May 2021 Volume 3

FORGING RESEARCH

results indicated that using a blind hole or no hole at all could significantly reduce the punch’s tensile stress level and, therefore, decelerate the initiation and propagation of cracks. The negative shear stress in the z-x plane on the punch bottom surface was the action of the force on the surface. When the hole was blinded or removed, this negative shear stress became smaller. Since removing the hole in practice was impossible, the study results recommended that the blind hole be adopted but be as shallow as possible when enough holding force could be provided to prevent severe stress concentration in the bottom punch surface. The punches adopting this modification saw a 5x increase in their service lives. 6.4 Finite element modeling of tooling stress in a high-speed hot forging process Modern high-speed hot forging lines can produce precision forging components at a very high production rate. Continuous operation at elevated temperatures and repeated impact loads, however, can reduce tool life. This use case describes a thermal-mechanical coupled finite element analysis of the tooling temperature and stress fields in one forging station of a multi-station high-speed hot forging line to study how temperature and stresses affect the tools’ performance. In this modeling effort, a non-isothermal finite element model that allowed heat transfer among the workpiece and tools was built. The assembly of tools with a formed workpiece is shown in Figure 31. To study how the temperatures and stresses evolved in the tools, 10 forging cycles were simulated.

Figure 32. Evolution of temperature in tools (blue, gray and green areas) Compared to the upper die, the center punch retained a higher temperature (up to 900°C) because it contacted the workpiece much longer than the upper die did and was in a more enclosed space. The temperature fluctuation in the center punch was smaller, implying that the tool did not have enough time to dissipate the heat between the two consecutive cycles. High temperatures on the surface of a forging tool may reduce yield strength and lead to plastic deformation. A high temperature gradient from surface to core causes uneven expansion and contraction in the tool. Such temperature changes also create cyclic thermal stresses and strains on the tool surface, which helps create cracking via low-cycle fatigue (heat checking). Maximumprincipal stress is an important measure in determining tool fatigue life. In a forging cycle, the maximum principal stresses at points 1 and 2 of both tools (Figures 33c and 34c) were primarily compressive. The highest compressive stresses occurred at the end of the forging cycle when the material filled the cavities in the tools. It was unlikely that the tool life was affected by the mostly compressive stresses if the tool remained in the elastic range at the elevated temperature. Effective stress is usually a measure for evaluating whether the tool at these locations has undergone plastic deformation. In the upper die, the effective stress at point 1 (Figure 33d) varied significantly in the early cycles when the operation began. In the later cycles when the operation was stabilized, the effective stress gradually became higher during the forming, indicating that thermal stresses in this location played a bigger role. But unlike the upper die, the effective stresses in the center punch were the highest at the beginning of the operation and then gradually became smaller (Figure 34d). The initial high stresses may have occurred because the tool had the largest expansion at the beginning of the operation, when the on-tool temperature jumped more than several hundred degrees. Since the center punch was a solid tool, the highest temperature and the largest thermal expansion occurred when the material flew in and filled around the corner of the punch. A large increase in thermal expansion tends to make the ejection more difficult, a leading cause of high stress in the tool. Based on the simulated temperatures and stress profiles of these two tools, it was recommended that, in addition to a regular inspection of the tools for any sign of plastic deformation or heat checking, cooling to the corners of the tools should be considered and modifications of some tooling features should be made.

Figure 31. Tooling assembly with formed workpiece Figure 32 shows that the temperature increased as the number of forging cycles increased. Among all the tools, the upper die (Figure 33) and center punch (Figure 34) were examined most closely because they were subjected to the highest temperature and stress and the largest temperature and stress gradients. The simulation revealed that the temperatures in both the upper die and center punch increased along the operation to a fairly high level and fluctuated in each forging cycle (Figures 33b and 34b).

FIA MAGAZINE | MAY 2021 83