November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

2. Energy Flow in a Forging Press The energy flow in the forging press can be divided into three groups: The energy flow into the billet, the energy flow into the die set, and the energy flow into the press frame. These energies are converted into plastic strain and elastic strain energies. The plastic strain energy is used to alter the geometry of the billet as well as altering the properties of the deforming material. Upon completion of the forging stroke, the elastic strain stored as potential energy in the die set and press frame causes the frame and the die set to spring back. The potential energy stored in the die set and the press frame is usually left unused. This study explores innovative ways to harness this energy. Could this energy be harnessed to power an actuator? Could it be tapped via piezoelectric materials to generate electricity? Could it be used to enhance tribological conditions at the tool-workpiece interface? etc. Strategies for harnessing the elastic strain energy requires a clear understanding of how strain energy density is distributed in the die set and the press frame. To determine the potential energy stored in the press system we first examine the forging load needed to make a pinion gear via the finite element simulation and then subject the forging press under such loading to determine the potential energy stored in the press and later we explore ways to use this energy. Forging of the pinion gear shaft is used as an example to study the energy flow. Figure 1a shows the dimensions of the pinion gear while the forging sequence is given in Fig. 1b. This figure also shows the effective strain distributions for the extrusion stage, coning stage and the heading operation. DEFORM2Dwas used to simulate this forging process. The temperature for the initial billet made of AISI 1045 steel was prescribed at 900 oC whereas the die temperature was set at 300 oC. A shear friction factor of 0.25 was prescribed at the interface between the die and the workpiece. The coning operation was carried out prior to the heading operation to prevent buckling and maintain concentricity. The FE simulation resulted in a maximum forging load of 70 tons during the heading operations.

Die life can also be enhanced through thermochemical surface treatment. Die coatings for both cold and hot forging operations have been developed over the past 20 years. Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) [15 19] are the major methods for diffusing chemical elements into the lattice of the tool material. Prestressing of dies used in forging and extrusion is a common practice in the forging industry. Inducing compressive stresses on the die container which houses the die inserts ensures that the die inserts will exhibit minimal tensile stresses during forming. The conventional prestressing container normally consists of single or double stress rings [2]. Depending on the required tolerances, the induced compressive stress may be too low. To further increase the stiffness of the die assembly STRECON Technology has developed strip-wound radially prestressed containers. The stiffness of the prestressed containers using a tungsten carbine core can reach 500 GPa. Substantial increase in the stiffness of strip-wound prestressed containers compared to that of steel containers greatly influences the longevity of the die [20]. Prestressing the die using the stress ring may not be sufficient to reduce the formation of tensile stresses along critical regions of the die surface during forging. There are few ways that local tensile stresses could be reduced: Dies can be split at locations where higher tensile stresses are exhibited. Another way is to design the die assembly with variable shrink fit interference. I.e., the insert can be machined with tapered surfaces or profiled geometry such that upon assembly with the stress ring, a non-uniform compressive stress distribution is induced at the die insert-ring interface [21]. The above methods for enhancing the life of the forging tool have limitations. This paper discusses a different approach for improving tool-life: manipulating the elastic strain field of the die during the forging cycle. The forging cycle can be divided into two steps: The forward stroke, where forging takes place and the ejection stroke, where the forged part is disengaged from the tooling. Tool workpiece disengagement can exhibit substantial tool-workpiece “residual contact pressure,” with potential for galling and tool wear. This study explores innovative tooling designs that are capable of reducing or eliminating the residual contact pressure, via manipulation of the elastic strain field of the dies during the forming cycle. This paper is divided into four sections. Section 1 gives a brief introduction of the techniques commonly used to enhance tool life in the forging industry. Section 2 examines the energy flow in a typical forging press, and highlights possibilities of harnessing the potential energy stored in the press system during each forging cycle. Among other possibilities of utilizing this energy, this section provides a detailed account on how the stored potential energy could be used to enhance the forging operations. Section 3 presents a case study whereby manipulation of elastic strain energy is used to reduce billet-die contact pressure during part ejection. Reduction of billet-die interface can enhance tribological conditions by reducing die wear. Section 4 provides concluding remarks.

FIA MAGAZINE | NOVEMBER 2020 33