Micro-mechanism And Control Of Elastic-plastic Contact And Wear For Extrusion Forming Process

Extrusion technology has the advantages of high efficiency, good quality and low-energy consuming, which has high practical value in the aspect of technology and economy. Extrusion technology has been widely used in machinery, instruments, electrical apparatus, light industry, aerospace, ship, military industry, and other industrial sectors, which has become one of the indispensable means of processing in metal forming field.However, working conditions are complicated in extrusion forming process and die service life is short, which constrain the application and development of extrusion technology. With the development of new materials and products, higher demands are proposed for die service life. Die service life is a comprehensive technology problem. And wear is the predominant factor which affects the die service life, especially at high temperature the failure is caused by wear in over 70% cases during extrusion forming process. Die wear is not the natural characteristic of material, but related to working conditions, die and workpiece material, surface monograph, contact type, lubrication mode and other complicated factors. Die wear is a nonlinear dynamic problem of multi-factor coupling, which involves thermal (temperature) - force (friction) - Chemical (lubricant). The wear is related to plasticity, tribology, metallography, chemistry, thermodynamics and other subjects of cross-knowledge, and has not formed a unified theoretical system. Therefore, the research of wear mechanism and wear control method for extrusion die have important theoretical and practical significance for improving the die service life, enriching and developing the tribology of plastic processing.In the aspect of the micro-mechanism of die wear, friction mechanism and calculation method of wear were explored, and the transient friction temperature rise of asperity was analyzed. Wear optimum design of extrusion die cavity and wear control method were researched to improve die service life. Specific research work is as follows:1. Based on Hertz contact theory, the model of microscopic elastic-plastic contact was established and friction mechanism was analyzed by adhesive friction theory. Effects of geometrical monograph of microcosmic rough surface on friction coefficient were researched. Then, combining the elastic contact Hertz theory and the fundamental theory of thermal conduction, the transient temperature rise distribution on asperity, which was caused by friction during relative movement between two rough surfaces, was calculated.2. Body temperature rise on die cavity surface caused by plastic deformation of workpiece and friction between die and workpiece was calculated by finite element method (FEM). Combining artificial neural network, the established neural network was trained with the simulation results as learning samples. The model was used to predict the body temperature rise of die cavity surface and improve the calculation speed of temperature rise, which lays the foundation for the establishment of temperature rise model of die cavity surface.3. Finite-element method, BP Neural Network and genetic algorithms were combined together to optimize extrusion die cavity. The method of B-spline function interpolation had been used to describe extrusion die cavity profile. The temperature, pressure and velocity field of nodes on the cavity surfaces were gained by FEM simulation. Wearing depths of extrusion die profile were calculated by modified Archard theory. The results were used as samples to train BP neural network, so that nonlinear mapping relation between reference point of die profile and wearing depth was established. In order to obtain uniform wearing depth, sequential quadratic programming and genetic algorithms were applied to optimize cavity profile of the extrusion die.4. Friction and wear properties of die surfaces, which were treated by magnetron sputtering and ion coating TiN, were studied by analogy experiment. The improving effect and mechanism of the wear resistance of coating by the adding rare earth element Y were investigated by scanning electron microscope (SEM) and 3-D topography instrument.