Effect of die surface topography on die-work interfacial friction in open die forging

Abstract

Friction between the tool-workpiece interface is a complex phenomenon. It affects the quality and the cost of the component produced and leads to die wear. The interface friction can be improved by introducing a lubricant. The lubricant is entrapped in surface troughs and is fed to the asperity tips in the course of deformation. High working pressures and severe deformation cause breakdown of lubrication. It is seen that die surfaces, with moderate finish, prove better than highly polished die surfaces. In the present work, the effect of directionality in grinding the dies has been studied. The die material was steel of grade H11. Two sets of dies had been prepared, using the same grinding wheel. One set, consisting of two dies, had been subjected to unidirectional grinding. Another, similar set of dies, had been provided with a criss-cross surface by grinding the surface at 45° to the first direction of grinding. The surface finish,( R a), as measured by the Talysurf instrument, was 0.11–0.12 microns for the criss-cross surface and 0.14–0.15 microns in the case of the unidirectional surface. For the same percentage deformation while using the respective sets of dies, it was found that the dies with the criss-cross ground surface, when compared to the set of dies having the unidirectional ground surface, required lesser forging loads. The value of m, the friction factor, was also lesser during the forging process, when the set of dies with the criss-cross surface pattern were used.