November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

extrusion was on the same order as the residual pressure. The loading characteristics observed in the tooling for the backward forward extrusion stage demonstrates that tool life can be affected by excessive stress on the die (fatigue failure) as well as by die wear. The die region CD on Fig. 18 is bound to exhibit higher wear rate as the same level of pressure is exhibited during the extrusion stroke and the ejection stroke. The wear rate at this critical region (CD) of the tooling could be minimized if the residual pressure is reduced as discussed later in section 3.2.

The plastic strain distribution obtained using this die setup can be seen in Fig. 19a, which is similar to the strain obtained from the conventional tooling shown in Fig. 13. In contrast to the conventional tooling, the strain distribution of the new tooling remains the same after ejection as before ejection (Fig. 19b). This implies that in the new tooling setup, no secondary deformation took place during ejection. Fig. 20 shows the extrusion load and ejection load. As expected, the ejection load with this new tooling setup is insignificant. This implies that during ejection, the workpiece barely touched the die. This is also evident from the residual pressure plotted in Fig. 21.

Fig. 17 Load curves for forward-backward extrusion stage

Fig. 19. Same strain distribution (a) before ejection and (b) after ejection [25].

Fig. 18. Die-workpiece interface pressure profiles for backward-forward extrusion [25]. 3.2. Tapered die assembly for Manipulating the Die Elastic Strain Field As discussed earlier, the extrusion tooling setup can be designed such that elastic strain energy is stored during the extrusion stroke and that as soon as the extrusion load is removed, the stored elastic energy is released, thus repositioning the die such that the residual contact pressure is at its minimum or eliminated. The storage of elastic strain energy is facilitated by having the die contract during the extrusion stroke. Upon release of the extrusion load, the die springs back, in accordance with Hooke’s law ( σ = Eε ), where E is the Young’s modulus and ε is the elastic strain. The workpiece that was also under compression will spring back as well, but because the yield stress of the deforming material is much smaller than that of the tooling, the magnitude of spring-back ẟ for the workpiece will be smaller than that of the die. The best tooling design should be such that ẟ die - ẟ billet > 0 along the contact surface, implying that the residual contact pressure is zero and the forged part can be ejected by merely overcoming the weight of the part.

Fig. 20. Load curves for the extruding stage for the new tooling setup [25]

Fig. 21. Pressure profiles for the extrusion stage for the new tooling setup. [The maximum pressure is the contact pressure before extrusion load is removed]

FIA MAGAZINE | NOVEMBER 2020 40