Marion Merklein

Characterisation of the flow properties of the quenchenable ultra high strength steel 22MnB5

Nowadays in the automobile industry more and more high strength steels find their application. For their processing innovative forming technologies, like the non-isothermal hot stamping process, are required. With regard to a reliable numerical process design the knowledge of the thermal and thermo-mechanical properties is essential. Within this paper an experimental setup will be introduced, which enables the determination of the materials flow behaviour according to the hot stamping time-temperature-characteristics. Further results of investigations on the thermo-mechanical flow properties of the quenchenable, ultra high strength steel 22MnB5 in dependency of the temperature, the strain and the cooling rate etc. will be represented.

New materials and production technologies for innovative lightweight constructions

Abstract Modern concepts for lightweight construction not only make necessary new and advanced materials but also the development of new production processes and new processing strategies is obligatory in order to be adapted successfully. This paper presents some present research topics of the Chair of Manufacturing Technology that are of interest for an innovative lightweight construction nowadays and in the future. These are laser-assisted bending and roll forming as an example for new production technologies and forming of aluminium tailored blanks or intrinsic aluminium blanks using a local heat treatment as an example for new processing strategies. Finally, the potentials for lightweight construction of a special class of materials, aluminium foams, are discussed.

Determination of tribological conditions within hot stamping

Innovative hot sheet metal forming technologies are gaining an increasing significance in the scope of application of more and more innovative high and ultra high strength steels in the automotive industry. With respect to a numerical process design beside the mechanical and the thermal material characteristics the friction coefficient represents an important input parameter for finite element (FE) simulation. Within the scope of this paper an evaluation method for the determination of the friction coefficient μ for the direct hot stamping process of boron-manganese steels will be presented. Therefore cup deep drawing tests at elevated temperatures following the time-temperature-characteristic of the hot stamping process are carried out. For the calculation of the friction coefficient the approach according to Siebel for the modeling of the maximum drawing force is used.

Laser forming of aluminium and aluminium alloys — microstructural investigation

Abstract This paper shows the microstructural development and changes in mechanical properties aluminium and aluminium alloys undergo during the process of laser forming. The resulting structures, grain structure and microstructure were analysed both quantitatively and qualitatively by optical and electron microscopy in order to examine the kind of existing structure zones and their extent. Zones with different substructures can be seen depending on the selected test parameters. The degree of substructural development seems to be a function of all influences and cannot entirely be explained by laser parameters and material characteristics. Obviously, during laser forming several different inhomogeneous particle and dislocation structures develop which are responsible for the strength of the whole composite on the one hand, and for the strength of individual small components of the whole on the other hand. Thus, hardness tests of laser-formed specimens prove the existence of different zones depending on the material and heat treatment.

Basic Investigations on the Hot Stamping Steel 22MnB5

Basic research concerning the material properties of the hot stamping steel 22MnB5 has been carried out. A survey is given about the as-delivered conditions with hardness tests, micrographs and flow curves. The process window of the austenitization time, before hot stamping can take place, is defined by austenitization tests. Also a new experimental set-up to detect the cooling rate in dependency on the contact pressure is presented. In addition to that the cooling experiments were simulated with ABAQUS and the heat transfer coefficient for each contact pressure is determined by inverse modeling.

Finite Element Simulation of Deep Drawing of Tailored Heat Treated Blanks

Abstract Using a laser system for local heat treatment, it is possible to adjust the mechanical properties of aluminum blanks in a restricted area by influencing the precipitation structure to manufacture so called Tailored Heat Treated Blanks (THTB). These blanks are characterized by a distribution of the mechanical properties adapted particularly for the forming conditions during deep drawing. This paper presents a finite element (FE) based procedure to determine adequate laser parameters for the heat treatment process to enhance the forming limits. Both FE and experimental results show the improved process compared to the conventional drawing of homogeneous aluminum blanks.

Mechanical Properties and Plastic Anisotropy of the Quenchenable High Strength Steel 22MnB5 at Elevated Temperatures

Within the scope of this paper, the formability of the press hardenable steel 22MnB5 will be investigated with regard to its anisotropic properties at elevated temperatures under the processing conditions of hot stamping. Two different experimental setups have been realized, one at the University of Erlangen-Nuremberg using conductive heating, and the other one at the University of Padova using inductive heating. Both of these equipments enable the characterization of the material anisotropy behavior by performing uniaxial, hot tensile tests in the range of hot stamping temperatures.

An inverse approach to the numerical design of the process sequence of tailored heat treated blanks

While in industries lightweight construction gain an increasingly significant role and as weight reduction is often done with aluminum sheets, advanced production technologies have to be developed to be competitive for this evolution. Since steel sheet metal parts cannot be substituted directly with aluminum due to its minor formability, the usage of so called tailored heat treated blanks (THTB) is presented in this work. THTB are locally heat treated aluminum blanks from the 6,000-series alloy which exhibit a specific strength pattern optimized to the forming operation leading to a significantly improved formability for the manufacturing of complex aluminum car body parts. The enhancement of the formability is reached by a local heat treatment before the forming process. Due to the strong interdependency of heat treatment and forming operation, a numerical investigation of the process sequence is a prerequisite for a cost-effective usage of the THTB. An inverse approach on basis of a finite element simulation enables the determination of process parameters for an optimized THTB, thus having an effective and efficient engineering method for this technology.

Time Dependent FLC Determination Comparison of Different Algorithms to Detect the Onset of Unstable Necking before Fracture

The ISO standard 12004-2:2008E for the determination of forming limit curves based on the section method was approved in 2008. About 4 years of measuring experience in different laboratories has shown advantages and weaknesses of the standard and is leading to some minor changes in the specification. In the years from the development of this standard until today a further technical development of the optical measuring devices occurred, so that it is now possible to determine forming limit curves using the time history of the test. This procedure of determination is referred to a time dependent technique and could be the basis of the ISO 12004 part 2 proposal worked out by the work group Erweiterung FLC ISO 12004 of the German group of the IDDRG. This publication recapitulates existing work which was carried out from the IDDRG work group regarding the determination of forming limit curves for sheet metal materials. On one hand known issues with the current section based approach are discussed and on the other hand it deals with a comparison of different algorithms to determine the FLC from the time history of the Nakajima test using strategies to identify the instant of onset of instable necking. The different time dependent algorithms [ utilised are automatically selecting the area where necking is leading to fracture and then analyze the time history of such points using the first or the second time derivative of the true major strain, or of the true thinning strain using methods like: correlation coefficient (modified method based on [2]), gliding correlation coefficient, linear best fit (modified method based on [3]) and gliding difference of mean to median. The resulting experimental FLC points are compared with the results from the section technique described in ISO 12004 part 2 and with the maximum strain values measured in each test. Further a large number of forming limit curves were determined and used for a comparison of these different methods to define the most promising time dependent algorithm, which was selected as a suggestion for the working group defining the new proposed ISO standard 12004 part 2.

Combined tube and double sheet hydroforming for the manufacturing of complex parts

Modern lightweight construction, especially in the automotive industry, requires more and more complex components, which can be manufactured in one process step using the hydroforming technology. The combination of the tube and double sheet hydroforming is a new forming process, where a tube and two blanks are formed simultaneously in a die cavity, combining the advantages of both hydroforming variants. This paper deals with the fundamental considerations and investigations related to connection between tube and double sheet. The finite element analysis and laboratory trials were used in order to design the shape of the die cavity and to avoid wrinkles, material tearing and the collapse of the tube section during forming. The paper will also illustrate an analytical model for the prediction of the edge shape in the constrained bulging of a rectangular cup together with several technical solutions, which enabled a complete forming of the investigated part. Finally, the definition of a hydroforming material factor based on the analytical model of the hydraulic bulging process enables the right choice of sheets with different material strength and thickness for the hydroforming of hybrid components.