November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

3. Manipulation of Elastic Strain Field in the Die In this section we present a case study to demonstrate how elastic strain energy stored in the die assembly can be used to reduce residual contact pressure between the die and forged part, thus enhancing tribological conditions. Forging of a constant velocity (CV) joint is used for demonstration. The forming sequence for manufacturing the CV joint consists of a forward extrusion stage, an upsetting stage, a backward-forward extrusion stage, and a sizing stage. Details on forming sequences can be found in [24]. The first three stages are usually performed warm or hot, whereas the last stage is usually performed cold. For this analysis, the geometry will be considered axisymmetric, implying that a preform without grooves is considered as the final geometry (Fig. 10).The simulations were carried out using the 2D DEFORM software package. An AISI 1045 was used for this part, under warm forming conditions. The flow stress of this material was obtained from the DEFORM software database. The interface between the stress ring and the die was prescribed by a shrink fit of 0.1mm. A shear friction factor of 0.25 was specified at the die-billet interfaces. The temperature for the billet and the die were set at 900 ° C and 300 ° C respectively. Only two forming stages will be presented: forward extrusion and forward-backward extrusion.

Fig. 11: FE model forward extrusion [Conventional die assembly][25]

Fig. 12: FE model forward extrusion [Taper die design assembly][25] 3.1 Conventional extrusion die assembly

Figure 13a shows effective strain at the end of the extrusion process in the conventional die setup. A maximum strain of 2.7 is exhibited in the material and the strain varied from 0.1 to 0.27 on the surface of the extruded part along its length. During ejection, a secondary deformation takes place: as clearly seen in Fig. 13b, the effective strain on the surface of the extruded stem is now about 2.7 along the length. Secondary deformation is not intended, but it occurs because the die opening springs back after removal of the extrusion load. This deformation is thus the consequence of residual pressure at the die-workpiece interface.

Fig. 10. CV Joint preforms and dimensions used in the FE model. Figure 11 shows FE model for the conventional forward extrusion die assembly and Fig. 12 shows the forward extrusion in a tapered die assembly. In the tapered die assembly, the die can slide along the tapered boundary surface. Fig. 12 shows that the die insert slid by 0.7mm during the extrusion stroke. The forging load and size of the die insert will dictate the level of elastic strain energy that can be stored in the die insert. The differences in the residual contact pressure at the die-billet interface between the conventional and the newdie setups are discussed to elucidate the benefits of manipulating elastic strain field.

FIA MAGAZINE | NOVEMBER 2020 38