November 2020 Volume 2

FORGING RESEARCH AND TECHNOLOGY

2.2Potential energy stored in the forging die assembly during each forging cycle Elastic strain is also stored as potential energy in the die set. We first examine the mechanics of a conventional die assembly shown in Fig. 5 and later examine the mechanics of a new die architecture that can concentrate elastic strain energy density in specific parts of the die assembly that is shown in Fig. 6. The conventional forging die (Fig. 5) has a compound die arrangement, with one stress ring and a die press-fitted together. A forging cycle will be regarded as consisting of a “forging stroke” and the ejection stroke.” The tool-contact pressure evolution along the boundary of the deforming body is a major factor in wearing the die and punch. Due to the elastic deformation characteristics of the tool and the billet, it can be shown that the contact pressure is exhibited during both the forging stroke and the ejection stroke. During the forging stroke, the diameter of the die will expand to D+∆d. As soon as the forging load is removed, the expanded die will spring back to D, causing a secondary deformation to the forged part. This is the reason for the occurrence of residual die-workpiece contact pressure. The tool contact pressure also has a cascading effect in intensifying thermal and tribological loads. For example, in hot forming the heat transfer from the billet to the die can result in tempering of the die surface. This may shorten the die life as adhesive wear rate is depended on the softening of the outer surface of the dies [22, 23]. The architecture of the new die set (Fig. 6) which is bound to concentrate potential energy in certain regions of the die assembly is conceived such that during the forging stroke, the die will shrink as the tapered die slides along the surface of the tapered stress ring. The force to push the die downward can be expressed as f (µ, Ps, P, P*) , where P and P* are contact pressures acting on the vertical die wall and tapered section, respectively (Fig. 6). The architecture of the tooling is such that upon release of the forging, the compressive elastic strain field in the die will relax, causing the die to expand (spring back) to where it retains a gap on the order of few microns between the die and the workpiece. If the gap is created along the whole tool-workpiece interface, residual contact pressure will be zero. Else, negligible contact pressure will be exhibited at the interface. Through optimization of the strain energy density distribution, the residual contact pressure during the ejection stroke could be eliminated.

Fig.3. Equivalent Stress in Press Frame

Fig.4. Strain Energy Density in Press Frame

FIA MAGAZINE | NOVEMBER 2020 35