The influence of flash design on material flow and tool pressure in closed-die forging: a practical example

Abstract

The closed-die hot-forging of components for the automobile industry involves heavy forces, high energies and severe wear conditions in the die. After a relatively short period of time in service — corresponding to the production of about 5000 components — the dies must be re-engraved. A conventional die is usually re-engraved twice and consequently the tool can only be used for the production of about 15 000 components. The cost of the tools amounts to about 10% of the total expenses for closed-die forged products. The material yield in closed die forging is low: the material losses are about 30%, the main part of which constitutes the flash. Considering the total expenses for a product, the cost of the material makes up about 50%. From the above, it is clear that the material yield and the tool life are two factors of utmost importance in considering the economy of a closed-die forging process. In the present work, plane-strain numerically-optimized upper-bound solutions for various flash designs are presented for the closed-die forging of a spindle. Metal flow just before and just after filling of the die cavity is analysed. The theoretical results are compared with experimental results obtained by the use of plasticine. It is concluded that from the material-yield point of view it is favourable to use a flash land with a V-notch. When the material flows through this kind of flash gap the notches are filled with material and intense shear takes place within the material itself. This kind of braking or restraining device is found to improve the material yield by restricting the material flow out into the flash gap. After die filling, however, the mean die-pressures are much heavier than those for dies with conventionally lubricated flat flash-lands. V-notched flash-lands are recommended provided that it is possible to stop the deformation process just before die filling. This statement pre-supposes, however, that the initial shape and weight of the workpiece can be accurately controlled and reproduced.