Abstract: Stringer sheets are bifurcated parts that possess, compared to flat sheet metal parts, a higher stiffness due to their higher geometrical moment of inertia. Currently, the common way of forming spatially curved stringer sheets is hydroforming. This article shows the feasibility of forming stringer sheets by using solid tools with short process times, which is more relevant for the industrial application. A die-bending process with a slot in the punch for the stringer is investigated. Since buckling of the stringer is one of the occurring failure modes, depending on the stringer height, an analytical estimation of the critical stringer height is carried out while considering the bending angle, bending radius and sheet thickness. The subsequent numerical and experimental investigations show a good agreement with the analytical estimation. Finally, a stiffness test is carried out with stringer sheets of different stringer heights. The result of this test underlines the motivation of forming buckling-free stringer sheets with stringers as high as possible. The normalized stiffness increases with rising stringer height until buckling occurs. At this point the stiffness values begin to fall with growing stringer height.
Abstract: For a deeper understanding of the forming behaviour of perforated aluminium sheets (Al99.5) in micro deep drawing processes, tensile and stretch forming tests are performed at forming speeds of 0.1 mm/s and 0.25 mm/s. The sheets have thicknesses of 100 µm and 200 µm, a perforated rectangular hole geometry of 300 µm x 300 µm and a bar width of 40 µm. During stretch-forming tests the flow characteristics and failure mechanisms within the mesh structure are determined. An isotropic forming behaviour with low ductility is observed. The maximum stretch-forming depth is increased at increasing perforated area. The mesh structure failure is peripherally obtained due to notch effects, friction and micro related size effects within the mesh structure. Moreover, the results show that cracks and necking within the mesh structure occur only tangentially to the punch movement due to high uniaxial stresses caused by the forming process.
Abstract: Seamless rings are applied in several industrial sectors and are mainly produced by radial-axial ring rolling. Ring climbing is one of the most occurring process errors leading to a distortion of the ring’s cross section. This paper presents the determination of the influencing factors and their impact and correlations on the process error of ring climbing via Design of Experiment. The highest impact on the climbing height of a ring has the adjustment of the rolling table and the guide rolls. Relatively low but still noticeable is the influence of the process parameters of the rings displacement and the unequal axial rolling speeds. With this knowledge it will be possible to develop, test and implement a rolling strategy that may reduce or avoid ring climbing successfully.
Abstract: Cold forging enables industrial mass production of steel based components characterized by high strength and precision. The present study focuses on the FE-based analysis of a forward extrusion process. The investigated process is the so called “Samanta”-process. In practice, this is also refered to extrusion in package. During the forming, multiple blanks are pressed sequentially through a die. The results reveal the process-specific "crown-shaped" area in the upper end region of the components. Furthermore, the tribological conditions reveal a great influence on the resulting component properties.
Abstract: A more precise numerical simulation of sheet metal forming processes leads to a demand for more detailed material characterisation. Hence, it is advisable to consider the strain rate reliant and anisotropic material characteristics. There are various common sheet metals that have beside of an anisotropic a more or less distinct strain rate dependent material behaviour. With regard to these material characteristics, for a more detailed numerical prediction of a sheet metal forming process, it is necessary to include the aspect of deformation velocity. A characterisation of the strain rate dependent hardening behaviour for the two common sheet metals DC04 and AA5182-O is performed under tensile as well as shear load and their behaviour is compared after v. Mises equivalent stress and strain. The two strain rate models from Norton-Hoff and Tanimura are calibrated on basis of the experimental data and their applicability for the investigated materials is evaluated. The calibration of the strain rate sensitive models showed for both materials a very good comparability, respectively.
Abstract: The prediction of microstructure evolution in addition to the macroscopic material strength, material flow and temperature evolution is becoming increasingly important as more and more complex materials, with properties that are heavily influenced by their microstructure, are being used. This in turn requires refined microstructure models to be parameterized. Compared to flow curve models, the experimental effort for the parameterization of microstructure models increases due to the inclusion of grain size and recrystallization effects. Therefore plenty of experiments are usually performed to fully characterize the material at hand. The increasing versatility of testing machines, like dilatometry with easily variable temperatures, in addition to the growing expenses that go along with increasing the number of experiments for high cost materials, leads to the question whether performing all those experiments is really justified. In this paper the microstructure model StrucSim is parameterized for the nickel-base alloy Inconel 718 and coupled online with a finite-element (FE) simulation to predict the material behaviour during double compression tests. StrucSim combines multiple constitutive equations into a single consistent material model representing also the microstructure. Therefore these constitutive equations are parameterized to their respective metal-physical phenomena to find the initial parameters for StrucSim. Afterwards the set of final parameters is determined by optimizing the initial parameters using the StrucSim algorithm interconnecting the constitutive equations to define a reference model. The reference model is later compared to different final parameter sets parameterized based on reduced experimental data. Beforehand the reference model is coupled with the FE software Simufact.forming to simulate double compression tests and compare them to experiments as a validation of the reference model. Here forces are predicted with a mean deviation (root of the sum of squared relative errors) of 7.6 % and grains sizes with a mean deviation of about 8 μm from the measurements. Afterwards the influence of reducing the available data during parameterization of StrucSim is investigated to evaluate the possibility of reducing the experimental effort. It is shown that when using only 50 % of the data the quality can be maintained with the reduced model. When simulating the double compression tests a comparable deviation regarding the forces and grain sizes is achieved. Reducing the number of experiments by 50 % during materials characterization therefore appears feasible.
Abstract: In this Study a Heat Transfer Model in Combination with Experimental Tests is Used to Determine the Portion of Plastic Work that is Converted into Heat (also Known as the Taylor-Quinney Coefficient, Inelastic Heat Fraction or IHF and Generally Noted β) during the Deformation of Two Modern Automotive Advanced High Strength Steels (AHSS) DP600 and DP1000. Therefore, Uniaxial Tension Tests were Performed under Vacuum in a Deformation-Dilatometer and the Temperature was Captured by Fine-Wire Thermocouples on Three Different Points on the Surface of the Tensile-Specimen during the Plastic Deformation. Afterwards, a Heat Transfer Model was Used to Calculate the Heat Loss at the Points of the Temperature Measurements and they were Accounted in the Final Energy Balance to Determine the Fraction of Plastic Work Converted to Heat. the Results Show that the Fraction of Plastic Work Converted into Heat is Decreasing from 1 to 0.21 over a Tensile Strain Range of 0 to 0.18. Finally, a Finite Element Model of the Tensile Test was Used to Show the Improvement of the Determined Factor in the Calculation of the Temperature Field Compared to the Classical Assumption that β Equals to 0.9.
Abstract: Within deep drawing processes, welding represents an innovative approach to optimising the branched process chains which entail uneconomic process steps in production and transport lines. Previous applications of thermal joining processes in presses required a downstream process step for joining standardised functional elements such as nuts. Within the scope of this publication, a weldable tool system is presented which offers the possibility of welding a deep drawing component to an automatically added non-standardised holder in a single-step deep drawing process without additional dwell time in the bottom dead point. In order to realise this innovative tool system, the interdependencies of deep drawing and projection welding are considered to enable a splash-free welding on flat and curved component areas, such as the rounding of a punch edge. Based on experimental research a special concept for the tool kinematics of welding electronics is drawn up which is based on press kinematics. In addition, this article also deals with electric insulation and the forming forces which have an impact on the welding electrodes integrated into the active surface of the forming tool. Thus, the joining process becomes independent from the type of press.
Abstract: During deep drawing process compressive stresses are induced in the sheet metal, which can cause undesirable wrinkling. On the other hand a high sheet thickness and high local surface curvature counteract the wrinkling tendency by stabilizing the blank. To investigate these factors influencing the wrinkling initiation experimentally, a modified and the requirements customized test form based on a simple test geometry, the so called Yoshida buckling test, is created. This enables to take into account surface curvature in the region of instability.