Flash-reduced forging is a promising alternative for the forging of complex heavy-duty parts. The use of flashless preforming operations is one possible approach in achieving this. Avoiding flash in preforming by keeping dies completely closed during the forging operation is the main challenge in flashless forging, especially when the parting line of the die is located at the center of the part. In order to do this, an advanced closing mechanism that completely locks the dies mechanically without the use of any spring mechanism was developed.
According to Euroforge’s 2013 worldwide survey, nearly 19 million tons of closed-die forgings were produced by over 2,900 forging plants globally. In order to remain competitive in this endeavor, it is necessary to reduce production costs, the main part of which includes material and energy costs.
Material costs represent up to 50% of total production costs. Therefore, a reduction of material reduces production costs in two ways: by decreasing the material costs and by decreasing energy demand for the whole forging process (i.e., for heating and forming). The flash ratio for complicated geometries like crankshafts can reach 50% or greater. Consequently, there is a huge potential for cost reduction by material savings.
Flashless precision forging technologies for complex, long pieces were developed for the project “Process Chain for the Production of Precision Forged High Performance Components” in our collaborative research center. Based on simulations and trials of the newly developed forging sequence, this process was adapted to the industrial production of a two-cylinder crankshaft.
With the new flashless preform process (Figure 1), the ability to reduce the flash ratio of complex final parts can result in a competitive advantage and a cost reduction. Enterprises from Germany, Romania, Spain and Turkey worked together on this project, which was supported by funding under the Seventh Framework Programme of the European Union to secure the achievement of the project goals.
Because of the high cost of forging dies and their high rate of wear, a process for a reduced-flash forged crankshaft is needed to boost cost efficiency. Therefore, our proposed process for forging a two-cylinder crankshaft enables a forging company to perform flashless preforming operations and a flash-reduced conventional final forging instead of forging that is flashless at every step. The flash in the existing forging sequence amounts to 54% of the forging’s weight and can be reduced to less than 10% by using flashless preforming operations. To achieve this, flash generation during preforming operations had to be eliminated. Using an advanced die-closing mechanism instead of conventional springs is one promising way to do this. The mechanism developed is presented in this article.
Forging of Complicated High-Duty Parts
Forging is an important manufacturing process. For the hot forging of steel, the billet is heated above its recrystallization temperature (850-1250°C). After its initial heating, the billet is forged in either one step or in a multistep process to its final geometry. The number of necessary steps for reaching the final geometry depends on the geometry’s complexity.
In recent years, several developments in forming technology have greatly changed the hot forging of steel. These developments include improvements in tooling and press technologies. For example, the design of the flash gap has a big influence on the material flow within the die.
A new tool concept that alters the material flow during the forming process by using a movable flash break has been developed. Improved process developments and designs increase the profitability and quality of forging processes. In the past, flashless precision forging processes have been industrially established for rotationally symmetric parts such as gearwheels and simple, long flat parts such as connecting rods.
For this project, we developed a flashless forging sequence in the laboratory for a simple two-cylinder crankshaft.
Closed-Die Flashless Forging for Preforms
In flashless forging, the workpiece is completely enclosed by the die and no flash is generated. Die design and process variables must be carefully controlled for this to occur. The comparative advantages of flashless forging relative to forging with flash (a partially enclosed workpiece) are material savings and the elimination of post-process trimming (clipping) operations. The main disadvantage of this process is its inherent inflexibility on billet volume fluctuations, which can either lead to an overload of the dies and the press or, conversely, incomplete die filling. Another is that the billet must be positioned exactly in the die to avoid loading out-of-position, which would generate shape defects and require increased forming forces.
Conventional Tool Concepts for Flashless Forging
The tools for flashless closed-die forging generally consist of a lower and an upper die and one or more punches (Figure 2). During the process, the workpiece lies completely enclosed in the gravure, and the punches move into the gravure to impress the hot material. After the complete filling of the gravure, the punches drive back and the dies open.
This concept is implemented by a lower monoblock die, which is like a conventional forging die but without a flash gap. The upper die assembly consists of two die segments –
the upper die matrix and the punch – and a die-closing mechanism with springs (Figure 3).
Due to the large required closing forces, disk springs or gas springs are generally used. The type of spring used depends on the available space between upper plate and upper die and the required stroke of the punch. The upper-die parts are assembled on an upper plate that is directly connected to the ram of the press. As the ram moves downward, the upper-die assembly comes into contact with the lower die and closes the die cavity without executing any forming operation. Ongoing ram movement pushes the punch into the upper-die cavity and performs the forging operation. To keep the dies closed during the relative movement of punch and upper-die matrix, springs are assembled between the upper plate and the upper-die matrix. To keep the disk springs in position, separate guides in the center of the disk springs were used.
The calculation of the spring forces required to close the dies during forming are determined by forging simulations using finite element analysis (FEA). The forces in the horizontal plane (x- and y-direction) of the upper die, which result from the increasing inner pressure of the die during forming, are taken into account in calculating the required spring force.
Requirements of Die-Closing Mechanisms
Flash-reduced closed-die forging faces more challenges than conventional forging processes. A flashless preform, for example, cannot use the flash to compensate for process tolerances, such as the billet mass or the positioning of the billet in the die cavity. Changing these parameters within the process changes the required spring forces. Consequently, the design of the closing dies and punches is more complex than the design of conventional forging tools.
There is a persistent danger of forming a thin flash in the gaps between upper and lower die in the closed-die forging process. During the forging process, the closing force induced in the dies has to be greater than the inner pressure. As the dies heat by preheating and heat transfer from hot billets, however, the temperature of the springs also increases, which decreases their expected service life.
The application of gas springs also has limitations due to the restricted operating temperatures. Being independent concerning the variation of spring force due to thermal influences is the main aim of the alternative die-closing concept. So, the alternative concept has to accommodate the relative movement of punch and upper die without using the springs.
Clamping the dies with a clip in the closed position is a feasible option. The disadvantage of clamped dies is the risk of damaging the tool if the clamping does not release the dies when the press opens the tool. The use of a restraining guide on the clip will avoid die-damaging risks. Further, clamps must provide a complete and precise closing of the dies. An imprecise machining of the clip will cause a small gap between the upper and lower dies, which might cause a thin flash.
Advanced Mechanical Die-Closing Mechanisms
Because the requirements for closing dies in lateral extrusion processes and flashless closed-die forging are similar, these concepts were taken into account in developing an advanced closing mechanism. These concepts could be divided into two groups: mechanical closings driven by the main working press and closing mechanisms working with additional gears.
To achieve a cheap and tough closing mechanism, a simple design without any additional gear or supporting spring is required. Most of the concepts work with supporting springs or additional supporting hydraulic power units, so we chose the simplest closing mechanism with the fewest moving parts.
The basic concept is a clamping clip laterally assembled to the dies. To avoid the problems of damaging the die while opening the tool, the clip is designed as a lever (Figure 4). This lever and its center of rotation are positioned in the upper die assembly. The rotational movement is provided by a control rod mounted on the upper plate.
With the downward movement of the press ram, the upper die assembly first touches the lower die. With further ram movement, the control rod rotates the closing lever to the closed position. To provide further movement of the punch, which is connected to the upper plate, the control rod must move further downward without rotating the lever.
This constraint necessitated a sliding-rail design in which the control rod moves laterally downward and keeps the lever in a closed position. The slide rail is a restraining guide of the closing lever without springs. The hook of the lever for closing the upper and lower dies is designed with a beveled geometry, which helps ensure precise closing of lower and upper dies.
Figure 4 shows the closing mechanism design concept for flashless closed-die forging. The bolts for rotating the lever and the movement of the control rod in the sliding rail are the weak spots, so the strength calculations for these parts have been done very carefully. Accordingly, it was found that a bolt diameter of 20 mm is necessary to provide a sufficient closing force of 60 tons. Depending on the required closing force, the diameter of the bolts can be increased or more bolts can be added. Due to the compact design, there is no interference between the closing mechanism and the handling of the billet in the opened die.
Conclusion and Outlook
For this project, a crankshaft was chosen as the sample part. Since the geometry of crankshafts is complex, achieving a finished forging with much-reduced flash was a very challenging task. It was decided that by increasing the quality of the preform product, the flash of the final forging could be substantially reduced. Therefore, a new forging sequence and mechanism for flashless preforms was designed.
The overall aim of the project is to have a newly developed process chain for forging a two-cylinder crankshaft using two flashless preforming steps followed by a flashless multidirectional forming step and the flash-reduced final forming.
Achieving an optimal mass distribution in the preforming operation, which is required for forging the final part with reduced flash, had to focus on avoiding flash in the preforming operations. To achieve this, a tough closing mechanism was designed, built and assembled. Forging trials were performed with this new closing mechanism. These trials resulted in a reduction in final-product flash from 54% by weight to less than 10%, which resulted in a material savings of 3.4 kg per crankshaft.
The Institut für Integrierte Produktion Hannover (IPH) is a non-profit organization providing research and development, consulting and training in industrial engineering. For additional information, visit www.iph-hannover.de. Omtas Otomotiv Transmisyon is a Turkish supplier of forgings for light and heavy commercial vehicles, personal cars, industrial engines, agricultural machinery, tractors and other equipment. For additional information on this project visit www.reforch.eu. Co-author Dr.-Ing. M. Stonis is the contact for this article. He may be reached at firstname.lastname@example.org.