Click to enlarge Forging/piercing die assembly is designed to fit in a standard forging press.

In modern business, the efficient use of resources, the optimization of processes and managing costs are key competitive factors. Those in the metal-forming industry know that the pressures of competition are always increasing and that the implementation of these competitive factors are paramount to staying in business. Furthermore, these continuous competitive pressures force the metal-forming industry to integrate advance manufacturing processes into their production cycles to reduce the costs of production and optimize plant efficiency and product quality.
 

Click to enlarge   Figure 1. Cross-sectional schematic of flashless forming and punching tool

One way of improving forging operations that helps achieve these key competitive factors is to reduce operating time and material usage during the forging process. Flashless precision forging permits both. Since flash does not occur, the requisite amount of raw material is reduced. The near-net-shape product formed omits the need for extensive machining. Also, forged parts with cavities, holes or apertures are often warm punched and, in many cases, simultaneously deflashed in a second process step. This second step extends the processing and handling time, as well as the cost of production, since an additional tool is required for the punching process.

Through the use of a combined forming and punching process, parts can be formed without flash and punched in a single stroke. This results in a decrease in both handling time and production costs since an additional tool is not required. Using a combined precision forming and punching process, high grades of surface quality can be achieved in the through holes, thus superseding additional finishing steps. The combined forming and punching process is applicable to rotationally symmetric parts as well as parts with a distinct longitudinal axis.

Combined Shaping and Piercing Forging Processes

Click to enlarge Figure 2. Manufacturing stages

The tooling concept for the combined flashless forming and punching process was developed for rotationally symmetric parts at Germany’s Institut für Integrierte Produktion Hannover gGmbH (IPH) within the auspices of the German Research Foundation (GRF). GRF originally funded a project in 2005 that studied “combined shaping and punching processes,” which was followed by a study completed in 2009 that adapted the original concept for use on parts with a longitudinal axis.

The die assembly (measuring approximately 21 inches long x 27.5 inches deep x 28 inches high) is designed to fit into a standard forging press. For these trials, a Müller Weingarten PSH 265 screw press was used. The tool consists of an upper die, an upper punch (combined forming and punching die), gas-filled springs, a lower die, a lower punch and a release mechanism (Figure 1). The steel grade for use in the tool was specified as 1.2365 (32CrMoV12-28) and that for the parts was 1.7131 (16MnCr5).

During the closure of the upper and the lower dies, a first flashless metal-forming operation is performed. Next, the upper punch starts moving while the lower punch remains stationary. Gas-filled spring devices keep the tool assembly closed. As soon as the die cavity is filled, the inner die pressure increases rapidly. Once the default force for the lower punch is exceeded, a downward movement of the punch is activated by a release mechanism. The residual material, or the “web,” that remains at the cross section of the upper punch is pushed downward by the upper punch, which moves until the workpiece is completely pierced. The part is thus completed (Figure 2).

During the upstroke of the ram, the springs release tension but hold the upper die and the forged part in position as the upper punch is lifted and pulled out of the forging. Only when it has reached its initial position relative to the upper die will the dies open and permit removal of the workpiece. Concurrently, it must be ensured that the lower punch remains in position until the tool assembly is opened and the workpiece is removed. This prevents the web from being pushed back into the forged part.

The release mechanism ensures the complete filling of the die cavity. It releases the lower punch with a force that is determined by the growing inner pressure in the die after the entire filling of the die cavity. It consists of two cutters and a variable number of pins, which are sheared by the cutters.

Simulations

The design of this forging sequence was facilitated by material-flow simulation programs using finite element analysis (FEA) techniques. The objective of the simulations was to prove that the tool can resist the occurring and recurring stresses of the forging process and to show the degree of part deformation.

Two different models of the forming process were simulated. In the first model, the closing of the dies occurs without any deformation of the part. All of the part forming and the piercing is made by the upper punch. In the second model, the closure of the dies starts the flashless metal-forming operation. Subsequently, the forming and piercing process is completed by the upper punch.

The simulation for the first model predicted the maximum stress in the die assembly to be a maximum of 3,500 N/mm² (507,500 pound/inch²), which occurs at the head of the part. However, the yield strength of the hot working tool steel is about 790 N/mm² (114,550 pound/inch²). These results make it impossible to form the part in one forging step.

The second model included a flashless workpiece deformation during the closure of the dies. This operation decreases the occurring stresses significantly. The stresses drop to a maximum of 520 N/mm² (75,400 pound/inch²) while the upper punch moves downward and finishes the forming and piercing process. During the movement of the punch, the gas-filled springs keep the upper and the lower dies in position.

Wear of the forming and piercing punches is predominantly determined by the thermo-mechanical stress. This stress can be reduced by a modification of the punch geometry.

Test Evaluations

Click to enlarge Figure 3. Hardness values for forged parts

Initially, the process parameters, such as the workpiece and tool material and the forming temperature, remained constant. During the second phase of the project, only the geometries of the upper punch were modified.

The working temperature of the part was fixed at 1200°C. The workpiece material was defined as 1.7131 and the tool material as 1.2365 coated TiB₂. The forging temperature for the tool was set at 110°C. Temperatures were monitored by a pyrometer and a thermographic camera.

To identify the influence of punch geometry, four different cutting-edge geometries were tested. For each punch geometry, 15 parts were forged for statistical evaluation. A combination of different tests was then performed on the parts to prove the feasibility of the combination process:

• surface roughness
• dimensional stability
• hardness
• grain orientation
• microstructure

The hardness measurements on the forged parts were performed with the CV 6000MA hardness tester from CV Instruments. The Rockwell Operation was applied (DIN EN ISO 508-1, hardness scale B). The hardness measurement was executed at four positions on each forged part – three positions near the piercing and one position at the end of the part – to determine the possible influence of the combined process on the hardness of the forged parts. The raw parts have a hardness of 86 HRB. The average value of the forged parts is between 94-97 HRB (Figure 3).

Conclusion

The feasibility of a combined forming and punching process to produce near-net-shaped parts was successfully demonstrated within the scope of two research projects. Additionally, it was demonstrated that a forged and pierced part could be formed in one operation. The studies were conducted with only one punch. Due to the complex nature of existing part geometries (e.g., axle stubs), the method should be expanded to explore the use of multiple punches.

An opportunity for further research is the serial operation of the tool concept, such as a study of the tool’s industrial application spectrum for fast-moving horizontal presses. High-volume parts, such as pinions, produced on fast horizontal presses would be a good subject for further research. The tool concept would have to be modified with regard to its applicability for horizontal presses.

The authors would like to thank the German Research Foundation for its financial support of the project described in this article. Dipl.-Ing. Judith Kerkeling may be reached at tel.: +49 511 – 27976-375, kerkeling@iph-hannover.de; Dipl.-Wirt.-Ing. Karsten Müller at tel.: +49 511-27976-335, kmueller@iph-hannover.de; Dr.-Ing. Dipl.-Oec. Rouven Nickel at tel.: +49 511-27976-119, nickel@iph-hannover.de; and Prof. Dr.-Ing. B.-A. Behrens at tel.: +49 511-27976-300, behrens@iph-hannover.de