Creation of High-Strength Structures and Joints

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Authors: Bernd Arno Behrens, Sven Hübner, Anas Bouguecha, Julian Knigge, Kathrin Voges-Schwieger, Katrin Weilandt
Abstract: The emphasis, in respect of content regarding the here presented project, lies within the production of localized reinforcements, by means of transformation-induced α’ martensite formation in solid and sheet metal components. During the forming process of metastable austenitic steels, high-strength martensite areas, next to ductile austenitic regions, are to be adjusted to enable the production of load-adapted components. To this end, extensive basic analyses are also necessary in order to determine the description of the mechanical behavior of α’-martensite structures, as well as to determine the extension of the numerical simulation as regards the structural change. The results achieved within the area of steel forming include the development of a temperable deep-drawing die (T = -35 °C until T = 100 °C) that carefully facilitates structural conversion at a constant forming-degree. Moreover the crash performance, based on transformation-induced martensite structures is improved. So-called Forming Curves (FCs) were developed as a new approach towards the material characterization of structured steel. In bulge forming components, comprised of chrome and nickel steels as well as manganous hard steel, martensite was specifically generated under the use of differing forming parameters. The tool design was aided by Finite Element Analysis (FEA). Moreover, fundamental simulations were carried out in order to calculate the structural change. The modification and extension of a semi-analytical model of the material followed so that the martensite content could be calculated in the previously examined sheet components, as in the massive forming.
Authors: Mehdi Asadi, Nicole Schulze, Heinz Palkowski
Abstract: This research deals with processes leading to local strengthening effects in Advanced High Strength Steels (AHSS). Dual phase (DP), retained austenite (RA) - both hot and cold rolled - and complex phase (CP) steels have been investigated to examine the effect of thermal and mechanical processing parameters on local properties. For this purpose, a method has been investigated to achieve local strengthening, namely local deformation and local heat treatment. Samples were locally deformed by bending and embossing processes. A local deformation with defined pre-strains leads to enhanced hardness and strengthening. A subsequent aging treatment leads to a further increase in mechanical properties. Local heat treatment was applied using a laser and an electron beam. Following local heat treatment with selected parameters, the microstructure of the surface and the cross section as well as the mechanical properties were evaluated by light optical, scanning as well as transmission electron microscopy, hardness measurement, tensile testing and thermal modelling. It can be stated that with partial heat treatment, local high strengthening can be produced. At lower heat treating temperatures, this effect could be attributed to bake hardening. With increased heat treating temperature, the initial microstructure near the surface is affected. A model can be improved, which defines the correlation between the influencing parameters and the local properties. The influence of over-aging in locally strengthen regions has been studied. For this investigation, parameters are stable to locally adjust the strengthening effect. Partial strengthening of AHSS by local deformation or local heat treatment can open up new fields of applications for locally using the strengthening effect to only influence relevant areas of interest, thus providing the potential for saving energy and designing the component’s behaviour.
Authors: Olga Sokolova, Adele Carradó, Heinz Palkowski
Abstract: The production process and the forming behaviour of locally reinforced steel/polymer/steel (316L/PP-PE/316L) hybrid sandwich composite materials (SMS) have been investigated. The effect of simple plate reinforcements with different size, shape and geometry on the forming limits of SMS was studied. As a local reinforcement, the simple solid steel and mesh steel plate inlays with central and edge positions were chosen instead of a polymer core as a sandwich laminate. In order to increase the adhesion properties between the metal and polymer layers, corona discharge and plasma preliminary surface treatments were applied prior to the sandwich production. Both, deep drawing and stretching cup-forming tests were performed in order to analyse the forming behaviour as well as the failure of SMS with and without local inlays subject to different tensile loadings. The influence of the local reinforcement on the bending behaviour was determined by three and four-point bending processes. Stress-strain curves and thinning behaviour of SMS with local reinforcements under the different forming loads were determined using digital image correlation via photogrammetry. The forming behaviour strongly depends on the quality, geometry and size of the local plate inlays. Owing to the different positions of reinforcement as well as to the different polymer content around of inlays, failure of SMS by bending and drawing differs. The sandwich samples with mesh reinforcement demonstrate better formability by drawing and bending than that of samples with solid plates. In order to minimise the loss in formability of sandwich samples during deep drawing, the size of the centred reinforcement has to be larger than the punch diameter.
Authors: Tillmann Plorin, Dirk Bormann, Torsten Heidenblut, Friedrich Wilhelm Bach
Abstract: Owing to their mechanical properties, metallic foams possess the outstanding ability to considerably improve a structure's stiffness and energy absorption capacity with low increases in weight. In the research results from the sub project A4 "Foam filled, rolled, closed profiles” of the CRC 675 "Creation of high strength metallic structures and joints by setting up scaled local material properties" introduced here, both the manufacture as well as the reinforcement of magnesium foams, which are produced by means of powder metallurgy, are described. The potential for increasing their strengths using reinforcements are demonstrated and the results of mechanical tests are presented. In addition to this, research results are presented which have contributed to achieving the main objectives of developing a combined technology for producing profiles which are locally reinforced using magnesium foam. The developed technology is characterised by integrating the foaming process into the roll forming process.
Authors: Mitja Schimek, Dirk Herzog, Dietmar Kracht, Heinz Haferkamp
Abstract: Nowadays, high requirements are being placed on producing lighter automobiles with a higher strength. To achieve graded strength properties and to improve the rigidity of high strength thin steel sheets, the side effects of laser joining processes can be used. Local physical and geometrical effects which have previously only been observed as side effects can be purposefully used to increase the rigidity and strength of sheet metal structures. By using a focused laser beam with a diameter of several tenths of a millimetre, bead-on-plate and overlap welding seams have been produced. The energy needed to produce this kind of welding seam can be limited to a small area of the workpiece. In comparison to other procedures, the basic material characteristics are retained after welding, the main reason for this being localized heat input. The continual development of laser beam sources to provide higher output powers has extended their spectrum of use in the field of joining technologies. One aim of the research is to produce local physical and geometrical effects with two different laser systems, on the one hand, with an Nd:YAG laser with a maximum output power of 4 kW, and on the other hand with a Yb:YAG laser with an maximum output power of 3 kW. Bead-on-plate and overlap welding seams were produced to demonstrate that rigidity and strength can be increased in metal sheets. The investigations were carried out on two high strength steels H340LA with two different zinc coatings (D and ZE) and TRIP700. The sheets were tested using tensile tests, 3-point bending tests and fatigue strength tests. During the tests, metallographic analyses were carried out. Seven different specimens were investigated, one without bead-on-plate welding seams, and six with different welding geometries, expect the material H340LAZE which was investigated with three various welding geometries. To analyse the complex stress status, investigations of the residual stress and the fracture were done. Calculations of the seam volume were done to be able to calculate the tensile strength for linear bead-on-plate welding seams and the maximum force for bending overlap welding seams. The tensile and bending tests showed that higher forces were needed before failure occurred, e.g. bending the specimens. Furthermore, the investigations showed that the strength of the specimens with welding seams increased, in comparison to the specimens without welding seams. Another result of the experiments is that there is a dependency between the fatigue strength and the position of the seam relative to the direction of the testing force.
Authors: Bernd Arno Behrens, Thomas Hagen, Andreas Klassen, Julian Knigge, Jens Mielke, Insa Pfeiffer
Abstract: A promising approach to handling low ductile aluminium alloys in a forming process is forming under superimposed hydrostatic pressure. The influence of superimposed hydrostatic pressure on the flow stress as well as on the formability for various hydrostatic pressures and temperatures was analysed [15, 3, and 7]. By increasing the formability of the workpiece, larger local plastic strains could be achieved. The results reveal highly increased formability at superimposed pressure of 85 MPa for workpieces from thermosetting alloy AlSi1MgMn (EN AW 6082) in comparison to those from self-hardening alloy AlMg4.5Mn0.7 (EN AW 5083). As a general tendency, the self-hardening alloys show a lower increase in formability when forged under superimposed pressure. But additionally, a charge-dependent influence of macro- and micro defects on the crack resistance was detected for alloy AlMg4.5Mn0.7. By evaluating damage models in finite element models the damage occurring in cold forming processes under superimposed hydrostatic pressure was predicted.
Authors: Berend Denkena, Bernd Breidenstein, Luis de Leon, Jan Dege
Abstract: Novel manufacturing technologies for high-strength structural components of aluminium allow a local modification of material properties to respond to operational demands. Machining and finishing processes for changing material properties like deep rolling or rubbing are to be combined to a single process step. The intention is the controlled adjustment of the component’s properties by the modification of its subsurface. For that purpose the essential understanding of the interaction mechanisms of the basic processes turning, deep rolling and rubbing is necessary. Influences of the tool geometry as well as of the process parameters on the material properties are investigated. The results will be extended by parameter studies within numerical simulations. Thereafter, combinations of the basic processes in process sequences are analyzed to their ability to modify the subsurface properties. In consideration of these results, a prototypic combined turn-rolling tool is developed
Authors: Volker Wesling, Antonia Schram, Henning Wiche
Abstract: Besides weldable component geometries for the high-frequency welding process also possible process and system induced activated material reactions during discontinuous high-frequency welding are presented in this paper. Among others such material reactions can be a locally limited thermal influence on the base metal, defined plastic derformations during the upsetting process as well as grain refinement in the weld seam, comparable to thermomechanical treatment during rolling for increasing strength or ductility.
Authors: Berend Denkena, Hans Christian Möhring, Wolfram Acker, Evgeny Zaretskiy
Abstract: High-strength structure components can be built by the application of an advantageously designed geometry. These structures are challenging to manufacture due to their complex shape. This motivates the design of a new system for quality assurance of the geometric properties of these structures. Firstly, different measurement approaches and their usability for the measurement of high-strength structure components are discussed. Then an optimized solution is suggested. A pattern projection method is introduced, whereas the patterns adapt locally to the measured surface. The basic object of the research are methods of the optic three-dimensional measurement of structural elements with the help of adaptable pattern projection that allows to adjust the measuring process to the local geometry and reflecting characteristics of structural elements. This technique takes care of the special properties of high-strength structure components within the production process. It is described, how a measurement speed can be achieved that can catch up with a common production frequency without interrupting the flow of the parts. Furthermore, the measurement can take place in an industrial environment, because it is relatively tolerant to movements of the measured structure. Therefore, the measurement procedure is divided into two steps. Within the adaptation-step the luminescence and the resolution of the projected pattern is adapted to the measured object. The actual picture is taken and calculated in a second step in order to be robust against vibrations. The result of the measurement is an optimized point cloud of several million points. The paper concludes with a description of the data processing procedures necessary in order to evaluate the correctness of the measured structure.

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