May 2021 Volume 3

FORGING RESEARCH

Remember that while a coarse mesh may not yield accurate simulation results, an overly fine mesh may not indefinitely increase the simulation accuracy and can cause intolerable computational time. It is not meaningful and can be expensive to use a very fine mesh for a slight accuracy gain. A middle ground should be sought in which the required accuracy can be achieved and the cost of modeling is acceptable. The material models to be used in the simulation, including the flow curves and physical and mechanical properties, need to be closely representative of the steel grades of the workpiece. The material type – either elasto-plastic (cold forming, stamping or deep drawing) or rigid-plastic (hot bulk forging) – should be determined at this time. The fracture properties of the workpiece need to be set up properly for material removal such as piercing. Although it is not common, gravity and inertial forces from the mass of the workpiece sometimes need to be assigned to the workpiece when their effects cannot be neglected. 5.3.2 Tooling When constructing tool objects, the primary consideration is whether the tools are rigid or deformable. A rigid tool does not undergo any size or shape change, nor is it susceptible to any thermal effect. Computational costs are low. If the simulation needs to study the interaction between the tools and workpiece or to assist in the tool design, modeling the tools as deformable, mostly elastic bodies is more appropriate. With deformable tools, the risk of high stress concentration or stress risers around the tool corners or the influence of tool deflection and distortion on the workpiece’s dimensions and geometry can be assessed. The computational cost when the tools are deformable can be much higher. Tool feed rate (tool travel speed) does not seem to be a factor in cold forming, but in hot forming it plays a deterministic role in selecting either a static, quasi-static or dynamic modeling technique because of the increased strain rate sensitivity of the workpiece material at an elevated temperature range. 5.3.3 Boundary conditions Mathematically, a finite element model can converge and deliver meaningful results only when boundary conditions are applied; otherwise, the solutions of the differential equations are not unique. Physically, a model without constraining boundary conditions to limit its degrees of freedom will fly in the space. Generally, there are two types of boundary conditions. One type is to regulate the motions assigned and forces applied to the workpiece and tools. The motional boundary conditions, either fixed or moving, are usually called essential boundary conditions. The force boundary conditions are called natural boundary conditions .The other type is to define the contact mechanisms, such as friction and heat transfer, among the workpiece and tools. In defining the essential boundary conditions, a workpiece can be either under-constrained or over-constrained. The under-constraint to the workpiece causes difficulties in simulation convergence and the over-constraint stiffens the workpiece, resulting in less deformation and exaggerated stresses in the workpiece. In some

cases, defining boundary conditions that are exactly the same as the actual conditions can be challenging. Multiple models might need to be built with varying boundary conditions to compare the impact of the settings on the end results. In defining the natural boundary conditions, caution must be exercised when using point forces in the model to represent the distributed forces in the actual process. The point forces should not only provide the equivalent magnitude, but also avoid generating or adding unwanted forces and moments. To mitigate the contact problems of finite element modeling, a number of friction theories are available. The most commonly used are the Coulomb and shear friction models. The friction coefficients in cold forming, hot forming, lubricated or dry conditions are all different and should be properly selected. How the friction characteristics are defined in the model can affect the forming force, workpiece size and shape, and even grain flow. In defining the contact pairs, the master and slave surfaces must be reasonably assigned. Conventionally, the tools are always assigned as the master surfaces and the workpiece is always assigned as the slave surface. A large difference in the element sizes on the master and slave surfaces can make it difficult for the simulation to run smoothly and therefore should be avoided. 5.3.4 Thermal conditions The setup of thermal conditions is required primarily in hot forming and forging process modeling. If the simulation is designed to study the product’s shape, an isothermal model is usually satisfactory. The isothermal condition means that the temperature assigned to the tools is the same as that assigned to the workpiece and is maintained as a constant. The isothermal condition does not allow any heat exchange between the workpiece and tooling, but it does prevent any local heat loss in the workpiece as seen in cases where the temperatures of the tools are much lower than that of the workpiece. If the size and shape changes in the tools are of interest or the influence of these changes on the workpiece is the focus, a thermo-mechanical coupled model involving heat transfer is appropriate. In this case, the thermal properties of the tools – for example, the linear thermal expansion, conduction, convection and radiation – must be defined. Since the tools need to be meshed in a thermo-mechanical simulation, the model is more complex and will take a longer time to run. In hot forming and forging, the temperature distributions in the tools can significantly affect tool life and how the cooling channels are designed. High temperatures on the tool surface may reduce the yield strength of the tool, leading to plastic deformation. High temperature gradients from the surface to the center of a tool cause uneven expansion and contraction in the tool. In production, the working cycle-induced temperature fluctuation in the tools creates cyclic thermal stresses and strains on the tool surface, which prompts cracking initiation via low-cycle fatigue (heat checking). If the manner in which the workpiece microstructures change during hot forming or heat treatment is to be studied, then the workpiece phase transformation data (e.g., TTT and CCT curves and volume changes of the phases) must be included in the material

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FIA MAGAZINE | MAY 2021