Keywords: taper upsetting, hot upset forging, finite element method, forging dies.

In hot upset forging, the part is generally formed by a series of operations. Taper upsetting stages are commonly used as preforms. The geometry of the die cavity for the taper preforms can easily be determined from the dimensions of the hot part at the related stages; however the outer dimensions of the header die are determined by using some empirical formulae. In this paper, a commercial finite element code coupled with thermal analysis has been used to simulate hot upset forging process and to examine effects of the process on header die wall and base thickness values for the taper preform stages.

Most of the time, a single stage is not sufficient to obtain the desired geometry due to the process limitations. Taper preforms are used to reduce the height of the part gradually, without buckling injury. The operation sequence design, the geometry of the parts at the intermediate stages and the related die design are realized depending on the well-established design rules which were rationalized by Gökler [1] based on recommendations given in several publications [2,3,4].

In close die forging, the parting plane is located along the largest cross section of the part. Gripper and header dies are designed by considering the parting line location. An empirical formula for determining the outer diameter of the header die is suggested based on header die cavity dimensions in [4]. However, there is no suggestion found in the literature for the base thickness of the header die. Determining the proper die dimensions is vital in reducing the cost. Otherwise, excessive amount of expensive die material is used; or a weak die construction with the wall and,or base thicknesses of the header die less than necessary may cause failure. The wall or base thicknesses of the header die should be determined based on the stresses on these during the forging process. In this paper, the stress distribution analysis and failure prediction of header die during hot upset taper forging by using finite element method (FEM) is presented. Four node quadrilateral axisymmetric elements are preferred which are suitable for coupled thermal analysis. The material behaviour is assumed isotropic, the work hardening characteristic of the material is incorporated and the von Mises criterion is used. The gripper dies are assumed as rigid whereas workpiece and the header die are modelled as deformable bodies. Constant shear friction model is used for the contact surfaces.

As a case study, the first taper upsetting stage of a part which requires three stages to be formed, has been examined. The radial clearance between the header die and the guide hole limits the maximum deformation of the header die walls in the analysis. When the radial deformation of the header die is more than the radial clearance, the header and gripper dies jam. In other words, the outer wall of the header die must not touch to inner surface of the guide hole during the process.

Several analyses have been realized for the different base thickness and die wall thickness values [5]. The minimum outer diameter is determined by using the empirical formula given in [4].

The stress distributions in the header die have been examined for different critical sections. The equivalent stresses reach the maximum values on the die cavity surface at each particular section and decrease toward the outer surface of the die. The maximum values at the inner surface have been observed at the fillet in the header die cavity and at a region of the cavity surface close to the parting plane.

The analyses have been repeated for different outer diameter and the base thickness of header die values. The maximum von Mises stress values for the most critical points and success-failure status of the process are tabulated. It is observed that, the local stresses increase with increasing outer diameter and base thickness values of the header die. This is due to an increase in the rigidity of the header die.

In this study, analysing the effects of the hot upset forging process on the header die wall and base thicknesses for the taper preform stages and the fillet radius in the header die cavity has been sucessfully realised. In the analysis, the stress distribution within the header die and the risk of failure have been examined. The excessive deformation of the header die was found to be the main concern for determining the outer dimensions of the header die during the process. The allowable deformation limit for the header die in the radial direction determines the minimum outer diameter of the header die. It has been found that the empirical formula provides a conservative value for the outer diameter of the header die and it is possible to reduce the die wall thickness and consequently the cost of the die material after verification using the finite element method. It has also been seen that the variation of the base thickness of the header die does not have a considerable effect on the stresses created in the header die.

1

M.I. Gökler, "Computer Aided Sequence Design for Hot Upset Forging", Ph.D. Thesis, University of Birmingham, UK, 1983.

2

Metals Handbook, ASM, 1985.

3

A.N. Bruchanow, S.K. Rebelski, "Gesenkschmieden und Warmpressen", Verlag-Technik, Berlin, Germany, 1955, (In German).

4

M.V. Storozhev, "Technologicheskii Spravocknic po kovke i obemnoi Shampovke", Mashgiz, Moscow, 1959, (In Russian).

5

M. Ceran, "Finite Element Analysis of Hot Upset Forging Dies", Master Thesis, Middle East Technical University, November 2002.

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