| Grain refinement is an effective method to improve the mechanics properties of magnesium alloys.Severe plastic deformation techniques(SPD)such as equal channel angular pressing(ECAP),accumulative roll bonding(ARB)and high pressure torsion(HPT)have been successfully used to refine the microstructure of magnesium alloys.In comparison with other SPD techniques,multi-directional forging(MDF)has wider prospect for industrial applications because of its merits of producing fine-grained material with simple process and little cost.However,due to the close-packed hexagonal structure with limited slip systems,magnesium alloys exhibit lower forgeability and thus there are few researches being conducted on the MDF process of magnesium alloys until now.For the purpose of refining microstructure and improving the mechanics properties of magnesium alloys,typical Mg-Al-Zn alloys AZ61,AZ80and Mg-Zn-Zr alloy ZK60 were selected as the research objects,and the microstructural evolution models of the alloys were developed on the basis of hot isothermal compression tests.The hot workability of the alloys was studied based on the processing map theory and the proper parameter range for the MDF process was identified by means of finite element(FE)simulation.Finally,the effects of the MDF process on the microstructure and mechanics properties of magnesium alloys were studied by experiments.A method for determining the expression of the critical strain for dynamical recrystallization(DRX)was put forward on the basis of the Lassraoui model and the DRX kinetic equations for the studied alloys were also established.The two-stage Lassraoui model reflects the work harding-dynamical recovery behavior and the strain softening induced by DRX and the precision of the model was determined by the condition where the second stage begins i.e.,the critical strain where DRX triggers.However,there are few researches carried out on the identification of the critical strain.For the purpose of improving the goodness of fit between the value calculated by the model and the experimental data,the related parameters of the critical strain was identified on the basis of the Lassraoui model and the DRX kinetic equations for DRX of the studied alloys were established.A method for recognizing the microstructural supporting point for processing map was put forward by tracking the deformation history of different points in the sample during hot compression.Hot compression test is a common method of investigating the workability of materials.However,the temperature field and deformation field of the sample are non-uniform due to the contact boundary and friction,which also lead to the non-uniform of the microstrucuture.For the evaluation of the workability of the materials,the flow stress data obtained from hot compression tests was needed for developing the processing map and the deformed microstructures were needed for support the analysis results of the processing map,whereas the non-uniform of the microstructure under the set deformation condition leads to the difficulty for supporting the analysis results of the processing map.In order to identify the point which experiences the closest preset deformation condition for microstructural observation with the purpose of supporting the analysis results of the processing map,the deformation history of different points in the sample during hot compression were traced and the most suitable point for microstructural observation supporting the analysis results of the processing map was identified.The hot workability of extruded magnesium alloys AZ61,AZ80 and ZK60 was evaluated based on the processing map theory and microstructural observations on the characteristic point.According to the distribution characteristics of the power dissipation efficiency of the studied alloys and the results of microstructures observation,the microstructural evolution mechanisms under various deformation conditions were identified and the optimal processing windows for the studied alloys were also obtained.Based on the coupled analysis of the prediction results of Jonas',Semiatin',Prasad',Murty',Gegel'and Alexander'flow instability criteria and the results of microstructures observation,the suitability of the instability criteria for the studied alloys was examined and the flow instability regions which should be avoided during hot working were also identified.A new strategy for MDF with an increased strain rate was put forward and the proper parameter range for the MDF process was also identified on the basis of the finite element(FE)simulations integrated with the processing map.In comparison with the traditional isothermal MDF and the MDF with a decreased temperature,there is no critical grain size for the MDF with an increased strain rate,and the troublesome operations caused by temperature change during the MDF with a decreased temperature were also avoided.The proposed MDF process is simple and thus more suitable for industrial applications.In order to avoid the occurrence of the flow instability during MDF process,the FE simulation software integrated with the processing map was used to simulate the distribution of the temperature,strain,strain rate,and flow instability region of the sample during MDF processing,and the proper parameter range for MDF process was identified.The MDF experiments with an increased strain rate was successfully completed and the effect of the MDF process on the microstructure and mechanical properties of magnesium alloys were studied by means of metallographic observation,mechanical property test,and scanning electron microscope analysis.The microstructural examinations of the samples after MDF process reveal that as the deformation progressing,the microstructure of the alloy was gradually refined and homogenized.Magnesium alloy AZ61 with a homogeneous fine grain structure was obtained after MDF process under the temperature of 320 ?,whereas the dynamic precipitation of the quadratic phase occurred during the following deformation owing to higher strain rates.Through tensile experiments,it was found that the mechanical properties of the alloy increased gradually with the refinement and homogenization of the microstructure.The mechanical properties of magnesium alloy AZ61 such as the yield strength,ultimate strength and elongation to failure improve from 122 MPa,183 MPa and 4.5%to 241 MPa,303 MPa and 13%after MDF process under the temperature of 320 ?,whereas the mechanical properties of the alloy decrease sharply when the dynamic precipitation of the quadratic phase occurred. |
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