[Show abstract][Hide abstract] ABSTRACT: Hot stamping is a well-established process in car manufacturing today. However, the resulting mechanical properties of a hot stamped part and its behaviour during a crash are still open questions. The usual procedure includes destructive experiments to determine the mechanical properties resulting from the forming and quenching process. The gained information is then used for crash simulation. Using images from micrographs to determine the proportion of bainite and martensite resulting from the hot stamping process has proved to be difficult, as these structures are fairly similar and hard to distinguish.
[Show abstract][Hide abstract] ABSTRACT: Metal forming is not only shaping the form of a product, it is also influencing its mechanical and physical properties over its entire volume. Advanced analysis methods recently enable accurate prediction of these properties and allow for setting these properties deterministically during the forming process. Effective measurement methods ensure the setting of these predicted properties. Several real examples demonstrate the impressive achievements and indicate the necessity of a paradigm change in designing products by including manufacturing-induced effects in the initial dimensioning. This paradigm change will lead to lightweight components and serve environmentally benign designs.
[Show abstract][Hide abstract] ABSTRACT: The realization of lubricant free forming processes is motivated by an increasing demand for resource efficiency and higher environmental standards. Further potentials are the reduction of production steps and time. The dry conditions lead to an intensive interaction between tool and workpiece. Increasing friction and wear are the consequences. One approach to face these challenges is the implementation of tailored tool surfaces. Within this study the behavior of different tool surfaces under dry and lubricated conditions is investigated. In this regards, a flat strip drawing test was conducted to determine the friction coefficients depending on the tool surface properties resulting from various machining processes. Furthermore, a surface characterization before and after the experiments was performed to gain knowledge about the wear mechanisms. The tests series with dry strips lead to a higher friction level. Furthermore, the results show that the friction can be reduced when the grinding marks are orientated transversal to the drawing direction. A further friction reduction is achieved when applying polished tools.
International Journal of Precision Engineering and Manufacturing-Green Technology 04/2015; 2(2):131-137. DOI:10.1007/s40684-015-0017-8
[Show abstract][Hide abstract] ABSTRACT: This paper deals with a manufacturing process for sheet metal components with integrated functional elements by sheet-bulk metal forming. In a single forming stage a cup-shaped base body with thin-walled features is deep drawn and its geometry is calibrated by upsetting. The fundamental numerical and experimental investigations include the analysis of the die filling behaviour, part geometry and mechanical properties due to strain hardening with respect to the variation of the blank layout, forming force and material strength. Finally, the process limits are identified and approaches for their enhancement by the application of tailored surfaces are presented.
[Show abstract][Hide abstract] ABSTRACT: The investigated material is the copper-free Al–Zn–Mg-alloy EN AW-7020 (AlZn4,5Mg1) in the artificial aged T6 temper. In this temper the 7xxx series alloys show a high strength level, but also a high stress corrosion sensitivity. Therefore, the retrogression and reaging (RRA)-treatment has been developed to combine stress corrosion resistance with high strength. The aim of this study is the description of the course of the mechanical behavior influenced by a RRA treatment. The changes of the mechanical properties are represented by changes in the microstructure as recorded by thermal analysis. Therefore typical mechanical properties and thermal analysis curves are determined according to temperature and time in each of the retrogression annealing and of the RRA-treatment. The temperature for retrogression annealing was between 220 and 360 °C. Reaging was performed on material at 130 °C for 16 h and 160 °C for 5 h after a storage of 7 days at room temperature. The results of the thermal analysis are generally described on the curve of the T6 tempered material and transferred to the changes by the retrogression temperature and the reaging parameter. The results show a main influence of the precipitation structure and mechanical properties after the retrogression and reaging treatment by the temperature of the retrogression annealing. For all investigated parameters, an overaging of the material after the RRA-treatment is recognizable.
Production Engineering 04/2015; 9(2). DOI:10.1007/s11740-014-0593-4
[Show abstract][Hide abstract] ABSTRACT: The on-going trend to lightweight construction leads to a special focus on plane lattice structures as an alternative for solid metal plates. They demonstrate similar mechanical properties while taking up only a fraction of the normal material input and are thus economically favourable. Additionally, they fulfil functional and design aspects and therefore are used by several industries like the automobile. Nevertheless, the two most common types of lattice structures-perforated metal plates and expended metal plates-Are either waste intensive or uneven and hence require additional rolling of the metal plate. Therefore, within this contribution a new and innovative approach for the production of plane lattice structures will be presented. The manufacturing process thereby consists of two steps. At first, a specially designed pattern is cut into metal plates via a laser. Subsequently, the plates are formed under uniaxial tension to realize the lattice structures. Based on the cutting length, cutting space and the row space different blanks with tailored lattice structures can be produced. From the experimental results first guidelines for the design of suitable patterns are derived. The investigations will be performed with precipitation hardenable aluminium AA6014.
[Show abstract][Hide abstract] ABSTRACT: Focus of this paper is to model the plastic forming behavior of AA6082, in order to develop the numerical FE analysis of the friction stir welding processes and the simulation of subsequent forming processes. During the friction stir welding process, the temperatures reached can range up to 500 C and have a fundamental role for the correct performance of the process, so the material data has to show a temperature dependency. Because of the tool rotation a strain rate sensitivity of the material has to be respected as well. In this context, the general material characteristics of AA6082 were first identified for different stress states. For the uniaxial state the standard PuD-Al used in the automotive industry was applied, for the shear state the ASTM B831-05 was used and for biaxial states the ISO 16842 was exploited. To characterize the plastic flow behavior of the AA6082 at elevated temperatures, tensile tests were performed according to DIN EN ISO 6892-2 from 25 °C to 500 °C with a strain rate from 0.1 s-1 up to 6.5 s-1.
[Show abstract][Hide abstract] ABSTRACT: Resource efficiency, design oriented accuracy and lightweight properties are demands on modern sheet metal forming parts in the automotive sector. The use of new materials leads to additional challenges on the numerical design of forming processes. During these forming processes the material undergoes different strain states that cause non-linear strain paths. Since the numerical prediction highly depends on the identified characteristic values of the material, an exact characterisation of the material behaviour is essential. Especially obtuse angles of the stress vector trigger a recovery of the material by returning stress. Besides, a relaxation of the material is investigated during holding a constant strain level. The effect of relaxation lead to an altered material behaviour that appears in a reduction of the beginning of plastic yielding. In addition, a kinematic hardening behaviour as under cyclic loading and load reversal, known as the Bauschinger effect, occurs after the relaxation of the stress and results in a reduced beginning of plastic yielding by loading in the same direction as the introduced pre-strain. Within this research work the effect of relaxation is investigated for two materials, AA5182 and DP600, with an initial sheet thickness of 1.0 mm. These materials are typically used for internal and accordingly functional parts in the automotive sector. The relaxation of the material is analysed with different holding times of a constant pre-strain at different levels of straining. The release of the material is studied by subsequent uniaxial tensile tests after pre-straining with the same load condition. Moreover, the influence of the named effects is shown by comparison of the translation of the yield loci.
[Show abstract][Hide abstract] ABSTRACT: The recent development of new lightweight sheet metal materials, like advanced highstrength steels or aluminium alloys, in combination with an increasing component complexity provides new challenges to the numerical material modelling in the FEM based process design. An auspicious approach to improve the quality of the numerical results - most notably in springback analysis - is the modelling of the so called Bauschinger effect achieved through implementation of kinematic hardening models. Within this paper the influence of the stress state and the level of prestrain on the numerical simulation result of the advanced high strength steel DP-K45/78+Z will be analysed. For this purpose, a parameter identification of the kinematic hardening law according to Chaboche and Rousselier is performed at different pre-strains on the basis of experimental data from tension-compression tests as well as cyclic shear tests. Finally, the identified parameters are validated in a comparison between numerical and experimental results of a cyclic bending test.
[Show abstract][Hide abstract] ABSTRACT: Due to the increasing number of electronics in several industrial sectors, especially in the automotive industry, there is a rising demand for flexible and adaptable electronic systems with high functional density and resilience. An efficient method for producing such parts is the encapsulation of metal inserts, for example lead frames, by means of assembly injection moulding. Often such parts are exposed to water and moist at the place of action. Thus, one major challenge is to provide electronics with durable media tightness in a severe environment. The research work covered in this paper focuses on the embossing of surface structures in metal inserts for subsequent assembly injection moulding. The influence of geometrical parameters of the embossed profile on both the material flow and the accuracy of the created structure are investigated. For this purpose experimental as well as numerical results are presented. Furthermore, the performance of embossed inserts in subsequent assembly injection moulding is analysed.
[Show abstract][Hide abstract] ABSTRACT: Solid Bonding based welding processes allow to obtain defect free joints with low residual stress and low distortion. However, the engineering and optimization of solid bonding processes is difficult and requires a large number of time and cost consuming test trials. In this way, proper numerical models are essential tools permitting effective process design. The aim of this research was the comparison of the material process conditions during two different manufacturing processes taking advantage of the same metallurgical phenomenon, namely solid bonding. Linear Friction Welding, used to weld non-axisymmetric components and Accumulative Roll Bonding, used to increase the mechanical properties of sheet metals, were considered. Numerical models were set up, validated and used to design the process by studying the complex material behavior during the solid bonding of different aluminum alloys. An implicit approach was used for the Linear Friction Welding and Accumulative Roll Bonding processes, leading to the understanding of the main process variables influence on the field variables distribution and the occurrence of actual bonding.
[Show abstract][Hide abstract] ABSTRACT: Global competition as well as social and scientific megatrends strongly influence the modern car manufacturing industry. One of the most important approaches is the implementation of lightweight constructions. Therefore, the usage of high performance materials with tailored properties gains importance. For safety-relevant components such as automotive passenger cells it is necessary to minimize deformation to reduce the risk of injury for the vehicle occupants during a car accident. Thus, hot stamped high-strength steels have been established. High-strength and low formability of this kind of materials represent new challenges for joining technologies. One possibility to join high-strength steels is the newly developed shear-clinching technology. Due to the use of a combined cutting and joining process, the connection of dissimilar materials with high difference in strength and formability can be achieved. Further research to ensure process reliability and to improve the strength of the joint is required. One possible approach for this is the numerical investigation of the material flow during the joining process. Therefore, the definition of process parameters for the finite element model is necessary. A big impact on the quality of the results has the accuracy of the used friction values. As established testing methods are not suitable for modeling the rather complex tribological system between the joining partners of the shear-clinching process, an innovative testing method is needed. Studies in the field of sheet-bulk metal forming already demonstrated the applicability of the ring compression test for sheet metals. This paper presents a concept for the adaption of the ring compression test to the specific needs of the investigated shear-clinching process. The numerical identification of the friction coefficients is validated by experimental data and first results are qualified by experimental and simulative shearclinching joints.
[Show abstract][Hide abstract] ABSTRACT: The dissolution of co-clusters in AlMgSi-Alloys by a short term heat treatment can be used to locally adjust the mechanical properties of a blank for a following forming operation. This approach is known as Tailored Heat Treated Blanks (THTB) and allows to significant enhance the forming limits of AlMgSi-Alloys. However, the dissolution of co-clusters leads to the observation of the Portevin-Le Chatelier (PLC) effect during deformation. The results are stretcher strain marks at the surface which are a limitation for potential applications of THTB. In contrast to AlMg-Alloys, a critical strain rate above which no PLC effect occurs is not observed for the investigated alloys. This paper investigates various influence factors on the occurrence of the PLC effect for different AlMgSi-Alloys and presents an approach under which conditions THTB can be used in applications with high demand on surface quality.
[Show abstract][Hide abstract] ABSTRACT: Within this paper a numerical study of the Continuum Damage Mechanics based damage model Lemaitre in commercial software LS-DYNA is performed in order to correctly predict failure in terms of crack occurrence within small curvature bending of AHSS steels. A strain based calibration method is used for the effective adaption of the Lemaitre model to the bending operation, which is based on the comparison and adaption of the numerically calculated and the experimentally measured deformation field on the outer surface of the bent specimen. Within this method the material dependent damage parameter S is systematically varied in the simulation in order to represent maximum major strain. The new method is proved by numerical simulation of experiments provoking crack initiation using smaller bending radii. It can be shown that failure in terms of crack initiation can be correctly predicted by the model with the damage parameters, which were determined by the method of strain based calibration and an additional optimisation of the parameter Dc. Thus, within this study a user friendly and effective way for the application of Lemaitre damage model to small curvature bending processes of AHSS steels is developed.
[Show abstract][Hide abstract] ABSTRACT: By applying bulk forming processes on sheet metals, thin-walled functional components with locally restricted wall thickness variations can be manufactured by forming operations. Using tailored blanks with a modified sheet thickness gradient instead of conventional blanks, an efficient controlling of the material flow can be achieved. One possible process to manufacture these semifinished parts is a flexible rolling process. Based on an established process strategy new results for steels of differing strength and work-hardening behavior are presented in this paper. The influences of each material on the resulting process forces and blank properties regarding the same target geometry are discussed. The tailored blanks are hereby analyzed by their geometrical dimensions, like sheet thickness, and their mechanical properties, e.g. hardness distribution. Additionally, the possibilities of processing these tailored blanks in a deep-drawing and upsetting process are presented with a hereby focus on the residual formability of the tailored blanks.
[Show abstract][Hide abstract] ABSTRACT: The forming limit diagram (FLD) is at the moment the most important method for the prediction of failure within sheet metal forming operations. Key idea is the detection of the onset of necking in dependency of different strain behaviours. Whereas the standardized evaluation methods provide very robust and reliable results for conventional materials like deep drawing steels, the determined forming limits for modern light materials are often too conservative due to the different failure behaviour. Therefore, within this contribution a new and innovative approach for the evaluation and analysis of material behaviour will be presented. Thereby the complete strain history during the test is evaluated using a pattern recognition-based approach in combination with an optical strain measurement system. The basic procedure as well as the first promising results are presented and discussed.
[Show abstract][Hide abstract] ABSTRACT: Proper friction control during a sheet metal forming process can positively influences the quality of the final product. The effects of textured tooling surface and the rolling direction of the strip surface with respect to the sliding direction, the sliding velocity, and the contact pressure on friction coefficient were investigated in this study with Aluminum 6111-T4 sheet and D2 tool steel. A flat-on-cylindrical setup was used to measure the friction force by pulling a strip sample across a tooling sample. The tool steel has laser-textured micro wedge-shaped dimples. The results showed a reduction in friction coefficient by using a textured tool. In addition, in the tests, the relative motion direction parallel to the sheet rolling direction leads a lowest friction coefficient compared to other orientations.
Tribology International 01/2015; 81. DOI:10.1016/j.triboint.2014.09.001 · 1.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To design the indirect hot stamping process, a finite element method (FEM) based prediction of the part geometry and the mechanical properties is required. In case of indirect hot stamping processes, producing car body parts with tailored properties, cooling paths occur causing diffusionless and diffusion controlled phase transformations. The volume expansion caused by the phase transformation of face-centred cubic (fcc) into body-centred cubic (bcc) and the martensitic formation of body-centred tetragonal (bct) leads to transformation induced strains that are important for the calculation of overall stresses in hot stamped car body parts. To calculate the strain and stress state correctly, it is necessary to model the diffusionless and diffusion controlled phase transformation phenomena, taking into account the boundary conditions of indirect hot stamping processes. The existing material models are analysed and extended in order to improve their prediction accuracy in calculating the amount and distribution of ferrite, perlite, bainite and martensite during the whole process of annealing. For industrial use the new approaches are implemented in the FE-code LS-DYNA 971 (Livermore Software Technology Corporation, 2006).