Key Engineering Materials Vols. 410-411

Abstract: Oxidation in hot stamping of ultra high strength steel sheets was prevented by coating the sheets with an oxidation preventive oil. For four types of oxidation preventive oil, the degree of oxidation under natural cooling of the heated sheets without forming was first evaluated. The oil that forms a liquefied film at elevated temperatures exhibited high oxidation prevention, and this oil was chosen for a hot bending experiment. Hot hat-shaped bending of the coated sheets using resistance heating was carried out to examine the properties of the products. The bending load was markedly decreased, the shape accuracy of bent products was very high, the surface roughness was similar to that of the sheet, and the hardness was about 1.5 times larger than that of the sheet before the bending due the die quenching. In addition, the layer remaining on the surface of the formed product could be removed using phosphoric acid. It was found that the hot stamping operation using the oxidation preventive oil is effective in the precision forming of ultra high strength steel sheets.

Abstract: This study investigates the effect of preheat temperature, blank holder force and feed rate on the formability of polypropylene based Fibre-Metal Laminates. Finite element method combined with Design of Experiments was used to determine the influence of the forming process parameters. The design of experiments was used to identify the relative influence of each process parameter considered in this study. A reduced set of coupled structural-thermal simulations using Ls-Dyna were carried out using a L9 orthogonal array. Simulations were carried out on the forming of domes. It was found that the blank holder force has the greatest influence to increase the minor/major ratios followed by feed rate and pre heat temperature. A more thorough investigation of preheat temperature illustrated an optimum preheat temperature at 130 °C.

Abstract: The present work has been undertaken in order to investigate the possibilities of improvements in the production technology of aluminium drawn-parts. Past technology does not guarantee the production of high quality elements. Aluminium is difficult to use in sheet-metal working processes, mainly because of a susceptibility to the formation of aluminium „build-ups” on the tools, i.e. on the punch and die. The “build-ups” can be limited or even completely eliminated by a proper selection of the technological lubricants and proper surface treatment of the tools (e.g. surface coatings). In order to determine the influence of some technological lubricants and anti-adhesive layers on frictional resistance occurring during the sheet-metal forming process a typical “strip-drawing” test has been carried out. 1050A aluminium strips with thickness of 0.5mm have been drawn between two steel die inserts. Tests have been carried out with lubrication and in dry conditions. Additionally, two anti-adhesive coatings - TiN and Cr coatings were tested. Friction coefficients for different frictional pairs have been determined. In order to analyse how friction impacts on the course of sheet-metal forming process a numerical simulation based on the finite element method was performed. A four-stage stamping process of an aluminium cup was analysed. The value of the friction coefficient was changed in order to take into account frictional resistance. Thickness of the cup walls was assumed as one of the parameters determining the usefulness of the drawn-part because it determines its strength.

Abstract: In this paper, some technological problems (e.g. low drawability, high susceptibility to galling, spring-back) occurring in the sheet-titanium forming process are discussed. A numerical simulation of the stamping process was carried out with the Adina System v.8.3 based on the finite element method. The effects of friction, the holding-down force and tool geometry on the course of the stamping process were analysed. The mechanical and technological material data needed for the calculations were determined experimentally. The friction coefficients for the frictional pair: ”titanium – tool steel” for different lubricants and antiadhesive layers determined in the “strip drawing” test were found. The role played by lubrication and antiadhesive layers in preventing titanium “build-ups” on the tools is presented. The calculated results were then confirmed experimentally.

Abstract: To improve the low formability that HSS sheets exhibit at room temperature, innovative forming technologies like the hot stamping process are currently applied. In order to avoid scaling and decarburization during the heating step, metal sheets coated with a specially developed Al-Si coating are utilized. In the present work the coating characteristics in terms of morphology, surface roughness and tribological behaviour are investigated as function of heating temperature, holding time and cooling rate that are typical of hot stamping processes.

Abstract: The paper presents the approaches followed by two labs – LFT at the University of Erlangen-Nuremberg (Germany) and DIMEG at University of Padua (Italy) – in evaluating formability limits of 22MnB5 sheets when processed under hot stamping conditions. Details about the two testing apparatuses and the testing procedures are outlined, and the results in terms of Forming Limit Curves FLC compared and critically commented.

Abstract: The V-ring indenter geometry (angle, height and position) was investigated by using the finite element method (FEM) to theoretically clarify the mechanism and its action in the fine-blanking process. The FEM simulation results indicate that very small or very large V-ring indenter angles, heights, and positions cause difficulty in the rotation of the material-flow and that the hydrostatic pressure is generated with great difficulty in the blanked material; therefore, crack formation occurs easily. The application of a suitable V-ring indenter angle, height, and position significantly suppresses the formation of rotating flow, which results in increased hydrostatic pressure, and crack formation is consequently prevented.

Abstract: A hot gas bulging process of an aluminium alloy tube using resistance heating, set into a forming machine, was developed. The tube was rapidly heated by the electrifying to increase the formability and to decrease the flow stress. The tube was bulged by thermally expanding the air sealed in the tube without control of internal pressure during the forming. Hot gas bulging of an aluminium alloy tube without and with axial feeding was performed. The effects of the initial internal pressure and the current on the expansion ratio of the tube were examined. The decrease in temperature around the contact with the electrode was prevented by inserting a stainless steel ring having low thermal conductivity and high heat generation between the copper electrode and tube, and thus the bulging length was increased. It was found that the hot gas bulging is effective in heightening the formability of the aluminium alloy tubes.

Abstract: Bent metal tubes find a widespread application in many industrial sectors. Among different bending processes developed for the manufacturing of these components, three-roll bending is characterized by a high flexibility, as only one toolkit per tube diameter is necessary to form the required bending radius. In this type of forming process the part geometry is obtained by means of a relative movement of the die (setting roll) towards the fixed tools (bending and holding roll) with simultaneous feeding of the tube. This study describes the FE-model developed for the three-roll bending and presents first results of numerical investigations conducted on steel tubes made of carbon steel St37. By the FE-analysis great attention is paid on the modeling of the stiffness of the tool, on the description of the kinematics of the setting roll as well as on the characterization of the material behavior for the simulation. The results of the numerical investigations are compared with experiments conducted with a CNC-bending machine available at the Chair of Manufacturing Technology of the University of Erlangen. As a main criterion for the validation of the FE-model the radius of the tube at the extrados and the bending angle are chosen. The geometry of the part is measured by means of both optical and tactile measuring devices.

Abstract: Seamless tubular components, such as stepped tubes, conical tubes and box shape tubes, are mainly produced in tube hydroforming process. In forming the components in this process, complete filling the die corners is very difficult. In this paper, the mechanism of improvement of die corner filling in a proposed tube hydroforming die was investigated. The FEM simulation results showed that the material flow and stress distribution could theoretically clarify the die corner filling in the proposed die. Also, the comparison of the die corner filling between the new die and a conventional die was explained. In order to verify the simulation results, some experiments were performed.