PSI - Issue 38

Martin Killmann et al. / Procedia Structural Integrity 38 (2022) 212–219 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

218

7

At an interference of 3‰ the maximum contact pressure is 790 MPa for γ = 12.5° and 2920 MPa for γ = 2.5°. The contact pressure significantly rises for high interference fits. At 9‰ and γ = 2.5° the maximum pressure amounts to 9470 MPa. In order to prevent plastic deformation of the die, lower values should be achieved. The compressive strength of the used die material ASP2023 is about 3000 MPa. Therefore, an angle of γ = 10° should be used when working with high interferences of 9‰ . 3.3 Number of critical areas In the previous section it was identified, that the decrease of the tensile stresses while using gaps is more pronounced for the elliptical part geometry. The main difference between the geometries lies in the number of critical areas. Therefore, Fig. 7 shows the influence of the number of functional elements of the maximum tensile stresses at process end with and without gaps. At the circumferential position of each functional element, a gap is placed on the outer die wall. Simulations with different gap angles α were conducted for all number of elements and the best results used for the graph. Firstly it can be observed, that the die stresses without gaps decrease with an increasing number of functional elements. This can be explained by an easier material flow, since the material has more areas where it can flow freely and less areas with direct contact to the tool, where outward material flow is impossible. Regarding the die stresses with gaps, the decrease in the critical tensile stresses compared to the setup without gaps is more pronounced for a lower number of functional elements. While the stresses with gaps are about 300 MPa lower than without them for three functional elements, the decrease is lower with about 100 MPa for four functional elements. For five to eight elements the decrease lies under 50 MPa, while at nine and ten functional elements the stresses with gaps are even higher than without them. To enable an effective use of gaps, an adequate distance in circumferential direction must be ensured between the area of constant interference and the omitted area, so that the lever arm is sufficient to induce a bending stress.

2500

α

n α( ) 3 30 4 25 5 20 6 10 7 10

With gaps Without gaps

MPa

500 Tangential stress σ t 1000 1500

8 5 9 5 10 5

0

3 4 5 6 7 8 9 10

Number of functional elements n

Fig. 7: Influence of the number of functional elements on the gap effect

4. Conclusion Cold forging dies are subjected to high cyclic loads. In combination with an increase in geometrical complexity, this makes fatigue failure of tools a major challenge in the economical use of cold forging. In the tools, local elements create tensile stress peaks. To counteract this, an approach for the local prestressing of forging tools was presented. By inserting gaps in the outer wall of critical die areas, the reinforcement pressure is locally taken away. A bending stress induced by the adjacent areas of full interference locally increases the compressive prestress at the inner die wall. This leads to lower tensile stresses at process end and prospectively to a longer tool life. The investigations