Keywords: cold upset forging, cold forging, cold heading, tapered preform, finite element analysis.

In this paper, effects of the tapered preform shapes on the final product in cold upset forging are presented. Different preform geometries with different upset ratios are considered for the same final part geometry. The overall upset ratio, , where is the initial billet diameter, and is the unsupported billet length. The elastic-plastic finite element method is used to examine die filling and determine the flash volume in the final stage for different preform geometries.

Upset forging is usually carried out with sequence of stages where there exist some process and design limitations [1,2,3,4]. Preform should provide a well-suited shape for the succeeding operation. Especially in preforming stages, no flash is desired. To reduce the cost, it is also necessary to have minimum quantity of flash in the final stage. In general practice, considerable number of trial and error approaches are used in industry and the tapered upsetting method in preforming is widely used.

The rules governing upset forging aim to achieve the desired geometry with a minimum number of preforming stages by ensuring maximum enlargement in diameter and reduction in height of the stock in each preforming stage without any failure. It has been shown that the numerical simulations depending on Gökler's suggestions [1] result with better quality of preforms, eliminating the buckling injuries and the suggestions for hot upsetting related to upset ratio and preform geometry can be used for cold upsetting [5].

An elastic-plastic finite element analysis has been applied in this study. The material is assumed to be isotropic and work hardening characteristics of the material are incorporated. The workpiece material has been selected as C45 steel with Young's modulus of 210 GPa and the Poisson's ratio of 0.3. The gripping dies are modeled as rigid bodies and remain in place during the forming steps of the analysis. The heading dies are also modeled as rigid bodies and move towards the gripping dies during the analysis. Four noded axi-symmetric elements have been used, since the geometry of the part analyzed is axi-symmetric.

In the finite element analysis of deformation of tapered performs, the commercially available software package program, MARC/AutoForge has been used. Large amounts of non-uniform deformation usually produce finite element meshes so distorted that the results become unreliable, hence remeshing has been used in the analysis, which is available in the particular software.

In order to see the effects of the preform shapes on the final product, several upset forging processes with different preforming cases are examined [6]. In the first case, where Gokler's design suggestions are used, well-filled die cavities are observed at both of the preforming stages and the final stage and hence the part without any flash is obtained. In the second case, flash formation along the parting plane and unfilled sections of die cavities at the corners of the final geometry have been observed. In each subsequent case, where amount of deformation left to the final stage increases, it is noticed that the flash amount increases gradually while unfilled volume increases. In order to make comparison between the cases, where reduction ratios were decreased gradually, volumes of the flash formations, which is equal to volume of the unfilled die cavities, are calculated and compared.

It has been observed that, well-filled final products are obtained when the preform shapes and die cavities are designed according to the design rules, which is the first case. On the other hand, defected products with the unfilled sections have been encountered for the other cases, where intermediate stages are redesigned by taking different reduction ratios and therefore higher amount of deformation left to the final stage.

It can be concluded that the rules for taper preform geometries, which offer maximum enlargement in diameter and maximum reduction in height of the billet in intermediate stages and a minimum amount of forming in final stage [1,2], have been verified.

1

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

2

M.I. Gökler, T.A. Dean, W.A. Knight, "Computer Aided Sequence Design for Hot Upset Forging", Proceedings of the 22nd International Machine Design and Research Conference, Manchester, England, 457-466, September 1981.

3

W. Naujoks, D.C. Fabel, Forging Handbook, 4th edition, The American Society for Metals, Cleveland, Ohio, USA, 1948.

4

Etchells Product Charts (Trade literature), Etchells Machinery Limited, Stafford Road, Darlston, Wednesbury, West Midlands, England.

5

M.I. Gökler, H. Darendeliler, N. Elmaskaya, "Analysis of Tapered Preforms in Cold Upsetting", International Journal of Machine Tools & Manufacture, 39, 1-16, 1999. doi:10.1016/S0890-6955(98)00021-2

6

Ö. Dogan, "Finite Element Analysis of Effects of Tapered Preforms on Final Product in Cold Upsetting", M. Sc. Thesis, Middle East Technical University, Ankara, Turkey, September 2000.

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