European forgers produced 6 billion tons of forgings in 2012, which equals about 22.5% of the world’s forging production[Eur12]. In forging, material and energy costs are among the primary elements of total production costs. The material costs total up to 50% for parts made of steel[Rit09]. Besides the cost of forging the part, the subsequent machining process also is responsible for a major part of the final production costs. This is true as well for pistons used in combustion engines, which are high-performance parts with high requirements for mechanical and thermal properties. Especially in the current market, where performance and efficiency are set by the auto industry’s performance standards, there is a need for new and innovative approaches to meet these requirements.

Producing pistons

Currently, pistons for cars are usually are aluminum castings. But, in general parts that must withstand high mechanical loads like crankshafts or con-rods for internal-combustion engines usually are forged. Pistons made of aluminum also are forged to increase the mechanical properties of the part but they are more expensive.

In recent years some research has been conducted on forging and use of steel pistons[Emm11]. The main advantage of steels pistons is that they can be produced in smaller dimensions than pistons made of aluminum, while keeping the same or even improving the mechanical properties (see Figure 1). Therefore, steel pistons allow smaller engine blocks and increase the efficiency due to higher possible compressions and less friction. Today, steel pistons are used in trucks, but in the near future it is most likely they will spread to the commercial automotive industry.

After the forging process, the pistons are machined. Functional surfaces, a cooling channel, and holes are established. During machining, larger areas of the pistons, especially near the pin bores are cut by chip removal. The volume in this area is about 10% of the total volume of the steel piston. To achieve the pin bores a first clamping has to be done. To be able to perform this clamping operation, the parts have to be prepared by a high-precision machining operation that is both time-consuming and expensive.

A pre-forging of the areas of the pin bores allows significant material savings and simplifies the first clamping operation. However, these pin bores are located orthogonally to the direction of the forging and represent an undercut (see Figure 2). Such an undercut cannot be produced by a usual (i.e. unidirectional) forging process.

Objectives of the project

To address these challenges a research project was initiated at IPH – Institut für Integrierte Produktion Hannover, to determine the feasibility of a multidirectional forging operation to produce the pin bores as undercuts. In the course of the research project the multidirectional forging will be analyzed, first using modern Finite Element Analyzes (FEA) software, Forge NxT.

FEA software is widely used in the forging industry and provides an easy and reliable way to predict forging process results. IPH is an expert in the field of FEA simulations and offers its competence to forging industry, e.g. for optimization of forging processes.

After a suitable forging sequence – without folds or other defects – has been identified, different parameters that have an influence on the forging sequence will be investigated. Examples of these parameters are the temperature of the billet, the speed of the forging press, or the geometry of the punches used to forge the undercut.

Once an optimized parameter set has been found, the information gained will be used to construct and produce the corresponding forging tools. Then, these tools will be mounted on an industrial-scale eccentric press and tested in an industrial environment. The project plan calls for forging a trial batch of 500 parts.

Once the forging trials are finished, the dies as well as the parts will be analyzed. Die wear is an important factor and the dies will be analyzed to evaluate abrasive die wear as well as wear on the moving elements. The overall quality and especially the feasibility of the first clamping operation directly after the forging process will be assessed. Additionally, the grain flow and mechanical properties will be determined.

Also, a guideline will be written based on the experience gained in the forging trials. This guideline will enable forging companies to use the forging of undercuts for different forging parts, such as suspension arms or hinges that have similar bores located orthogonally to the forging direction.

Finally, the economic feasibility of the process will be determined and the new process evaluated by a comparison to the current forging process.