May 2021 Volume 3

FORGING RESEARCH

models. Usually, the phase transformation data for a metal or steel can be found in metals handbooks or through dilatometry testing. 5.4 Requirements for process and design engineers An engineer who is doing modeling work is always encouraged to build an understanding of both the forming processes and the modeling methods through learning and practicing. Lack of either is the reason the models occasionally provide no practical or usable results. Engineering knowledge in the following areas is essential in performing metal forming modeling work. ·Mechanics: Elasticity, plasticity, mechanics of materials, principles of heat transfer. ·Material: Physical andmechanical characterization, formability, metallurgy and thermal treatments. ·Modeling: Finite element formulations, solution methods, suitability and limitations. ·Metal forming process: Process principles, suitability and limitations, forming sequence and workpiece design, and equipment selection and capability. Building a finite element model is a tedious task that can take hours or sometimes days to complete. The model should be built as simply as possible by ignoring minor or insignificant factors or features via so-called idealization. A sensitivity study can be conducted to study the impact of a few important variables on the behavior, robustness, trend and outcome of a model. An engineer should never be encouraged to pursue building a perfect model. This practice does not deliver proportionally more superior results relative to the extra hours expended. A critical step after a model is executed is for the engineer to use his/her common sense and extensive experience to validate the modeling results and give a sound explanation. The numerical error often may be in the same order of magnitude as that of the solution, especially in the simulation of precision metal forming processes. In this case, caution must be exercised on how the results are read. Confidence will be gradually built after the modeling results repeatedly agree with the actual production data or the modeling continuously provides solutions to the product and production issues. 6. Applications This section gives several examples of applying finite element modeling techniques to serve the objectives described in Section 5.1. All models were run in either the DEFORM or the ABAQUS finite element programs. 6.1 Finite element analysis-assisted product design in a skew rolling process The skew rolling process (Figure 17) evolved from the transverse rolling process. It is a unique hot forming process that produces near net-shape cylindrical or annular metal components with the ability to contour the outside diameter (Figure 18). One of the main characteristics of this process is that the axes of the rolls are arranged obliquely to each other with a small angle. The workpiece is continuously formed from a tube shell into forgings through the

helical grooves machined on the forming roll surfaces. Skew rolling has a high production rate and is able to form intricate geometries.

Figure 17. Schematic view of skew rolling

Figure 18. Examples of skew rolled bearing products However, the design of the forming rolls for skew rolling is very challenging work, involving a complex combination of the geometric variations, metal flow and stress distribution, and requiring intensive calculations (see the sample forming roll shown in Figure 19). A lack of process knowledge meant that the designs had to go through a number of expensive trial iterations, effectuating a long lead time and high product development costs [10]. At the time of process development, the capability and stability of 3D nonlinear finite element programs had significantly improved and computers had become more powerful in terms of their speed and capacity. The rapidly emerging parallel computing technique further enhanced the computation power. These advancements made the finite element methods capable of assisting the skew rolling designs. (Note: In all the finite element models, the workpiece was considered a rigid-plastic material, while all forming rolls and the mandrel were rigid.)

FIA MAGAZINE | MAY 2021 79