Objectives
The main goals of this project are
a) To develop numerical procedures able to simulate multi-step sheet metal forming processes and
b) To develop an automatic design tool able to predict the ideal number and shape of the multi-step progressive forming tools;
The benefits of such design tool developed within this project are: (i) overall increase in project productivity; (ii) design simulation and virtual die tryouts that would identify forming problems like splits, cracks and wrinkles; (iii) these common problems on metallic parts such as springback, wrinkling, buckling instabilities, flow localization and fracture, are intended to be avoided; (iv) improved quality and reliability to ensure accuracy of the data for “right the first time” design of the real experimental tool.
The problem of simulation of multi-step sheet forming process corresponds to the resolution of a direct problem. However, the automatic design of the forming tools corresponds to the resolution of an inverse problem in which the final shape of the stamped sheet is already known and the geometry/shape of the tool that leads to the desired result becomes the required information.
From previous works of the R&D team [PT1, PT5], it can be seen that inverse problems can be solved with the aid of an optimization methodology coupled with FEM analysis. Considering that each FEM analysis is very time consuming, the choice of the optimization method should be done wisely. A way of optimisation in combination with time-consuming function evaluations is using approximate optimisation algorithms, of which response surface methodology (RSM) can be representative.
The shape of the final tool will be found considering parameterized geometry. However, the results obtained by such methodology are always limited to the geometric possibilities offered by the variation of the parameters that define/parameterize the shape. In this project, this limitation will be overtaken by the use of a non-parameterized definition for the tool surface geometry as well.
At the end of the project, it is expected to obtain a numerical tool able to efficiently simulate multi-step analysis. It is also expected to have other numerical tool that, together with the previous tool, is capable of automatic design the progressive forming tool, including the ideal minimum number of steps and avoid forming problems.
Research plan and methods
The design of forming tools, and specifically progressive tools, can be still seen as a “trial-and-error” practice, based on previously obtained experience by engineers, mainly due to complexities inherent to plastic forming processes (large deformations, friction, springback and wrinkles in formed parts, to name a few). In industries, such as the automotive, where complex and innovative parts are constantly required at the shortest time possible, this practice can lead to large economical costs and, consequently, lost of competitiveness.
It is well known that the design of multi-step sheet metal forming process is rather difficult. Even small errors may cause significant quality problem. In recent years, Finite element Analysis (FEA) has being considered as an essential tool for the design. However, even with recent tecniques, FEA encounters problems when multi-step processes are analysed. This fact is mostly due to the repeatly springback and trimming processes. During a multi-step process simulation, and before setting the deformed part on new tools, it is necessary to remove the external load remaining from previous steps from the FEM model, initiating springback phenomena. A similar situation exists when the cutting or trimming processes are analysed: part of the sheet is removed and unequilibrated forces appear on the cutting (trimming) line.
Other of the major problems in the design of multi-step (or progressive) tools is knowing when to divide the overall process (and the minimum number of required steps) in the multi-step processes. Today, the die maker divide the process when the formability results in failure. Moreover, the division is performed right before failure takes places. However, this procedure is inefficient and can lead to premature failure in the subsequent step. The knowledge of How the design of the intermediate steps affect the forming quality is not yet understood and it is of upmost importance for the die makers.
Therefore, the main goals of this project are
a) To develop numerical procedures able to simulate multi-step sheet metal forming processes and
b) To create an automatic numerical tool able to design the ideal number and shape of the multi-step progressive forming tools.
c) Experimental validation of the previous tools and development of an optimized experimental methodology to design the required forming tool.
The benefits of such design tools developed within this project are:
(i) overall increase in project productivity;
(ii) design simulation and virtual die tryouts that would indentify forming problems like splits, cracks and wrinkles;
(iii) these common problems on metallic parts such as springback, wrinkling, buckling instabilities, flow localization and fracture, are intended to be avoided;
(iv) improved quality and reliability to ensure accuracy of the data for “right the first time” design of the real experimental tool;
(v) calculate the optimal number of steps required for the forming of complex parts;
(vi) automatic design of initial, intermediates and final tools shape;
(vii) predicting the favourable process parameters, such as punch force, etc.
For the first goal, the strategy that will be used to predict springback will be performed in only one step, removing all the tools simultaneously and forcing the blank sheet to attain equilibrium. In this strategy, named “One Step Springback”, all the constraints imposed by the tools will be eliminated at the beginning of the unloading phase. To perform the trimming/splitting phases, the strategy consists firstly, in evaluating the elements that are to be eliminated/kept with the trimming procedure, and then adjusting the boundary’s remaining affected elements to the desired geometry. This adjustment is done by a node stretching technique, for two projection schemes, with optimization of the final element shape at the boundary. The final stage is performed as springback is calculated by letting the part relax.
The simulation of multi-step sheet forming process corresponds to the resolution of a direct problem. However, the automatic design of the forming tools corresponds to the resolution of an inverse problem in which the final shape of the stamped sheet is already known and the shape of the tool that leads to the desired result becomes the required information. In multi-step processe, tgis methodology will be employed considering that each operation is considered as an inverse problem. The overall problem can be defined as a multiple dependent inverse problem (with interdependent subproblems).
From previous works of the R&D team [PT1, PT5], it can be seen that inverse problems can be solved with the aid of an optimization methodology coupled with FEM analysis. The two main families of optimization algorithms generally applied in engineering are the evolutionary algorithms (EA) and the gradient-based methods (see [PT1]). Both families can be used in together, in parallel or in cascade [PT5] in demanding industrial problems. However, considering that each FEM analysis is very time consuming, the choice of the optimization method should be done wisely [PT5]. A way of optimisation in combination with time-consuming function evaluations is using approximate optimisation algorithms, of which response surface methodology (RSM) can be representative. RSM is based on fitting a low order polynomial metamodel through response points, which are obtained by running FEM calculations for carefully chosen design variable settings and finally optimising this metamodel. The Project team have large experience in using different optimization methods in inverse problems [PT5].
Considering the works done until now (see bibliography review), the shape optimization of metal forming tools almost always considers the geometry parameterized (parametric shape). The results obtained by such methodologies are always limited to the geometric possibilities offered by the variation of the parameters that define the shape [21]. This is a limitation considering that the task of choosing the parametric variables is very complex without knowing the solution of the inverse problem. In this project, this limitation will be overtaken. A non-parametric solution (a nodal based solution) together with filters will also be used.
All numerical work has no benefit if no experimental validation is performed. Therefore, a large set of experimental tests will be performed with the collaboration of the forming tool manufacturer named P.J. Ferramentas (www.pjf.pt) and at the GIFT at Pohang University of Science and Technology (POSCO, Korea). The experimental tests will consider simple part geometries and real demanding parts.
All mechanical properties will be monitored in these tests by the use of an advanced 3D optical metrology system. All results are presented in a fine resolution mesh created from the determination of the 3D coordinates and reflecting the surface of the measured object. The measuring system operates independent of the forming process and provides full-field results with high local resolution, for small as well as for large components.
The experimental results will be also used as verification/validation of FE simulations. Together with the numerical simulation of forming, optical measuring systems have significant potential for quality improvement and optimization of development time for products and production.
In all the period of the project, special attention will be made to dissemination of the obtained results and developed tools to both scientific and industry community, in order to establish scientific and industrial milestones of the ongoing work.
In this kind of works, and due to the large technological and research complexity involved, it is mandatory to establish contacts and collaborations with renowned entities, both research centres and Universities. As a result, the efficiency and the global quality of the work developed can then be guaranteed. For the effect, it will be made frequent contacts with three well-known institutions in the research field: (a) LTAS-MC&T – Continuum mechanics and Thermodynamics, University of Liège, Belgium and (b) Limatb – Laboratoired’Ingénierie des Matériaux de Bretagne – Université de Bretagne Sud, France. The research collaborators in the mentioned institutions are Jean-Philippe Ponthot and Philippe Pilvin, respectively.
At the end of the project, it is expected to obtain a numerical tool able to efficiently simulate multi-step analysis. It is also expected to have other numerical tool that, together with the previous tool, is capable of automatic design the progressive forming tool, including the ideal minimum number of steps and avoid forming problems.
A multidisciplinary research team composed by experts from mechanical technology and computational mechanics will perform this project, guaranteeing that there will be very useful work produced in industrial and scientific terms.
Institutions
GRIDS, TEMA, University of Aveiro
CEMUC, FCTUC, University of Coimbra
P.J. Ferramentas, Lda
Project team
António Gil d’Orey de Andrade-Campos (PI)
Luis Filipe Martins Menezes
Marta Cristina Cardoso de Oliveira
José Luis de Carvalho Martins Alves
Rui Oliveira Martins
Pedro Barros
Elisabete Ferreira
Ana Maia