A finite element analysis of cold-forging dies using two- and three-dimensional models

Abstract The cold-forging process analysed in this paper deals with the production of a hexagonal shape on the head of a bolt. The process utilises a die known as a “Standard Trim Die” which is forced at velocity onto the workpiece to form the bolt head. This process is a combination of cutting and forging. By definition, a cold-forging operation occurs at a temperature below the recrystallisation range of the metal being forged. Due to this fact very high forging loads are required, which in turn cause very high stresses within the die material. These high stresses can cause die failure due to overloading or fatigue. The main focus of this analysis was to predict the level of these stresses during the forging operation, and if possible to find the optimum operating conditions which would increase tool life. The analysis consisted of creating a range of different trim die geometry’s using AutoCAD/mechanical desktop and importing them into the finite-element analysis (FEA) package DEFORM. Due to geometrical considerations and computational limitations the initial tests consisted of two-dimensional (2D) models. Extensive analysis of these results enabled a more accurate simulation of the forging operation using three-dimensional (3D) models. Full elasto-plastic FEA, including contact and friction, was performed in order to produce the most realistic results possible for the stress distribution within both the tooling and workpiece. The FEA analysis indicated that the highest stress concentration occurred within the body of the tool and not along the contact surfaces as might first be expected. Results showing the variation in tool stress as a function of changes to the trim die cutting edge corner geometry are also presented. From these results it was possible to predict the corner fillet radii required to give the optimum geometry and hence produce the lowest stress concentration within the trim die.