Failure analysis of cold forging dies using FEA

The results presented within this paper represent part of a larger collabor ated investigation being conducted by Trinity College, Dublin and an indust rial partner, The particular cold forging process analysed was the method u sed to 'trim' a hexagonal shape on the head of a fastener. The fastener hea d geometry is achieved by forcing the die, known as a trim die because of i ts function, onto the workpiece, whereupon a combined forging and cutting a ction produces the desired well-known hexagonal shape for the head. The siz e of the trim die modelled was that for an M6 fastener. The trim die materi al was taken as M2 high-speed steel. When the trim die reaches the end of its stroke, a knockout pin shears off the excess trimmed material. A finite element analysis package called DEFOR M (R) was implemented to simulate the trim die forging process. DEFORM util ises Cockroft and Latham's fracture criteria to calculate the damage induce d within the workpiece material during the process. Elements are deleted fr om the model when they exceed a specified damage value. The trim die geomet ry, if incorrect, can cause premature shearing of the waste material during forging. This premature shearing has a detrimental effect on tool life and the forging machinery. The relationship between the trim die geometry and its final stopping distance, the consequent induced stresses and the energy required to shear off this excess material was investigated. Finally the e ffect of altering the damage value C was analysed, see Eq. (1). From this i nvestigation, conclusions as to the optimum trim die shape and final stoppi ng distance, which would facilitate increased die life, were obtained. (C) 2001 Elsevier Science B.V. All rights reserved.