bluuebunny/crossref_metadata_2025 huggingface dataset:
The Influence of Through-Thickness Material Property Variation on Stretch Forming Springback
Authors: McMurray Robert J. , Leacock Alan G. , Brown Desmond
Published: March 2011

Stretch forming is commonly used in the manufacture of low-volume convex aircraft panels. The gentle curvature and high degree of stretch leads to relatively low levels of elastic recovery post forming. Previous investigations [1] have shown that there is a large difference in the measured springback depending upon which side of an Alclad aluminium sheet is in contact with the tool surface. Initial investigation into the clad layer properties revealed no accountable variation in either thickness or strength. Instead the asymmetric springback behaviour was attributed to material property variations through the alloy core of the sheet. An analytical method for the prediction of the asymmetric springback incorporating the through thickness property variation is presented. Material strengths at specific through thickness positions were determined using a procedure based on residual strength measurements. The predicted results correlate well with the experimental data, which had initially demonstrated the asymmetric springback
Transformation in Austenitic Stainless Steel Sheet under Different Loading Directions
Authors: van den Boogaard Ton , Krauer J. , Hora Pavel
Published: March 2011

The stress-strain relation for austenitic stainless steels is based on 2 main contributions: work hardening and a phase transformation from austenite to martensite. The transformation is highly temperature dependent. In most models for phase transformation from austenite to martensite, the stress triaxiality plays an important role also. The sensitivity to triaxiality is often investigated based on uniaxial compression and tensile tests. To validate the common formulation for triaxiality dependence of the martensitic transformation, a series of experiments is performed with the Twente biaxial tester for sheet material. A number of deformation directions are prescribed between plane strain and simple shear. Uniaxial tensile tests were performed at different temperatures to get a temperature corrected reference curve for the martensite–strain relation. The current results for typical stress states in sheet forming do not show the dependency on the triaxiality that is given in literature. This means that for sheet forming simulations, changes in stress state affects the martensitic transformation less than expected from tension–compression experiments.
Numerical Simulation of Forming Limit Test for AZ31 at 200°C
Authors: Nebebe M. , Bohlen Jan , Steglich Dirk , Mosler Jörn
Published: March 2011

This work is concerned with numerical analyses of the forming behavior of magnesium at elevated temperature. For that purpose, a thermodynamically consistent, rate-dependent, finite-strain elasto-plastic constitutive model is presented. This model captures the stress differential effect as well as the anisotropy of magnesium. Furthermore, the change in shape of the yield locus (distortional hardening) is also taken into account. This constitutive law, together with its parameter calibration based on uni-axial tensile tests, is finally combined with the localization criterion originally proposed by Marciniak and Kuczynski and applied to the simulation of forming limit test. Comparisons to experiments show the excellent predictive capabilities of the model.
Modification of the Mechanical Anisotropy in Extruded AZ31 Sheets
Authors: Grittner N. , Engelhardt M. , Hepke M. , Bormann Dirk , Behrens Bernd-Arno , Bach Friedrich Wilhelm
Published: March 2011

Due to its low density and good mechanical properties Magnesium holds a high potential for design applications. The investigations discussed herein focus on the use of extruded magnesium sheets as semi-finished products e.g. for subsequent processing by roll forming. Special interest is given by the hcp-structure of magnesium which reduces forming abilities. Extrusion experiments using AZ31 were carried out to investigate the influence of different die geometries (S = sharp-edged and R= radius), billet temperatures (TB= 350°C and 390°C) and extrusion speeds (vext= 2.31 m/min and 9.24 m/min) on mechanical properties, grain structure and texture of said alloy. Two types of sheets with the dimensions of 80 x 1 mm and 80 x 2 mm respectively were produced using the institutes 10 MN extrusion press make SMS MEER. Sheet material with a thickness of 1 mm rolled from cast billets has been used for reference. Microstructural analysis focused on recrystallization behavior and grain size as well as grain distribution within the sheets. Tensile tests in extrusion or rolling direction as well as 45° and 90° to the same showed a strong influence of the specimen orientation on the mechanical properties. Additional Erichsen cupping tests demonstrated the stretch-forming capacity of the different test set-ups.
Modeling Laser Heating for Roller Hemming Applications
Authors: Kleeh Tobias , Merklein Marion , Roll Karl
Published: March 2011

In order to improve fuel efficiency, the use of lightweight materials in the automotive industry is continuously increasing. The AA6XXX and AA7XXX aluminum alloys in particular are the subject of significant attention. The aim is to use them in the same way as other structural materials such as conventional steel. Unfortunately their formability presents major challenges. These alloys lack sufficient formability for bending/hemming operations in particular. To overcome this, forming can take place at elevated temperatures. In this study, a combined laser-assisted roller hemming process is set up. Therefore, a 4000 W Nd:YAG-laser with a wavelength of 1096 nm is used. The first step involves qualifying the process window for laser heating. Several parameters are defined and effects on temperature and surface quality for AA6014 are detected using the design of experiments. The second step involves setting up a finite element model of the heating process. Different modeling strategies for laser heating are compared and a new approach is presented. The laser heating source is modeled by a semi-circular Gaussian temperature distribution and is validated through comparison of the temperature with laser heating experiments, showing very good agreement in the range from 600-4000 W. Finally, the validated thermal model is coupled with the roller hemming simulation and investigations into the thermal process window are performed.
A Predictive Model for Tolerance Verification of Bent Sheet Metal Parts
Authors: Nguyen Thi Hong Minh , Duflou Joost R. , Kruth Jean Pierre
Published: March 2011

This paper presents a complete model including method and result for tolerance verification of bent sheet metal parts. The method uses a mathematical model to take into account aspects influencing the dimensional accuracy of linear and angular dimensions of bent parts. The model was implemented using a set of experimental data to estimate the dimensional accuracy for dimensions of interest of a specific part. Comparing the prediction data with the measured results allowed validation of the model.
Incremental Bending of Ultra-High-Strength Steels
Authors: Määttä Antti , Mäntyjärvi Kari , Karjalainen Jussi A.
Published: March 2011

Utilisation of ultra-high-strength steels (UHS) has increased, particularly in the automotive industry. By using these materials vehicle structures can be lightened. However, one of the problems of UHS is weak formability. Materials fracture easily with small bending radii and the minimum bending radii are rather large. In this study, the tested materials were complex phase (CP) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250). The steels were incrementally bent with a press brake in the rolling direction and perpendicular to it, and the final bending angle was 90 degrees. The incremental bending angles were 150°, 130°, 110° and 90°. The punch was unloaded after every incremental bending step. The test materials were bent with different bending radii. The aim was to find the minimum bending radius which produces an acceptable bend. Every incremental bend was compared with a bending performed in the traditional manner. The aim of this study was to examine how well the results of incremental bending compare to roll forming. In addition, clarification studies of when the bend started to fracture were made. It is well known that steels are more efficiently bent by roll forming compared with traditional bending. The results presented in this study demonstrate that incremental bending does not produce better results than traditional bending. Nevertheless, it has been shown that the examined steels can be bent incrementally against manufacturer’s recommendations.
Numerical Simulation of Electron Beam Welding Process of Inconel 706 Sheets
Authors: Lacki Piotr , Adamus Konrad , Wojsyk Kwiryn , Zawadzki Marcin
Published: March 2011

Welding operation of aircraft engine sheet part will be analyzed in this paper. The sheet part is made of narrow Inconel 706 sheet pieces. During manufacturing process first sheets undergo the process of bending. Subsequently they are welded to produce the final shape. Finite element analysis will be used to model welding operation. The thermal field and its impact on the stress field will be analyzed. The produced results will be used to design the actual welding process. Sheets will be welded using electron beam welding, EBW, method. This method is characterized by high concentration of power which instantly melts metal. As a result small HAZ is produced and comparatively small distortions are introduced. EBW process is characterized mainly by three input parameters: beam voltage, beam current and welding speed. The goal of numerical simulation is to identify the values of input parameters that produce full-depth fusion zone. As a guideline for simulation the actual dependency between input parameters and weld pool geometry will be taken from calibration data for EB welding unit. Calibration was performed using 18-8 steel. Partial least square method will be used to project those data on Inconel 706 alloy.
Prediction of Surface Quality due to Chatter Vibration in Rolling of Thin Steel Strip Using ALE Finite Element Method
Authors: Niroomand Mohammad Reza , Forouzan Mohammad Reza , Salimi Mahmoud
Published: March 2011

Chatter is one of the most encountered vibration problems in high-speed rolling of thin steel sheets in tandem mills. Due to its negative cause on product quality and maintenance costs, chatter is a technical and economical problem in rolling mills. To study chatter in rolling, it is necessary to set up models for the rolling process as well as the mill stand. Existing models of rolling process are analytic and are based on many simplifying assumptions. In current work the finite element method is utilized for the first time to model the chatter vibrations in rolling, which relaxes many of these assumptions. The model has the benefit of mass translation to the computational region by using Arbitrary Lagrangian Eulerian (ALE) technique, which tremendously reduces huge computational requirements of common Lagrangian models. Four chatter mechanisms in rolling are reviewed in this paper, but the presented model is updated for simulating the negative damping effect by means of some online velocity sensors and signal filtering method in applying the ALE boundary conditions. Using this new model, single stand negative damping mechanism of chatter and its effect on the sheet surface is understood more accurately. Simulation results are in comprehensive agreement with the experimental and industrial observations.
Analytical and Numerical Calculation of the Force and Power Requirements for Air Bending of Structured Sheet Metals
Authors: Malikov V. , Ossenbrink Ralf , Viehweger B. , Michailov Vesselin
Published: March 2011

Structured sheet metals with regular bumps offer higher stiffness compared to smooth sheet metals. They can be produced by a hydroforming process. The application of the structured sheet metals, however, is inhibited by the lack of knowledge for the subsequent processing steps. In this paper, the force and power requirements for air bending of structured sheet metals are calculated with a Finite Element Simulation (FE) and an analytical approach. In the first step, the hydroforming manufacturing process of the structured sheet metals is simulated in order to predict the exact geometry and the change in the material properties. Following, air bending simulations have been done taking into account the results of the hydroforming simulation. The FE-Simulations have been carried out with the software package LS-DYNA. The simulation models are validated with the optical displacement measuring system ARGUS and by a series of bending tests. For the analytical calculation the model based on the bending theory is adapted by simplifying the cross section of the structured sheet metals. The results of the FE-Simulation, the analytic calculation and the experiments are compared. The advantages and disadvantages as well as the application areas of the considered methods are indicated.
Simulation of Super-Plastic Forming Based on a Micro-Structural Constitutive Model and Considering Grain Growth
Authors: Farzin Mahmoud , Jafari Nedoushan Reza , Mashayekhi Mohammad
Published: March 2011

Constitutive models based on dominant mechanisms in hot forming are proposed. These models consider inter-granular deformation, grain boundary sliding, grain boundary diffusion and grain growth. New stress-strain rate relationships are proposed to predict deformation due to grain boundary sliding and grain boundary diffusion. Beside a Taylor type polycrystalline constitutive model, a visco-plastic relation in conjunction with a yield function is used to predict inter-granular deformation with much less computational costs. The proposed models are calibrated with tensile test data of AA5083 at . The calibrated models closely fit simple tension experimental data for various strain rates and strains. Then as an example the models are used to simulate a tray forming experiment. Dome heights and tray thicknesses at various positions during forming time can well predict experimental observations.
Computer Aided Modelling of Rubber Pad Forming Process
Authors: Del Prete Antonio , Papadia Gabriele , Manisi Barbara
Published: March 2011

Rubber pad forming (RPF) is a novel method for sheet metal forming that has been increasingly used for: automotive, energy, electronic and aeronautic applications [1]. Compared with the conventional forming processes, this method only requires one rigid die, according to the shape of the part, and the other tool is replaced by a rubber pad [1]. This method can greatly improve the formability of the blank because the contact surface between the rigid die and the rubber pad is flexible. By this way the rubber pad forming enables the production of sheet metal parts with complex contours and bends. Furthermore, the rubber pad forming process is characterized by a low cost of the die because only one rigid die is required [2]. The conventional way to develop rubber pad forming processes of metallic components requires a burdensome trial-and-error process for setting-up the technology, whose success chiefly depends on operator’s skill and experience [4][5]. In the aeronautical field, where the parts are produced in small series, a too lengthy and costly development phase cannot be accepted. Moreover, the small number of components does not justify large investments in tooling. For these reasons, it is necessary that, during the conceptual design, possible technological troubles are preliminarily faced by means of numerical simulation [4],[6]. In this study, the rubber forming process of an aluminum alloy aeronautic component has been explored with numerical simulations and the significant parameters associated with this process have been investigated. Several effects, depending on: stamping strategy, component geometry and rubber pad characterization have been taken into account. The process analysis has been carried out thanks to an extensive use of a commercially finite element (FE) package useful for an appropriate set-up of the process model [7],[8]. These investigations have shown the effectiveness of simulations in process design and highlighted the critical parameters which require necessary adjustments before physical tests.
Experimental and Numerical Investigation of the Effect of Anisotropy on Springback in U-Bending Process
Authors: Daiezadeh Vahid , Elyasi Majid , Rahmani Behzad , Bakhshi-Jooybari Mohammad
Published: March 2011

In this research, the effect of anisotropy on springback in CK67 steel sheet in U-bending process was studied by finite element simulation and experimental approach. The finite element software, ABAQUS/EXPLICIT 6.7, was used for simulation. In order to verify the simulation results some experiments were performed. The obtained results indicated that increasing the bending angle to the rolling direction resulted in an increase in springback. The results of the experiments were compared with simulation results. There was significant agreement between simulation and experiment.
Simulation of Heat Assisted Forming of Commercial Aluminium Alloy AA5182
Authors: Govindarajan Rajarajan , Zubeil Martin , Siefert Kathleen , Ageorges Christophe , Roll Karl
Published: March 2011

To improve the formability of commercial aluminium alloy AA5182, a new heat assisted forming method is used. The process sequence of this method is; cold forming (pre-forming, 90 to 95% of final shape), heat treatment and cooling it down to room temperature and final cold forming. AA5182 is a non-heat treatable alloy and hence heat treating a strain hardened non-heat treatable aluminium alloy leads to lose in strength and gain in plastic recovery. Therefore by heat treating a preformed part and then following it up by further forming stages, it not only gains the lost strength back but also shows increased formability. This behavior is particularly useful in forming more complex automotive interior body parts. To accurately simulate this method, modeling the effect of heat treatment is important. Initial investigations on tensile tests showed that the degree of pre-forming in combination with heat treatment is directly proportional to plastic recovery. Which means, the more the pre-strain is the more the recovery becomes viable. Based on this, a new algorithm has been developed and implemented in LS-DYNA to capture the effect of heat treatment. Finally experimental investigations were carried out on a cross die deep drawn cup to validate the developed simulation model.
Influence of the Adaptive Remeshing during Simulations of Sheet Metal Forming Processes
Authors: Giraud-Moreau Laurence , Cherouat Abel , Borouchaki Houman
Published: March 2011

In this work, an adaptive remeshing scheme is presented in order to simulate with accuracy sheet metal forming processes. During simulations of metal forming processes, large plastic deformations with ductile damage occur and severe mesh distortion takes place after a few incremental steps. Hence frequent remeshing of the part must be performed in order to carry out the finite element analysis. The necessary steps to remesh the damaged structure during the simulation of the sheet metal forming process are given. The adaptive remeshing based on refinement and coarsening techniques, is controlled by geometrical and physical size maps. This remeshing strategy has been coupled with a projection method in order to avoid problems of contact between the part and the rigid tools. The influence of the remeshing is studied on numerical examples which show the capacity of the proposed procedure.
FE-Simulation of the Heat Transfer by Defined Cooling Conditions during the Hot Stamping Process
Authors: Svec Thomas , Grüner Martin , Merklein Marion
Published: March 2011

Increasing demands regarding security aspects and light weight construction lead to the application of advanced high strength steels (AHSS) and ultra high strength steels (UHSS) in the automotive sector. Due to high process forces and the reduced formability of these steel grades within cold forming new manufacturing technologies like the hot stamping process are required. Furthermore, crash-performance plays an important role in the automotive industry. Therefore functional optimized components are necessary. Hence, actual research work within the community is focused on manufacturing components with local adjusted mechanical properties. One of the strategies to realize the contradictorily requirements regarding energy absorption and structural integrity is the Tailored tempering process where the cooling rates are adjusted by controlled heating or cooling of different tool zones within the hot stamping process. Thereby knowledge concerning the influence of the different heated tool parts on the heat transfer and the resulting mechanical properties is necessary. Furthermore, the applicability and the accuracy of the calculation approaches used for characteristic values like the heat transfer coefficient in the FE-based simulation have to be analyzed and evaluated. Due to this experiments with a tool which exhibits a heated and a cooled zone were performed according to the Tailored tempering process. During the experiments contact pressures and tool temperatures in the heated tool part were varied and analyzed regarding the influence on the heat transfer. Furthermore, the heat transfer coefficients were calculated and verified by a numerical model built according to the experimental setup and the accuracy of the model was evaluated by the comparison of characteristic values calculated from the experimental and numerical process data.
Consideration of Elastic Tool Deformation in Numerical Simulation of Hydroforming with Granular Material Used as a Medium
Authors: Grüner Martin , Merklein Marion
Published: March 2011

The use of high and ultra high strength steels in modern bodies in white raises steadily since the 1980’s. This trend is caused by the consumers’ wish of low fuel consuming cars with an increased passenger’s safety. The processing of these steels brings new challenges e.g. high flow stresses and a low formability at room temperature or high tool loads. These challenges can be resolved by warm forming at temperatures up to 600 °C reducing the flow stresses and increasing formability. For the production of complex parts that can not be produced by deep drawing hydroforming is an appropriate technology which can also help to reduce the number of parts and thus the weight of the body in white. Nowadays typical fluids used for hydroforming are only temperature stable up to about 330 °C so that it is not possible to combine the benefits of warm forming and hydroforming. Media like gases and fluids tend to leakage during the process which can only be avoided by a sealing or high blank holder forces. A new approach is the use of ceramic beads as medium for hydroforming at elevated temperatures. Building up a heatable tool for hydroforming with granular material used as medium makes it necessary to consider thermal conductivity so that there is a need of thick insulation plates. These insulation plates show high elastic deformations affecting the blank holder forces during the forming process. Measurements of the compressibility of these plates and implementation in numerical simulation allow a significant increase of the prediction accuracy of the model. A comparison of real part geometry and numerical results from models with and without consideration of elastic deformation will be given.
Advanced Friction Modeling in Sheet Metal Forming
Authors: Hol J. , Cid Alfaro M.V. , Meinders T. , Huétink J.
Published: March 2011

The Coulomb friction model is frequently used for sheet metal forming simulations. This model incorporates a constant coefficient of friction and does not take the influence of important parameters such as contact pressure or deformation of the sheet material into account. This article presents a more advanced friction model for large-scale forming simulations based on the surface changes on the micro-scale. When two surfaces are in contact, the surface texture of a material changes due to the combination of normal loading and stretching. Consequently, shear stresses between contacting surfaces, caused by the adhesion and ploughing effect between contacting asperities, will change when the surface texture changes. A friction model has been developed which accounts for these microscopic dependencies and its influence on the friction behavior on the macro-scale. The friction model has been validated by means of finite element simulations on the micro-scale and has been implemented in a finite element code to run large scale sheet metal forming simulations. Results showed a realistic distribution of the coefficient of friction depending on the local process conditions.
A Knowledge-Based System for Manufacturability Assessment of Deep Drawn Sheet Metal Parts
Authors: Naranje Vishal , Kumar Shailendra
Published: March 2011

In this paper a knowledge based system (KBS) for checking manufacturability of deep drawn sheet metal parts is described. For the development of proposed system technical knowledge acquired from different sources of knowledge acquisition is framed in the form of production rules of ‘IF-THEN’ variety and then coded using AutoLISP language. For consultation, the user loads the system into the prompt area of AutoCAD. The proposed system generates friendly prompt eliciting from the user for data pertaining to the job at hand. The system output includes recommendations on suitability of geometrical features of the part for required manufacturing operations. The system is ﬂexible and its knowledge base can be extended and modiﬁed as old manufacturing facilities are discarded or newer ones are acquired in a particular enterprise. Effectiveness of the proposed system is demonstrated by taking an example of an industrial sheet metal part. The low cost of implementation of proposed system makes it affordable for small and medium scale sheet metal industries.
Sheet Metal Forming Process Design Rules Development
Authors: Del Prete Antonio , Papadia Gabriele , Primo Teresa
Published: March 2011

Finite element analysis (FEA) is a powerful tool to evaluate the formability of stamping parts during process and die design development procedures. However, in order to achieve good product quality and process reliability, FEA application has to be performed many times exploring different process parameters combinations. Meanwhile, it is very difficult to perform an exhaustive process design definition when many parameters play a fundamental role to define such a complex problem. So, under the needs of reduction in: design time, development cost and parts weight, there is an urgent need to develop and apply more efficient methods in order to improve the current design procedures. For a generic component it is clear how its shape, among several parameters, has a direct influence on its feasibility. Starting from this assumption, the authors have developed a new approach grouping components upon their shapes analyzing component formability within a given “component family”. Nowadays, it exists only a process designer “sensitivity” that produces a ranking upon shape/feasibility ratio. Having as reference industrial test cases, the authors have defined appropriate shape parameters in order to have dimensionless coefficients representative for the given geometries. In particular, the components have been classified using a parameters set defining similarity families: related to geometrical aspects and to constitutive material. From the geometrical point of view the following parameters have been defined: family name, shape factor, punch radius-thickness ratio, die radius-thickness ratio, while for the constitutive material a code has been defined. FEA has been extensively used in order to: define, investigate and validate each shape parameter with a proper comparison to the macro feasibility of the chosen component geometry. The feasibility configuration definition, for a given shape, has been made through an appropriate study of the influence of each process variable on the properly process performances.
On the Sustainability Evaluation in Sheet Metal Forming Processes
Authors: Ingarao Giuseppe , Ambrogio Giuseppina , Di Lorenzo Rosanna , Micari Fabrizio
Published: March 2011

In sheet metal forming processes there is still a lack of knowledge in the field of environmental sustainability mainly due to the need of a proper modeling of issues and factors to be taken into account. Such topic is, nowadays, a urgent and remarkable issue in manufacturing and the main concerns are related to more efficient use of materials and energy. What is more, the estimation of environmental burdens of forming technologies is very complex to be accomplished since it is essentially process-dependent. This means that when comparing, for instance, a traditional forming process with an innovative one, there are some peculiar aspects to be considered; actually, processes can be rather different each other in terms of tooling, operative parameters and so on. In this paper, a first modeling effort is presented in order to compare, from a sustainability point of view, a classical stamping process with an incremental forming one. In particular, a frustum of pyramid part is considered and a quantitative analysis of the process energy consumption was developed. The paper aims to provide some sustainability guidelines to promote discussion on limitations, advantages, savings, drawbacks offered by different technologies within sheet metal forming field.
Enhancing Incremental Sheet Forming Performance Using High Speed
Authors: Ambrogio Giuseppina , Filice Luigino , Gagliardi Francesco
Published: March 2011

Flexible sheet metal forming processes represent one of the most relevant industrial issues of the scientific research. Incremental Sheet Forming is one of the most promising answers for many production scenarios. In particular, it becomes competitive when the production lot size decreases and the production variability increases. The process is basically set up on numerically controlled machines: a blank is clamped at its border and progressively deformed by a punch that moves according to a proper tool path program, reproducing the final part shape. Thus, the manufacturing time is directly dependent on the tool path length. Up to now, this aspect is one of the reasons why a systematic industrial application is not permitted. To overcome this drawback, an experimental investigation was planned in order to evaluate how the process is affected changing the cycle time. More in detail, an extended experimental investigation on the influence of process speed (i.e. tool rotation speed, tool feed) and other process parameters was executed taking into account a relatively simple 3D component. An accurate analysis of the obtained parts was performed, with particular attention to the thinning distribution that, of course, influences the material failure. Finally, the surface quality was also measured as an output variable.
Robot-Based Incremental Sheet Metal Forming – Increasing the Geometrical Accuracy of Complex Parts
Authors: Kreimeier D. , Buff B. , Magnus C. , Smukala V. , Zhu J.
Published: March 2011

This paper describes new developments in an incremental, robot-based sheet metal forming process (Roboforming) for the cost-effective production of sheet metal components for limited-lot productions and prototypes. The paper presents strategies in robot based incremental sheet metal forming for the force controlled forming of complex parts. These parts can consist of features such as steep flanks or convex/concave alternating surfaces and they are mostly formed with a local support tool which substitutes a full die. The strategies were developed in a cooperative project funded by the German Federal Ministry of Education and Research and the German Research Foundation. Approaches to increase the part accuracy of complex parts are presented. One approach concentrates on a servo loop, consisting of sensors and a programming system. It guarantees higher part accuracies by measuring the deviations between a formed part and its target geometry. These deviations are used to derive corrected tool paths. The abdication of a partial or full die leads to a larger influence of the free compliant sheet area surrounding the formed part. Because of that the geometry shifts away from the forming tool and it cannot be formed completely. Another approach to increase the part accuracy by reinforcing this free sheet area is also presented.
Finding the Best Machine for SPIF Operations - a Brief Discussion
Authors: Marabuto S.R. , Afonso Daniel Gil , Ferreira J.A.F. , Melo F.Q. , Martins Miguel A.B.E. , de Sousa Ricardo J. Alves
Published: March 2011

Single Point Incremental Forming (SPIF) given its easy implementation and absence of dedicated tooling is a promising manufacturing technology concerning the production of customized products, low batches or prototyping of ready-to-use parts. The range of application is wide, covering many materials and virtually unlimited geometries. Indeed, current process boundaries are more related to machine limitations than to the procedure itself. In this paper, research is carried out on the state-of-the-art of existing SPIF machine technology, in order to determine an appropriate configuration for an incremental forming equipment that overcomes such limitations. A comparative analysis is carried out to evaluate the different types of currently used equipment: adapted milling machines, serial robots and purpose built machines. Comparison parameters include among many others the maximum payload, tool path flexibility, stiffness and overall cost of the machine, based on information gathered on publications mainly from the last decade. Alternatively, other solutions used for different technological processes and assembly operations, such as precision positioning, are also taken into account. Based on the comparison of all solutions, and on the objectives of the current project carried out at the University of Aveiro, it is concluded that an equipment with parallel kinematics, driven by hydraulic servo-cylinders, could be the best choice to achieve the established goals.
The Simulation of Robot Based Incremental Sheet Metal Forming by Means of a New Solid-Shell Finite Element Technology and a Finite Elastoplastic Model with Combined Hardening
Authors: Kiliclar Yalin , Laurischkat Roman , Reese Stefanie , Meier Horst
Published: March 2011

The principle of robot based incremental sheet metal forming is based on flexible shaping by means of a freely programmable path-synchronous movement of two tools, which are operated by two industrial robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the geometry’s contour in lateral direction. The main problem during the forming process is the influence on the dimensional accuracy resulting from the compliance of the involved machine structures and the springback effects of the workpiece. The project aims to predict these deviations caused by resiliences and to carry out a compensative path planning based on this prediction. Therefore a planning tool is implemented which compensates the robot’s compliance and the springback effects of the sheet metal. Finite element analysis using a material model developed at the Institute of Applied Mechanics (IFAM) [1] has been used for the simulation of the forming process. The finite strain constitutive model combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamical setting. It is based on the multiplicative split of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening which is widely adopted as capable of representing the above metal hardening effects. The major problem of low-order finite elements used to simulate thin sheet structures, such as used for the experiments, is locking, a non-physical stiffening effect. Recent research focuses on the large deformation version of a new eight-node solid-shell finite element based on reduced integration with hourglass stabilization. In the solid-shell formulation developed at IFAM ([2], [3]) the enhanced assumed strain (EAS) concept as well as the assumed natural strain (ANS) concept are implemented to circumvent locking. These tools are very important to obtain a good correlation between experiment and simulation.
Accuracy Improvement in Single Point Incremental Forming through Systematic Study of Feature Interactions
Authors: Behera Amar Kumar , Vanhove Hans , Lauwers Bert , Duflou Joost R.
Published: March 2011

Previous studies have shown that feature detection and part segmentation are useful tools to generate compensated toolpaths for single point incremental forming leading to improvement in accuracy of manufactured parts. However, in most practical applications, features do not occur by themselves. Rather, they occur in combination with other features, and the presence of the neighbouring features influences the behaviour of the feature of interest. The final shape of the formed part depends on the interaction between the features. In this study, an attempt has been made to generate a complete taxonomy of common features relevant for incrementally formed parts. This taxonomy is then utilized to generate a matrix of feature interactions, and to classify them as feasible or not. From the subset of feasible feature interactions, a number of cases are analyzed to illustrate the effect of the interactions on the magnitude and nature of inaccuracies resulting in uncompensated parts. Strategies to use the knowledge of the interaction between these features to improve the accuracy of the manufactured parts are then discussed with the help of experimental case studies.
CAM-Solution for Two Robots Based Incremental Sheet Metal Forming
Authors: Kreimeier D. , Zhu J. , Smukala V. , Buff B. , Magnus C.
Published: March 2011

Robot based incremental sheet metal forming (Roboforming)is a new dieless forming process, which is suitable for cost-effective manufacture of prototype parts and small batch sizes.The principle of Roboforming is based on flexible shaping through a freely programmable path-synchronous movement of two industrial robots. These two robots, which are connected to a cooperating robot system, hold respectively a forming and a supporting tool. Similar to other incremental forming methods, the final shape is produced bythe movement of the forming toolalongthe lateral direction and its gradual infeed in the depth direction. In Roboforming, there are twodifferent strategies for the synchronous movement of the supporting tool, eitheralong the outer contour onbacksideof the sheet or directly opposed to the forming tool building a forming gap.The second strategy can be combined with a force controlled method to increase the surface quality and geometricaccuracy. MThe most existing CAM systems used in numerous incremental forming approaches are only applicable for milling machines. In this paper, with the use of self-programmed postprocessors and an Application Programming Interface (API) in a CAM system, movement programs for two cooperating robots can be generated for both forming strategies to produce sheet metal parts with different sizes and complex freeform structures. This CAM-solution for Roboforming is validated bythe forming experiments.
Tool Path Strategies for Single Point Incremental Forming
Authors: Bagudanch Isabel , Pérez-Santiago Rogelio , Garcia-Romeu Maria Luisa
Published: March 2011

In the last few years the interest in Incremental Sheet Forming (ISF) has considerably grown due to the possibility of obtain small production batches and high customized products. Despite the increasingly knowledge of this processing technique, there are still some important process parameters that require further development. One of the most important parameters when experimental or numerical studies of ISF are carried out is the tool path programming. The decision of the strategy that will be followed can affect the accuracy, the surface finishing, the forming forces, etc. The analysis of these relationships has been studied for several authors using different tool path. In the present paper different tool path strategies for the same geometry are compared. The geometry has two inclination wall angles. Also, different commercial softwares are employed to develop the tool path of the desired geometry. The main objective of the paper is to identify some indicators that can assist in the choice of which strategy is the best, not only depending on the complexity of the geometry or the software used but also on qualitative criteria. With all this work it is possible to provide some useful guidelines that permit to establish a selection criterion of the tool path, which will be very interesting before developing an experimental work.
Shape Distortion and Thickness Distribution during SPIF Processes: Expermental and Numerical Analysis
Authors: Buffa Gianluca , Ingarao Giuseppe , Fratini Livan , Micari Fabrizio
Published: March 2011

Single Point Incremental Forming (SPIF) is a quite new sheet forming process which offers the possibility to deform complex parts without dedicated dies using only a single point tool and a standard 3-axis CNC machine. The process mechanics enables strains much higher than traditional sheet forming processes, but particular attention must be given to the final part geometrical accuracy. In this paper the capabilities of a dedicated explicit numerical model are quantitatively analyzed on pyramid-shaped parts. In particular a comparison between experimental and numerical results is reported. Three different shapes at the varying of the stamping angle were considered and the final shape was acquired, through a white light triangulation based optical scanner, after the removing of the punch and of the clamping fixture. LS-DYNA commercial code was used to simulate the three different case studies, and an explicit loading-implicit unloading approach was implemented in order to simulate also the shape distortions due to springback phenomenon. The effect of several numerical parameters on the final shape and the thickness distribution were investigated. In particular numerical results were analyzed at the varying of the material constituve law, of the shell element type and of the number of integration points along the thickness.
Measurement of Critical Dimensions of Cold Roll Formed Sections Using Digital Image Processing
Authors: Williams Ian , Shirvani Bez , Mourier Jean Michael
Published: March 2011

A research investigation is presented which discusses the practicality of using several image processing and knowledge based techniques for the measurement and classification of cold rolled steel sections. Image analysis techniques can be applied to many different applications and assessing the quality and the accuracy of cold roll formed steel sections is no exception. The operations detailed within this paper are both traditional image processing methods and novel neural network based techniques which are combined together to give a bespoke alternative to the manual processing currently employed to test these sections. The results show the suitability of using image analysis and image processing to aid in the quality control of cold steel roll forming and initial tests have demonstrated great potential for this work.
Pressure Field Stabilization in High-Voltage Underwater Pulsed Metal Forming Using Wire-Initiated Discharges
Authors: Eguia Iñaki , San José Juan , Knyazyev Mykhaylo , Zhovnovatyuk Yaroslav
Published: March 2011

Electrohydraulic forming of sheet metals is characterized by the usage of large transient hydraulic pressure generated in underwater current discharges. Pulsed power underwater discharges are often categorized as being non-repeatable in terms of pressure map replication. The work described here presents the improvements made in terms of pressure stabilization based on process parameter optimization pertaining to impact electrohydraulic forming. The work consists of a comparative study showing the differences obtained in terms of pressure fields, when discharges initiated by high-voltage breakdown (wireless discharges) and initiated by copper and aluminium wires at otherwise equal test conditions are compared, for a conical discharge chamber. Characteristic pressure maps belonging to the three analyzed discharge conditions are presented and the criteria for a quantitative comparison are set: maximum pressure value, relative pressure scatter and arithmetic mean deviation for a test pressure field. The maximum pressure value characterises the limitation in the sheet material thickness that could be formed at this pressure. For all 3 types of tests the obtained maximum pressure value is nearly equal with a little bit higher level for discharges initiated by aluminium wire. The relative pressure scatter provides information about uniformity of the pressure distribution along loaded area. While wireless tests showed low uniformity with average relative pressure scatter of 33 %, the application of copper and aluminium wire reduced non-uniformity down to 28 and 24 % respectively. The most important effect of the wire introduction has to do with the great increase of stability (repeatability) of pressure fields observed, characterised by a decrease of the arithmetic mean deviation of pressure along a pressure loaded area.
Process Design for Hydroformed Tailored Blanks Technology through CAE Techniques
Authors: Del Prete Antonio , Papadia Gabriele , Manisi Barbara
Published: March 2011

Ecological awareness and economic analysis force industry to decrease the weight of transportation vehicles and to achieve a higher product quality with a reduction of production costs [1]. Lightweight constructions made out of Tailored Blanks (TBs) and advanced manufacturing technologies, like sheet metal Hydromechanical Deep Drawing (HDD), help to reach these goals [1]. From this point of view, HDD techniques have been largely accepted by the industry [2] for the production of components characterized by: complex shapes, good surface quality and small residual stress. In this work, starting from previous studies of the same authors about hydroformed components with a redrawing area [3] [4], an original approach based on Thickness Percentage Reduction (TPR) distribution has been implemented to design a particular TBs for HDD applications. Numerical and experimental results about the studied test case have been allowed the verification of their correlation as well as the necessary reliability of the implemented process simulation methodology.
Micro Warm Coining of Stainless Steel Sheets Using Electric Conductive Heating
Authors: Zhao Kun Ning , Wietbrock Burkhard , Hirt Gerhard
Published: March 2011

The motivation of this study is to investigate micro warm coining of metals with high strength (e.g. austenitic steels) in the field of fabricating micro functional surface structures. The current micro cold coining technology, which uses punches made out of steel, is limited to soft metals, such as Al and Cu. Conducting micro cold coining on steels, leads to high tool wear and bad form filling. The approach of this study is integrating electric conductive heating into a micro coining system to realize micro warm coining of stainless steel. This paper presents the experimental and numerical analysis of micro warm coining technology using the material stainless steel 1.4401. A coupled thermal-mechanical model in the commercial Finite-Element-code ABAQUS is used to help analyzing the micro warm coining process. The results are summarized as follows: (1) Open die micro warm coining has been realized. The form filling achieved was 56%, which was three times larger than the form filling of cold micro coining on the same material. (2) Both the experiments and simulations showed that faster processing, by increasing the punch velocity, resulted in a slightly improved form filling (5% from 5 mm/s to 100 mm/s). (3) Surface chilling hindered the filling of thin ribs.