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Large components, for instance in the energy industry, mining and heavy machinery, are produced from high weight cast as ingots followed by open-die forging. Besides achieving a certain final geometry and microstructure, one of the main objectives during the forging process is the elimination of casting defects, like voids from the solidification shrinkage. This process is divided in the two stages of void closure and void healing. During the healing of the closed void a solid bond is established at high temperature. In literature void closure in open die forging is thoroughly investigated. Concerning the healing by solid bond generation there are only few studies related to voids in open die forging but there is substantial literature related to bond formation in roll hot bonding and diffusion bonding. Most of this work however determines the bond strength after cooling to room temperature. Concerning future appropriate modelling of the closure and healing process in open die forging, it is important to decide, whether a bond, which was established in one forging stroke, would be strong enough to withstand the following strokes. As a first step in this direction, this paper experimentally examines the bond strength directly after bond formation under conditions typical for open die forging strokes. The results quantitatively confirm the expected influence of forming temperature, surface enlargement, holding time and oxide films.

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.

Improving tool life is one of the main challenges and research objectives in closed-die forging. One approach, first proposed in (Rosenstock et al. 2015 ) uses an inexpensive and easy-to-exchange sheet metal die cover, which is placed on the die surface during forging. By using this cover, die wear can be reduced since mechanical and thermal loads mostly affect the die cover instead of the die itself. Numerical and experimental investigations (Wolfgarten et al. 2015 ; Yu et al. 2016 ) have proven the general key features of the concept and the positive impact on the lifetime of the forging die. However, these studies have shown that the applicability of the concept and the corresponding lifetime of the die cover are strongly dependent on the investigated die and die cover geometries. For simple 2D die cover geometries, where the die cover can be produced by a simple bending operation, the maximum die cover lifetime of seven forging cycles was achieved. Since these geometries offer low structural stability, the die covers are prone to deformation or folding during forging. Hence, this work investigates the application of die covers in complex geometries. For this purpose, a 3D die cover geometry was proposed that offers higher structural stability and less challenging contact conditions regarding the material flow during forging. The die cover successfully experienced 40 forging cycles without visible distortions or folds. Regarding the thermal loads, the maximum temperature and the temperature amplitude measured in the forging die were reduced by 40 °C. Based on the experimental results, a numerical simulation model was built and validated regarding the temperature evolution and forging forces. The numerical model indicates a significant reduction in the mechanical loads on the dies through the application of the die cover.

Twin-roll strip casting of steel provides a resource and energy efficient way to produce thin hot strips directly from the liquid phase. Clad metals offer less costly alternatives to monolithic alloys for a wide range of applications, but their various production routes are extensive, expensive, or slow. In order to exploit the strengths of twin-roll strip casting to provide a possibly more cost and energy efficient production route for clad thin strips, research into the expansion of twin-roll strip casting process is conducted. The aim of the current research is the combination of steel with copper. For this purpose, a prefabricated cladding strip of commercially pure copper is inserted into the twin-roll strip casting process. Bonding between the copper strip and the cast steel strip (DC01) is realized by exploiting the process heat. The bonding zone of the clad strip is subsequently analyzed under the optical microscope and in the electron micro probe analyzer. The imagery shows an irregular bonding interface with straight and locally altered regions alternating. These irregularities can be classified into four groups based on their morphology and suspected forming mechanism. Bond strength and formability of the clad strips is qualitatively examined in rolling and bending tests. Rolling was possible without delamination and a total height reduction of approx. 40% while defect-free bending of 2 mm and 3 mm thick specimens was possible up to a bending angle of 90° for a bending radius of 5 mm.

Friction is a critical influencing factor for a variety of forming processes, as it affects, for example, the required forming force. Complex models for the numerical description of friction often have two or more model parameters but lack appropriate calibration methods since calibration schemes developed for one-parameter models are not applicable. The objective of this work is to develop an evaluation method based on inverse modeling of the conical tube-upsetting test in order to allow for the parametrization of a two-parameter friction model, providing a unique solution for the model parameters. It is based on a comparison of the specimen’s outer contour for several points in time throughout the forming process according to the finite element model of the test. An optimization algorithm minimizes the deviation between the experimental and the simulated contour by adapting the friction model parameters. A two-parameter model is used that considers normal stress as well as relative velocity. First, purely numerical investigations show the necessity of a model adaption due to insufficient data. The modeling scheme is therefore adapted to consider data from two tests with different relative velocities. The results suggest a unique solution for the determination of the friction model parameters for purely numerical studies as well as for experimental conditions, comparing with the evolving contour of the conical tube-upsetting test specimen. Thus, this study presents a promising approach for the calibration of two-parameter friction models.

Various industries, including mechanical engineering, utilize steel rings featuring variable cross-sectional profiles, such as eccentric rings. Presently employed methods for producing eccentric rings possess drawbacks like restricted geometries, significant material wastage or uneven microstructures. The radial–axial ring rolling process serves to create seamless rolled steel rings with near-net-shaped cross-sections. A novel technique involves achieving eccentricity by dynamically adjusting the mandrel’s position during the ring rolling process. This method’s fundamental feasibility has previously been showcased using a blend of oil clay and a labor test bench. Transferring the possibility of manufacturing eccentric rings on industrial radial–axial ring rolling mills would expand the product range of ring manufacturers without encountering drawbacks associated with existing manufacturing processes. The objective of this paper is to demonstrate the basic feasibility of the concept of an industrial radial–axial ring rolling mill. In the first step, FEA simulation studies were carried out to develop the rolling strategy and estimate the achievable eccentricity on the institute’s radial–axial ring mill. Subsequently, the rolling strategy was implemented on an industrial ring rolling mill with the help of a unique technology module programmed in C++. Finally, an eccentric ring was ring rolled and compared with the FEA simulation, and the reproducibility was demonstrated to be successful.

Iron–silicon alloys with up to 6.5 wt.% Si offer an improvement of soft magnetic properties in electrical steels compared to conventional electrical steel grades. However, steels with high Si contents are very brittle and cannot be produced by cold rolling. In addition to solid solution hardening, it is assumed that the B2- and DO3-superlattice structures are responsible for the poor cold workability. In this work, two cast strips with 6.0 wt.% Si were successfully produced by the twin roll strip casting process and cooled differently by secondary cooling. The aim of the different cooling strategies was to suppress the formation of the embrittling superlattice structures and thus enable further processing by cold rolling. A comprehensive material characterization allows for the understanding of the influence of casting parameters and cooling strategies on segregation, microstructure and superlattice structure. The results show that both cooling strategies are not sufficient to prevent the formation of B2- and DO3-structures. Although the dark field images show a condition which is far from equilibrium, the achieved condition is not sufficient to ensure cold processing of the material.

In forming simulations, flow curves are cardinal inputs to predict features, such as forming forces and material flow. The laboratory-scale experiments to determine them, like compression or tensile tests, are affected by deformation heating, restricting direct flow curve determination. In principle, the current analytical and inverse methods determine flow curves from these tests, but while the analytical methods assume a simplified temperature profile, the inverse methods require a closed-form flow curve equation, which mostly cannot capture complex material behavior like multiple recrystallization cycles. Therefore, the inverse piecewise flow curve determination method “FepiM” previously developed and published by the current authors is extended by introducing a two-step procedure to obtain isothermal flow curves at elevated temperatures and different strain rates. Thereby, the flow curve is represented as tabular data instead of an equation to reproduce complex flow curve shapes while also compensating the effect of inhomogeneous temperature profiles on the flow stress. First, a flow curve at the highest temperature is determined. In the second step, using this first flow curve as a reference, the flow curves at lower temperatures are obtained via interpolation. Flow curves from conventional compression tests for aluminum and copper in the temperature range of 20–500 °C are predicted, and it is shown that these flow curves can reproduce the experimental forces with a maximum deviation of less than 1%. Therefore, the proposed new piecewise method accurately predicts isothermal flow curves for compression tests, and the method could be further extended to highly inhomogeneous methods in the future.

Refractory linings are, in addition to loads due to corrosion and creep processes, particularly affected by thermomechanical stresses caused by the restricted thermal expansion of the lining. These stresses can occur within individual components as well as in bricks and can lead to plastic deformation, cracks, and material failure. Thus, comprehensive knowledge about thermomechanical behavior is mandatory for an accurate prediction of occurring stresses to design load‐optimized linings. This paper presents a method for utilizing refractoriness under load (RUL) tests to determine a temperature dependent static Young's modulus for refractories. In a first step, RUL tests with a negligible load are carried out in order to determine the materials temperature dependent thermal expansion coefficient. Afterwards, several RUL tests with higher loads are carried out. The measured data of change in temperature and length are then corrected by the thermal expansion and used to construct elastic lines for several temperatures, where the elastic slope is determined using the change in length and the respective load. Thus, Young's modulus for several temperatures can be determined. The obtained values for the Young's modulus are then compared to resonant frequency damping analysis measurements and validated using a finite element (FE) model of the RUL test. This paper presents a method for utilizing refractoriness under load (RUL) tests to determine a temperature dependent static Young's modulus for refractories.

Incremental Sheet Forming (ISF) is a manufacturing technology for individualized and small batch production. Among the opportunities this technology provides there is the possibility of a short ramp-up time and to cover the whole production chain of sheet metal parts by using a single reconfigurable machine set-up. Since recent developments proved that manufacturing of industrial parts is feasible, finishing operations such as flanging and trimming gain importance, which are an integral part of manufacturing process chains of many sheet metal parts. This paper analyses the technological capabilities of performing flanging operations by ISF. Due to the localized forming zone and the absence of surrounding clamping devices, ISF exhibits a different material flow than conventional flanging processes. In this paper, the influence of the tool path characteristics, the flange length as well as the flange radius is analysed in order to establish a process window and to compare it to the process limits of conventional flanging operations. Since geometrical deviations occur when flanging operations are performed by ISF, a new adaptive blank holder is developed, which acts in the vicinity of the forming tool and reduces unwanted deformation outside the primary forming zone. The experimental results show the benefits of the adaptive blank holder with respect to geometric accuracy. The established process window and the adaptive blank holder hence contribute to the applicability of incremental flanging operations, such that ISF can be used for all forming and flanging operations along the process chain.