Numerical Simulation Techniques For Incremental Bulk Forming And The Applications In Process Design Of Multi-Pass Stretching

In order to precisely control geometry and property of forging products, it is necessary to rationally design process schedules and accurately predict the forging performance. Great opportunities for this goal have arisen due to achievements in numerical simulation technology and computer technology over the recent decades. However, precise control of product geometry and property demands more than what the state-of-the-art can provide. For example, some incremental bulk forming processes are composed of many operations. Complete numerical simulations of these processes are too costly or unpractical, which makes it hard to fully design process schedules with numerical simulations. Therefore the problems, including how to improve applicability of numerical simulation for analysis of incremental bulk forming processes and how to rationally use numerical simulation in design of process schedules, have important scientific and engineering values. This paper researches on general techniques for numerical simulation of bulk metal forming, special techniques for numerical simulation of incremental bulk forming and process design of multi-pass flat-tool stretching, which yields some technical improvements and some new techniques as follows.The principles for reduction of pre-assigned rigid zone DOFs are presented, and the algorithms for solving nonlinear rigid-viscoplastic finite-element equations and for identifying unknown rigid zone are improved. In order to reduce the system DOFs in analysis of incremental bulk forming, two principles are derived for the first time: the principle of reduction of functional integral region based on the condition that the contribution of the rigid zone material to the total energy functional is zero, and the principle of condensation of rigid-zone nodal DOFs based on the condition that the motion of the rigid zone material is of rigid body type. In order to enhance iteration robustness, the direct iteration algorithm and the Newton-Raphson iteration algorithm are both improved. An approach is proposed for automatic identification of unknown rigid zone. It uses the information of deformation history to update the current reference average effective strain rate and is adaptive to different deformation conditions.Some key techniques for efficient and robust simulation of incremental bulk forming are discussed. In incremental bulk forming, deformation occurs in the local region while heat transfer occurs in the whole region. According this characteristic, two mesh systems are adopted to improve computation efficiency for the thermo-mechanical coupled analysis of incremental bulk forming. The whole mesh is used for temperature field computation and the sub mesh is used for velocity field computation. A technique of reduction of pre-assigned rigid zone DOFs is proposed for generation of the sub mesh and for mapping of the two mesh systems. A“penetration”criterion and a“near”criterion are used for search of dynamic contact boundaries, and a special“penetration”case is considered in the algorithm for adjustment of the contact boundary nodes. Two interpolation approaches, which are linear piecewise interpolation and logarithmic piecewise interpolation, are proposed for handling discrete flow stress data.By introducing the concept of zero-thickness surface element layer from the grid-based method, two methods are respectively proposed for automatic regeneration of unstructured hexahedral mesh and layered hexahedral mesh. In regeneration of unstructured hexahedral mesh, a new mesh is produced by surface coverage of the old mesh with zero-thickness element layer(s). For a class of incremental bulk forming processes such as stretching, saddle forging and radial forging, a layered hexahedral mesh model is proposed. The new mesh is generated and improved by insertion of quadrilateral element layer(s), surface coverage of the old mesh with zero-thickness element layer(s) and subsequent mesh smoothing and boundary fitting. In data transfer from the old mesh to the new mesh, extrapolation along the inside edge and extrapolation along the element diagonal are respectively used for two types of boundary nodes. When calculating local coordinates of the new nodes in the old elements, a particle swarm optimization algorithm is adopted to improve computation efficiency and robustness. The results of two examples show that the new meshes produced by the proposed methods have good quality and the data transfer procedure leads to small accuracy loss.On the basis of these principles and techniques, the software XFORM is developed with the capability of finite element simulation of incremental bulk forming. The effectiveness of the software is validated by some numerical examples and a physical experiment. The results show that: (i) for finite element simulation of general bulk metal forming, the accuracy of XFORM are close to that of the commercial finite element software DEFORM and, XFORM has advantage on convergence speed of nonlinear iteration but lack on bandwidth optimization; (ii) in the finite element simulation of a stretching process, the efficiency is improved by about 62% with no significant loss of accuracy after using the technique of reduction of pre-assigned rigid zone DOFs and this result validates effectiveness of this technique for finite element simulation of incremental bulk forming.By using an analytical method and finite element simulation, process design of multi-pass flat-tool stretching is discussed. Firstly, an analytical method is presented for flat-tool stretching based on the Markov variational principle. In this method, a set of transient kinematically admissible velocity fields are established with considering the influence of friction and rigid ends, and then the reduction in a stretching bite is analyzed with incremental method. In each incremental step, the Markov variational principle is used for approximate velocity field solution, and a Lagrangian mesh is used for numerical integration and configuration update. Secondly, with the analytical method, a fast process planning algorithm is developed for multi-pass flat-tool stretching. This algorithm can produce feasible pass schedules which satisfy the requirement of final dimensions. Thirdly, by using XFORM as the analysis core, a continuous finite element simulation platform is developed for multi-pass flat-tool stretching. This platform includes the rigid zone DOFs reduction technique and the layered hexahedral mesh regeneration technique. Fourthly, the operations of press and manipulator during stretching are analyzed from the process schedule point of view, and then a type of code is proposed for describing the press-manipulator linkage operations. This type of code can be used for precise control and automation of stretching process. With the process planning algorithm and the finite element simulation platform, a stretching process for production of an alloy block is designed and a code for press-manipulator linkage operations is generated. The results show the feasibility of using these techniques in industry.