In a standard forging process the ram of the forming press moves downward vertically, the upper dies makes contact with the billet and forms it until the desired (pre-) form is achieved. But, the pin bores are located orthogonally to the press movement, so the forming direction has to be changed by 90° to a horizontal direction. Such a redirection of the press movement can be done by the use of multidirectional forging tools. The basic functionality is shown in Figure 3. Such tools have been developed for flashless and flash-reduced forging of steering links, con-rods, and crankshafts[Lan15, Beh10].

In a multidirectional forging operation the vertical movement of the press is redirected into a horizontal movement using wedge drives. Outer wedges are driven by the press ram. As the ram moves downward, the wedges make contact with sliders that move on a base plate. By such a system the change of the movement of the press is possible. Hence, it is basically possible to use this method to forge undercuts.

However, in such a process the upper and lower dies have to be kept closed while the sliders form the part. Therefore, the movements of the upper and lower die and the press have to be decoupled, which is one of the challenges of such a process.

Developing the forging sequence, tools

Forge NxT 1.0 simulation software is used develop the forging sequence. Based on CAD-data of the forged part, preforms and the initial billet dimension were derived first. Afterward, the sequence simulations of the preforms were performed and analyzed. Adjustments and optimizations were made iteratively, based on the results of the previous simulations to avoid folds or areas where a missing filling of the cavity occurred.

Typical forging parameters for a hot forging process were assumed in the simulations. The initial temperature of the billet was set to 1,200 °C, the temperature of the dies was set to 150 °C, and a water-graphite lubricant (µ=0.3) was used. The pistons are formed in 42CrMo4 (AISI 4137), so the corresponding flow-curves are used in the simulations. The initial dimensions of the cylindrical billet were 60 mm in diameter and 50 mm high.

The first forging operations are similar to a standard forging process but special attention was necessary in the development of the forging of the undercut. For example, determining the thickness of the piston walls was an important detail to ascertain (see Figure 4).

As shown in Figure 4 different wall thicknesses lead to different issues. On the left side the thickness of the wall initially was big, with a width of 20 mm. This results in too much material in the upper areas of the part, which leads to folds. A part with folds will have to be disposed as scrap.

On the other hand a thin wall thickness of a width of 5 mm leads also to folds. Additionally, there is a missing filling of the cavity present, which has to be avoided, too. The final solution was to chamfer the thickness of the wall from top to bottom in the final forging, with the thick part at the bottom. Such a chamfered wall is suitable for forging the undercuts without folds or a missing filling of the cavity.

Figure 5 shows a forging process sequence that consists of four stages. First, the billet is upset. Later, the outer diameter is used to center the billet in the die of the preform. In the preform the areas of the walls of the piston and the area of the combustion chamber bowl is preformed. In the final forging the part is finished except at the areas of the pin bores. Last, the multidirectional forging process is used to forge the undercut in the areas of the pin bores.

The final forging step is the one with the highest loads and a maximum press force of about 4,000 kN. About 500 kN are required for each slider to pre-forge the pin bores. Compared to the conventional process that is the current standard for industry, the material consumption in this process could be reduced by about 7%. The flash-ratio is about 6%.

Based on the forging sequence developed the forging tools are being manufactured now. As the multidirectional forging tool is the most complex one, it was developed first. Currently there are two possible variants under consideration to keep the dies closed. The first possibility would use disk springs that can be compressed, and the second would use a new closing mechanism based on a guide bar. After the construction of the tools they will be manufactured and tested on an industrial press.