Constrction Of Polymorphous Crystal Model And Mechanical Simulation

The numerical simulation technology combining the metal material crystal plasticity theory and the meso-scale crystal geometric model provides a new idea for further studying the plastic deformation mechanism of materials and guiding the material forming process.The reliability of the numerical simulation results of crystal plasticity depends on the accurate establishment of the crystal plastic const itutive model,and the reasonable construction of polycrystalline geometric structure model with complex morphology also affects the simulation results.In the numerical simulation of meso-scale crystal deformation,researchers pay more attention to the research of crystal plastic deformation theory,and the research on the geometric model construction of polymorphic crystals in numerical simulation is less relatively.Voronoi diagram is often used to approximate the crystal structure of metal materials due to the polygonal structure characteristics similar to the actual crystal structure,but the grain size and shape in the crystal structure of real materials are various,and it is difficult to accurately describe the complex morphology of crystal structure by using Voronoi diagram directly.In this paper,the crystal model constructed by Voronoi diagram algorithm is optimized by analyzing and comparing Voronoi diagram and the spatial structure characteristics of metal polycrystalline.The construction methods of polymorphous grain model in metal materials are systematically studied.The polymorphic crystal structure model with different regularity and grain size is obtained by changing the position and density of Voronoi diagram seed points.Combined with the crystal plasticity theory,the numerical simulation of crystal deformation based on crystal plasticity finite element method is carried out for the constructed crystal model.The meso-mechanical behavior of crystal deformation is studied by stress cloud and stress-strain curve of crystal deformation.The main research works are as follows:(1)Optimization of Voronoi crystal model.Based on the grain boundary structure characteristics of metal materials,the crystal model generated by Voronoi diagram algorithm is improved.Using the improved crystal model generation algorithm,the four or more boundary structure existing in the initial Voronoi crystal model is modified to three boundary structure,so that the established crystal model is more consistent with the real crystal structure of metal materials.(2)Morphological control of crystal model.The crystal structure of real metal materials is complex and diverse.It is difficult to restore the crystal structure morphology when constructing the meso-crystal geometric model,but it is necessary to ensure the similarity of grain size,shape and arrangement.The offset factor (?) of seed points is set in the perfect equilibrium two-dimensional hexagonal crystal model,and the crystal model with controllable grain size and shape regularity is established by changing the value of (?).In view of the existence of multi-grain crystal geometry in the material,the grain size of crystal model is controlled by iteratively adjusting the seed points density of Voronoi diagram in some regions,and the crystal structure model of polycrystalline particle is constructed.(3)Construction of fiber grain model.The corresponding model of fiber grain structure in metal materials cannot be directly established by Voronoi diagram.According to the formation mechanism of fiber organization with cold-working process,the vertex coordinates of the initial Voronoi crystal model are transformed,and the vertex coordinates are amplified along the specified direction of space.The grains will be elongated along the direction of coordinate amplification to form fiber grains in the initial model.Based on it,the crystal model of grain fiber is established.The interface of the initial Voronoi crystal model is shifted and the grain boundary model with thickness is constructed by Boolean operation with the original crystal model.(4)Mechanical simulation of crystal model.The stress cloud and stress-strain curve of uniaxial compression deformation of crystal are obtained by the crystal plasticity finite element simulation of three-dimensional crystal model.The results show that the mechanical properties of different crystal orientation grains are different.Some grains are not conducive to deformation due to crystal orientation,and the deformation stress accumulates in the grains to form stress-concentrated grains.The plane strain simulation of two-dimensional polymorphous crystal model with different grain sizes and shapes is carried out by using crystal plasticity finite element method.The simulated stress cloud and stress-strain curve results show that the deformation behavior of the polycrystalline model is basically consistent with the influence of grain size and shape on the crystal deformation of metal materials.