism of EPT.When EPT with a output pulse width of 50?s,a current density of 4.55×109 A/m2 and a processing time of 5min is applied,the microstructure and properties of the sample are improved,and the average grain size of AZ31 alloy is refined from 25?m to 1.05?m while the yield strength,tensile strength and the elongation of AZ31 alloy are 315MPa,430 MPa and12.9%,respectively.Convent heat treatment experiments were used to investigate the heat effect of EPT on the recrystallization of the rolled Mg alloys.The temperature rise of the sample during EPT is not sufficient to induce the recrystallization of deformed metal.As a result,we think that the recrystallization of the deformed metal is mainly induced by the athermal effect of EPT.In other words,the athermal effect of the electrical pulse is the main reason of the static recrystallization of the deformed Mg alloy. |
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