PSI - Issue 38

Martin Killmann et al. / Procedia Structural Integrity 38 (2022) 212–219 Killmann / Structural Integrity Procedia00 (2021) 000 – 000

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good mechanical properties, which can be tailored depending on their applications (Tekkaya et al. 2015). Within metal forming, forging operations are used for the production of stout components like gears (Jafarzadeh et al. 2012), fasteners (Hsia and Shih 2015) or drive shafts (Ku 2018). In comparison to hot forging, cold forging processes enable a higher geometrical accuracy and better surface quality of the produced parts (Hirschvogel and Dommelen 1992). Since the flow stress of the formed parts is not reduced by heating, high forces and tool loads are a challenge, especially for the forming of complex part geometries (Engel et al. 2011). Small radii and local elements impede the material flow and lead to higher forming forces (Jafarzadeh et al. 2012). In combination with non-rotationally symmetrical part geometries, this leads to local tensile stress peaks (Körner and Pingel 2015). As a result of this challenging stress state, almost all complex cold forging tools fail due to fatigue (Tekkaya and Sonsöz 1996). This type of failure is especially critical, because it leads not only to costs for replacing the tool, but also to secondary costs following machine downtime (International Cold Forging Group 2002). Conventionally, cold forging dies are prestressed by pressing them into a reinforcement ring to counteract fatigue failure (International Cold Forging Group 1992). In order to counteract local stress peaks, a locally increased prestressing would be beneficial (Killmann and Merklein 2020). To generate an axial prestress, reinforcement rings with an adapted interference closure have been suggested (Groenbaek 1996). With this concept tool life could be increased from 30,000 to 200,000 produced parts in a forward extrusion process (Groenbaek and Nielsen 1997). The use of adapted interferences to create a locally increased tangential prestress has not been researched. Therefore, this paper aims to analyse the local prestressing effect of vertical gaps, which locally remove the interference between die and reinforcement. For this purpose, a model process with a local stress state is analysed for two different part geometries. Subsequently, parameters of the gaps are discussed for different interference fits between die and reinforcement. Finally, the effect of gaps is analysed depending on the number of critical elements in the tool. 2. Analysis of the model process To analyse high local tool loads, the closed-die forging of non-circular symmetrical parts is chosen as a model process. The process setup and the two analysed geometries are shown in Fig. 1. A round billet made of 16MnCrS5 is placed into the die and formed by vertical movement of the punch. Punch, counterpunch and the inner contour of the die have the same cross-section as the part that is to be formed. The tools materials are powder-metallurgical steels, specifically ASP2023 for the die and ASP2030 for punch and counterpunch. The die is prestressed by a reinforcement ring made of 1.2344 with an interference fit of 3‰. The produced geometries are an elliptical part and a cyclically symmetrical part with four functional elements. Both have a constant cross-section in order to not induce axial stresses. The analysis of the stress state is carried out with the software Simufact.Forming 15 (Hexagon 2020) using a numerical model, which has been introduced and validated in previous research (Killmann and Merklein 2020).

Elliptical

Functional Elements

20

20

Die Reinforcement Punch Billet Counterpunch

24

19

30 mm

16

All units in mm

16

Fig. 1: Model process