Bio: Gerhard Hirt is an academic researcher from RWTH Aachen University. The author has contributed to research in topic(s): Forging & Incremental sheet forming. The author has an hindex of 30, co-authored 335 publication(s) receiving 4463 citation(s). Previous affiliations of Gerhard Hirt include Saarland University & University of Bath.
Abstract: Non-oriented electrical steel sheets are applied as a core material in rotors and stators of electric machines in order to guide and magnify their magnetic flux density. Their contouring is often realized in a blanking process step, which results in plastic deformation of the cut edges and thus deteriorates the magnetic properties of the base material. This work evaluates the influence of the material’s grain size on its iron losses after the blanking process. Samples for the single sheet test were blanked at different cutting clearances (15 µm–70 µm) from sheets with identical chemical composition (3.2 wt.% Si) but varying average grain size (28 µm–210 µm) and thickness (0.25 mm and 0.5 mm). Additionally, in situ measurements of blanking force and punch travel were carried out. Results show that blanking-related iron losses either increase for 0.25 mm thick sheets or decrease for 0.5 mm thick sheets with increasing grain size. Although this is partly in contradiction to previous research, it can be explained by the interplay of dislocation annihilation and transgranular fracturing. The paper thus contributes to a deeper understanding of the blanking process of coarse-grained, thin electrical steel sheets.
Abstract: The magnetic properties of non-oriented electrical steel, widely used in electric machines, are closely related to the grain size and texture of the material. How to control the evolution of grain size and texture through processing in order to improve the magnetic properties is the research focus of this article. Therefore, the complete process chain of a non-oriented electrical steel with 3.2 wt.-% Si was studied with regard to hot rolling, cold rolling, and final annealing on laboratory scale. Through a comprehensive analysis of the process chain, the influence of important process parameters on the grain size and texture evolution as well as the magnetic properties was determined. It was found that furnace cooling after the last hot rolling pass led to a fully recrystallized grain structure with the favorable ND-rotated-cube component, and a large portion of this component was retained in the thin strip after cold rolling, resulting in a texture with a low γ-fiber and a high ND-cube component after final annealing at moderate to high temperatures. These promising results on a laboratory scale can be regarded as an effective way to control the processing on an industrial scale, to finally tailor the magnetic properties of non-oriented electrical steel according to their final application.
Abstract: A tailor-made microstructure, especially regarding grain size and texture, improves the magnetic properties of non-oriented electrical steels. One way to adjust the microstructure is to control the production and processing in great detail. Simulation and modeling approaches can help to evaluate the impact of different process parameters and finally select them appropriately. We present individual model approaches for hot rolling, cold rolling, annealing and shear cutting and aim to connect the models to account for the complex interrelationships between the process steps. A layer model combined with a microstructure model describes the grain size evolution during hot rolling. The crystal plasticity finite-element method (CPFEM) predicts the cold-rolling texture. Grain size and texture evolution during annealing is captured by the level-set method and the heat treatment model GraGLeS2D+. The impact of different grain sizes across the sheet thickness on residual stress state is evaluated by the surface model. All models take heterogeneous microstructures across the sheet thickness into account. Furthermore, a relationship is established between process and material parameters and magnetic properties. The basic mathematical principles of the models are explained and demonstrated using laboratory experiments on a non-oriented electrical steel with 3.16 wt.% Si as an example.
Abstract: Due to the nonlinear material behavior and contradicting application requirements, the selection of a specific electrical steel grade for a highly efficient electrical machine during its design stage is challenging. With sufficient knowledge of the correlations between material and magnetic properties and capable material models, a material design for specific requirements can be enabled. In this work, the correlations between magnetization behavior, iron loss and the most relevant material parameters for non-oriented electrical steels, i.e., alloying, sheet thickness and grain size, are studied on laboratory-produced iron-based electrical steels of 2.4 and 3.2 wt % silicon. Different final thicknesses and grain sizes for both alloys are obtained by different production parameters to produce a total of 21 final material states, which are characterized by state-of-the-art material characterization methods. The magnetic properties are measured on a single sheet tester, quantified up to 5 kHz and used to parametrize the semi-physical IEM loss model. From the loss parameters, a tailor-made material, marked by its thickness and grain size is deduced. The influence of different steel grades and the chance of tailor-made material design is discussed in the context of an exemplary e-mobility application by performing finite-element electrical machine simulations and post-processing on four of the twenty-one materials and the tailor-made material. It is shown that thicker materials can lead to fewer iron losses if the alloying and grain size are adapted and that the three studied parameters are in fact levers for material design where resources can be saved by a targeted optimization.
01 Nov 2021
Abstract: To produce cold-rolled steel strips with specific mechanical properties and surface roughness typically temper rolling is adopted. In most cases, a uniform roughness pattern on the strip surface is mandatory. Due to the wear of the textured work rolls, their surface roughness ( R a ) continuously reduces during the process, which should be accounted for process control. However, conventional temper rolling systems fail to guarantee a uniform surface roughness. In this work, the influence of strip tension on the imprinting of surface roughness during temper rolling is analyzed based on a multi-scale FE modeling concept to explore new ways for surface roughness control. This is done in simulation where, a macroscopic rolling model incorporating strip tension is coupled to a mesoscopic imprinting model and both models are validated using copper rolling trials. The influence of different thickness reductions, strip tensions and incoming strip's surface roughness on imprinting is modeled and compared. The numerical results reveal that a higher strip tension decreases the roughness transfer, which presents potential to control the roughness transfer ratio without changing other process parameters like the prescribed thickness reduction in the future.
Abstract: In this work, the dynamic process of oil droplets impacting and migrating on structured surfaces with imposed thermal gradients was investigated. It was observed that on an isothermal smooth surface, a lubricant droplet would impact, spread to a maximum diameter, and retract; while on a non-isothermal smooth surface, an asymmetric geometrical morphology of droplet was generated, accompanying with a migration process. Relevant dimensionless parameters were employed to evaluate the dynamic process, and the physical mechanism was revealed. Decorating surfaces with convergent microgrooves pattern could not only increase the maximum spreading diameter but also accelerate the migration process. These are beneficial for the heat exchange efficiency and lubrication performances. [DOI: 10.1115/1.4052779]
Abstract: Metal sheet forming is widely used in industrial field. Generally, the use of patchwork banks allows the manufacturing of a more uniform thickness of the formed parts. Often, they are obtained using welding techniques: in these cases, distortions and localized metallurgical transformations could have detrimental effects on the mechanical performance. Moreover, it is not possible to weld very thin patches. In this paper, an innovative approach based on bonded patches has been developed and investigated. In this manner, it is possible to adopt thinner patches, doing away with the problems arising from the welding process. Experimental tests showed the potentiality of using adhesives in forming processes to influence the thickness distribution. Moreover, the commercial code PAM-STAMP has been used for modelling the forming process. In this way, the effects of the thickness of the patch and the friction conditions on the strain state generated on the patchwork blanks have been investigated.
Abstract: The demand for new structural concept design of automotive parts has grown with weight reduction and improvement of crash safety in automotive industry. In this study, the established local patchwork hot stamping technology was used in structural design of hot-stamped automotive parts and expected to obtain maximum lightweight efficiency while maintaining crash performance by using local patchwork blanks instead of conventional reinforcement. Firstly, the technical feasibility of this technology was verified in an experimental method, and an optimization method was proposed to determine the optimal position and shape of local patchwork blanks. Then it was used for the improvements of two parts as examples, a top-hat channel created based on a conventional B pillar cross section and a B pillar. Finite element (FE) analysis models of these two parts were established based on the deformation of B pillar during full vehicle side impact and validated through experiments. It was confirmed that both the top-hat channel and the B pillar optimized have the same crashworthiness but became lighter compared to the original parts with reinforcement. Furthermore, the effects of local patchwork blank thickness, material strength and outer thickness on the optimal position and shape of local patchwork blanks and lightweight efficiency were investigated. Finally, the effectiveness of a hot-stamped B pillar with local patchwork blanks was verified through full vehicle side impact simulations. It can be concluded that the established local patchwork hot stamping technology and the proposed optimization method can be used as a dependable tool to design and manufacture hot-stamped parts with local patchwork blanks.
••01 Jan 2022
TL;DR: This work looks closely at some of the techniques used by the traditional smith to form sheets and finds that decision-making elements of these adapted processes are not yet as capable as their manual counterparts, suggesting there is still a lot the authors can learn from the traditionalsmith.
Abstract: The traditional metal smith has the remarkable capability to form a variety of part shapes from flat sheets using only a few universal tools. Such versatility is increasingly appealing to manufacturers who now seek to diversify part catalogues and reduce tooling costs. Despite this utility, the laborious, manual nature of these traditional techniques preclude them from meeting modern-day, high-volume demand. However, some techniques have served as starting points for the development of new flexible metal forming processes, either through creating new processes that closely replicate the traditional techniques or by automating the manual process. Here, we look closely at some of the techniques used by the traditional smith to form sheets and review automated adaptations of these processes. We find that decision-making elements of these adapted processes are not yet as capable as their manual counterparts, suggesting there is still a lot we can learn from the traditional smith. As such, we look both within and beyond the domain of metal forming at the technologies and the methods that can be used to capture the skilled actions of the smith and how the resulting data can be used to enhance the design and operation of mechanised variants.
Author's H-index: 30