Tool Making Innovations in Sheet Metal Forming

Transcription

1 Tool Making Innovations in Sheet Metal Forming M. Tisza 1, Zs. Lukács 1, P. Kovács 1 1 Univeristy of Miskolc, Department of Mechanical Engineering, Hungary Summary The increased competition in the automotive industry leads to a great model variety with increasing demand to shorten model cycles and to reduce lead times at a decreasing cost level. This requires a special response from car manufacturers to realise a more flexible and simultaneously more standardised manufacturing of vehicles applying new materials, new manufacturing methods, as well as more innovative die design and tool making activities, combined with shorter development and production times and lower manufacturing costs in tooling production, as well. This can be achieved by the increasing use of various methods of Computer Aided Engineering in tool and die design including sophisticated CAD systems for die design, and ever increasingly the various finite element analysis methods leading to an overall digital tool shop concept. This paper will introduce a simulation based die design concept implemented in tool-making for sheet metal forming with particular interest on car manufacturing. 1 Introduction Sheet metal forming is one of the most important manufacturing processes. This is particularly valid for the automotive industry, where sheet metal forming has an even more important key position. Since the automotive industry is one of the leading sectors in many countries, thus it is often the main driving force behind the sheet metal forming developments as well. The competition in car manufacturing is extremely strong leading to larger model variety and shorter model cycles. This competition also leads to very intense development activity to increase productivity and to reduce costs. This competition also leads to the application of new body concepts. New design concepts often require new materials, and new materials stimulate the elaboration of new innovative forming processes and new tooling concepts. Tool making had always a key role in sheet manufacturing, and this is even further emphasized with the need to implement more cost-optimized processes and production techniques for the realisation of quantity-optimized and variant-flexible tools for forming. The contributions of the tool making to the decrease of lead time and to reduce the production costs are indisputable, thus, the importance of innovations in tool making even further increases in the future. Higher functional requirements increased demands on lightweight constructions, which are often solved for example by the application of so-called multi-material body concept. In addition to new tool technology, multi-material construction principle also requires new tooling concepts. A further critical contribution of tool making to the objectives described above is to be found in the ongoing reduction in tool costs and with it a reduction in investment costs despite greater component and tool complexity coupled with higher material prices. An important tool development issue is the quantity-optimised tool concept for small production series. Depending on the quantity, in which a vehicle is to be produced, the optimal relationship between investment cost and production costs needs to be determined. Volume production requires tools and plants capable of manufacturing components requiring no reworking. Due to the large volumes, the required high investment costs amount to a small proportion of the cost of manufacturing a component. That is why, especially with small volumes, the investment costs need to be minimised. 2 Brief overview of main research tendencies in sheet metal forming as prerequisites behind tool developments As mentioned before, strong competition in car manufacturing led to the application of new body concepts requiring new materials, and new materials stimulated the elaboration of new innovative forming processes and new tooling concepts. Therefore, it is worth mentioning some recent developments in materials and sheet forming processes as the prerequisites of tool developments. 792

2 2.1 Material research and development Some decades ago, design engineers mainly focused their attention on structural and dimensional stability and durability. In recent years, the reduction of fuel consumption together with increasing comfort requirements led to the intensive development of innovative new materials. Enhanced stiffness together with weight reduction resulted in the development and wide application of various grades of high strength steels. Nowadays, several micro-alloyed and phosphorous-alloyed steels both with and without bake-hardening are frequently used. An increasing use of interstitial-free (IF) steels, dual-phase and TRIP-steels, as well as the ultra low and super ultra low carbon steels can also be mentioned. It can be observed that from the elaboration of various micro-alloyed steels in the midseventieth of the last century, there is a continuous pressure on material development leading to the appearance of new advanced steel materials practically in each five year [1]. Due to the increasing demand for environment friendly vehicles requiring reduced fuel consumption and weight, besides steel as structural material, aluminium alloys in automobiles are recently also widely used in car manufacturing for body-in-white production and their ratio will even further increase: these developments led to the elaboration of multi-material body concept [2]. 2.2 Innovative new forming processes New materials often require new, innovative sheet forming processes: in this respect, the automotive industry always played key role, and on the other hand, automobile as a mass production would be unthinkable without forming technology. They have a mutually advantageous effect on each other resulting in a synergic interdisciplinarity of various sciences. The rapidly emerging application of tailor welded parts, which are already used in most modern vehicles, is one of the most characteristic examples in this respect. Tailored blanks, where various sheets of different thickness or quality are welded together and formed subsequently to withstand various loads at different sections, are mainly due to the developments in laser technology. Nowadays, formability issues of tailor welded blanks are one of the most important questions, to provide significant load-bearing parts with reduced weight without any decrease of structural stability [3]. Among new, innovative forming processes hydroforming can be regarded as an outstanding one. Hydroforming is mainly used to produce hollow sheet or tube products often with complicated geometry to produce lightweight parts more economically [4]. There are many other new innovative forming processes recently introduced in sheet metal forming partly due to the latest results of materials development. Here, we just mention the new the renaissance of superplastic forming to produce complicated parts in a single operation for low volume production, and more recently the hot forming/press hardening of boron alloyed steels as a very promising field for the application of high strength materials in the automotive industry [5]. Since, this paper mainly aims dealing with tool making innovations, here we just mention these technological innovations and will rather focus on tooling aspects of forming processes, however, technological and tool developments are in inevitable interaction. 3 Tool making innovations: research and developments in the press shop It is obvious that tool making has a vital role in the development in sheet metal forming. Since stamping tool manufacturing is one of the most expensive fields in car manufacturing, it is of utmost importance to reduce the time and costs for tool design and manufacturing and to meet the concept right tools at first. In this respect, the application of various methods of Computer Aided Engineering (CAE) including the Computer Aided Tool Design and Computer Controlled Tool Manufacturing, as well as the integration of product-, process- and die development with numerical simulation (mainly with FEM modelling) in the whole development cycle are crucial for the competitiveness in sheet metal forming. Today, both the available computer technique and the various dedicated software packages developed particularly for the metal forming industry (in many cases in close cooperation with the automotive industry) may be regarded as the most significant developments in the last years. Due to these developments, tool making which was for a long time rather an art of toolmaker masters become a real science based engineering discipline. 793

3 3.1 Evolution of Computer Aided Engineering in tool design The Computer Aided Design (CAD) and Computer Aided Engineering (CAE) had and still have a tremendous impact on tool and die industry. At the beginning, CAD systems were mainly used only to create 2D and 3D die design drawings. Though, it was a significant development step from the old stamping design practice, however, this approach often resulted in unforeseen problems during tooling tryout, since, in this traditional solution CAD was only used during the die design phase. Thus, when it was completed, then the die was constructed followed by the tooling tryout to troubleshoot for stamping defects. If parts were produced without stamping defects, it was sent for production. If any defects occurred during the tryout, the die set had to be reworked. As a result, the die manufacturing time and cost of die making was significant [6]. Forming simulation was introduced into the stamping manufacturing process mainly to improve the efficiency of process planning. The reliability and accuracy in forming simulation have helped to find forming defects before the real die construction. By implementing simulation in the forming process chain, the major stamping defects can be minimized prior to the die construction stage. Hence, the tooling tryout process is smoother and the die set can be delivered for production much earlier [7]. An interesting simulation approach is realised at GE Manufacturing Engineering reported by Wang and his co-workers [8]. They defined sheet metal forming as a displacement or draw-in controlled manufacturing process which is particularly valid for automotive body panel production. Introducing the draw-in amount as a single manufacturing index that controls all forming characteristics (strains, stresses, thinning, etc.), stamping failures (splits, wrinkles, surface distortion, etc.). Draw-in Map is engineered for math-based die developments via advanced stamping simulation technology. Then the Draw-in Map is provided to die makers in plants as a road map for math-guided die tryout in which the die tryout workers follow the engineered tryout conditions and matches the engineered draw-in amount so that the tryout time and cost are greatly reduced, and quality is ensured. Similarly significant results are reported by Griesbach and his co-workers [9] achieved at the Audi Motor Co. Applying the recent results of material and tool innovations, as well as by the application of numerical simulation and Computer Aided Engineering techniques in tool making resulted in significant time and cost reduction in spite of the increasing complexity of sheet metal parts and manufacturing dies. For example, the production time for a die set for an outer side panel was reduced by 60% between 1996 and 2006, together with a 30-40% cut in the associated tool costs. 3.2 Simulation Based Die Design Stamping manufacturing process shortfalls can be further improved by the implementation of Simulation Based Die Design modules into simulation packages to enhance interaction with product designers. These implementations support die design process by solving stamping defects during the die development process. Therefore, they even further reduce the lead time of die design and construction, and also the associated cost. Today, in most stamping simulation packages, Simulation Based Die Design module is embedded in the system. For example, in the AutoForm one of the leading sheet metal forming simulation packages this is the so-called Die Design module, which is associated both to AF-OneStep inverse simulation module providing fast feasibility analysis and to AF-Incremental module suitable for detailed numerical process simulation. This Die Design module strongly supports process and die design engineers to quickly derive forming die surfaces, including binder and addendum from the part geometry. It allows the user to easily design and re-engineer the die tooling using numerous automated functions provided in the software. The implementation of Simulation Based Die Design supports die design engineers to further reduce iteration time for tooling design and development cycles as well as the associated cost. 3.3 Integrated Process Simulation and Die Design In our practice at the University of Miskolc, Department of Materials Processing Technologies in co-operation with sheet metal forming companies, we elaborated an integrated system for process simulation and simulation based die design [10]. 794

4 Product design CAD part model Unigrapics NX UG-AF interface Feasibility study OneStep simulation AF-OS OK Die Design AutoForm Die Designer AF-DD Product redesign Failed UG-AF interface Process or die redesign In this system, the whole product development cycle is handled in a joint integrated CAD and FEM simulation environment (Fig. 1.). This solution requires efficient CAD and FEM packages and a special interface module to enhance the information and data exchange between CAD modelling and FEM simulations in both directions making possible the most efficient integration during the whole product development cycle. In our solution, we use Unigraphics- NX or CATIA as CAD system together with the AutoForm simulation package. The selection of the above two CAD systems may be reasoned first of all by the fact that both have a unique interface (AF-NX, and AF-CATIA), which provides the necessary and continuous interaction between the CAD and FEM simulation packages making possible a real integrated process simulation and die design procedure as shown in Fig. 1. This integrated solution will be illustrated through the example of an automotive part. The CAD model of the component to be manufactured created by the product design engineer is shown in Fig. 2. As it often happens in the automotive industry, the component has a symmetric counterpart (so-called left and right handed or double attached parts). This model is created in the Unigraphics NX CAD system as a solid model in the Product design module (see Fig. 1.), however, FEM systems dedicated for sheet metal forming usually require surface models. Therefore, before exporting the part model a surface model should be created. This function is well-supported in most CAD systems: even we can decide which surface (top, middle or bottom) will be exported into the surface model. Unigraphics is particularly well suited for this purpose, due to its UG-AF interface module which is associatively linked to the AutoForm software package providing a smooth, efficient and consistent data transfer between the two systems in both directions Feasibility of the component formability In most cases, process planning engineers would like to know right at the beginning whether the component can be Process planning Incremental simulation AF-IS AF-UG interface Die construction Unigrapics NX NC/CNC manufacturing Prototyping & Tool try-out (Optional) Line Production Die redesign Fig. 2. CAD model of the component to be produced manufactured with the planned formability operations. Therefore, after importing the CAD model of the component, first a fast feasibility study should be performed. In this system, it is done in the AutoForm OneStep (AF-OS) module. In the AutoForm, this formability analysis can be done even if we do not have UG-AF interface Fig. 1. Process Planning and Die Design in integrated simulation environment Die redesign 795

5 any, or just very little information on the forming tools. By this inverse, One-Step simulation, a quick decision can be made if any modification of the part is required. If this feasibility study is successful, the next step is the determination of the the optimum blank shape and sizes which is very efficiently supported by the system Determination of the optimum blank The optimum blank determination is particularly important when we have such a complicated part, where besides the expected intricate contour line, we have to consider the joined shape of left and right handed, double attached parts, as well. The precise determination of optimum blank is important from other points of view as well. As it can be seen from the CAD model of the part, a trimming operation is practically impossible when the part is already formed due to its complex 3D part boundary line. Therefore, we have to apply a blank geometry which does not need any trimming operation in spite of the complicated 3D forming operations, i.e. when the forming is finished, we have the required part boundary line within the prescribed tolerances. Furthermore, this optimum blank is also necessary for the determination of material utilization, which can be done in the Nesting and Blank-layout module. This is also an important starting information for the detailed, incremental forming simulation Die Design and detailed incremental process simulations Even the One-Step simulation resulted in good formability, the final decision on the whole process can be made only after performing a detailed incremental modeling particularly concerning the critical forming steps. For this detailed simulation we need very detailed knowledge on forming tools and process parameters. The active surfaces of the forming tools can be derived from the imported surface model of the component to be produced utilizing the many useful possibilities offered by the Die- Designer module to create the binder and addendum surfaces, as well as the so-called reference surface, Fig. 4. Simulation tool setup for the incremental process simulation Fig. 3. Reference surface to derive the tool surfaces which can be used to quickly derive the punch and die surfaces, as well (Fig. 3.). Obviously, even for the detailed, incremental simulation, we just need the tool surfaces of active tool elements (punches and dies), as shown in Fig. 4. Applying this simulation tool setup, incremental simulation can be performed with various process parameters and depending on the simulation results several modifications on the tool parameters can be also made. Thus, for example, we have to modify the binder surface to reduce the drawing depth and by this, the value of critical strains, or we have to change the tool radii at certain critical sections to avoid undesirable thinning, or splitting, etc. These modifications can be performed within the AutoForm package just returning back to the Die Designer module. During this incremental simulations, the effect of process parameters, and the necessity of applying special forming operations can be decided. For example, for this components defect-free parts could be produced only using so-called free cut-off lines in the middle of the double attached part to provide free deformation in the scrap region to avoid unacceptable defects on the parts as shown in Fig

6 Separating the left and right parts, the cut-off lines and splitting in the scrap zone are removed and we have the components with the required quality. After getting the defect free component with some tool and process parameter changes, we can complete the die design task on the basis of simulation results. For this purpose, the final reference geometry is exported back to the Unigraphics CAD system through the reverse AF-UG interface module to design the complete tool assembly. In this case, we have the reverse problem, as we had when importing the part model. For the tool design we need solid models: however, the exported reference geometry is a surface model, which should be converted to a solid one. Fig. 5. Defect free double attached part with cut-off lines 4 Conclusions Computer aided engineering has a vital and central role in the recent developments in sheet metal forming concerning the whole product development cycle, and particularly in the tool design phase. The application of various methods and techniques of CAE activities resulted in significant developments: the formerly trial-and-error based workshop practice has been continuously transformed into a sciencebased and technology driven engineering solution. In this paper, an integrated simulation based die design concept has been described. Implementing this concept in tool making practice, it results in shorter lead times, better performance and significant reduction in associated costs. Acknowledgements This research work was financed by the Hungarian Academy of Sciences (MTA) and the National Science Foundation (Ref. No.: OTKA NI 61724). Both financial supports are gratefully acknowledged. References [ 1] Wagoner, H. W.: New Developments in sheet metal forming: materials, tools and machinery, Journ. of Mat. Proc. Techn., 1997, [ 2] Tisza, M.: Recent developments in sheet metal forming with special regard on the automotive industry, Advances in Materials and Manufacturing Engineering, 2007, v [ 3] Waddel, W., Davies, G. M.: Laser welded tailored blanks in the automotive industry, Weld. Met. Fabr., 1995, [ 4] Lücke, H. U., Hartl, Ch., Abbey, T.: Hydroforming, Journ. of Mat. Proc. Techn., 2001, [ 5] Bayraktar, E., Isac, N., Arnold, G.: Study of forming parameters of deep-drawable steel sheet in automotive industry, Journ. of Mat. Proc. Techn., 2005, [ 6] Tang et., Lee, W. C., Pierre, S. St., He, J., Liu, K., Chen, C. C. : CAE Based Die Face Engineering Development, Numisheet 2005, Proc. of American Institute of Physics, 2005, [ 7] Tisza, M., Gál, G. G., Lukács, Zs.: Rapid Parametric Process Design using FEM, Advanced Materials Research, 2005, v. 6-8., [ 8] Wang, C. T., Zhang, J. J., Goan, N.: Draw-in Map: A Road Map for Simulation Guided Die Tryout and Stamping Process Control, Numisheet 2005, Proc. of American Institute of Physics, 2005, [9] Waltl, H., Vette, W., Griesbach, B.: Toolmaking for Future Car Bodies, IDDRG 2007, Győr, Hungary, May 2007, Proc. of IDDRG 2007 (ed. Tisza, M.), [10] Tisza, M., Gál, G. G., Lukács, Zs.: Integrated Process Simulation and Die Design, Int. Journ, of Material Forming, DOI /s