Assessment of forming parameters influencing spring-back in multi-point forming process: A comprehensive experimental and numerical study

Like all sheet metal forming methods, one of the main characteristics of parts formed by multi-point forming is dimensional deviation caused by elastic recovery that is known as spring-back. In this paper the effects of material property, sheet thickness and anisotropy ratio along with process parameters such as elastic layer thickness, elastic layer hardness and number of punch elements on spring-back are studied utilizing finite element simulations and experimental tests. Experimental tests are carried out under various conditions by forming V-shaped and Sin-shaped geometries. Aluminum alloy 3105, stainless steel 304 and pure copper were used as sheet materials for experiments. Likewise, black rubber with shore A hardness of 50 and polyurethane with hardness of 65 and 85 were allocated as elastic layers. The Abaqus® commercial code is employed for finite element simulations. The definition of yield behavior of utilized sheet materials is fulfilled by using three yield criteria of Barlat-89, Hill-48 and Von-Mises. Since the Barlat-89 is not adopted in Abaqus, VUMAT and UMAT user defined subroutines are provided and integrated with explicit simulation of forming process and implicit simulation of spring-back phenomenon respectively. The results indicate that parameters such as material property, blank thickness and anisotropy affect spring-back in multi-point forming. Also the thickness and hardness of elastic layers are novel ideas that should be considered in order to minimize the spring-back. In general, using the elastic layer with minimum possible thickness and greater hardness beside the maximum number of pins leads to minimum spring-back.     

Z形梁冲压回弹畸变的形成机理研究及控制

采用冲压成形分析软件Dynaform对汽车Z形梁类零件的成形过程和回弹进行了有限元模拟分析,研究了分析结果中各截面应变不均的问题,得到了应变不均的分布曲线。分析了传统方案的特点,提出了改进方案,并对改进方案的应变分布进行了分析。曲线显示压料面与梁的法兰边高度差越大,应变分布不均严重。并对新方案的关键变量进行了分析,找到回弹值最均匀的方案,同时单个截面的最大回弹值减少到1°以内。最终按此优化的型面加工模具,并成功获得了合格产品。     

Spring-back characteristics of fiber metal laminate (GLARE) in brake forming process

The brake forming process has been considered as a feasible method for producing fiber metal laminate (GLARE) stringer. However, the spring-back developed during brake forming leads to serious problems in the final dimensional tolerance of the stringer. A series of experiments were performed to examine the effect of tool design and process parameters on the spring-back of GLARE. The parameters studied include punch radius, punch speed, forming load, and forming temperature. This paper shows that both design and process parameters can significantly affect the amount of spring-back. Scanning electron microscopy (SEM) was also carried out for the observation of delamination or cracking in the bent zone.     

一种高形状精度U形件的弯曲模优化设计

介绍了一种高形状精度U形件的弯曲成型方法。分析了U形零件的工艺特点及检验要求,对零件的形状精度进行了合理评估。在充分考虑卸载后的弹复变形后,对模具结构进行了优化设计。与改进前的弯曲模相比,改进后的凸模为带有凸筋的结构,在成型过程中集成了弯曲与镦挤的复合变形,使U形件原本以凹模为成型基准变成了以带筋凸模为成型基准,改进后模具结构更加简单,零件的形状精度得到保证,外观轮廓清晰,满足了质量要求。     

U型钢弯曲回弹试验及回弹规律数值分析

通过对25U型钢弯曲回弹前后实际曲率半径与型材弯曲理论公式数值计算结果的对比、分析,结果表明:25U型钢在弯曲半径大于1 000 mm时,随着弯曲半径的减小,所对应回弹量增大,回弹前的弯曲半径与型材弯曲理论公式数值计算结果之间存在很大的误差;当弯曲半径小于1 000 mm时,随着弯曲半径的减小,所对应回弹量变化不大,并且与型材弯曲理论公式数值计算结果之间的误差较小。实际应用中以1 000～2 500 mm曲率半径为主,因此需在这一范围内对型材弯曲理论公式进行修正,修正后的型材弯曲理论公式数值计算结果与实验值相吻合,可以用于计算25U型钢加工设备的控制量,指导实际生产。     

Influence of blank profile on the V-die bending camber process of sheet metal

The finite element simulation is now widely used in the design of stamping tools. A trial and error procedure has been replaced by a simulation in which defects associated with sheet forming processes are predicted and evaluated. This paper aims to clarify the process conditions of the V-die bending of a sheet metal. It provides a model that predicts not only the correct punch load for bending, but also the precise final shape of the products after unloading. An incremental elastic-plastic finite element computer code, based on an updated Lagrangian formulation, was developed to simulate the V-die bending of sheet metal. In particular, the assumed strain field (ASF) element was used to formulate the stiffness matrix. The r-minimum technique was used to deal with the elastic-plastic state and solve contact problems at the tool-metal interface. A series of experiments were performed to validate the formulation in the theory, leading to the development of the computer codes. The predicted punch load in the finite element model agrees closely with the experimental results. The whole history of deformation and the distribution of stress and strain during the forming process were obtained by carefully considering the moving boundary condition in the finite element method .A unique feature of this V-die bending process is the camber after unloading. The computer code successfully simulates this camber. The simulation was performed to evaluate the effects of the size of the blank on the camber process. The results in this study clearly demonstrate that the computer code efficiently simulated the camber process .     

The Application of Neural Networks in the Prediction of Spring-back in an L-Shaped Bend

The purpose of this work is to establish an effective prediction of the spring-back of material during the processing of an L-shaped bend. FEM-simulation of an L-shaped bend is carried out for various thicknesses of material, various punch-round-radii and die-round-radii. The spring-back depends on the shape of the bend-die and the mechanical properties of the material. The results of spring-back from FEM-simulation are then input to a neural network to establish a model for the L-shaped bend variables. The neural network is composed of a number of functional nodes. Once the L-shaped bend para-meters (material thickness, punch-round-radius and die-round-radius) are given, the bend processing performance (spring-back) can be accurately predicted by this developed network. A simulation annealing (SA) optimisation algorithm with a performance index to obtain a perfect L-shape can search for the optimal bend processing parameters. A satisfactory result was achieved based on a demonstration of simulation and on practical experience, showing that this is a new and feasible approach for use in the control of spring-back of materials.     

Fabrication of radar absorbing structure (RAS) using GFR-nano composite and spring-back compensation of hybrid composite RAS shells

The fiber-reinforced composite materials have been advanced to provide excellent mechanical and electromagnetic properties. The radar absorbing structure (RAS) is such an example that satisfies both radar absorbing property and structural characteristics. The absorbing efficiency of RAS can be obtained from selected materials having special absorptive properties and structural characteristics such as multi-layer and stacking sequence.

In this research, to develop a RAS, three-phase composites consisted of {glass fiber}/{epoxy}/{nano size carbon materials} were fabricated, and their radar absorbing efficiency was measured on the X-band frequency range (8–12 GHz). Although some of GFR (Glass Fiber–Reinforced)-nano composites showed outstanding absorbing efficiency, during their manufacturing process, undesired thermal deformation (so called spring-back) was produced. The main cause of spring-back is thought to be temperature drop from the cure temperature to the room temperature. In order to reduce spring-back, two types of hybrid composite shells were fabricated with {carbon/epoxy} and {glass/epoxy} composites. Their spring-back was measured by experiment and predicted by finite element analysis (ANSYS). To fabricate desired final geometry, a spring-back compensated mold was designed and manufactured. Using the mold, hybrid composite shells with good dimensional tolerance were fabricated.     

Optimization of spring-back in creep age forming process of 7075 Al-Alclad alloy using D-optimal design of experiment method

This paper attempts to model and predict the spring-back for creep age forming of a 7075 Al-Alclad alloy using statistical analyses based on a design of experiments method. Time and temperature were chosen as effective variables for determining spring-back in the creep age-forming process. The D-optimal design of experiments method facilitated statistical analyses and the extraction of a mathematical model for determining spring-back in the experimental variables domain. The spring-back of the specimens was calculated using a numerical procedure based on the pure bending theory. Analysis of the variances for spring-back showed that temperature was the most effective variable in the creep-age forming process. Additionally, a mathematical model and the response surface of the spring-back showed that to decrease spring-back, the significant variables should be in the upper level. The spring-back in the creep age-forming process was optimized for a 7075 Al-Alclad alloy in the optimum mechanical properties region.     

某大梁成型工艺控制研究

通过某汽车后大梁零件的成型分析,详细讲解了高强度钢板回弹的影响因素.如何在成型过程中尽可能减小回弹的影响,包括模具结构、产品的几何特征、辅助机构等相关因素都需要考虑,通过大梁的成型过程总结出了高强度钢板梁类工件回弹分析及控制的一般常用方法.