Analysis of stress pins for the local prestressing of cold forging tools

Abstract

The trend towards lightweight design leads to an increasing demand for sophisticated part geometries with high functional integration. In order to use the advantages of cold forging regarding the time- and resource-efficient production of high-quality parts, high local stresses causing fatigue failure in geometrically complex tools have to be controlled. The objective of this manuscript is to analyse the use of stress pins for a local influence on the stress state within forging, especially in non-circular symmetrical cold forging dies. For this purpose, a closed-die forging process for elliptical parts is designed and analysed regarding the die stresses. Distinct areas with local compressive and tensile stresses occur in the process. To counteract the tensile stresses critical for fatigue failure, the effect of stress pins pressed into the die creating a local prestress is analysed. Around the pins, compressive radial and tensile tangential stresses occur. While large pin diameters, interferences and close positioning to the tensile area lead to an increasing prestressing effect, too high values of these parameters cause a detrimental superposition of tensile process and pin stresses. If used correctly, there is high potential to improve the stress state and tool life especially for locally stressed complex tools.

A Systematic Review of Factors Affecting the Process Parameters and Various Measurement Techniques in Forging Processes

“…Thus, the correct application of stress pins can significantly contribute to longer tool life of forging dies. [ 144 ] The authors further devised another approach of prestressing by development of gaps in the outer walls of dies. The adjacent areas induced bending stresses, locally enhancing the prestressing at inner walls, leading to reduction in tensile stresses and increased die life.…”