Initiation And Healing Models Of Pores And Numerical Simulation In Plastic Deformation

Pore is one of the common defects in metal materials. Under applied loading, the pore defect may initiate, expand and coalesce to a macro crack, which leads to failure of components in nuclear energy, electric power, chemical industry. Therefore, the research of initiation, expansion and healing of the pore defect is important for developing plastic processing, diffusion bonding and powder sintering technology. The main contents of this thesis are as follows.First, by means of building finite element model based on the real microstructure, the effect of second phase particles on the damage and fracture behavior was studied. Meanwhile, the evolution of porosity in plate rolling was also studied. For the20%SiCp/Al composite, the results indicated that pores are initiated at the interface of particle and the base metal in the tensile deformation. The pores expanded along the interface and lead to a macro crack. It can be seen from the simulation result that fracture of base metal, debonding of the interface and fracture of the particle all exist on the fracture. Besides, the component properties have a significant influence on the fracture behavior of the composite. The simulation results indicate that few pores were generated when the interface is strong; more pores initiated when the particles are strong. The simulation results of rolling show that the edge crack of the SiCp/Al composite plate is caused by the tensile stress, which is produced by non-uniform deformation and leads to the damage of microstructures when the rolling reduction is25%.Second, the effect of porous defect on the high temperature deformation behavior has been studied by the tensile test at different temperatures and strain rates. The results indicated that in the range of800℃～1200℃and0.04～4s-1, there are dimples on the fracture morphology of the1%porous316L stainless steel. The dimple became small and closed to each other as the strain rate increasing. On the other hand, the size of the dimple increased as the temperature increased. The large dimples on the fracture are formed by original pores in the material.The porous defect healing kinetic model during heat treatment was established according to diffusion and probability theory. The size distribution of pores was considered in the probability model. The computed results using probability model were more closed to experimental results than the deterministic model. Besides, the necessary healing time and temperature can be determined from porous healing diagram which is constructed by probability model.Based on Kuhn plasticity theory of porous materials, the densification model during uniaxial compression was established considering the influence of temperature. The computed result is more precise than that of Kuhn model according to experimental results.The healing behavior of porous defect, the microstructure and the deformation behavior influenced by porous defect have been studied systematically by the heat treatment experiment, isothermal compression test and heat treatment test after deformation. The results indicate that hot compression can significantly improve the homogeneity of the pore size, and help to enhance the relative density of the material in varying degree according to different compression temperatures. The effect of pores on the compression behavior can either be the impediments of dislocation movement or be the nucleation sites of the recrystallization according to the difference of deformation temperatures. Besides, Deformation energy, residual stress, surface energy, and heat energy are the driving force of densification during subsequent heat treatment of hot forging. The deformation energy is the main driving force in pore healing process. The grain boundary interacts with the pore during heat treatment. Some of the pores were healed when the grain boundary is migrating.The crack healing kinetic model was established considering plastic deformation, creep deformation and diffusion theory. It is indicated from the result that the effect of grain boundary diffusion on crack healing is decreased as the grain growing in high temperature process. The healing time and temperature can be determined from the crack healing diagram, which is constructed according to the crack healing kinetic model. The calculated results agree well with the experimental results.