To predict accurately the forming limit in sheet metal forming, the combination of FE simulation with tension tests is adopted in this paper to determine the material constants
p and
C in a ductile ...fracture criterion (DFC), which is advanced by the author. Forming limits of bore-expanding, hemispherical punch bulging and deep drawing (cylindrical, square-cup parts) are predicted by means of the DFC. Comparison of the results predicted by the DFC with experimental values shows that the precision of forming limit predicted by material constants obtained by the combination method is more accurate than that predicted by material constants obtained by the tension method, and that the critical punch stoke and the fracture initiation position in forming processes above mentioned are predicted accurately by the DFC.
In this paper, pulsed attractive electromagnetic force between two coils has been introduced for supplying blank holding force in sheet metal forming process. First, the requirements on the pulsed ...blank holding force and constraint conditions on the system were specified. Then, a design flow of the coil system fulfilling the requirements was presented. Based on the design, a prototype of a pulsed electromagnetic blank holder was fabricated. Numerical and experimental investigations were carried out to validate the feasibility of the designed system. Results show good agreement between the measured and simulated results of discharge currents and magnetic fields. The system generates a pulsed blank holding force with a magnitude of 930 kN and a rise time of 4.56 ms, which fulfills the specified requirements.
How to perform sheet metal forming in a rapid and cost-efficient way, especially for the small-batch productions, has been a challenge to the manufacturing industry for a long time. Incremental sheet ...forming (ISF) has vast potentials in rapid and economical production of small-batch thin-walled parts, but so far, the technology is mainly applied in a limited forming range of planar sheets, most with a deformation mode of bi-axial stretching. To establish a more complete forming chain based on ISF and then take full advantage of it, this paper proposed a new methodology called “Multi-directional Incremental Sheet Forming (MISF),” particularly aiming to accomplish the non bi-axial stretching formation of thin-walled structures such as flanging, curling, and localized forming of planar sheets and hollow profiles. The method can not only extend the application of conventional ISF, but also can fabricate some special-shaped shell components which are hard to integrally form by any other ways. Thus, it can provide a strong support to realize the lightweight of structures. The progress review of MISF was presented, and the mechanical features during forming, including the complex coupled elastic-plastic deformation, buckling and post-buckling, the combination of constrained and free deformations, etc., were summarized. Moreover, key items of defect control were addressed.
In sheet metal forming simulations using finite element method, the coefficient of friction according to Coulomb is normally assumed to be constant for the whole tool region. Recent research has ...demonstrated that under contact conditions typical for sheet metal forming, the coefficient of friction is strongly dependent on the contact pressure and the slip-rate between the interacting partners as well as on the contact temperature. A friction law based on the approach of Filzek is proposed in this research paper and implemented as a user subroutine in
Abaqus
. Moreover, a novel user subroutine coupling is developed which enables the visualization of local coefficients of friction for every contact node. For the very first time, this enables the in situ visualization of local coefficients of friction in sheet metal forming simulations using the FE system
Abaqus
. Multiple evaluation algorithms are presented as well. The presented friction model and the user subroutine coupling are validated using experimental data of an industrial deep drawing process. The proposed combination of the friction modeling and the in situ visualization of coefficients of friction enables the identification of friction hot spots. Based on such analysis, the tool and die making industry can realize tribologically optimized tools either by applying special coatings in areas of low or high friction, or by choosing different materials for tooling inserts, or by changing the type as well as the amount of the lubricant. Whereas the presented friction model is tailored to sheet metal forming and developed using the commercial code of
Abaqus
, the visualization coupling methodology is transferable to any manufacturing process and various FE systems supporting
Fortran
programming language.
This paper deals with simulation methods for predicting exact crack position of advanced high-strength thick steel sheets under the stretch-bending condition. The material properties related to ...anisotropy and hardening behavior were investigated from the tensile test, the loading-unloading and the tension/compression test. The stretch-bending test incorporating DIC ARAMIS system was performed for various punch radii in order to investigate the crack position and the fracture strain. Three simulation methods were established according to the material model and the element type for finding key points for the stretch-bending crack prediction. Simulation results show that the contact pressure effect should be considered for predicting the accurate stretch-bending crack phenomenon. A proposed simulation method can be effectively utilized to the sheet metal forming process design of automotive high-strength chassis parts.
For sheet metal forming simulations, it is frequently sufficient to consider only the plane stress conditions. However, for certain applications like bending with small radii, thick sheets or special ...forming processes, e.g. hemming, wall ironing and roll forming, neglecting the stresses σ33, σ13 and σ23 is not a workable solution to obtain accurate and reliable results. Hence, the full stress state has to be taken into account to improve the simulation quality.
Within the work presented, a roller hemming process is simulated by considering the full stress state through continuum elements and anisotropic 3D yield functions. The parameters for the anisotropic 3D yield models Hill48 and Yld2004-18p used here are identified by experimental measurements as well as a crystal plasticity modelling approach. The results are used to assess the effect of incorporating the full stress state and the influence of various anisotropic 3D yield functions on roller hemming simulations. The material considered is a sheet made out of DX56D deep drawing steel.
Increasing the flexibility of manufacturing processes is regarded as key challenge for modern production facilities to meet the requirements of market economy. An option to achieve a wide production ...variety of forming processes is to realize geometrical independence between tools and products. If not bound to tool shape, force transmission from machine to product can be transposed by friction. Because strain is determined by the size and shape of the forming zone, it is hence of crucial interest how the zone develops over the ongoing process. This study presents an analytical approach to study the forming zone of a frictionally engaged, i.e. force fitted, bending process. The model depends parametrically on the mechanical material properties as well as the kinematic parameters with regard to bending angle, clamping pressure and friction. An estimation of the material portion, that is drawn out of the initially clamped area is given by the analytic process model which can thus be deployed to describe strain within the bent arc. For validation purposes, the force fitted in-plane bending process is implemented in a numerical simulation based on solid elements. A significant influence of the material’s hardening properties on the forming zone is observed from the models. With regard to the process, the parameters clamping force and friction coefficient were identified as important influences. With these achievements, the present approach is intended to deliver transferrable knowledge for forming processes, in particular about the areas where tool clamping applies, and thus the impact of strain on geometry, such as the beginning and ending transition zones of a bent arc. In contrast to numerical approaches, the presented analytical model does not require significant computing effort and, with regard to its equations, allows to identify the main influences on strain generated within the forming zone of force fitted bending.
Sheet metal forming boundaries are established using the forming limit diagram (FLD). The Nakajima and Marciniak tests, which are based on stretching a material using a punch, are the most commonly ...used methods for determining the FLD or fracture forming limit diagram (FFLD). The results are usually evaluated by calculating local strain, strain rates, specimen thickness reduction or fracture strain. When the amount of experimental material is insufficient, miniaturization of the testing specimens may be a solution. However, the interchangeability of the results for standard and miniaturized specimens has not been proven yet. In this study, the Nakajima tests were performed using standard and sub-sized specimens made of manganese–boron steel 22MnB5, commonly used in the automotive industry. Afterwards, four FLD/FFLD evaluation methods were applied and compared. The miniaturized specimens yielded higher strain values, which was explained by the varied ratio of material thickness/punch diameter and different bending conditions. The highest compliance of the results was recorded for the standard and miniaturized FFLD.
The use of high-strength sheet material applications in the automotive industry has become widespread due to their high strength/weight ratios. The formability, however, of these high-strength sheet ...materials is limited at room temperatures (RTs). In this study, the first of its kind to be conducted, experimental research was performed on the formability of HC420LA grade sheet material using the warm deep drawing (WDD) method. Temperature control is the most important parameter in WDD. In this method, a new temperature control system was designed and manufactured to increase the formability of HC420LA grade sheet material. The limiting drawing ratio (LDR) for a 1.2-mm sheet thickness of HC420LA grade sheet material, which is 2.14 at RT, increased to 2.47 after applying this method. For a 1.5-mm sheet thickness of HC420LA, LDR, which is 2.15 at RT, increased to 2.59 after applying this method. Percent increases of the drawing ratios for 1.2 and 1.5 mm thicknesses were 15.42 and 20.45 %, respectively. A maximum reduction of 15 % in thickness was obtained in formed cups. Lastly, the microstructures of the warm cup’s punch corner region and wall and bottom regions were investigated under an optical microscope. The results showed whether any changes occurred in the microstructures and mechanical properties.
This research work primarily focused on investigating the effects of changing rotational speed on the forming temperature and microstructure during incremental sheet metal forming (ISF) of AA-2219-O ...and AA-2219-T6 sheets. Tool rotational speed was varied in the defined range (50–3000 rpm). The tool feed rate of 3000 mm/min and step size of 0.3 mm with spiral tool path were kept fixed in the tests. The sheets were formed into pyramid shapes of 45° draw angle, with the hemispherical end forming tool of 12 mm diameter. While the sheets were forming, the temperature variation due to friction at the sheet–tool contact zone was recorded, using a non-contact laser projected infrared temperature sensor. It was observed that the temperature rising rate for the T6 sheet during ISF is higher as compared to the annealed sheet, thereby showing that the T6 tempered sheet offers higher friction than the annealed sheet. Due to this reason, the T6 tempered sheet fails to achieve the defined forming depth of 25 mm when the rotational speed exceeds 2000 rpm. The effects of rotational speed and associated rise in the temperature were examined on the microstructure, using the scanning electron microscopic (SEM). The results reveal that the density of second phase particles reduces with increasing speed reasoning to corresponding temperature rise. However, the particle size in both tempers of AA2219 received a slight change and showed a trivial response to an increase in the rotational speed.
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