Key Engineering Materials Vols. 554-557

Abstract: The global fuel crisis and increasing public safety concerns are driving the automotive industry to design high strength and low weight vehicles. The development of Dual Phase (DP) steels has been a big step forward in achieving this goal. DP steels are used in many automotive body-in-white structural components such as A and B pillar reinforcements, longitudinal members and crash structure parts. DP steels are also used in other industrial sectors such as precision tubes, train seats and Liquid Petroleum Gas (LPG) cylinders. Although the ductility of DP steel is higher than classical high strength steels, it is lower than that of classical deep drawing steels it has to replace. The low ductility of DP steels is attributed to damage development. Damage not only weakens the material but also reduces the ductility by formation of meso-cracks due to interacting micro defects. Damage in a material usually refers to presence of micro defects in the material. It is a known fact that plastic deformation induces damage in DP steels. Therefore damage development in these steels have to be included in the simulation of the forming process. In ductile metals, damage leads to crack initiation. A crack is anisotropic which makes damage anisotropic in nature. However, most researchers assume damage to be an isotropic phenomenon. For correct and accurate simulation results, damage shall be considered as anisotropic, especially if the results are used to determine the crack propagation direction. This paper presents an efficient plasticity induced anisotropic damage model to simulate complex failure mechanisms and accurately predict failure in macro-scale sheet forming processes. Anisotropy in damage can be categorized based on the cause which induces the anisotropy, i.e. the loading state and the material microstructure. According to the Load Induced Anisotropic Damage (LIAD) model, if the material is deformed in one direction then damage will be higher in this direction compared to the other two orthogonal directions, irrespective of the microstructure of the material. According to Material Induced Anisotropic Damage (MIAD) model, if there is an anisotropy in shape or distribution of the particles responsible for damage (hard second phase particles, inclusions or impurities) then the material will have different damage characteristics for different orientations in the sheet material. The LIAD part of the damage model is a modification of Lemaitre’s (ML) anisotropic damage model. Modifications are made for damage development under compression state and influence of strain rate on damage, and are presented in this paper. Viscoplastic regularization is used to avoid pathological mesh dependency. The MIAD part of the model is an extension of the LIAD model. Experimental evidence is given of the MIAD phenomenon in DP600 steel. The experimental analysis is carried out using tensile tests, optical strain measurement system (ARAMIS) and scanning electron microscopy. The extension to incorporate MIAD in the ML anisotropic damage model is presented in this paper as well. The paper concludes with a validation of the anisotropic damage model for different applications. The MIAD part of the model is validated by experimental cylindrical cup drawing wheras the LIAD part of the model is validated by the cross die drawing process.

Abstract: Abstract. Bending effects, especially for Advanced High Strength Steels (AHSS), are known to influence the material formability when stretching and bending is combined in sheet forming. Traditional formability measures (e.g. the conventional forming limit curve (FLC)) fail to reliably predict formability when bending is added. Consequently, in order to take full advantage of the available forming potential of AHSS sheets in industrial applications and to ensure a reliable failure assessment at the same time, current research is focusing on the experimental characterization and modeling of the influence of bending effects on the AHSS sheets formability in forming scenarios of combined stretching and bending. It is expected that aside parameters such as bending radius or strain ratio, individual deformation scenarios of combined stretching and bending may affect the material formability too. Due to tool geometry and the resulting material flow in deep drawing various complex scenarios of combined stretching and bending can occur. For example, a material element is subjected to a complex deformation history of in-plane stretching with subsequent stretch-bending over a cylindrical tool contour, followed by unbending under tension. Another material element of the same drawing part presumably starts also with in-plane stretching but is consequently stretch-bent over a doubly curved tool geometry. Consequently, comprehensive knowledge on the stretch-bending deformation scenarios prevailing in deep drawing is crucial for a more reliable formability assessment. This work aims to identify and characterize the stretch-bending deformation scenarios to occur in different complex deep drawing parts (i.e. B-pillar, cross-die test) and small scale tests (i.e. Angular Stretch-Bend Test (ASBT)). For this reason, this investigation uses an innovative approach recently developed by some of the authors and published elsewhere to categorize the stretch-bending scenarios in industrial deep drawing processes. The approach consists of a stretch-bending categorization schema and a procedure to categorize the forming scenarios in deep drawing parts using data of finite element (FE) simulations. Results of the categorization can directly be plotted on the FE mesh of the deep drawing part (i.e. map type plot of deformation scenarios). The categorization approach mentioned uses results of conventional shell-type FE forming simulation and is therefore applicable in an industrial environment. The FE forming simulation results of the parts selected were analyzed using the stretch-bending categorization approach to identify which stretch-bending scenarios occur in deep drawing parts, to quantify which scenarios to prevail and to show that the conventional ASBT does not represent all the relevant deformation scenarios that prevail in typical deep drawing parts. Furthermore, with the use of experimental observations from real part forming, the stretch-bending scenarios which are the most critical (i.e. the scenarios under which failure occurs) in forming the cross-die geometry are identified. Results of these analysis are presented and discussed in detail.

Abstract: Single point incremental forming (SPIF) is a modern method of forming sheet metal, where parts can be formed without the use of dedicated dies. The ability of SPIF to form a part is based on various forming parameters. Previous work was not accomplished with the help of design of experiments (DOE), thus reducing the number of parameters varied at any time. This paper presents a Box-Behnken experimental design, which develops the numerical plan, formalizes the forming parameters critical in SPIF and analyse data. The most critical factors affecting SPIF were found to be wall inclination angle, incremental step size, material thickness and tool size. The main effects of these parameters on the quality of the formed parts were studied in detail. Actually this work aims to “optimize the thinning rate and the maximum force by considering the tool diameter and the vertical pitch as unknown parameters for two different wall angles and thicknesses”. To this purpose, an optimization procedure based on the use of response surface methodology (RSM) and genetic algorithms (GA) have been proposed for application to find the optimum solutions. Finally, it demonstrated that the developed methods can solve high non-linear problems successfully. Associated plots are shown to be very efficient for a quick localization of the region of the search space containing the global optimum values of the SPIF parameters.

Abstract: This paper discusses the application of sheet hydroforming technology to the forming of deep draw aluminum automotive body panels. Currently, the amount of aluminum in vehicle architectures is somewhat limited due to cost and also the inability to incorporate common body panel design to aluminum sheet due to lower formability. Typical aluminum sheet has approximately about 30~40% of the formability of comparative steel grades. Automotive designers have been hampered by this fact and have not been able to successfully introduce aluminum sheet for wide range of panels. Sheet hydroforming, however, has a formability advantage over many types of forming methods. This paper will discuss those advantages and show some successful applications to the automotive industry.

Abstract: In this work, an off-line compensation procedure, based on an elastic modelling of the machine structure coupled with a Finite Element Analysis (FEA) of the process is applied to Robotized Single Point Incremental Forming (RSPIF). Assuming an ideal stiff robot, the FEA evaluates the Tool Center Point (TCP) forces during the forming stage. These forces are then defined as an input data of the elastic robot model to predict and correct the tool path deviations. In order to make efficient the tool path correction, the weight of three numerical and material parameters of the FEA on the predicted forces is investigated. Finally, the efficiency of the proposed method is validated by the comparison between numerical and experimental geometries obtained with or without correction of the tool path.

Abstract: FINITE ELEMENT AND EXPERIMENTAL INVESTIGATIONS OF THE MULTI-POINT FLEXIBLE HYDOFORMING. N. Selmi*, H. BelHadjSalah* *Mechanical Engineering Laboratory (LGM), National Engineering School of Monastir (ENIM), University of Monastir, Avenue Ibn El Jazzar 5019, Monastir, Tunisia. naselmi2002@yahoo.fr, hedi.belhadjsalah@enim.rnu.tn. ABSTRACT Multi-point flexible forming (MPF) process is relatively recent flexible techniques [1], instead of the conventional fixed shape die sets, the basic idea in this process, consist to form the sheet metal between a pair of opposed matrices of punch elements, by adjusting the height of the punch elements [2]. Production of many parts with different geometry will be possible, just by using one same device and the need to design and manufacturing of various dies will be avoided that lead to great saving in time and manufacturing cost specially in the field of small batch or single production. The hydroforming process is attractive compared with conventional solid die forming processes, the basic idea consist to suppress one tool of two forming tools (punch or die), which is replaced by hydraulic pressure, only one tool is necessary to define the final shape of formed sheet. The multipoint flexible hydroforming, proposed in this paper, is an original process which combines the hydroforming and the multipoint flexible forming [3], to obtain a synergy of the advantages of both processes. The new process, subject of this work, is a combination of the last described processes that keep the whole flexibility of the basic multipoint flexible forming (with two dies), by using, only at one side, a single multipoint die to perform completely the final part shape, the fluid pressure is applied on the other side of the sheet metal part and substitutes advantageously the second die. Firstly, investigations were carried out by numerical simulation, to quantify, the effect of the most influent parameters on the process performances, and to highlight the ability of this new process, in the production of complex forms, as well as its contribution in quality, placed with regards existing flexible processes. Secondly, to prove the feasibility and to carry out a valuable experimental investigation of the multipoint flexible hydroforming, an experimental prototype was designed and realized, and successful doubly curved shell shape parts were obtained by the new process testing set up. The part profiles and the thickness distribution were in agreement with those obtained by numerical investigation furthermore, numerical investigation for efficient methods to suppress the dimpling phenomenon and edge buckling were confirmed by experimental investigation. From investigations it appears that the parameters attached to the discreet character of the multipoint tool, have an important effect on the quality of the final metal sheet product, such as, the punch elements density, the punch elements extremity curvature radius, the blank and the elastomeric interpolator thicknesses. From simulation results, it emerges essentially, that an adequate setting of parameters can upgrade the thickness distribution, reduce the residual stress and attenuate the dimples. References: [1] Zhong-Yi Cai, Shao-Hui Wanga, Ming-Zhe Li, (2008), Numerical investigation of multi-point forming process for sheet metal: wrinkling, dimpling and spring back, Int J Adv Manuf Technol (2008) 37:927–936. [2] Zhong-Yi Cai, Shao-Hui Wang, Xu-Dong Xu, Ming-Zhe Li (2009), Numerical simulation for the multi-point stretch forming process of sheet metal, journal of materials processing technology 209 (2009) 396–407. [3] N. Selmi, H. Bel hadj salah, Simulation numérique de l’hydroformage à matrice flexible, 7éme journées scientifiques en mécanique et matériaux JSTMM2010, Hammamet 26-27 novembre2010.

Abstract: The reduction of the maximum temperature is one of the main goals in the research activities dedicated to hot sheet stamping, thanks to the benefits it can produce in terms of energy consumption and die wear decrease. New steel grades are being expressly developed with the aim of reducing the austenitization temperature without losing the mechanical characteristics and the formability shown by the conventional Usibor 1500P™. In the present work, the flow behavior of four new steel grades is investigated by means of hot tensile tests at varying thermo-mechanical conditions. Results are presented and discussed in terms of obtained mechanical and ductility characteristics.

Abstract: Micro forming processes are very well suited for manufacturing of small metal parts in large quantities and micro deep drawing provides a great application potential for the manufacturing of parts with complex shapes. But size effects like changed tribology and material properties usually result in smaller process windows for micro forming operations. Process caused wear as well as large inaccuracy in manufacturing of micro forming tools is responsible for geometrical deviation of the tools from nominal size. Both influences can have essential impact on the process window size and process stability. A better understanding of the influence of tool geometry on process stability can help to improve and optimize process control in micro forming. In addition, a quantitative judgment of the impact of wear and manufacturing inaccuracy will be possible. Therefore, in this study, the impact of different tool geometries on the punch force in micro deep drawing was investigated. Significantly varied tool geometries were punch diameter, drawing gap, punch and drawing die radius and shape of the die edge. FEM simulations as well as experiments were used to determine tool geometry influence on the punch force of a micro deep drawing process. Hereby, it was possible to classify each geometry variation regarding its impact on the punch force and therefore on one important parameter of the process stability. Results show that the greatest impact on the punch force was caused by modifications of the punch diameter and variation of the drawing gap. Changes in punch or drawing die radii proved to be of minor importance.

Abstract: Incremental Sheet Forming (ISF) is able to produce highly customized products at a reasonable manufacturing cost and it has gain importance in the last years, becoming the focus of interest for many researchers and institutions. Some recent publications have revealed an increasing interest in forming thermoplastic materials. There is a tremendous amount of effort put in developing a model that may describe the equilibrium hysteresis and rate-dependence of thermoplastic materials in ISF. This paper will present a brief review of the most common constitutive equations that are able to model the behaviour of glassy polymers. It will be shown that by using a small number of material parameters defined in the Marlow model, it is possible to accurately predict experimental data collected on samples of PVC subjected to simple uniaxial test performed at room temperature. Additionally, some parts have been formed with ISF in order to verify whether the material is incompressible or not. It can be concluded that Marlow model might be used in future work to model the ISF manufacturing process.

Abstract: This study investigated the twisting phenomenon in curved hat channel products made of dual-phase 980-MPa-class high-tensile-strength steel sheets. The stroke returning deep drawing (SRDD) method was proposed to deal with twisting. In this new method, after the punch reaches the bottom dead point, it returns to a certain drawing height without the blank holder being removed. With the application of the SRDD method, twisting hardly occurred, but sidewall curl increased. A two-step SRDD was then proposed to reduce the sidewall curl of SRDD products. In the two-step SRDD method, a stroke returning process is carried out in two steps under different conditions. The results showed that the two-step SRDD method reduced the sidewall curl and twist simultaneously.