Prof. Dr.-Ing. Bernd-Arno Behrens

Function:
Managing partner
Phone:
+49 (0)511 279 76-119
E-Mail:
info@iph-hannover.de
vCard:
vCard
ResearchGate:
http://www.researchgate.net/profile/Bernd-Arno_Behrens

Publications

In the automotive industry, aluminum forged parts must fulfill lightweight and heavy duty performance requirements. The generation of thin flash between die halves and in the small gaps between the die and punch must be prevented during the flashless forging process in completely enclosed dies. However, thin flash formation is neither predictable nor preventable.

A numerical model is developed based on finite element analysis to investigate and predict the generation of thin flash in aluminum flashless precision forging processes. The significance and effects of the main influencing input parameters, including billet temperature, forming velocity, and width of gap, on different resulting parameters are evaluated. Among all resulting parameters in the established numerical model, hydrostatic pressure and the forming force in the main forming direction have been identified as the most suitable for predicting thin flash generation.

aluminum forging, forging in completely enclosed dies, flashless forging, FEA

High temperatures up to 1280 °C and high pressures during the forming opperation lead to strong tool wear in forging processes. Increasing tool wear can lead to very high costs. By experiments conducted at the Institut für Integrierte Produktion in Hanover the correlation between tool wear and lot size in hot forging processes was verfied. The findings will help companies to optimise maintenance procedures and therefore reduce cost in the future.

forging, steel, tool wear, lot size

A low energy demand and a fast processing time are required in each industrial process for the production of crankshafts. Crankshafts have a very complex geometry and are forged with a high percentage of flash compared to other forging parts. Recent research showed the feasibility of a flashless forging of crankshafts. One way to forge a flashless crankshaft within three steps is to use cross wedge rolling, multi-directional forging and final forging.

This paper presents the investigation results of the influence of the cross section area reduction in cross wedge rolling on different parameters at multi-directional forging. First, the state of research, the process development and tool design of cross wedge rolling and multi-directional forging are described. Then a parameter study will be presented and the influence of the cross section area reduction on flash generation, billet temperatures, forming degree, forming forces and effective strain are shown. Generally, flash generates because a rotation-symmetric billet is forced into an asymmetric movement. The influence of an increasing cross section area reduction leads to a decreasing amount of flash at the bottom of the crankwebs.

multi-directional forging, cross wedge rolling, crankshaft, parameter study, forming angle

A main target in automotive engineering research and development is currently to reduce fuel consumption and CO2 emissions. Therefore in this project lightweight design was combined with material design in order to produce more efficient structural components. The joining process for tubes of steel and aluminum by laser brazing was investigated to create a joint area that is highly formable. These steel-aluminum joints were afterwards hydroformed, at which steel and aluminum parts were formed in a single step. This process is called "IHU-THT" and can provide lightweight components with excellent mechanical properties.

FEA, hydro forming, tailored forming

Most of today’s technical parts and components are made of monolithic materials. These mono-material components produced in established production processes reach their limits due to their respective material characteristics. Thus, a significant increase in production quality and efficiency can only be achieved by combining different materials in one part. Bulk forming of previously joined semi-finished products to net shape hybrid components that consist of two different materials is a promising method to produce parts with locally optimized characteristics. This new production process chain offers a number of advantages compared to conventional manufacturing technologies. Examples are the production of specific load-adapted forged parts with a high level of material utilization, an improvement of the joining zone caused by the following forming process and an easy to implement joining process due to the simple geometries of the semi-finished products.

This paper describes the production process of hybrid steel parts, produced by combining a plasma-transferred arc deposition welding process with a subsequent cross wedge rolling process. This innovative process chain enables the production of hybrid parts. To evaluate the developed process chain, coating thickness of the billet is analysed before and after cross wedge rolling. It could be shown, that the forming process leads to an improvement of the coating, meaning a more homogeneous distribution along the main axis.

process chain, plasma-transferred arc deposition welding, hybrid parts, cross wedge rolling

Different challenges arise in cross wedge rolling hybrid parts depending of the material arrangement (serial or coaxial) which need to be investigated fundamentally first.

In cross wedge rolling of serial components, the controlled forming of the joining zone is the greatest challenge. The forming behaviour of the component halves is different, depending on the flow stress of the materials used. In order to allow the forming process to be carried out in a controlled manner, the forming behaviour was first analysed with regard to the displacement and quality of the joining zone, and then possibilities were determined with which the forming can be effected in a targeted manner. For this purpose, the influencing parameters (workpiece temperature, forming speed, cross-section reduction, shoulder and wedge angle) were determined systematically using the Finite Element method, and the investigations were then verified experimentally. In order to influence the forming behaviour the investigations include structural measures (e.g. unequal tool halves) as well as process-related parameters (e.g. unequal temperature distribution).

Cross wedge rolling of coaxial components has other challenges due to the component construction. The aim is to be able to specifically influence the course of the thickness of the applied coating during the forming. Therefore finite element simulations were carried out to determine the influencing parameters. By a systematic investigation of the test parameters according to the DoE method, the layer thickness before the deformation as well as the cross-section reduction are parameters with the greatest influences on the course of the layer thickness after the deformation gave. The results obtained were subsequently verified in experimental tests.

cross wedge rolling, steel, aluminum, joining zone, coating thickness

In recent years, the requirements for technical components have steadily been increasing. This development is intensified by the desire for products with lower weight, smaller size and extended functionality, but also higher resistance against specific stresses.

The superior aim of the Collaborative Research Centre 1153 "Tailored Forming" is to develop potentials for hybrid solid components on the basis of a new process chain by using joined semi-finished workpieces.

This paper presents the approach and first results of selected subprojects for semi-finished workpiece production by composite extrusion presses, for forming the hybrid semi-finished products by means of cross wedge rolling, die forging and extrusion, and numerical failure prediction of the joining zones. This provides an overview of possible lightweight strategies in the area of bulk forming by the use of pre-joined semi-finished workpieces.

tailored forming, semi-finished workpiece production, forming, cross wedge rolling

For lighter and less consuming car engines the uncercut forging of a steel piston the process has to be designed at first. Therefore the process had been set up in FEA simulations and developed until the final forging sequence was found.

FEA, forging, forge, undercut, multidirectional

Hybrid forging combines forming of bulky and sheet metal elements in one process step. During the forming of the bulky and sheet metal elements a joining operation is initiated by the energy provided by the forging operation. Thereby component areas with high loads can be designed using a bulky element whereas areas with lower loads can be designed using a sheet metal element. In consequence, significant weight reductions as well as energy savings within the forging process are achievable. The paper presents the development of a hybrid forging process, using a control arm as demonstration part. By the aid of Finite Element Analysis computations the interactions between the main process parameters and the target value process quality are being derived. It will be shown that the bulky element’s shape has a major impact on further process parameters and that the temperature is crucial for material bonding.

FEA, hybrid forging, bulge forming, sheet metal forming

A low energy demand and a fast processing time are required in each industrial process for the production of crankshafts. Crankshafts have a very complex geometry and are forged with a high percentage of flash compared to other forging parts. Recent research showed the feasibility of a flashless forging of crankshafts. One way to forge a flashless crankshaft within three steps is to use cross wedge rolling, multi-directional forging and final forging.

This paper presents the investigation results of the influence of the forming angle in cross wedge rolling on different parameters at multi-directional forging. First the state of research, the process development and tool design of cross wedge rolling and multidirectional forging are described. Then the parameter study will be presented and the influence of the forming angle ? on flash generation, billet temperatures, forming degree, forming forces and effective strain are shown. Generally, flash generates because a rotation-symmetric billet is forced into an asymmetric movement. The influence of a rising forming angle leads to a higher amount of flash at the bottom of the crankwebs.

multi-directional forging, cross wedge rolling, crankshaft, parameter study, forming angle

To reduce production costs of forged parts, different approaches are possible. Especially for valuable materials like titanium, material costs represent a large part of the production costs. Therefore, reducing the initial material can decrease the total costs significantly. In order to identify the potential for improvements, an existing forging sequence was investigated.

For a titanium hip implant, a new forging sequence was developed. To reduce the initially needed material, cross wedge rolling as a preforming operation and die forging with flash brakes was investigated. The influence of the different stages on the final result was analysed and presented in detail. To increase the prediction accuracy of the newly developed flash-reduced forging sequence and to decrease iteration loops of die designs, feasible simulation parameters considering the boundary conditions of the forging environment were investigated. This is done using Finite Element Analysis (FEA), considering form filling, process stability, die stress and press forces. Using cross wedge rolling and die forging with flash brakes, the newly developed forging sequence reduces the flash rate significantly from 69 % to 32 %.

cross wedge rolling,forging, flash-reduced, finite element simulations, flash brakes

In multistage hot forging processes, the preform shape is the parameter mainly influencing the final forging result. Nevertheless, the design of multistage hot forging processes is still a trial and error process and, therefore, time consuming. The quality of developed forging sequences strongly depends on the engineer’s experience. To overcome these obstacles this paper presents an algorithm for solving the multi-objective optimization problem in designing preforms. Cross wedge rolled preforms were chosen as subject of investigation. An evolutionary algorithm is introduced to optimize the preform shape taking into account the mass distribution of the final part, the preform volume and the shape complexity. A crucial factor in preform optimization for hot forging processes is the amount of flash. Therefore, an equation for improving the amount of flash is derived. The developed algorithm is tested using two connecting rods with different shape complexities as demonstration parts.

preform optimization, forging, evolutionary algorithms, cross wedge rolling

In order to enable small and medium-sized enterprises to use cross-wedge rolling in the future, existing barriers have been eliminated. For this purpose, a method was developed to enable the design of cross wedge tools using software support. For two demonstrator components, hip implant and common rail, cross-wedge rolling processes were designed. With the cross-wedge rolled preforms, flash-reduced forging sequences could be designed for both demonstrator components. In order to be able to roll the parts industrially at low cost, a cross wedge rolliing machine was designed, manufactured and built at the forging company. The complete process chain of the hip implant was successfully tested.

cross wedge rolling, forming machine, ressource efficiency, hip implant, common rail

In multistage hot forging processes, the preform shape is the parameter mainly influencing the final forging result. Nevertheless, the design of multistage hot forging processes is still a trial and error process and therefore time-consuming. The quality of developed forging sequences strongly depends on the engineer's experience. To overcome these obstacles, this paper presents an algorithm for solving the multi-objective optimization problem when designing preforms. Cross wedge rolled (CWR) preforms were chosen as subject of investigation. An evolutionary algorithm is introduced to optimize the preform shape taking into account the mass distribution of the final part, the preform volume and the shape complexity. The developed algorithm is tested using a connecting rod as a demonstration part. Based on finite element analysis, the implemented fitness function is evaluated, and thus the progressive optimization can be traced.

preforming optimization, hot forging, evolutionary algorithms, cross wedge rolling

Flash-reduced forging is a promising alternative for forging complicated high-duty parts. With a new process chain, the ability to reduce the existing flash quota of complex high-duty parts can make the difference in the competition and reduce the costs compared to flashless forging. The European Union is funding a research project which deals with the improvement of the forging sequence of a two-cylinder crankshaft by using flash-reduced forging. To increase the forecast quality of simulations using Finite-Element-Analysis for a future process chain design, the conventional existing process chain is simulated with FORGE3 and compared with industrial forging trials. Furthermore, a variation of simulation parameters has been used to get the significant influence parameters, fitting the results of these forging trials.

Forging, crankshaft, flash reduced, investigation simulation parameters

An artificial neural network is developed to predict the form filling in forging sequences. This model uses various cross sectional area properties to improve the form filling prediction and to estimate simulation results. The effect of different cross sectional area properties on the prediction quality of the form filling is shown and the best way to predict the form filling is chosen.

FEA, forging, preform optimization

A numerical model using FEA is developed to investigate the generation of thin flash in aluminum forging. Significance and effects of the influencing parameters temperature, forming velocity and width of flash gap on the thin flash generation are shown by statistical analysis carried out with the established model. Experimental trials have been made to verify the model.

FEA, aluminum, forging, flashless forging, thin flash

The most common method of bulk forming processes is closed die forging with flash. In these processes a surplus of material is used to ensure a complete filling of the cavity of the forging die. The surplus material is driven out of the die through the flash land, thus the design of the flash land has a major influence on the filling of the die. All dimensions of the flash land are typically fixed during the manufacturing process of the die and can not be changed within the forging process. By use of a moveable flash gap that can be actively changed during the forging process the material flow can be altered. This permits to improve the filling of the cavity. In this paper a moveable flash gap for a hot forging process is described and the influence of such a system on the filling of the die cavity is determined. This is done by a comparison to a conventional forging process with a fixed flash land. Furthermore, the results of experimental trials are compared to results of corresponding FEA simulations. Additionally, the influence of the initial billet temperature is investigated. Experimental trials showed that the moveable flash gap has a distinct influence on the material flow. The higher the flash ratio, the bigger is the influence of the moveable flash gap. The moveable flash gap is designed as a flash brake of a height of 2 mm. Its usage lead to differences in height of the parts up to 4.5 mm, which correspond to 16.6% of the parts height, compared to parts forged with a fixed flash land. If the forging temperature is decreased from 1200 °C to 1000 °C, the influence of the moveable flash gap is reduced. The average differences in height are about 0.5 mm (about 3%).

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

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.

closed-die forging, closing mechanism, flashless forging, forging process, preforms

The most common bulk forming process is closed die forging with flash. One goal of the industry is to reduce flash. For geometrically difficult parts like crankshafts flash reduction can be achieved by flashless preforming and flash-reduced final forging. The corresponding process design is challenging and defects like an insufficient cavity filling often occur in final forging. A controlled, moveable flash gap enables the alteration of the material flow, increasing the filling of the cavity again. In this paper, the flashless preforming for crankshafts and the influence of a controlled flash gap on cavity filling are described.

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