Research On Microstructure Optimization Design Methods In Forging Processes

Metal plastic processing is the fundamental manufacturing industry for national economy. For the reason of intense market competition and rapid updating of the products, the requirements of development period, cost and quality for products are increasingly improved. The design level of processes and dies are more recognized by industry, especially for the precision forming. With the development of computer and info-technology, the modern manufacturing industries are undergoing a profound revolution about simulation, virtual manufacturing and computerized manufacturing. The new characteristic requires digitized and scientific method and according software for the plastic forming processes and dies design. The traditional and empirical method of the design could not satisfy the requirement. The study and development on the numerical simulation and optimization technique are needed to satisfy the digitized requirement of the manufacturing processes and designs.With the development of the computer technology and the reliability of metal forming theory, the numerical simulation of forging processes can be realized on the computer. With the method of numerical simulation, many information of the metal forging such as metal flow, stress or strain and so on can be obtained, the load and energy needed in plastic forming process can be decided, the distribution of the stress, strain and temperature can be obtained in both workpiece and dies, and the forming status of the workpiece, residual strain, defect, grain size and orientation of the grain in the workpiece can be predicted. The simulation gives a powerful tool for the processes and die designs.The evolvement and distribution of the microstructure in the forging process have a direct influence on the mechanical properties of the final parts. So, among the various parameters of the forging process, the simulation and optimization of the microstructure are pivotal and integrant. The preform shape is corresponding to the shape of the final parts. It directly controls the reliability of metal. And it directly influences the shape of the final part and the capability of the microstructure. So the preform design becomes the crucial aspect of controlling the quality of the product. Thus, the optimal perform design is the most efficient way in the optimization design during forging processes.In recent years, the theory of shape optimization and sensitivity analysis has been introduced into processes and die designs of bulk forming process from the fields of the structure optimization. The problem of the design is regarded as the problem of the optimization for the processes and die designs based sensitivity analysis. It is on the basis of forward FEM simulation, combined with optimal algorithm to realize the optimal design for the processes and dies. It fully utilizes the information provided by the FEM simulation and can automatically realize the optimal designs. So, it is a new method for the processes and die designs.The research and development on the numerical simulation and optimization technique for bulk forming is carried out. The optimal methods based on the sensitivity analysis and genetic algorithm for the die designs are researched and constructed. Based on the above analysis, the multiple objective preform optimal design of the metal forging is studied in this dissertation. The research work is carried based on the visco-rigid plastic finite element method, the sensitivity analysis and the genetic algorithm. The optimal object is the preform die shape of the metal forging. The preform die shape is represented by the cubic B-spline. Therefore, the coordinates of the control points of the B-spline are used as the optimal design variables. The optimal objective is to obtain such a final forging that integrity quality involved the shape of forging and the uniformity of the microstructure of the forging is high. Based on the above analysis, an optimization design method of the shape and microstructure in forging process is proposed in this dissertation. And multi-objective optimization designs for the forging processes are carried out using above two methods.The establishment of optimal model in forging process is investigated, which includes choosing proper design variables, constructing mathematical model of objective function and selecting proper optimization methods to solve the mathematical model. Mathematical models of various optimal objectives in forging process are given in detail. The selection principle of design variables in forging process is specified. The preform forging shapes and preform die shapes, as design variables, are compared. The method of using cubic B-spline to represent the preform die shapes is specified. The optimization methods used in optimal design for forging process are also compared.The single objective optimization is the foundation of the multiple-objective optimization. Therefore, the research of the single objective optimization prior to the multiple-objective optimization is very important in the optimization. The precision shape of the forging is the basic requirement of the forging's quality. So, the optimization aimed to obtain the precision shape of the forging, net shape or near net forging, is carried out first. The different areas between the desired shape and the actual shape of the forging are used as the optimal objective. Then, the objective function is given. The sensitivity of the objective function respect to the design variables is developed. The sensitivity of the nodal coordinate and the nodal velocity with respect to the design variables are developed too. The velocity sensitivity boundary conditions are specified. The formulations of flow stress sensitivity are given. The equations of nodal temperature sensitivity are deduced in detail.The distribution of the microstructure in the forging has direct influence on the mechanical properties of the forging. So, it is important to control and optimize the microstructure to improve the quality of the forgings. Based on known model of microstructure evolution and FEM sensitivity analysis method, the simulation and optimization to microstructure in forging process are carried out. The goal is to achieve uniform recrystallized grain size after deformation. The least-square deviation between the average grain size and the actual grain size is used as the objective function. The optimal design is carried out for the forging process. The sensitivity equations of the objective function and grain size with respect to the design variables are deduced in detail. The procedure of the optimal microstructure design in forging process is given. The simulation system for microstructure distribution is established.The optimal preform design aimed to obtain the net shape or near net shape and more uniformity microstructure distribution of forging at the same time is carried out in this dissertation. The objective function for the multiple-objective optimization is put forward. The sensitivity of the objective function with respect to the design variables is developed. The sensitivity of the shape sub-objective and the microstructure distribution sub-objective with respect to the design variables is given. The selection of the weighting factor is presented. The software for multiple objective optimization of the preform design of the metal forging is developed. An example to optimizing the preform design of an H-shaped forging in axisymmetric deformation is done. The total objective, the shape design sub-objective and the microstructure distribution sub-objective versus the optimization iteration number is given. The different shapes of the preform dies and the final forgings at the various optimization iteration steps are given. The different microstructure distributions of the final forgings at the various optimization iteration steps are given, too. The optimization results show that the function of multiple-objective optimization is feasible.Genetic algorithm is widely used in many fields according to its simple ideas and easy realizability. The multiple-objective optimal method whose optimal objective is to involve the net shape of forgings and the microstructure of forgings is put forward. The multiple objective optimization function which is linearly combined by two sub-objective functions is given. The formula of the shape sub-objective function and the microstructure distribution sub-objective function are described in detail. The perform die shape is represented using the cubic B-spline. The software for optimal design method of the microstructure in forging process based on preform design using genetic algorithm is developed. The macro process parameters optimal design of the cylinder upsetting process and the microstructure optimal design based on perform shapes of an H-shaped forging in plane strain deformation realize the optimization of the final microstructure distribution and fine grain size. The initial billet dimension and deformation amount in performing process are also optimized. The optimal arithmetic is proved greatly credible.Taking H-shaped forging component as an example, an experiment is contrived for validating the microstructure simulation and optimization software developed in this dissertation. Dies used in the experiment and the experimental scheme are designed in detail. The experiment is carried out for applying the data testifications for the dependability of the computer results. The comparison of the experimental results and the optimization results shows good consistency. And thus, is testifies the stability and realizability of the multi-objective optimization design theory and system based on the microstructure and perform shapes in forging processes.