Key Engineering Materials Vols. 504-506

Abstract: Bending can be considered one of easier sheet metal forming processes. In fact, it represents one of the basic variants of applied deformations to metal blanks. However, the numerous research contributions dedicated to sheet metal bending that have been published over the past decade and the constant stream of announcements by R&D departments of machine constructors are strong indications that not all research challenges related to sheet metal bending have been done. This paper reports the developed activity carried out to design a bending testing rig characterized by: a working horizontal axis, a maximum bending length equal to 200 mm, a maximum applicable force equal to 80 kN. A partitioned blankholder has also been designed to allow bending operations on tailored blanks. Moreover, a Graphical User Interface hollows to set up the process parameters and the acquisition of testing data (Temperature and/or Force as function of the process time or punch stroke). CAE tools application had a strategic role to develop the best layout and to find the optimum solutions for the process variables tuning. CAE techniques have allowed to investigate and verify different layout solutions both for the bending process and the structural components of the tooling.

Abstract: Abstract. A method for the prediction of cross-sectional distortions of hollow sections under the action of combined bending and stretching has been developed. The model is based on classical deformation theory of plasticity combined with detailed interpretations, making the simplifications necessary to develop a practical closed-form solution while retaining basic mechanisms. The model provides the ability to assess the effect of material properties, tool and section geometry on deformation of individual cross-sectional members. The applicability of the model has been verified with a series of carefully controlled laboratory experiments done in an industry-type rotary stretch bending machine. The findings show that the main parameter with respect to (magnitude of) nominal deformations is the width of the cross section, whereas flange thickness is the main parameter with respect to controlling dimensional accuracy. Comparing the experimental and analytical results, the agreement is remarkably good - in most cases comparable to what could be achieved with more elaborate FE analyses - despite the simplifications necessary to develop the model. A new tool design concept denoted Flatness Limit Curve has been introduced, and used to demonstrate the general applicability of the approach.

Abstract: Industrial application of magnesium alloys is increasing in the last decade due to the very high mechanical strength to weight ratio they present. Typical applications are aeronautic parts but their use in the automotive industry is also growing. The bigger the reduction of the vehicle weight is, which can be obtain using these alloys, the bigger the energy saving will be in the near future. However, the formability of these alloys is poor and they are very difficult to be formed at room temperature. Several works have been presented by different authors on the positive influence the temperature increase has to form magnesium alloys. Normal process temperature is about 200-250°C. In this paper warm incremental forming of magnesium alloys is investigated. The work focuses in the formability limits of the alloys and the identification of the optimal process parameters. Aiming to obtain a homogeneous temperature in the forming areas, hot fluid is used as heating media. Experimental results for different temperatures are presented and final microstructures are related to the possible deformation mechanism.

Abstract: Incremental sheet metal forming (ISF) had a great interest in the scientific community, in the last years. A common opinion is that ISF has not to be considered as an alternative to conventional stamping but has to be regarded as a process able to work materials in a new way. Furthermore, ISF could be a suitable alternative to manufacture some “hard to work materials”. Among them, Titanium plays a relevant role. Today Titanium is usually worked by superplastic forming (SPF) or hot forming (HF) in case of simple shapes. However, both the processes are very slow and expensive. In a previous work the authors showed how it is possible to form Titanium alloys using ISF combined with a local heating. However, heating suggests also to analyze energy consumption. The process does not requires large forces but is really slow. Thus, the different heating sources can have a deep impact on the global energy performance. The paper is a first attempt to consider the process in a wider view, looking at the energy consumption as a primary issue. In particular, a comparison among different heating methods was carried out.

Abstract: The effect of localized laser hardening on the dimensional accuracy of incrementally formed steel sheets has been studied. By dynamically heating by means of laser beam scanning (500W Nd:YAG) the temperature of the sheet reaches the austenization temperature and by subsequent self-quenching a hard martensitic structure will form. Using FE modeling, a laser power setting of 202 W, scanning velocity of 600 mm/min and beam diameter of 6 mm were selected as optimum processing parameters for transformation hardening. Hardness tests were performed in order to investigate the hardness profile along the depth and width of the laser hardened zone. Experimental results reveal that generation of a selectively hardened martensitic band, formed by transformation hardening, can increase the accuracy of the incrementally formed part.

Abstract: In the present work, the upper-bound approach is used to study the deformation zone of single point incremental forming of truncated cones. The velocity field and the dissipated power of the process are achieved using an assumed deformation zone and streamlines defined by Bezier curves. The tangential force acting on the tool is attained by optimizing the presented upper-bound solution. Then, influences of the effective parameters including vertical pitch, initial thickness, tool diameter, and wall angle on the tangential force are investigated. In order to validate the presented upper-bound solution, predicted tangential forces are compared with experimental data available in literature. The comparison shows an appropriate agreement between them.

Abstract: In this paper, authors present a highly flexible tooling system based on reconfigurable multi-point thermoforming (MPTF) methodology, which has been developed within an EU-granted FP7 project. The MPTF technology employs an actuated-punch matrix to dynamically configure a controllable tool working surface through digitally adjusting relative displacement of each punch in the matrix. Novel MPTF methods have been proposed through re-changing configurations of actuated-punch tooling system according to rapid thermoforming principles and relevant cladding applications. The tooling system includes an industrial-scale prototype of an MPTF tooling integrated with functional CAD/CAE/CAT software interfaces. The numerical simulation with an explicit FEM predicts the unexpected deformation defects of dimples and wrinkles regarding to discrete contact boundaries between punches and the sheet blank. Innovative techniques of variable blank-holder and deformable cushion have been implemented to suppress wrinkling and eliminate dimpling effectively. The tooling system has been successfully applied to manufacture complex double-curved panels, which are described as application examples. Compared with conventional fixed moulds, the flexible tooling offers robust, rapid and re-changeable means to make mould-less manufacturing large freeform panels.

Abstract: Local damage models are known to produce pathological mesh dependence in finite element simulations. The solution is to either use a regularization technique or to adopt a non-local damage model. Viscoplasticity is one technique which can regularize the mesh dependence of local damage model by incorporating a physical phenomenon in the constitutive model i.e. rate effects. A detailed numerical study of viscoplastic regularization is carried out in this work. Two case studies were considered i.e. a bar with shear loading and a sheet metal under tensile loading. The influence of hardening / softening parameters, prescribed deformation rate and mesh size on the regularization was studied. It was found that the primary viscoplastic length scale is a function of hardening and softening parameters but does not depend upon the deformation rate. Mesh dependency appeared at higher damage values. This mesh dependence can be reduced by mesh refinement in the localized region and also by increasing the deformation rates. The viscoplastic regularization was successfully used with a local anisotropic damage model to predict failure in a cross die drawing process with the actual physical process parameters.

Abstract: The strain induced martensite formation is known to be sensitive to the stress state [1]. The amount of martensite formed varies if the same amount of load is applied in different states of stress. For example, martensite formed is maximum in tension, minimum in compression and somewhere in between, in shear. Martensite could also originate due to elastic stress, or in absence of any mechanical energy, solely, by change in temperature (thermal martensite). Thus, it is the aim of this project to understand the different kinds of martensite that originate due to different processing paths and then, to understand how the precipitation behaviour is affected by the process of arriving at the martensite. Basically, it is to understand how the dislocation substructure varies under various stress states and thermal states, and how it affects the kinetics and type of precipitation in metastable austenitic stainless steels. This work is carried out with a high alloy, metastable and precipitation hardenable stainless steel called Sandvik NanoflexTM. Formation of strain induced martensite in semi austenitic metastable stainless steels is strongly a function of strain rate. It is also a function of stress state: in this study a planer planar shear state and an axial tensile state of stress are compared. The morphology of martensite formed in tension is different from that in shear. Predominantly sheared samples show rippled structures with ridges and valleys where as predominantly tensile deformation creates samples with planer planar laths separated by crevasse-like boundaries. The morphological difference in the martensite formed under different stress-states, creates different shape of precipitates in semi austenitic metatstable stainless steels. The predominantly sheared samples show roundish precipitates where as predominantly tensile deformed samples show precipitates with a core. Difference in the dislocation substructure is thought to be the root cause of such morphological differences in the martensite and precipitates formed through different stress states.

Abstract: For the manufacturing of large quantities of profile-shaped products, the roll forming process represents one of the most effective metal forming technologies. During this process, the sheet metal will be formed into a desired cross-sectional profile using successive pairs of forming rolls. This process is well known as a very complex process in industry because of the multiplicity of the process and design parameters. For that reason, the optimization of roll forming processes using numerical methods like the finite element method is very complex and time-consuming. In this paper, a numerical method will be introduced to accelerate the simulation and to optimize the roll forming process. The newly developed algorithm will be illustrated and validated by analyzing the roll forming process. The details of the FE-model and the numerical algorithm will be described. Furthermore, the results of the numerical simulation with and without the application of the numerical algorithms will be compared. Finally, the process will be optimized using the newly developed method.