Key Engineering Materials Vols. 554-557

Abstract: High strength steel sheets are increasingly used in automotive body parts with the aim of weight reduction, but their use urgently requires further improvement in sheet forming technology to overcome difficulties such as poor formability, dimensional inaccuracy, etc. On the other hand, servo press facilities are becoming increasingly used in industry and many attempts are being made to bring out their characteristic features for enhancing the formability of high strength steel sheets. Although some of these attempts have been successful in finding the advantages of servo presses for improving formability and dimensional accuracy, the mechanisms of such improvements have yet to be clarified in conjunction with the mechanical properties of the materials used. One of the most remarkable features of the servo press lies in its flexibility in slide motion control. It is thus effective to investigate the relevance of strain rate sensitivity of a material to the mechanism of improvement in formability enabled by the flexible slide motion of the servo press. However, very few studies have been carried out with material testing, material modeling, and numerical analyses combined with experimental verifications. In this study, Norton’s creep model was implemented in the FEM solver in order to take into account visco-elasto-plastic deformation including stress relaxation behavior. Parameters for the visco-elasto-plastic material model were identified through physical measurements and FEM simulations of uniaxial tension and crosshead displacement dwell tests, as shown in Fig. 1. The identified material model was applied to sheet forming simulations of an automotive body part and validity of the model was examined by comparing with stamping experiments using a servo press with a variety of slide motions. Numerical results with the identified material model showed the same tendency with respect to the slide motions as the experimental results. Stress relaxation behavior was found to be an important factor for improving formability enabled by modifying the slide motion.

Abstract: The last decade has shown an expanding interest towards incremental sheet forming (ISF). The present paper focuses on a wide variety of materials used in ISF with emphasis on material behavior and post forming properties. Material behavior knowledge is essential for improving process control and accuracy, post forming properties are essential for introducing ISF for wider industrial use. Included in the study are: a ferritic and four austenitic stainless steel grades, a deep drawing steel, as well as copper and silver. The two latter materials have been widely used for crafted products trough centuries and still have their place in high-end products. The suitability of the different materials for ISF is studied and compared. A comprehensive testing procedure is performed to gain information about behavior during and properties after deformation. The material property evolution is also compared to other forming methods to further enhance understanding of the process. The materials can be divided into stable and metastable materials. Stainless steels are an increasingly important class of alloys that are widely used in industry today. The structure of austenitic stainless steels is metastable which may lead to phase transformation. Strain induced martensite phases can form during plastic deformation of these steels. The formation of martensite affects significantly their mechanical behavior by enhancing the work hardening. Transformation impedes the property prediction as it is linked to the process conditions which may vary during forming. The stable materials lack phase transformation but show both diverse and similar material property evolution as a result of ISF.

Abstract: Incremental sheet metal forming is a new process to manufacture sheet metal parts and it is becoming a remarkable technology for fast prototyping and small lot production because of the advantages of this technology such as process flexibility, product independent tooling and higher formability. On the other hand, limited maximum drawing angle, relatively coarse surface roughness, low geometrical accuracy and long forming time are common disadvantages of the process. Furthermore, it is affected by process parameters which are tool diameter, forming velocity, spindle speed, forming geometry and depth, etc. Toolpath strategy which is used to form sheet metal by CNC machine has a key role among these parameters. The present study has been undertaken in order to investigate the suitable toolpath strategy which is developed for metal cutting by commercial CAD/CAM software to increase geometrical accuracy and decrease thinning and forming time. For the intended purpose, seven different toolpath strategies which are rough and finish strategies were used to form a truncated frustum by using one millimeter thick S235JR steel alloy. The effect of each strategy on the surface roughness, geometrical accuracy and thinning distribution of formed product was studied by measuring thickness, drawing angle and depth of formed parts. Therefore, formed parts scanned by 3D laser scanner and STL files of parts were generated then STL files were converted into CAD file. CAD data of parts was used for measurements. The measurements showed that not only forming movements but also transition movements along the tool path affected the geometric accuracy and thinning distribution, surface roughness and forming time of formed parts. On the other hand it was observed that rough strategies were given good results as finish strategies and tool paths generated by CAM software need to be editing for better geometric accuracy, thinning, forming time and surface quality.

Abstract: This paper describes new developments in an incremental, robot-based sheet metal forming process (‘Roboforming’) for the production of sheet metal components in small batch sizes. The dieless kinematic-based generation of a shape is implemented by means of two industrial robots which are interconnected to a cooperating robot system. Compared to other incremental sheet metal forming machines, this system offers high geometrical form flexibility without the need of any part-dependent tools. The industrial application of incremental sheet metal forming is still limited by certain constraints, e.g. the low geometrical accuracy and number of formable alloys. One approach to overcome the stated constraints is to use the advantages of metal forming at elevated temperatures. For the temperature input into the sheet metal, there are different approaches like heating with warm fluids, a laser beam or using direct resistance heating. This paper presents results of the research project ‘Local heating in robot-based incremental forming’, funded by the German Research Foundation (DFG), where the heating of the current forming zone by means of direct resistance heating is examined as a variation of the Roboforming process. In order to achieve a local limitation of the heating on the current forming zone, the electric current flows into the sheet at the electric contact of the forming tool and the sheet metal. Thus the forming tool is part of the electric circuit. In current literature Authors report about results from experiments using single-point incremental forming, where the forming tool and the clamping frame of the sheet are connected to the power source. In order to further limit the heating on the forming zone, a new approach will be presented in this paper, where a second tool is used to support the forming and heating process, as both tools can be connected to the power source, making a current flow through the rest of the sheet and the clamping frame unnecessary. With the use of two tools the current flow and thus the heated zone of the sheet can be manipulated. Additionally the advantages of the supporting tool, already shown in forming at room temperature, such as increased geometrical accuracy and maximum draw angle can be used. Starting with a description of the new process setup for steel forming at about 600 °C, results of experiments evaluating the influence of the supporting tool on the forming process at elevated temperatures and the resulting geometrical accuracy will be presented in this paper. Therefore, different process parameters as forming temperature, cooling and relative positioning of the both tools have been varied.

Abstract: Friction spinning is an innovative incremental forming process for the manufacture of tailor-made components with defined functionally graded properties. The process is characterized by the use of friction sub-processes for self-induced heat generation that can be employed for the defined thermo-mechanical treatment. Due to this in-process heat treatment, it is possible to extend the forming limits and achieve more complex geometries as well as favorable part properties. One very interesting application for friction spinning is the sealing of tube ends. There are a lot of conceivable application fields, including the substitution of soldering or welding operations in chemical engineering. Another interesting field is the use of this incremental forming process for industrial or automotive applications such as the manufacture of very slim cylindrical cups. An advantage of this method is the feasibility of defined control of the thickness distribution in the bottom and side wall area. This is supported by a new tool system with a pivoting forming tool. The pivot movement is controlled by a process control system. This system makes it possible to achieve different contact conditions between the tool and the workpiece during the process so as to attain a defined influence on the material flow and hence to enhance the attainable bottom wall thickness compared with previous fixed-angle tools. This tool concept thus offers an opportunity to improve the properties of the components as well as to manufacture new and complex geometries, such as hollow, fully closed roll type parts.

Abstract: Recently, new sheet metal forming technique, incremental forming has been introduced. It is based on using a single spherical tool, which is moved along CNC controlled tool path. During the incremental forming process, the sheet blank is fixed in sheet holder. The tool follows a certain tool path and progressively deforms the sheet. Nowadays, numerical simulations of metal forming are widely used by industry to predict the geometry of the part, stresses and strain during the forming process. Because incremental forming is a dieless process, it is perfectly suited for prototyping and small volume production [1, 2]. On the other hand, this process is very slow and therefore it can only be used when a slow series production is required. As the sheet incremental forming process is an emerging process which has a high industrial interest, scientific efforts are required in order to optimize the process and to increase the knowledge of this process through experimental studies and the development of accurate simulation models. In this paper, a comparison between numerical simulation and experimental results is realized in order to assess the suitability of the numerical model. The experimental investigation is realized using a three-axis CNC milling machine. The forming tool consists in a cylindrical rotating punch with a hemispherical head. A subroutine has been developed to describe the tool path from CAM procedure. A numerical model has been developed to simulate the sheet incremental forming process. The finite element code Abaqus explicit has been used. The simulation of the incremental forming process stays a complex task and the computation time is often prohibitive for many reasons. During this simulation, the blank is deformed by a sequence of small increments that requires many numerical increments to be performed. Moreover, the size of the tool diameter is generally very small compared to the size of the metal sheet and thus the contact zone between the tool and the sheet is limited. As the tool deforms almost every part of the sheet, small elements are required everywhere in the sheet resulting in a very high computation time. In this paper, an adaptive remeshing method has been used to simulate the incremental forming process. This strategy, based on adaptive refinement and coarsening procedures avoids having an initially fine mesh, resulting in an enormous computing time. Experiments have been carried out using aluminum alloy sheets. The final geometrical shape and the thickness profile have been measured and compared with the numerical results. These measurements have allowed validating the proposed numerical model. References [1] M. Yamashita, M. Grotoh, S.-Y. Atsumi, Numerical simulation of incremental forming of sheet metal, J. Processing Technology, No. 199 (2008), p. 163 172. [2] C. Henrard, A.M. Hbraken, A. Szekeres, J.R. Duflou, S. He, P. Van Houtte, Comparison of FEM Simulations for the Incremental Forming Process, Advanced Materials Research, 6-8 (2005), p. 533-542.

Abstract: The phenomenon of springback, which is ruled by strain recovery after removal of forming loads, is of remarkable interest in air bending of metal sheets. In this process, the final angle is affected by a number of parameters related to both process geometry (sheet thickness, die and punch radii) and material properties (elastoplastic stress-strain law); because of this, punch stroke has to be calculated in a nontrivial way and a number of input parameters should be taken into account. In this work the study of total load as a function of displacement is used to collect information about material stress-strain law; using this approach, load data may be exploited to fine tune the mathematical description of the material and, finally, to improve springback prediction. A customized press brake able to measure both displacement and force during bending was fabricated for this purpose. The press brake is equipped with a control system algorithm able to collect material information directly during the initial stage of the bending process. These collected data are used to feed a model based on a FEM simulation and the model output is the final punch displacement suitable to obtained a specific bending angle after unloading. The program utilized for the simulation is Deform 2D. Preliminary tests were executed on metal sheets having different thickness.

Abstract: Single point incremental forming (SPIF) belongs to the branch of incremental sheet forming processes that enables plastic deformation of blanks without resorting to any specific die or punch. The main characteristics of SPIF determine its appropriateness for producing small batches or single products, being the medical implants one of the key potential fields of application, due to the need of product customization to each patient. Customization is particularly important for obtaining preoperative implants because it allows a significant decrease in the overall surgery time in conjunction with a higher level of flexibility to ensure the required shapes. This results in an improved final product either in aesthetic as well as in functional terms.

Abstract: European manufacturing companies are currently facing increased competition as a result of intensified globalization in the market and supply base. One strategy to meet this challenge is to develop and manufacture higher quality products at reduced cost. Metal forming is a typical manufacturing operation where improved technology can create advantages in the market place through higher value-added products. In the automotive industry, for example, improved shaping capabilities of profiles will improve product functionality, while reducing system cost due to reduction of part count and subsequent assembly operations such as welding. In addition, improved dimensional accuracy will provide benefits in terms of reduced quality costs and, sometimes, eliminating downstream processing steps such as calibration or machining. Rotary draw bending is typically used to manufacture profile-based shapes bent at tight radii with reasonable dimensional accuracy. The advantage of this process is low operational cost combined with relatively high flexibility, particularly with regard to bend angle. On the other hand, the disadvantage associated with the method is limited abilities to control local distortions of the cross section without taking special actions such as applying external stretching or complex tooling that ultimately would increase the investment and operational cost. The objective of the present paper is to identify the most important factors that influence cross-sectional distortions and quantify their impact on dimensional accuracy in draw bending, by performing a series of experiments in an industry-type draw bender. In order to accommodate different cross-sectional geometries, a flexible, modular tool concept was developed. AA6xxx aluminum alloy profile with different cross-sectional geometries (width, depth, thicknesses), single and multi-camber, were bent at different radii and bend angles, while measuring local distortions of the cross sections. The results have been presented in diagrams denoted flatness limit curves, showing the impact of various geometry (and material) parameters on local deformations of individual cross sectional members. The results show that the flange width (i.e. the free span between webs) is the main factor with regard to distortions, followed by wall thickness and bending radius and, finally, depth of the cross sections. Material parameters seem to have limited effect for the alloy tempers investigated. Attempts have been made to interpret the mechanisms associated with the development of local cross sectional distortions with the purpose of developing a practical design tool based on analytical calculations. The very first results show reasonably well agreement in the cases when local buckling of the internal compressive flange is less predominant.

Abstract: Blanking of sheet-metal is an important forming process in the automotive industry for the manufacture of mechanical components. The final component shape, obtained at the end of bending or deep-drawing processes, often has sharp edges due to the blanking operation. Edge Rounding by Punching (E.R.P.) of safety components is necessary to avoid cutting the belt material. In addition to removing the sharp edges, the punching results in work hardening of the material in the rounded zones which results in an increase in the local resistance of the material. In this study, a High Strength Low Alloy steel (HSLA S500MC) is tested with the aim of analysing the residuals fields in the chaining of blanking and edge rounding processes. The mechanical behaviour of the sheet material is investigated by means of tensile tests and Vickers micro-hardness measurements. Numerical simulations are performed using a ductile damage criterion. The experimental residual stress fields are characterised and compared to numerical results, in view of predicting the in-service behaviour of the component. Specimens with rounded edges are compared to specimens that were not submitted to the rounding operation. It is shown that (E.R.P.) improves the component resistance, therefore justifying the use of this process in the manufacture of automotive safety components.