The production of connecting elements is a key area of cold forming in Germany, but is facing growing challenges due to shorter product life cycles. In particular, the energy-intensive profile rolling of complex components is currently limited by empirical, cost-intensive process designs and a lack of digital models. The finite element method (FEM) as a digitalization option offers great potential here for predicting process errors and efficiently mapping process limits using virtual simulation and damage models, such as the Mohr-Coulomb criterion. This can increase the competitiveness of small and medium-sized enterprises (SMEs).

Motivation

With a total annual mass of 777,000 tons and a turnover of €3.82 billion, the production of connecting elements represents a significant proportion of the components manufactured in Germany using cold forming technology.

Advancing digitalization and the associated shortening of product life cycles pose new challenges for the manufacturing industry, especially small and medium-sized enterprises (SMEs). This trend requires shorter, more efficient, digital and sustainable product development processes. Cold forming in particular, as an energy-intensive process with high machine and tool costs, must adapt to these developments. High quality, strength and output as well as low unit costs with high quantities of the components produced are at odds with the aforementioned requirements.

Profile rolling offers great potential for increasing efficiency and flexibility in production. However, the use of profile rolling for new, more complex components such as modern connecting elements with grooves or special fitting surfaces has so far been limited due to challenges. Process design is currently based primarily on empirical, iterative methods, which leads to high costs. Expensive tools are often produced multiple times with iterative improvements (Figure 1, left).

The finite element method (FEM) is able to overcome these challenges by providing a virtual representation of the forming process (Figure 1, right). However, previous studies show that the application of FEM in profile rolling is still under development. In particular, the prediction of process defects such as internal cracks and necking represents a major challenge. Long simulation times and the process limits that still need to be researched make widespread application difficult. Damage models offer the possibility of mapping these process limits by predicting damage.

Approach

The aim of this project is to develop failure mechanisms such as internal cracks and necking during profile rolling using the finite element method (FEM) with integrated damage models (Mohr-Coulomb damage model). In order to be able to map the process design using FE simulation, a simulation is first created and optimized on the basis of profile rolling (see Figure 2). At the same time, a model test is developed to specifically introduce damage (internal cracks) into cylindrical test specimens. This will be tested in industrial rolling experiments. At the same time, the rolling tests are simulated. Through metallographic examinations for internal cracks and the comparison of the stress states at the same location in the simulation, support points can be taken with which the damage equation of the Mohr-Coulomb criterion can be calibrated and integrated into the simulation. In the next step, a parameter study is carried out with the simulation extended by the damage model in order to be able to determine the process window by simulation.

The process window is used to determine which geometries can be used to roll grooves without possible damage. To validate specific influences of the solver in the simulation, the same simulation is modeled and carried out with “QForm UK” in addition to the “Simufact Forming” software. The determined, FEM-based process window is then validated experimentally to confirm whether the simulated process limit also exists in reality. The developed calibration test is also evaluated and the simulation model is further optimized. The knowledge gained can also be transferred to other damage mechanisms such as necking or overrolling.

Acknowledgement

The research project is part of the IGF project IGF 22806 N/1 of the Forschungsgesellschaft Stahlumformung e.V. (FSV). This project is funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) via the German Federation of Industrial Research Associations (AiF) as part of the program for the promotion of joint industrial research (IGF) on the basis of a resolution of the German Bundestag.

We would like to thank all industrial partners, the BMWK, the AiF and FSV, who support the research project IGF 22806 N/1 in the project support committee (PA).

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