November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

Fig. 15. Tool-workpiece interface pressure profiles for the forward extrusion stage [The maximum pressure is the contact pressure before extrusion load is removed] [25] The FE model for the forward-backward extrusion stage is shown in Fig 16a. Prior to this forming operation, an upsetting stage was carried out to widen the head. This stage is not discussed here. The boundary conditions set for the simulations are the same as for the extrusion stage discussed above. Fig. 16b shows that the workpiece experienced considerable deformation, reaching a maximum effective strain of 4.77.

Fig.13 Effective strain (a) before ejection, and (b) after ejection. Figure 14 shows the forming and ejection load. The sharp reduction in the cross-section causes the load to increase rapidly to its maximum value, close to 240 tons. After reaching its maximum, the load stabilizes and slightly decreases with stroke because the die-workpiece contact surface is also decreasing. At this moment, maximum pressure is being exerted on the die, as shown in Fig. 15. When the extrusion load is removed the die springs back and thus compresses the workpiece. The pressure the die exerts on the workpiece when no load is being applied is referred to here as residual pressure. This pressure which is also given in Fig 15 is mainly what causes the ejection stroke to require load. In this case study, the residual pressure is about 30% of the maximum pressure induced during the forging stroke. As the ejection progresses, the contact surface area decreases. This can increase contact pressure in certain locations, as dictated by the overall elastic strain energy stored in the die.This behavior can also be reflected in the ejection load profile. As the ejection progresses, the land of the die (B-C, Fig 15) will exhibit significantly larger residual pressure up to the end of the ejection stroke. This region is thus bound to endure significant die wear.

Fig.16. Forward-backward extrusion stage: (a) FE model before and after extrusion, (b) strain distribution [25]. As seen in Fig. 17, this operation required reasonably low forming load at start, to allow the hot material to deform. When the material touched the die and the backward extrusion started, the load increased steadily up to the point where the cavity was about to be completely filled, and then the load spiked. The peak load to fill the die cavity was 700 tons. The maximum load and maximum pressure at the die-workpiece interface necessary to complete the operation were substantially higher than in the previous cases. The high loads and pressures resulted in a substantial deflection of the die, which upon spring-back led to large ejection load. As seen in Fig. 17, the force required to remove the workpiece starts near 40 tons and decays slowly. Figure 18 shows the maximum pressure profile and residual pressure at the tool-workpiece interface. The beveled region (BC) did not exhibit any residual pressure. However, region DC exhibited a large residual pressure of the order of 500 MPa (72.5Ksi). At this region, the maximum pressure induced during the forward-backward

Fig. 14. Load curves for the forward extrusion stage of the CV joint

FIA MAGAZINE | NOVEMBER 2020 39