May 2021 Volume 3

FORGING RESEARCH

· Forming steps in sheet forming. Sheet-metal forming processes typically consist of many steps, from blanking to the final finished product. In each step, the strains in the workpiece must be maintained within the forming limit to prevent localized necking, a plastic instability in tension, from happening. The process factors affecting formability are: · Reduction ratio. The reduction ratio is an accumulation of the strains exerted on the steel before it becomes a finished product. A sufficient reduction is needed in the workpiece to heal porosities, break the inclusions and alleviate the segregations. All of these are inherited from ingot casting. · Working temperature and speed (Section 4.5). · Friction. Friction, a result of high shear stresses at the interface of the workpiece and tooling under a high compressive forming force, is needed to deform the workpiece into the desired shape. Excessive friction, on the other hand, increases the deformation force, makes deformation non-uniform, retards metal flow and increases defects and fractures. It also has a profound impact on tooling life and equipment capacity requirements. In some circumstances, friction can change the grain flow patterns in the workpiece as well. To maintain tooling life and geometry and achieve consistent product quality, friction must be kept in check. Lubrication is the most commonly used method in cold and hot forming operations to control friction. Selecting the right type of lubrication and defining the amount and interval of lubrication application are of critical importance. Other means, such as improvement of the tooling and/or workpiece surface finish, can also reduce friction. 5. Metal FormingModeling 5.1 Roles and limitations of metal forming modeling Finite element metal forming modeling has evolved historically from a number of approximate analytical methods including, for example, the slab method, the slip-line field method and the upper- bound method. However, the degree and sophistication of metal forming modeling suitable for industrial applications was available only after the invention of digital computers. Readers can find many good books that discuss in depth the rich history and fundamentals of finite element theory, so the author does not touch on them in this article. Metal forming is a unique field in finite element modeling.The basic requirement of metal forming modeling is that it be able to simulate, replicate and examine what happens in the workpiece and tooling as if they are in the actual forming process. Metal forming modeling (mostly finite element analyses) can be employed to assist product and process engineers in every phase of the processing cycle, from development to production, with clear advantages (Figure 15) over the results that historically were achieved by trial-and-error methods or based on anecdotal experience. The modeling can be applied in the conceptual development phase – as an integral component of computer-aided design (CAD) and computer-aidedmanufacturing (CAM) – to study project feasibility,

select the correct forming process, design the tooling and preform, calculate the forming force, seek the right forming machine and conduct a cost analysis. This virtual tool is versatile and flexible. The tooling and preform configurations and forming parameters can be changed easily and rapidly. In the trial and prototyping phase (after the initial concept is validated), the modeling can formalize the forming scheme, tooling prints, blank form and process parameters through the iterations and interactions of the modeling work and physical testing so that a defect-free product with the desired size and shape is attained. It has been demonstrated that applying modeling in process development phases can substantially reduce the cost and lead time. The modeling can also play a critical role in the production phase to help troubleshoot and identify the root causes of production issues and product defects and possibly provide remedial solutions. A unique feature of process modeling is that it provides an opportunity to review any moment of the entire forming simulation to help understand how defects or imperfections are initiated and propagated.

Figure 15. Roles of process modeling in manufacturing process development Although forming process modeling has advanced dramatically in recent years, it is not without limitations. Challenges remain, especially when the analysis is not isothermal and involves heat transfer, microstructure and phase transformations, or when it needs to model multiple deformable objects involving elasto-plastic formulations (for example, to study tooling-workpiece interactions or tooling deformation). In the modeling software, the challenges are rooted in the intricate nonlinearities and boundary conditions of metal forming modeling, implementation of accurate and sophisticated solution methods and an accurate material response through appropriate constitutive equations. In the application, the challenges are rooted in the lack of actual data for the modeling.This is especially true for the material data; for example, the mechanical and thermal properties, microstructural and phase transformation properties, flow curves, work-hardening and strain rate sensitivity, to name a few. Often the exact material data for the metals or steels to be modeled can only be obtained by extensive testing, which is difficult to do for most engineers. Therefore, adopting similar material data from handbooks or other sources is generally acceptable.

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FIA MAGAZINE | MAY 2021