November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

3.3 Comparison between conventional and tapered die design assembly The residual pressure profiles for the conventional and new designs at the start of the ejection process are shown in Fig. 22. The conventional die exhibits residual pressure average of about 300 MPa (43.5KSI) along region ABC. In the new design, the pressure is almost zero everywhere except a small region. Without good lubrication, an interface pressure of 300 MPa can increase friction drastically, leading to die wear. Therefore, the new tooling setup has the potential to increase tool life substantially. Due to reduction in the residual stress, the ejection load decreased significantly as seen in Fig. 23. Integrating area under the load curves can quantify the total energy savings. This case study shows an energy saving of 15% from the new die setup. Reducing residual pressure also reduces heat transfer from the workpiece to the die. In hot forging/extrusion, die-workpiece contact time is minimized to prevent die thermal softening, which can reduce die life. Thus, by reducing the residual contact pressure, the amount of heat that can be transferred to the die will be reduced. In turn, the potential for die softening can substantially be reduced.

the sleeve and stress ring to form a single die assembly. Having a steady die assembly will minimize press set-up issues and any die assembly process control. Of critical importance, however, is to ensure that the interface between the die insert and the sleeve is well lubricated to allow relative movement of the die insert. Since back and forth movements of the die insert is key to this method, the die taper angle should be chosen appropriately to ensure that there is no locking. 4. Concluding remarks This paper has examined the energy flow in a typical forging press and explored the possibilities of harnessing the potential energy stored in the press system during the forging cycle. By concentrating the potential energy in certain part of the tooling it is possible to conceive a forging die design architecture that minimizes the residual contact pressure at the die-workpiece interface during the ejection stroke. A segmented forging die set up with a tapered die that slide along a tapered stress ring was conceived. To determine the viability of the proposed tooling design, finite element forging/ extrusion simulations for a constant-velocity joint and a pinion gear blank were carried out. The major conclusions drawn from this study are as follows: • The die elastic strain energy can be manipulated by segmenting a die into a tapered insert and a sleeve. During the extrusion stroke, the tapered die insert is allowed to move downward, whereas the sleeve is constrained. Since it is moving on a tapered wall, compressive strains are induced on the die insert, thereby storing elastic strain energy. During ejection, the die insert sprang back, allowing the forged part to be released with minimal load. For the simulation cases studied, a die taper angle of 7 ° was sufficient to reduce the contact pressure to insignificant levels. • The finite element simulations showed that the residual pressure of 400 MPa – 500 MPa exhibited in conventional dies can reduced substantially by using the new die setup. Without good lubrication, this level of interface pressure can dramatically increase friction, leading to die wear. Therefore, the new tooling setup has the potential to increase tool life. • Reduction of residual contact pressure results in reduction of heat transfer from the workpiece to the die. This condition is critical in hot forging, where die-workpiece contact time needs to be minimized to prevent die thermal softening, which is bound to reduce the hardness of the dies and possibly decrease die life. Acknowledgment The authors would like to acknowledge Forging Industry Education Research Foundation (FIERF) for supporting this work.

Fig. 22. Comparison in residual pressure between conventional and new tooling setup for forward extrusion [25].

Fig. 23. Comparison in ejection load between conventional and new tooling setup for forward extrusion [25]. Although the proposed method results in a substantial reduction in the residual pressure and reduction in the billet ejection load, there are few limitations: It requires precisionmachining of the tapered die insert and sleeve. This method will not be suitable for long extrusion products due to the difficulty in concentrating enough elastic strain energy density in the die inert. Also the implementation of this tooling design in a forging press requires a few design aspects to be addressed: The die inserts should be firmly assembled together with

FIA MAGAZINE | NOVEMBER 2020 41