THE USE OF COMPUTER TECHNIQUES TO EVALUATE THE DRAWABILITY OF THE THIN SHEET METAL. Monika HYRCZA-MICHALSKA

Transcription

1 THE USE OF COMPUTER TECHNIQUES TO EVALUATE THE DRAWABILITY OF THE THIN SHEET METAL Monika HYRCZA-MICHALSKA Silesian University of technology, Katowice, Poland, EU, Abstract The drawability evaluation of thin sheet for stamping requires assessment of tools and techniques to modern ways measure and to evaluate complexity of the advanced construction materials characteristics such as mentioned thin metal sheets. Press forming is a complex issue to assess by applying classical methods of mechanical and physical properties study, such basic and technological tests. More and more are being developed comprehensive characterizations of charges for stamping like thin sheet metals, e.g. in the range of forming limit curves or diagrams. The development of those curves is possible through the use of image analysis techniques for indexing and distorted grids use digital software such as strain analysers AutoGrid system, and an older one - Asame system. However, extensive opportunities to develop methods of drawing, starting from cutting through the bending and shaping by using rigid tools, hydroforming or explosives, require computer support through product design and production technology. In this field are used several programs for modeling and simulation of stamping processes such as finite element method (FEM). The paper presents the application of the strain analyser AutoGrid to determine local strain distributions on drawpieces made of Inconel 625 alloy sheets. These strain distributions were compared with the simulation result of forming of the same shape drawpieces obtained with the use of software eta/dynaform 5.8. Comparison of qualitative and quantitative results showed good correlation with the actual simulation results of the drawing. The use of computer techniques in both cases facilitates the development of drawability properties characteristics and allows you to design and predict the results of stamping without most expensive production trials used to start the process of producing new stampings. Keywords: Computer techniques, strain analysis, drawability evaluation of thin sheet metal, numerical simulation, eta/dynaform 1. INTRODUCTION Nowadays computer science gives tools to materials evaluation for forming processes. Mainly the simulation software provides and shows result of forming process and design tools [4, 8]. However, in the development range of the material characteristics are using computer programs based on imagine analysis. They allow determining local strain distribution, forming limit curves [7] and foreseeing forming phenomena [1]. In the project Development of methods for computer-aided design process of stamping products for the aircraft industry, N R were tested analyser and forming process software described in 1.1 and 1.2 subchapter respectively. First results for application are made to evaluation drawability of 0.45mm thick Incontel 625 sheet. Such kind thin sheet metals are applied at aircraft industry. 1.1 Strain analyser AutoGrid The Silesian University of Technology have strain analyser AutoGrid by ViALUX. It provides a fast and accurate measurement to determination of forming limit curves (FLCs) of sheet metal materials. The system as a tool is directly connected to the testing machine Erichsen universal press (Fig. 1 a). Then the system gives precise access to the maximum strain value just before necking and subsequent cracking occurs. The software package allows to create FLCs from a set of specimens very effectively. Despite the

2 knowledge of the original undeformed grid size (size range between 1mm to 5mm), the system is able to determine a field of strain values in the range of 0.5 % to more than 100 % without any unambiguity. Images sequences of up to 24x4 frames per second can be recorded. In case of the standard testing velocity of 1-2 mm s, a punch depth resolution of less than 0.1 mm can be achieved. The system also features additional data inputs that can track different values from the testing machine. For example, punch stroke can be stored with each measurement. Automatic evaluation helps to generate complete forming limit curves. All process variables of differently shaped specimen are provided to determine reliable FLC data. Advanced software features can be used to process an export the data and to create reports. The system also fulfil the requirements of bulge or tensile testing [5]. Exported from strain analyser measurement data could be successfully applied to Dynaform software as material characteristics during material model data preparation. a) b) Fig. 1. Photographies of strain analyzers AutoGrid systems: a) stationery strain analyzer built on universal press Erichsen at the Silesian University of Technology; b) movable strain analyser in compact version used at industrial trials On the other hand the AutoGrid compact model of strain analyser (Fig. 1 b) has been developed in order to meet common user requirements for high mobility and flexibility combined with robustness and easy of use.the new solution in pictures recording is completely based on digital camera technology, and the use of standard netbook provides whole mobility. Four cameras are rigidly placed inside the measuring head allowing for constantly received data colecting without any setup and calibration stages. Operator manual handling recordings by pressing the button when calibration the object region - and 4 CCD cameras are triggered to take the photogrammetric view. The object working distance is indicated by a laser and the high brightness LED illumination switches automatically during the recording. The convenient AutoGrid user interface provides software control of exposure time and camera gain settings. The compact system takes advantage of high-resolution cameras recording 5.7 million pixels in total per snapshot. In connection with high-quality optics the system is able to take an area of up to 0.5x0.4 m² where up to grid points can be rapidly evaluated in one picture set. Single measurements can be easily stitched together yielding the analysis of whole components without size limitations [3]. This compact strain analyzer is widely used in industrial measurements and works well. Such kind industrial trials results are presented in experimental part of this article.

3 1.2 Eta/ Dynaform 5.8 software Eta/DYNAFORM 5.8 allowed us using finite element modeling for forming analysis, as well as displaying the forming results. This software is applied to simulate: single section draw with contact definition, drawbeds application and gravity loads; solid element double action draw; springback and springback compensation analysis; tube bending and hydroforming [2]. On the market are programs to simulate processes e.g. PAM- STAMP, AUTOFORM, MARC, etc.. Nowadays important is proper selection of software for the implemented forming processes. Also drawing tools are designed using CAD software to allow the application to CNC numerical control machine tools. Also gives easily convert files to the known formats used to control the 5- axis machines. However, recognize a geometric imperfection characteristic of the product in conjunction with the properties of the charge in the design process. Thus, the geometry of tools is often revised after testing movement, which is associated with a significant increase in production costs. This paper shows results for prediction of the pressing techniques using computer-aided design and numerical simulation method. Dynaform 5.8 was chosen to simulation procedure, which allowed showing manufacturing processes on each methods including hydroforming. 2. EXPERIMENTAL RESULTS OF APPLYING COMPUTER TECHNIQUES To cone industrial drawpieces are detailed implemented analysis of local strain distribution. The forming test simulations are made by using obtained the material data base and selected software. The first tests were performed in a discharge cone of the industrial-type metal alloy Inconel. Conducted a qualitative analysis of the results of the process of stamping die axially-symmetric using the method of forming a liquid (hydroforming). Quantitative analysis of plastic flow characteristics of the Inconel sheet metals 0,45 mm thick in this process was made using a portable measurement system AutoGrid. The charges for cones were 215 mm dimension discs with 2 mm square mesh subdivision. The cones were manufactured in hydroforming process for forming variant: 1B plane disc, forming with lubrication, 2A disc with scratches, forming without lubrication, 2B disc with scratches, forming with lubrication. Manufactured drawpieces are shown in Fig. 2 and results of measurements using strain analyser are shown in Fig. 3. a) b) c) Fig. 2. Photographies of cones manufactured in hydroforming process for forming variant: a) 1B plane disc, forming with lubrication, b) 2A disc with scratches, forming without lubrication, c) 2B disc with scratches, forming with lubrication

4 a) b) c) Fig. 3. The results of the quantitative analysis of local strain distributions using the Auto Grid measurement system: a) photograph of the cone drawpiece hydroformed in terms of lubrication and increased roughness (there were made cracks in the flange area) where pointed side surface of the cone presents measured area, b) material thinning distribution at the selected area, c) the same resolution, related to the forming limit diagram of tested sheet metal The study was made for precise analysis of influence of lubrication and surface roughness in hydroforming cone. First result for simulated data are shown in publications [5, 6]. Numerical simulation of cone hydroforming are done using eta/dynaform software. The modeled tools are shown in Fig. 4. Prepared simulations bring forming phenomena forecasting possibility with good correspondence to real process (see Fig. 5 and compare with Fig. 2).

5 a) b) Fig. 4. The view of modeled tool for cone hydromechanical forming: a) punch method, b) die method Fig. 5. Results eta/dynaform simulation - forecasting of forming phenomena appeared during cone hydromechanical forming using punch method Fig. 6. Results eta/dynaform simulation - thinning distribution at the cone drawpiece for punch method

6 Presented in Fig.6. thinning distribution at the cone drawpiece, it forecast results of hydromechanical forming with rubber membrane. Such kind manufacturing is very advanced and thanks to software options of Eta/DYNAFORM solutions is possible. 3. CONCLUSIONS Analyzing strain distribution on cone-shaped drawpieces obtained from numerical simulation and from industrial tests, one can conclude that the results of measurements of local strain distributions in the corresponding areas of numerical and experimental drawpieces are qualitatively comparable. The simulation results of hydroforming process using both: punch and die methods allowed to produce cones from Inconel sheet blank 0,45 mm thick using pointed out guidelines based on simulation result. According to strain measurement, the punch method is more safety manufacturing technique and it is recommended for Inconel sheet blanks forming. Despite quantitative differences in the results of simulations, they predict good representation of the shape of the cone and the virtually target cone can be used in its present form practically. Based on carried out tests Eta/DYNAFORM version 5.8 has been rated as a program well suited for simulation of hydroforming. However, advanced software options of Eta/DYNAFORM must be used to receive more precisely solutions. Especially advanced software options should be applied to model rubber membrane characteristic. The advisable computer techniques accelerate the thin sheet metal drawability evaluation. Presented software allow effectively forecast stamping results corresponding to the real processes frequently the qualitative and quantitative. ACKNOWLEDGEMENTS These research works were carried out within the framework of Developmental Investigative Project number N R financially supported by Polish The National Centre for Research and Development (NCBiR) in years REFERENCES [1] ASGARI, S.A., PEREIRA, M., ROLFE, B.F., DINGLE, M., HODGSON, P.D., Statistical analysis of finite element modeling in sheet metal forming and springback analysis, Journal of Materials Processing Tech., 203, 1-3 (2008), [2] Eta/DYNAFORM Application Manual Engineering Technology Associates, Inc [3] [4] HOJNY, M., Application of an integrated CAD/CAM/CAE/IBC system in the stamping process of a bathtub1200 S, Archives of Metallurgy and Materials, 55, 3 (2010), [5] HYRCZA-MICHALSKA, M., Drawability of advanced high strength steel and creep-resisting nickel super alloys, Steel Research International, special Edition 2010 Metal Forming Conference, 81, 9, (2010), [6] HYRCZA-MICHALSKA, M., Badania technologicznej plastyczności nadstopów niklu w procesie hydromechanicznego kształtowania stożka, Hutnik-Wiadomości Hutnicze, 78, 8, (2011), s [7] KIM, S.B., HUH, H., BOK, H.H., MOON, M.B., Forming limit diagram of auto-body steel sheets for high-speed sheet metal forming, Journal of Materials Processing Technology, 211 (2011), [8] SHERIFF MOHAMED N., MOHAMED ISMAIL M., Numerical design optimisation of drawbead position and experimental validation of cup drawing process, Journal of Materials Processing Tech., 206, 1-3 (2008),