Prof. Dr.-Ing. Bernd-Arno Behrens

Managing partner
+49 (0)511 279 76-119


By increasing the height of wind turbine generators the usable wind energy increases exponentially. However, tower weight increases accordingly. Current tower designs limit the tower height due to the explained correlation. The application of lightweight design concepts can lead to a significant weight reduction at comparable tower stiffness. As part of a research project, lightweight design concepts have been developed. These design concepts allow a mass reduction for onshore wind turbine towers up to 20 %. In this paper, the development and results regarding these lightweight concepts are presented.

xxl-product, large-scale, xxl, wind turbine, wind power, lightweight construction, tower constructio

Hydoroforming of titanium tubes was investigated by the Institut für Integrierte Produktion Hannover (IPH) and the Institute of Metal Forming Siegen. The basic research for designing a multi-stage process chain of hollow profiles made of titanium grade 2 was divided into three sub-items, the material model, the annealing treatment and friction value determination. The results were brought together in a simulation model, which enabled developing a multistage process chain.

hydroforming, FEA, titanium

Different billet dimensions or progressive die wear increase the geometric fluctuations of forged parts. This can lead to waste. Mostly this geometric fluctuations are compensated by increased billet masses which leads to lower efficiency in the manufacturing process. In a research project a controlled, moveable flash gap was examined, that enables the alteration of the material flow to increase the part quality.

forging, die design, material flow, FEA, flash land

The precision forging is energy and time saving. Therefore the basics of this topic are presented. On the one hand the material characteristics as temperature and yield stress development are shown. On the other hand tool geometry and construction instructions are given. Beyond these topics the principles of FEM, reachable tolerances are presented. Different types of crankshafts, a gear wheel and a pinion shaft are examples that were successfully investigated and even party produced in industrial environment. The goal of this chapter is to show the basic knowledge of precision forging.

precision forging, flashless

In this study two approaches in automotive lightweight construction have been combined: weight reduction based on material and based on structural design. Steel and aluminum are used for the first approach, and hydroforming is the method for design optimization by shaping. Steel and aluminum show different forming behaviors due to their characteristic yield stresses. The challenge in developing a hydroforming process for THTs is to provide feasible forming conditions for both the steel and the aluminum part. Forming tests were carried out on sheet metal specimens of laser brazed and laser soldered steel and aluminum to investigate the impact of specific parameters and to analyze the joint area’s forming behavior. An FEA simulation was set up in order to design a tube hydroforming process for THTs of laser joined steel and aluminum. Eventually, the hydroforming process for THTs will be designed and executed.

hydroforming, automotive, laser brazing, laser soldering, steel-aluminum, car body, engine downsizin

The project REFoCh is about resource efficient forging of complex high duty parts, e.g. crankshafts. Within this project founded by the European Union, a new process chain for a crankshaft will be developed with the aim of reducing the flash quota significantly. In the conventional forging sequence, 54 % flash occur, the aimed flash quota is 15 %. To reach this objective, a multidirectional forging step will be implemented in the new process chain in order to reduce the material significantly.

flash reduced, multidirerectional forging, cranskshafts

The material costs represent up to 50 % of the total production costs. By decreasing the flash ratio the material usage and production costs in forging operations can be reduced. For complicated forging parts the development of a new forging sequence is necessary to achieve a flash reduced forging sequence. This development was performed for a two-cylinder crankshaft. The new flash reduced forging sequence is using flashless preforming operations, an induction reheating of the preformed complicated work piece and a flash reduced final forging. With the reduction of the flash ratio from 54 % down to 10 % the total energy consumption was reduced and the competitive capacity of forging SME’s is increasing.

flash reduced, process chain, crankshaft, induction heating

Today most technical parts and components are made of monolithic materials. Nevertheless, the previously used monolithic materials reach their technological and constructive limits, so that an improvement of the component properties can be realized by hybrid parts. Forging of previously joined semi-finished products to net shape hybrid components is a promising method to produce functional adapted parts in a few process steps. This new process chain offers a number of advantages compared to other manufacturing technologies. Examples are the production of specific load-adapted forging parts with a high level of material utilization, an improvement of the joining zone caused by the followed forming process and an easy to implement joining process because of the simple geometries of the semi-finished products. This paper describes the production process of hybrid steel parts, which are produced by a combination of a deposition welding process with a subsequent hot forging (upsetting) or cross-wedge-rolling. It could be shown that the innovative process chain enables the production of hybrid parts whereby the forging processes lead to an improvement of the mechanical properties of the laser deposited material.

process chain, deposition welding, hot forging, cross-wedge-rolling

The usable wind energy increases exponentially by increasing the height of wind turbine generators. The material requirements and the tower weight increases disproportionately high by growing tower height. In current designs, the height of towers is limited. The application of lightweight design concepts in the production of wind turbine tower sections can lead to a reduction in the weight with the same tower stiffness. Therefore lightweight designs have great potential to increase wind turbine efficiency. In this paper, the results of the research for lightweight concepts and their implementation on towers and a guiding systematic approach are presented. As part of the research, design concepts have been developed. These design concepts allow the mass reduction, with constant stiffness, for the wind turbine towers on land (onshore). Several investigation loops of these lightweight designs were run. Different concepts of bionics, aviation, aerospace and automotive have been investigated for their suitability in wind turbine towers. A suitable concept was identified based on trapezoidal sheets. Using these sheets, the weight of towers can be reduced by 20 %.

xxl-product, large-scale, xxl, wind turbine, wind power, lightweight construction, tower constructio

In this paper the development of the new production technology hybrid forging is described. Hybrid forging combines forming of bulk and sheet metal-elements including joining. Using three model experiments the joining by material bond and by form closure is analyzed. The form closure works like a riveting and the experiments showed good results. But not in every area of the specimen a material bond was present.

hybrid, forging, sheet metal forming, joining technology, steel, adhesive bond

The strength/density ratio of aluminium can be increased significantly by the integration of ceramic particles in the metal matrix, so called Aluminum MMC. Because machining of MMCs is very expensive due to the high hardness of the ceramic particles precision forging of Aluminum MMCs allows the high quantity production of near net shape components with a strength/density ration. The paper describes the development of a precision forging sequence for a demonstration made of Aluminum MMC.

aluminium metal matrix composites, flashless forging, FEA

In a research project by the Institut für Integrierte Produktion Hannover (IPH) and the Institute of Metal Forming Siegen the basics for hydroforming titanium tubes were investigated. The aim of this research project was to develop a multi-stage process chain for hydroformed parts made of titanium. First the method-specific material properties as well as the tribological system were examined. The results were combined in a simulation model which enabled the process development. Finally, the developed process chain was successfully tested using a real component and the trials were compared with the simulation results.

hydroforming, FEA, titanium

Due to high mechanical loads the die cavity shows high wear in warm forging processes. Within this paper the ability of a hydrogenated amorphous carbon doped by 40 % chromium as wear protective coating for bulk forming processes will be analysed. Therefore billets with temperatures between 650 and 1200 °C have been forged on this layer. Thereafter the layers are analysed and the wear behaviour is evaluated.

coating, forging, forming

The early detection of defects in forging processes offers economic benefits. For example, workpieces that are forged incorrectly can be discharged from the process chain immediately and thus do not cause additional process costs, i. e. in a subsequent heat treatment. The development of a novel process monitoring using electrical current allows the identification of forging defects and underfillings.

forging, process monitoring

The higher mechanical loads in warm forming processes lead to increased mechanical wear of the dies. Using hard Diamond-like Carbon (DLC)-coatings can reduce this main disadvantage of warm forging. By an adequate doping of DLC with elements the temperature stability of these coating can be increased. Especially in areas with high relative movement the analysed coatings show advantageous behaviour compared to not coated dies.

diamond-like carbon (DLC), warm forming, bulk forming, wear

Forged parts have geometrical fluctuations for example caused by die wear. The investigation of a controlled, movable flash gap by FEA for compensation purpose showed how far the material flow can be varied during the forming operation. A potential to improve forging results is derived.

forging, die design, material flow, FEA, flash land

Hydroforming is often used to conduct forming of thin-walled hollow parts made of tubes. In order to form thick-walled hollow parts made of Aluminum a process has been developed, to form such parts. These parts are in a heated state. The forming has been made by help of an active fluid medium on a hydraulic deep-drawing press. For investigation purpose an example process has been designed for a trial part. This process has been analyzed by FEA and verified by laboratory tests.

hydroforming, forging, tubes, aluminium, FEA

Recent research dealt with the basic forgeability of aluminium-matrix composites produced under laboratory conditions. These research projects concentrated on metallurgy and, by deriving flow curves, on formability. In contrast to laboratory conditions, the industrial use requires large batches of raw materials and takes place under real conditions. Those conditions have not yet been analysed. This paper describes the combination of mechanical advantages of aluminium-matrix composites and a flashless forging process. The advantages can be outlined as higher part strength, compared to conventional forging parts with the same weight. Newly derived flow curves demonstrate formability and were used to evaluate basic forging parameters. The first forging trials in a conventional forging process using a high forging rate resulted in broken parts and therefore required a reduced deformation rate. The matrix slug material provides a higher deformation rate but has to be reduced, due to the use of MMCs. Using a conventional forging press (e.g. screw press) causes material failures likes cracks, because forging Al-MMCs requires a reduced forming speed of 20 mm/s. Reducing the forming speed, hydraulic presses are recommended. FE-analysis of the newly developed flashless forging process is described and depicts the basic forming operations (spreading, elongating and climbing) of the material in the forging die. Altogether a new strategy for a flashless forging process layout with Al-MMC is shown.

forging, flashless, aluminium, matrix, composite, characterization, upsetting, FEA, simulation, tool

In automotive industry, parts made of aluminum alloys are used with increasing frequency. During forging operations for the production of aluminum long flat pieces, defects like folds can appear. Especially internal folds are of interest, which are only evident in the fiber orientation and have a negative effect on the dynamic mechanical properties of the forged part. In forging, the forming operation can be realized either from one direction (uni-directional) or from several directions (multi-directional). The boundary conditions for multi-directional forging are described in this article. For a given tool geometry, multi-directional forging permits the realization of fold-free forgings, which has been shown to be not possible with uni-directional operations. A newly developed method based on Finite-Elements-Analysis (FEA) simulation helps with the design of the forming process and the determination of the appropriate tool geometry. A new algorithm integrates the computer-aided identification of internal folds. For a given process and tool geometry, the area with internal folds is adjusted, until the simulation shows no fold formation. It is shown, that by using this model, a dependable assessment and correction of forging tools and forming process and thus the realization of a fold-free forming are possible.

multi-directional forging, aluminum forging, internal folds, finite-elements-analysis, algorithm, to

A smart option to increase the energy yield of wind turbine generators is to increase its height. There is an exponential increase of the usable wind energy at enlarging the tower’s height, but also an exponential increase of the tower’s weight. The application of lightweight design concepts in the production of wind turbine tower sections may lead to weight reduction while keeping the tower’s stiffness at an equal level. Here the results of a study for lightweight concepts and their implementation on towers and a guiding systematic approach are being presented. The investigated design solutions proved successfully in bionic, aerospace and automotive applications. FEA simulations were used to compare the different structures and to estimate their feasibility. The investigation’s main result is a lightweight structure which provides weight reductions up to 20 %, by using lower wall thicknesses.

forging, genetic algorithm, preforming optimization