A STUDY OF FINE BLANKING PROCESS BY FEM SIMULATION. G. Fang, P. Zeng

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1 Key Engineering Materials Vols (2004) pp Online available since 2004/Apr/15 at (2004) Trans Tech Publications, Switzerland doi: / A STUDY OF FINE BLANKING PROCESS BY FEM SIMULATION G. Fang, P. Zeng Dept. of Mechanical Engineering, Tsinghua University, Beijing, , P.R.China Keywords: Fine blanking, FEM simulation, Ductile Fracture ABSTRACT Fine blanking process with V-ring was simulated with FEM. The geometric parameters of the die, the punch, the serrated ring and the sheet are modeled. In this paper, some other assumptions are made for the analysis. The workpiece is considered as elastic-plastic material, while the tools are defined as rigid bodies. The damage model taking into account the influence of hydrostatic stress is used to simulate material fracture in blanking. The stress status and forming process are analyzed. Authors also investigated the effect of distance from tooth to die edge on roll-over high. The simulation can reflect the laws of fine blanking process. 1. Introduction The blanking is a key process in precision instrument and electronic product manufacturing. To improve the precision of parts and forming qualities, the fine blanking (FB) is usually adopted. Fine blanking has the following features[1]: 1. Provides a clean-cut surface without the need for secondary machining such as shaving or milling. 2. Ensures stable production with high quality and precision. 3. Permits three-dimensional composite processing including coining, semi-shearing, bending, and drawing. 4. Assures better flatness than conventional press techniques. 5. The FB requires special presses featuring high accuracy and rigidity as well as precision tools. 6. The FB press uses three independently adjustable pressures (Triple Action Mode) for blanking operation The fine blanking is a constrained shearing operation that involves elastic deflection, plastic deformation and fracture of the materials. Compared with common blanking process, fine blanking is to make metal deform with more smooth cross-section and more precise dimension. In fine blanking process, some factors such as punch-die clearance, punch velocity, tool geometry and the mechanical properties of the materials influence the quality of cross-section and dimension precision. Selecting some different tool dimension and processing factors, we can obtain workpiece with the different quality. It is necessary that the study of forming mechanisms and laws in fine blanking. The fine blanking process is classical elastic-plastic large deformation, which is divided into All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, USA-13/01/15,03:32:36)

2 604 Advances in Fracture and Failure Prevention three phases with fundamentally different physical behavior and numerical treatment, that is elastic deformation, plastic deformation and fracture separation. In fine blanking process, the plastic deformation phase should be extended as possible, and the fracture phase should be shortened, even avoided. The difference between common blanking and fine blanking lies in fracture damage phase. Since Fritz Schiess designed and manufactured the first fine-blanking equipment in 1921, the fine-blanking technologies, which include workpiece geometries, tools, materials and machines, have been studied for 80 years. Many studies are based on experiments and production. For the stress and strain state in plastic deformation are complicated and inconstant, the theories studies on fine blanking, especial forming mechanisms, are not in-depth and thorough. Finite element method (FEM) is an effect technology to study metal forming. With FEM simulation results, designers can predict forming quality of important process parameters [2,3]. In the case of blanking operation, material separation often appears, and there are some difficulties for common FEM programs to simulate crack initiation and propagation. So far, some researchers have studied blanking process by FEM, and some valuable resulted have been gained. In this paper, material deformation, stress and strain distribution and the effects of V-ring geometry on deformation are studied by FEM. The elastic-plastic finite element analysis code DEFORM-2D was used, which is based on an implicit lagrangian computational routine. 2. MATERIAL DUCTILE FRACTURE CRITERIA So far, the effective method of predicting ductile fracture of metal is application of empirical criteria. The main effort has been placed not in developing a full mechanics analysis of ductile cracking, but simply on establishing criteria for predicting the fracture initiation sites and the level of deformation at which cracks will occur. Empirical ductile fracture criteria can be divided into three broad classes depending on the type of function used to express the criterion, and is usually stress and/or strain and strain path dependent. The stress-based functions typically involve effective, hydrostatic or shear stresses whereas the strain-based functions incorporate effective, hydrostatic or thickness strains. These criteria were often expressed as following: f 0 f (, e ) d e = C where f is the strain at rupture, e the effective plastic strain. If C exceeds the critical value, the material can be considered as ductile fracture. Different failure criteria that can be applied to predicted ductile fracture of the material subjected to excessive plastic deformation[4,5]. The failure process is essential to the selection of a suitable damage model. In the case of sheet blanking by shearing processes, numerous authors have studied the different physical mechanisms leading to the rupture, and proposed their own models. Ductile fracture in metals is known to be caused by the growth and coalescence of voids. These voids are holes in the material, caused by dislocation pile-ups, second-phase particles or other imperfections. Under the influence of plastic deformation, the voids can grow, until a number of voids coalesce to initiate a crack. The void-growth rate is influenced strongly by the hydrostatic stress during plastic deformation. So the model presented by Oyane was applied in simulation of fine blanking. f m (1+ A ) d = c 0 (2) (1)

3 Key Engineering Materials Vols where is the effective plastic strain, the effective stress, m is the hydrostatic stress, and A and c are material constants derived from some experiments. 3. NUMERICAL MODELS To study the fine blanking process with serrated ring (V-ring), some elastic-plastic FEM models are built, which are based on the axisymmetric fine blanking operation of a metal sheet with 6mm thickness. The geometric data are shown in Fig.1. The tool dimension and loads corresponding to Fig.1 model are showed in Table 1. Fig. 1 Geometry of fine blanking model Table 1 Tool dimension and loads of fine-blanking Parameter Value Outer diameter of blank holder (d b, mm) 140 Outer diameter of die (d do, mm) 140 Inner diameter of die (d di, mm) 70 Punch-die clearance (g, mm) 0.02 Fillet radius of punch (R p, mm) 0.05 Fillet radius of die (R d, mm) 0.35 Thickness of sheet metal (t, mm) 6 High of tooth (h, mm) 1.5 Distance of tooth (a, mm) 3.6, 4.6, Shape of tooth 45 R 0.15mm Workpiece material AISI 45 Adverse force ( F a, KN) 200 Blank holding force ( F b, KN) 500

4 606 Advances in Fracture and Failure Prevention In the simulation, workpiece material is AISI It is assumed that the material is isotropic and that the yielding behavior follows the Von Mises yield criterion. The following equation is used to model the material behavior. The flow stress, or instantaneous yield stress at which workprice material starts to flow, is influenced by temperature, effective strain, and effective strain-rate. The flow stress equation for material used in this paper is given with a constitutional model: = f (, &, T ) (3) where is the effective stress, is the effective strain, & is the effective strain rate, and T is temperature. The flow stress curves at 1 and 100 strain rate and at temperature 20 C The flow stresses at other strain state are obtained by linear interpolation. Other mechanical characteristics of the material AISI 1045 are listed as following: E=210000Mpa (elastic module) v=0.3 (poisson ratio) Contact between the workpiece and the dies using shear friction, which is used mostly for metal forming simulations. The frictional force in the constant shear model is defined by f s = mk (4) where f s is the frictional stress, k is the shear yield stress and m is the friction factor. This states that the friction is a function of the yield stress of the deforming body. 4. SIMULATION RESULTS AND DISCUSSION First of all, the case is simulated when the distance of V-ring and edge of die equals 3.6mm. Fig.2 shows the forming process that V-ring penetrates into the blank. The convex appeared when V-ring penetrated sheet. From the Fig.3, the hydrostatic stresses near the tooth were very high, which contribute to restrain or avoid crack initiation in the blanking. Fig. 2 Process of V-ring penetration into blank penetration depth 0.283mm penetration depth 1.431mm Fig.3 Hydrostatic stresses distribution near tooth

5 Key Engineering Materials Vols Fig.4 shows the effective stress (von Mises) distribution and their change in the blank during blanking process. From the results, we can see that the deformation area is more and more small and concentrated. For the V-ring and smaller clearance, the cracks and material fracture occur at the end of process. From deformed mesh shape of blank at stroke 5.814mm, we can find the forming profile is clean-cut and roll-over is small, which is the advantage of fine blanking. In reference [6], fracture occurs in common blanking can been simulated by FEM and fracture criteria. stroke=0.576mm stroke=2.376mm stroke=4.410mmm stroke=5.814mm Fig.4 Effective stress distribution in blank

6 608 Advances in Fracture and Failure Prevention Fig.5 Deformed mesh for blank at stroke 5.814mm Roll-over often occurs in blanking process, which is an indentation surface due to plastic deformation. In fine blanking, roll-over high is affected by thickness, geometry and properties of material, reverse press and V-ring factors. In this paper, the effect of distance a on roll-over high is simulated. The value of parameter a is defined 3.6mm, 4.6mm, 5.6mm and 6.6mm. In the Fig.6, the difference roll-over high corresponded to three value of a above are listed. From the results, the tendency is obvious, the more the value of a increase, the more roll-over h become great. Fig.6 Effect of tooth distance a on roll-over high h 5 CONCLUSIONS In this paper, under several factors setting, the fine blanking processes with V-ring are simulated. The following conclusions may be drawn: (1) After V-ring is penetrated material, crack initiation are restrain or avoid in the blanking due to the hydrostatic stress increasing. (2) the more the of value tooth distance increase, the more roll-over become great. 6 ACKNOWLEDGMENTS The authors would like to gratefully acknowledge the support of the Natural Science Foundation of China (No ). REFERENCE 1. S.B. Li, Stamping process technology, (Beijing, Mechanical Industry Press,1982, in Chinese) 2. A.M. Goijaerts and Y.W. Stegeman, J. Mater. Pro. Tech.,103 (2000) p Thomas Pytte, Ralf John and Michael Hoogen, Int. J. Mach. T. & Man., 40 (2000) p M. Oyane, T. Sato, K. Okimoto and S. Shima, J. Mech. Phys. Solids, 17 (1969) p F.A. McClintock, Trans. ASME J. Appl. Mech., 35 (1968) p Fang Gang and Zeng P. J.Mat.Pro.Tech.,vol.122(2002)p

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