Key Engineering Materials Vols. 504-506

Abstract: Development of new technologies and processes for small batch and prototype production of sheet metal components has a very important role in the recent years. The reason is the quick and efficient response to the market demands. For this reasons new manufacturing concepts have to be developed in order to enable a fast and reliable production of complex components and parts without investing in special forming machines. The need for flexible forming processes has been accelerated during the last 15 years, and by these developments the technology reaches new extensions. Incremental sheet metal forming (ISMF) may be regarded as one of the promising developments for these purposes. A comprehensive research work is in progress at the University of Miskolc (Hungary) to study the effect of important process parameters with particular emphasis on the shape and dimensional accuracy of the products and particularly on the formability limitations of the process. In this paper, some results concerning the determination of forming limit diagrams for single point incremental sheet metal forming will be described.

Abstract: Previous studies have shown that optimized tool paths based on behavior of individual features and feature interactions can be used to improve the accuracy of features in parts produced by single point incremental forming. These tool paths are generated with compensated CAD files of the part, which result from a prediction of deviations of individual features. However, in order to improve the accuracy of an entire part, it is important to systematically look at behavior of all the individual features and all feasible interactions between features. In this paper, the authors present a graph topology approach to integrating the effects of the behavior of all features present in a part. For any given part, a conceptual graph is constructed representing all the features and connecting them based on their spatial locations with conceptual relations. Next, all possible feature interactions based on the generated graph are analyzed, and the deviations due to the feasible interactions in an uncompensated test are predicted. Depending on the feature types and interactions present, a comprehensive strategy for accurate part manufacture is generated. This strategy may be composed of a selection of one or more complementary tool path strategies for compensating the anticipated deviations on the part. Case studies illustrating improvement in accuracy of parts produced by this technique are discussed next to justify the use of the graph based approach.

Abstract: With the use of two industrial robots, Roboforming is a dieless incremental forming process, which is developed by the Chair of Production Systems at the Ruhr-University of Bochum. Connected to a cooperating robot system, these two robots hold respectively a forming and a supporting tool. Suitable for rapid prototyping and manufacture of small batch sizes with low costs, this forming process is based on flexible shaping through the synchronous movement of two industrial robots. Different from other single point incremental forming (SPIF) methods, the supporting tool used here greatly increases the geometric accuracy and the limited draw angle. A new processing technology always needs the computer-aided planning and simulation, which could accelerate the whole process and also give users the possibility to analyse and improve the process. In this paper, the whole integrated process design is introduced. After the modelling of the target CAD geometry, a self-developed CAM solution is used to get both tools’ positions and orientations according to the points on the geometrical surface. Based on the different forming strategies used, the supporting tool can even be synchronously placed at different positions on the sheet backside. After the tool path planning, the paths are first inputted into a simulation environment, which is consistent with the settings in the pilot plant. The tool positions and each robot’s postures can be seen and validated during the simulation. Before the final forming experiment, the tool paths are also sent into another simulation model for the forming analysis with the use of FEM technology. With consideration of many real material properties like springback and the subsequent deformation, the formed CAD geometry from the simulation is compared with the target CAD geometry and the forming results can be forecasted.

Abstract: Forming tasks in Sheet Metal Prototyping are currently a balancing act between part flexibility and accuracy. In view of Asymmetric Incremental Sheet Forming (AISF), the part support is the decisive factor: Die-based processes such as TPIF are restricted to the given geometry of the part. On the other hand, the die-less variant (SPIF) is prone to a much more complex process-layout – once a similar accuracy needs to be obtained. Consequently, this requires a flexible die concept, supporting the part in the critical zones only. Within this article we meet this challenge by introducing the configurable tooling concept "FlexDie". This support tool comprises a construction kit for skeleton dies allowing for an adjustment of its geometry to almost any desired shape. Based on the solar cooker benchmark by Jeswiet et al., we show both the tooling-concept and the feasibility. The latter we discuss, based on the quality features geometric accuracy as well as surface quality. Both features are assessed with respect to the forming results obtained by use of a full-die. The accuracy resulting by applying the FlexDie is only slightly inferior to the parts formed by use of a full-die. However, the FlexDie allows for simple optimization of both, die and part geometry. In addition, compensation strategies by adapting the toolpath are still possible. In summary, the results show the feasibility of the FlexDie concept for industrial ISF tasks - even at very low production volumes.

Abstract: Flow curves coming from tensile tests together with constant young modulus are widely used by industry when modelling sheet metal bending processes. Unfortunately this modelization strategy based on previous mentioned variables is not accurate enough and leads to big errors due to the springback of the material. This drawback is even more important when high strength steels are bended. Aiming to reduce the springback errors, Ludwik’s hardening material models under bending deformation have been obtained for a Ti6Al4V alloy and for a MS1200 martensitic ultra high strength steel and their accuracy has been compared to the classical models obtained from tensile tests. First of all, classical Ludwik’s hardening models are obtained from tensile tests. Then, using a V-Bending test and inverse simulation, Ludwik hardening model parameters for bending are calculated using a Von Mises yielding criteria. As a result, a model able to represent the bending behaviour of these two materials more accurately without complex code modifications is achieved.

Abstract: In the automotive sector the application of advanced high strength steels (AHSS) for structural and safety relevant components plays an important role. Typical manufacturing processes concerning these parts are bending and cutting operations. However, the forming and cutting potential of these steel grades is different compared to conventional steels, as the process behaviour is changing. For an improved workpiece quality the fundamental knowledge of the damage and failure mechanisms is essential. This study presents a methodology for the analysis of AHSS in bending and out-of-plane shearing operations. Two micro alloyed high strength steels are investigated within this work. First results are presented concerning material characterisation by tensile tests, the material performance in air bending tests and the development of a modular punching tool. The study is closed by summarizing the damage behaviour along the process chain considering both bending and cutting. This shows the applicability of the presented methodology for analysing the process behaviour with respect to occurring failure.

Abstract: The use of ultra-high-strength steels (UHS) has become more and more popular within last decade. Higher strength levels provide lighter and more robust steel structures, but UHS-steels are also more sensitive to surface defects (e.g. scratches). Practically this means that the critical crack size decreases when the strength increases. The aim of the study was to study if the formula of critical crack size is valid on forming processes of UHS-steels. Surface cracks with different depths were created by scratching the surface of the sheet by machining center. Effect of the scratch depth was determined by bending the specimens to 90 degrees. Bents were then visually compared and classified by the minimum achieved bending radius. Test materials used were direct quenched (DQ) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250). Results from the bending tests were compared to the calculated values given by the formula of critical crack size.

Abstract: During the manufacture of metal parts, geometrical deviations can appear. The reasons for this can be a variation in the properties of the semi-finished product, or wear phenomena on the punch-bending machine itself or on the punch-bending tool. When geometrical deviations appear, the process parameters normally have to be adjusted manually. The choice of the most appropriate process parameters is currently based on the operator's experience. Unfortunately, this is a time-consuming and expensive procedure right at the early stages of a production scenario. In addition, the trend towards reduced part sizes with tight tolerances, made of high strength materials, is drastically increasing the requirements regarding the production process. In order to reduce the scrap rate and the setup time for production scenarios, it is necessary to implement corrective action during the process by means of a special control strategy. A self-correcting control strategy based on a closed-loop control approach is thus under development at the University of Paderborn. The first step in this strategy involved conducting simulations is to identify those process variables, e.g. the strength or the geometrical properties of the material, which have a significant influence on the process. Once correlations between input and output variables had been established, different self-correcting control strategies were set up. To validate the simulation and to test the quality of the self-correcting control strategies, a special experimental tool, mapping the most important bending operations, was constructed at the University of Paderborn. The experimental tool is equipped with an additional measurement device and can be operated on a universal testing machine. Finally, the self-correcting control strategies were tested under production conditions on the original tool in order to take any additional influences of the punch-bending machine into consideration. In this paper, recent investigations are presented that were conducted in a collaborative project at the University of Paderborn together with two industrial partners. The results of the correlation between the variables governing the process, the development of a suitable measurement method, and a first approach to a self-correcting control strategy are set out.

Abstract: Single Point Incremental Forming (SPIF) is a recent sheet forming process which can give a symmetrical or asymmetrical shape by using a small tool. Without the need of dies, the SPIF is capable to deal with rapid prototyping and small batch productions at low cost. Extensive research from both experimental and numerical sides has been carried out in the last years. Recent developments in the finite element simulations for sheet metal forming have allowed new modeling techniques, such as the Solid Shell elements, which combine the main features of shell hypothesis with a solid-brick element. In this article, two recently developed elements -SSH3D element [1, 2] and RESS3 element [3]- implemented in Lagamine (finite element code developed by the ArGEnCo department of the University of Liège) are explained and evaluated using the SPIF line test. To avoid locking problems, the well-known Enhanced Assumed Strain (EAS) and Assumed Natural Strain (ANS) techniques are used. The influence of the different EAS and ANS parameters are analized comparing the predicted tool forces and the shape of a transversal cut, at the end of the process. The results show a strong influence of the EAS in the forces prediction, proving that a correct choice is fundamental for an accurate simulation of the SPIF using Solid Shell elements.

Abstract: The ability to manufacture accurate parts in single point incremental forming is dependent on the capability to properly predict accuracy response surfaces of individual features and feature interaction combinations formed using uncompensated tool paths. Recent studies show that the accuracy profiles obtained are dependent on the choice of material used for forming, in terms of magnitude, geometric shape and nature of errors (under forming and over forming). In this paper, an attempt is made to capture the effect of material properties on the accuracy response surfaces. The response surfaces are modeled using Multivariate Adaptive Regression Splines (MARS), which is a non-parametric multivariate regression technique that helps generating continuous response surfaces. The MARS functions are based on process and feature specific geometric parameters. A set of features and feature interactions for which the response surface dependence on material properties is well predicted is used to illustrate the applicability of the MARS method for predicting the accuracy. An in-process stereo camera system is used to measure the displacement fields for different materials using digital image correlation (DIC) and understand the material dislocation mechanism. Improvements in accuracy for different sheet metal materials based on the predicted response surfaces are then discussed.