Optimization And Simulation For Forming And Densification Of Pure Mo Powder Materials During Equal Channel Angular Pressing

Severe plastic deformation(SPD) is a new research techniques of preparation for bulk ultrafine-grained(UFG) materials,and as a typical method of SPD, equal channel angular pressing (ECAP) has become a hot research topics related fields in material science and engineering for most rapid development and the very prospect of industrial application.Plastic forming and densification of powder metallurgy materials has been the difficulty of materials research in the field.Currently, the related study on powder material is just beginning, and there has poor plastic workability and complication of the deformation and densification mechanism for powder materials, especially there have no related reports about ECAP application with respect to the high melting point, low plasticity and hard deformed powder materials. Therefore, the deformation mechanism and the densification behavior of pure Mo powder materials in the ECAP process are a comprehensive and in-depth studied based on the method of combining basic theoretical research, the finite element numerical simulation with experimental study in this paper. All of these will provide the essential theoretical foundation for ECAP process research and application in the field.Contrary to characteristics of Mo and its powder materials, the influence of different factors on pure Mo powder sintered materials and its plastic compression deformation behavior have been studied. The flow stress increases with increasing strain rate and decreases with increasing temperature.The yield strength of material is the higher and more prone to rupture compared with the higher initial relative density,and increasing rate of hardness is not sensitive to the change of temperature, while the increase in temperature is beneficial to the reduction of yield strength. There is double impact of the pore and particle deformation and makes the material gradually dense but does not make fully dense for pure Mo powder sintered materials based on uniaxial compression.Based on "compressible continuum" assumptions of the powder material and analysis for various existing yield, the plastic constitutive equations adapted to pure Mo powder sintered materials are researched. Rigid-plastic constitutive equation of compressibility coefficient g is deduced and the quantitative relationship between the various parameters are given. The constitutive equations and their applications are reasonable analyzed,too. All of these will give available reference for the plastic forming process control and simulation analysis and enrich and expand the powder metallurgy generalized plasticity theory.On the basis of the comprehensive analysis of the slip line method, geometric derivation method and upper bound method to solve the plastic mechanics,the ECAP forming process is analytical resolved and the stress-strain field, velocity field, the extrusion pressure are quantitative analyzed.Considering three dimensions and the friction during the ECAP processing and using the upper limit of the theory, ECAP pressure is a comprehensive carried out and mathematical expression of the extrusion pressure is given.it will provide a theoretical basis of ECAP equipment selection and tooling design.In addition, for the "back pressure" on the ECAP process, the extrusion pressure with back pressure ECAP is solved and quantitative analyzed by using slip-line method,too.Thermodynamic coupled finite element model for powder material plastic forming is deduced,and based on different die structures and different initial conditions of pure Mo powder sintered materials, forming dense simulation is studied and distribution regularity of access extrusion pressure, stress-strain, the relative density and temperature field are obtained.The simulation results show that the shear deformation caused by the die corners is the forming and dense motivation,and despite ECAP deformation distributes the inhomogeneity but the ECAP processing for pure Mo powder material has a good densification effect.Die geometry shape is the key factor affecting the densification. On the basis of higher die strength and the allow of material processed,it is essential to choose a smaller die angles with the appropriate exterior angle improving the material flow of the outer corner and being satisfied with the amount of deformation and densification effect.Systematic analysis of influence of different process parameters to densification on pure Mo powder material.Appropriate to increase the friction can increase the extent of uniform deformation and densification,and the hardening properties of the material and the forming pressure will be reduced with increasing temperature and improve deformation homogeneity and compact effect but also adverses the organization refine.The pore is easy to be suppressed and acquire high degree of densification when initial relative density is smaller. Otherwise, there have good densification mechanism for grain crushing when the initial relative density is larger.When extrusion speed is large, the compact is better because of making the powder body particles squeeze, increasing deformation rate; and due to the extrusion stroke lengthened, forming a dense process more fully, there has well dense effect if extrusion speed is small.In the actual process, it is recommended to select a smaller value of the initial relative density,extrusion speed and moderate friction factor to protect the die equipment and to extend the equipment utilization,a reasonable choice of different process parameters should be considered during ECAP of pure Mo powder sintered materials.Multiple passes extrusion on various paths are simulated.The results show that the path BC after four passes extrusion completes a cycle and four surfaces are subjected to shear in the specimen, internal deformation is evenly distributed,so it is best extrusion path.That C path makes the deformation distribution tends to be symmetrical and uniform because of exchanging of the upper and lower contact surfaces. In view of uniformity of densification, path C is significantly better than path A after even (2n)passes extrusion, and the overall degree of densification of path A is higher than that of the path BA,too.Several common backpressure way and its role are described and simulation of two types BP-ECAP process are studied. The results show that forming dense is general proportional to the applied back pressure,However, that back pressure is too large will form a "dead zone" in die with a small exterior angle; Simultaneously there has too large extrusion pressure required, this will have serious implications for the ECAP forming die structure.It is conclusion of comprehensive comparison that the first class of the BP-ECAP design should be given firstly because of being more in line with the ECAP forming characteristics and more overall uniform for material deformation.On the basis of the theoretical foundation, numerical simulation studies, die and heating equipment are designed for pure Mo powder materials and experimental program is prepared. Extrusion experiments of the tube-powder sintered, tube-powder for ECAP and tube-powder sintered for BP-ECAP are carried out with comparative analysis of the finite element simulation results. After a single pass of tube-powder sintered ECAP.there is uneven distribution of deformation and density of the sample which is low relative density within near the bottom and the rest of the deformation and pore welding is better, while the sample overall has average higher relative density, it is confirmed that the ECAP has a strong densification effect. Microhardness of the specimen obtained has greatly improved depends on the extrusion path, extrusion temperature, the initial relative density.The ovservation and analysis of microstructure, scanning electron microscopy after multi-passes ECAP show that, ECAP has a powerful effect of the welding and refining effect of sintered powder materials. After a single pass, most of the pore of the sample deformation zone has been closed, specimen is close to the theoretical compaction density at the same time the grain has been significantly refined after2passes ECAP. Experiment results of2passes ECAP for tube-pure Mo powder show that there has smaller effect to deformation and densification besides occuring mainly particle rearrangement, elastic and plastic deformation behavior; the degree of densification of the specimen is significantly improved, the main deformation zone is near full density after2passes ECAP processing. A single pass BP-ECAP experiments using homemade simple back pressure equipment combined with the simulation results show,proper back pressure can increase the hydrostatic pressure and a greater shear deformation, promote particle crushing, melting, welding,and contribute effectively to the powder material densification. Reasonable range of back pressure for ECAP of pure Mo powder sintered materials is20-35Mpa when using the conventional right angle die.Finally, based on the orthogonal experimental design and numerical simulation,and innovative use of the Analytic Hierarchy of fuzzy mathematics and gray system theory, multi-objective optimization design of pure Mo powder sintered material during the ECAP process is studied. Results have proved that a reasonable choice of the die outer corner, forming temperature, the initial relative density, forming speed and the friction factor can significantly reduce the maximum damage value, obtain high hydrostatic pressure and make material significantly dense on the premis of the die inner angles to determine.