Numerical Simulation And Forming Process Optimization On Unsteady Bending Of Large-diameter Equal Wall-Thickness Elbow

As one of the key components for piping system, elbow is widely used in chemical, petroleum, natural gas, machinery and electricity, etc. With the rapid development and expansion of industry, large-diameter thick wall elbow is demanded more and more in practical engineering application. However, the processing methods of large diameter thick-walled elbow in our country is still in the early stages of development, mainly rely on the introduction of foreign technology and own research. In this paper, the forming process of squashing bending and differential extrusion is studied, and the forming process is optimized and improved.The forming process of large diameter thick wall elbow is divided into two steps——aquashing process for circular tube billet and bending process. According to the characteristics of the two processes, the forming molds and tube billet were designed by using PRO/E software. And then the forming processes of the two steps were simulated by using DEFORM-3D software and the forming results of different forming strategies were obtained. In the beginning of the study, based on the data of simulation results, the BP network combined with genetic algorithm was ultilized to predict the tube size and process parameters, obtaining a satisfactory result. The analysis of stress, strain, wall thickness changing, velocity, temperature, thermal stress and load curves during the deformation were carried out. And the deformation characteristics, stress and strain distribution situation of the main deformation area which influence elbow forming were gained. Through comparative analysis of three group simulation results under different pressure reduction, different squashing speed and bending velocity, the influence laws of different squashing speeds and velocities on tube deformation as well as the best process strategy were gained.The gained elbow basically complied with production requirements and the gained process parameters can provide a reference for practical production.For the differential extrusion process of large diameter thick wall elbow, the eccentric tube is designed to achieve differential flow between inner arc and outer arc of the tube, resulting in bending and forming elbow. The process involves a single step, using the hot extrusion with a mandrel method. According to the speed difference requirement converted by product specification and process features of differential extrusion, the tube billet and forming dies were designed by using PRO/E software. The finite element numerical simulation of forming process was carried out by using DEFORM - 3D software. The Stress-Effective, Strain-Effective, velocity and load curves were analysed and the forming features of differential extrusion process were studied. It was found that the formed elbow without section flattening, homogeneous wall thickness, which reached the size deviation requirements, and the residual stress is small. Studies show that through the eccentric tube design, the velocity deficiency between inner arc and outer arc of the tube can reach the expectations. Howrver, the bend curvature of the formed elbow failed to meet the requirements. The relative analysis of simulation results under different load paths showed effect regulation of different load speed curves on bend curvature of formed elbow, which can lay a solid foundation for further in-depth study.