ADDED VALUE OF SIMULATION TO METAL FORMING USING Finite Element Analysis
Challenges of Metal forming industry
In the current metal-forming product design and development paradigm, product cost, time-to-market, and product quality are three overriding issues, which determine the competitiveness of the developed products. In up-front design process, the first 20 % of design activities commits to more than 75 % of product development cost and product quality issues. How to conduct “design right the first time” is critical to ensure low product development cost, high product quality, and short time-to-market.
CAD, CAM, and CAE
To address these issues, state-of-the-art technologies are needed to support design solution generation, evaluation, and optimization in metal-forming product design and development. Traditionally, computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies provide approaches for representation and realization of design solutions physically.
However, how to generate design solution and conduct design solution evaluation and optimization is a non-trivial issue. In metal-forming product design and development, it is difficult to simultaneously address the design issues related to metal-formed part design, forming process determination, process parameter configuration, tooling structure design, material selection, prediction of the properties of deformed part, and finally the product quality control and assurance.
In addition, how to reveal, assess, and evaluate the interaction and interplay of different design variables or factors in the above-mentioned different stages and areas is another critical issue. Computer-aided engineering (CAE) technology fills this gap as it helps practitioners generate, evaluate, and optimize design solutions before the best design solution is feasibly and uniquely identified and practically and physically implemented.
In the current metal-forming product development paradigm, CAE simulation technology is one of the state-of-the-art technologies, which has been widely used in addressing the above-mentioned technical issues and will be used to solve the emerging bottleneck problems in the next upward trend of technology and product development.
In metal-forming processes, the plastic flow of materials and the deformation of tooling are the main physical behaviors of a forming system, which can be simulated by numerical approaches through modeling of the plastic flow of materials and the elastic or elastic–plastic deformation of tooling. The numerical results from simulation are correspondingly related to the physical content of the forming system being simulated. Currently, most CAE simulations employ commercial finite element (FE) simulation packages, in which the numerical technique, such as the finite element method, is the enabling and kernel technology.
Regarding the detailed applications of FE simulation in metal-forming product development, the focus is more on some individual issues such as revealing of deformation behavior, deformation loading, and flow scenario for process route determination and forming machine selection, stress and strain analysis for tooling design, and defect prediction and avoidance for product quality control and assurance
System input variables
On the other hand, it is well known that a metal-forming system comprises of all the input variables related to:
The billet geometry and material
The conditions at tool–material interface
The equipment used
The forming process
The forming configuration
Characteristics and requirements of the final products
Evaluation of the performance of a forming system thus needs to deal with these input variables. In addition, the interplay and interaction of these inputs are quite complex and the physical experiments to reveal them at design stage are not possible.
Even if it is feasible by using physical modeling experiment, it is also quite time-consuming, expensive, and not efficient.
FE simulation, however, provides an efficient approach for evaluation of a forming system designed as a whole and reveals the instantaneous behaviors and performances of the system in such a way the whole panorama of concern of the designed system can be systematically understood and evaluated.
The “output”, on the other hand, is the desirable information related to material flow and deformation velocity, flow-line distribution, advance of material flow front in forming process, and the magnitude and distribution of stress and strain in the deformation body. The outcome information is used to help design solution generation, evaluation, and optimization in terms of metal-deformed part design, process determination, process parameter configuration, tooling design and service life assessment, prediction and avoidance of flow and stress-induced defects, and dimensional accuracy and property-related product quality control and assurance.
In the traditional design practice, however, design conceptualization and solution generation are generally a trial-and-error process based on experiences and know-hows. The kinds of experiences and know-hows are usually acquired through long years of apprenticeship and skilled craftsmanship.
Therefore, the traditional design paradigm is time-consuming and error-prone and needs more experimental tryouts in workshop to verify the design, and further, it may involve more design changes. Moreover, if the designs of product, process, and the entire forming system are not satisfactory, it is also difficult to identify the reasons and pinpoint the root causes by the traditional approaches due to the complex interaction and interplay among a plethora of affecting factors, which are generally difficult to be revealed, explored, and quantitatively represented.
In net-shape forming process, there is no subsequent machining or working process needed to further shaping the geometries of the deformed parts. The difference of the geometries and
shapes between the deformed parts and the final products lies in the geometry compensation taken in design of the deformed parts via considering the elastic recovery and volume contraction in deformation process. The physical forming system covers feeding system, ejection mechanism, die structure, lubrication, and the selected forming equipment with auxiliary subsystems.
Springback cannot be avoided but can be minimized by several methods such as applying tension (part is subjected to tension while being bent. In order to reduce spring back bending may also be carried to reduce spring back bending may also be carried to reduce spring back bending may also be carried out at elevated temperatures), over-bending, and warm & hot forming. FEM is widely used in industry to predict metal flow and Springback. Based on the Springback predictions obtained from FEM, the tool geometries are virtually modified to compensate for the Springback before the tool is manufactured. Thus, tool manufacturing time and cost are significantly reduced.
Factors affecting the design of forming system
- Geometry, shape, and dimension of the metal-formed part
- Material, material micro-structure, and the properties prepared by previous working processes
- Process route, process parameters configuration, and design of billet and preform
- Die geometry, structure, and quality
- Friction and lubrication condition at the interface between work-piece and tooling
- Equipment capacity and working condition setting
The following figure shows for instance the extra cost induced due to the uncertainty in determination of the maximum deformation load for screw forming process. According to the
figure, a 30 % uncertainty in determination of the maximum deformation load could lead to the extra capital cost of equipment close to 90 % and the extra direct cost of the forged parts between 5 and 9 %, depending on the cost of materials.
In the traditional forming product development, a 30 % uncertainty in loading estimation is a common issue as the deformation load is generally estimated by empirical formula via simplifying or ignoring some low-priority factors. It is thus very difficult to give an accurate evaluation and prediction of these performance and outcome of the designed forming system.
The CAE simulation, however, provides an efficient, robust, feasible, viable, and pragmatic approach to support the design and development of metal-forming product, forming process, die set, and the entire whole forming system.
Performance and service life
It has become a standard tool to support metal-formed part design, process determination,
process parameter configuration, tooling design, and product quality control and assurance. By using FE simulation technology, the systematic evaluation and verification of design solutions via considering the intended performance of the forming system and the deformation behaviors of the deforming work-piece based on the predefined evaluation criteria thus becomes possible, feasible, and practical. The evaluation of design solutions needs to take into account the interactions and interplays of different design variables and further to identify the good, better, and best design solutions.
The simulation-enabled forming system design and evaluation is thus able to reduce experimental work, shorten time-to-market, and cut development cost.
In this process, the mechanical deformation behaviors, metallurgical phenomena, and thermal phenomena in the forming process should be well represented by the input data and information. On the other hand, the simulations of tooling deformation and the plastic flow of work-piece in forming process are conducted simultaneously, which is the so-called coupled simulation.