Hot stamping also known as press hardening is one of the most important processes for manufacturing components with strengths exceeding 1500 MPa. The process combines heat treatment and forming in a single operation. Heating is typically carried out in gas-fired roller hearth furnaces and, due to the AlSi coating, requires slow heating times of 6-10 min. For alternative heating methods such as resistance heating, only uncoated sheet material can be used. However, during rapid heating rates of more than 100 K/s, the sheets suffer from scaling, which makes costly postprocessing necessary. This study demonstrates that by using a protective atmosphere of nitrogen and monosilane, a process environment can be created that corresponds to the oxygen concentration of an extreme high vacuum. At the same time, this process atmosphere enables the in-situ coating of uncoated sheets with the process heat required for hot stamping. Furthermore, it is shown that coating materials can be applied which form intermetallic phases.

The lubrication of tools in forming technology primarily serves to reduce wear. Tool lubrication is therefore an essential factor in the optimization of processes with regard to their economic efficiency. There are different approaches that address lubrication on the semi-finished product side. In this contribution, the approach of lubrication through infiltration of porous semi-finished products produced by powder metallurgy is discussed. These semi-finished products should later constantly release lubricant during an extrusion process to enable a constant lubricating film. For this purpose, the porosities serve as reservoirs. The cylinders produced here consist of steel powder mixed with pressing aid to minimize friction during powder pressing. Powder pressing at different pressures was used to produce samples with different densities, which were then sintered. The density was determined by geometric and weight measurement. The semi-finished products were then infiltrated with lubricant. Different lubricants are examined and used to investigate the influence of the viscosity on the infiltration amount. Due to the temperature dependence, the viscosity was determined as a function of temperature. Infiltration was carried out in a lubricant bath with different infiltration times. The amount of the lubricant was measured by weighing to research the correlation between porosity and infiltration quantity. It was found that a lubricant with a high viscosity infiltrates in higher quantity, but retention is lower due to drip-off. In addition, the duration of infiltration at low viscosities has less influence on the lubricant quantity. The results published here can be used to design self-lubricating blanks depending on the desired lubricant quantity for a cold forming process.

Optical technologies are probably the key technologies of the twenty-first century. Traditional optical systems such as projectors, microscopes or laser optical systems still consist of numerous separate components which generate, manipulate or detect light. By combining various technologies, optical systems can be developed to integrate a large number of functions in a small space at low costs. Optical gratings, for example, are used in waveguides for light coupling or in optical sensors. The structure sizes of such diffractive optical elements are often smaller than one micrometer, which creates major challenges to manufacturing technology. The conflict of objectives between costs, quality, accuracy and functionality can be addressed by embossing in combination with an adaptive process control in real time. In addition to injection molding and laser engraving processes, micro embossing offers a cost-efficient option for the mass production of optical components with structured surfaces. The embossing process also provides a high degree of flexibility to be adapted to new optical materials without the need of extensive adjustments. An embossing device enabling to transfer micro- and nanostructures precisely and reproducibly onto semifinished optical products was developed as part of the cluster of excellence PhoenixD, funded by the German Research Foundation. This paper focuses on demonstrating both the positioning accuracy and the achievable quality of the embossed structures. The innovative combination of embossing processes and real-time process control opens up new chances for the production of high-quality optical components for a wide range of applications.

Medium manganese steels provide numerous benefits in hot forming, including reduced blank reheating temperatures and critical quenching rates compared to conventional boron-added steels. Moreover, their enhanced strength and ductility make them a promising material for lightweight components in the mobility sector. In this study, the flow behaviour of a novel medium manganese steel is characterised and modelled to enable the simulation of hot forming processes. A forming and quenching dilatometer is utilised for isothermal tensile tests at different forming temperatures and strain rates. The specimens undergo heat treatment prior to forming, following a process route that includes annealing, cooling, and reheating to replicate the heat treatment at the steel producer and the hot forming at the parts manufacturer. An in-situ optical measurement system is used to determine the strains with digital image correlation. The experimental flow curves are modelled using various phenomenological hardening laws. Finally, the applicability of the hardening laws is verified by the simulation of a tensile test that was not used for modelling. The best prediction accuracy was achieved by the modified Norton-Hoff law, which provided a root mean square error of 14.4% during model calibration and a low mean absolute percentage error of 1.3% during validation.

Hydrogen-based energy systems are considered a key pillar of the energy transition, yet the cost-efficient, mass production of metallic bipolar plates (BPPs) for proton exchange membrane fuel cells (PEMFCs) remains challenging, as conventional processes are limited by comparatively long cycle times and forming-related instabilities. This paper investigates the rubber drawing process as a cost-efficient manufacturing method for metallic bipolar plates, proposed as an alternative to the commonly applied hydroforming process, analysing the influence of pressing force, rubber hardness and thickness, tool modifications for varying pressure distribution, and the suitability of additively manufactured tool dies made from Maraging Steel 1 (X3NiCoMoTi 18-9-5) or ceramic-filled UV resin. The results show that precise and stable tool guidance, as well as a well-adapted tool setup, are required to achieve reproducible component quality; targeted adjustments of process and rubber parameters improved channel dimensional accuracy, but revealed limited forming capability in certain areas. Furthermore, concavely and convexly modified rubber dies reduced component warping in specific directions, and steel dies exhibited higher precision and less distortion compared to ceramic-filled UV resin dies. These findings highlight the potential of the rubber drawing process for cost-effective production of bipolar plates, while identifying key parameters for further optimization toward industrial-scale manufacturing.

In cold bulk metal forming, coatings based on zinc phosphate are commonly used for lubrication. This has a negative impact on the environment, negatively affects human health, and requires significant pre-and post-surface treatments. Powder metallurgical (PM) components are a promising alternative to zinc phosphate coatings due to the process related porosity of the workpiece which acts as lubricant reservoir. During the forming process, the lubricant stored in the pores is released and lubricates the tool and workpiece surfaces. For an efficient process design of such components, finite element method (FEM) is an effective tool to analyse forming and friction behaviour. To this end, a realistic material model is essential for accurate simulation results. Hence, in this work, the flow behaviour of PM semi-finished products is characterised by means of compression and tensile tests. The results indicate that the material exhibits different behaviour under compression and tension. In compression, the material demonstrates higher yield strength and flow stresses compared to tension. Additionally, inhomogeneity of the material distribution can be observed, characterised by a denser core and more porous outer regions. The porous outer regions make it suitable for storing lubricant for application in forming processes.

Industrial process monitoring has a significant impact on ensuring quality and efficiency, particularly in complex manufacturing processes such as cross-wedge rolling. This study evaluates the performance of four deep learning approaches—Artificial Neural Networks (ANN), Convolutional Neural Networks (1D-CNN), Autoencoders, and Recurrent Neural Networks (RNN) using Long Short-Term Memory (LSTM)—for detecting deviations during the cross-wedge rolling of hybrid workpieces. Given the challenge of a small dataset, data augmentation techniques are applied to enhance model training. The investigation aims to determine whether deep learning can effectively classify process deviations and which architecture performs best. The results indicate that 1D-CNN models achieve the highest classification accuracy within the test group. Additionally, various hyperparameters are analyzed to identify those with the most significant impact on model performance. The findings contribute to the optimization of deep learning-based process monitoring, highlighting the potential of 1D-CNN and Autoencoder models for industrial applications.

During the hot bulk forming of long parts, inhomogeneous distributions of deformations and temperatures occur. The gradients of these distributions lead to complex, overlaying residual stresses, which can cause critical geometric deviations and mechanical failures. Common finite element (FE)-simulations for designing a process are in principle capable to predict the thermal, mechanical and metallurgical effects, but require extended material models. Thereby, the total strain increment can be described through the partial strain components of the elastic, plastic, thermal transformation related and transformation plasticity strain. To allow the numerical prediction of the distortion of long hot formed parts, an experimental characterisation of the TRIP and backflow effects is presented for the steel 31CrMoV9. Time temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams are determined with JMatPro and verified by means ofmicrostructure analysis and hardness measurements. Based on these diagrams, the transformation plasticity is investigated through dilatometric tests whereby tensile and compressive loads are applied during the phase transformation. The martensite phase transformation showed the highest amounts of TRIP strains, whilst the bainite transformation exhibited lower strains but a high tensile backflow strain. For perlite the beginning of the phase transformation was delayed and its duration extended due to the induced loads.

For the reliable numerical simulation and design of compound forging processes involving dissimilar materials, an accurate representation of thermal boundary conditions is essential. In particular, the heat transfer coefficient (HTC) at the interface of the workpiece and the die strongly influences temperature distribution, material flow, and interfacial integrity. Despite its significance, the HTC is frequently modelled as constant in finite element (FE) simulation due to the lack of experimental data for forging-relevant conditions. Therefore, this study presents an experimental–numerical methodology for determining load-dependent HTCs representative for compound forging. A specialised test setup was used to reproduce the thermal–mechanical boundary conditions of hot bulk forming, by inducing contact pressures both below and above the flow stress of the workpiece material. Temperature histories were recorded using embedded thermocouples and analysed through an inverse numerical approach based on a one-dimensional (1D) finite element (FE) model. The influence of contact pressure, heating atmosphere, and lubrication on the HTC was systematically investigated for a S235JR specimen temperature of 600 °C. The results demonstrate a major pressure dependency of the HTC, whilst increasing for higher contact pressures. Oxide formation and lubrication were shown to significantly affect heat transfer behaviour, particularly while heating under atmospheric conditions. The presented approach provides process-specific HTC data that can substantially improve the predictive capability of numerical simulations for compound forging applications.

In hot forming processes of steel, oxide scale layers are formed in dependence of the occurring process conditions. As remaining oxide scale can weaken the formed workpiece, it has to be removed at the end of the process. Through this, the oxide scale formation can lead to a mass loss of up to 3

In industrial bulk metal forming processes the die wear has a major influence on the process stability and the overall economical profitability. In Tailored Forming processes, the semi-finished products consist of different materials, such as aluminium and steel. These hybrid semi-finished products are formed together. The predominant wear mechanism in steel forming processes is abrasive wear, whereas in aluminium forming processes mainly adhesive wear occurs. Due to the alternating material contact, the dies are exposed to alternating wear mechanisms. Although both wear mechanisms can separately be predicted numerically using models such as the approach of Archard, interactions between the mechanisms are not taken into account. The present work is therefore dedicated to investigating the wear effects during a hybrid bulk forming process. Therefore, first upsetting experiments with mono-materials were carried out. The experiments were then repeated whilst alternating the workpiece materials each stroke and then compared to the first trial. Finite element (FE) simulations were used to recreate the upsetting processes numerically. The wear depth was determined numerically through an adapted simulation model to take the alternating wear mechanisms into account. The numerical results were compared to the experiments to analyse the interactions of the wear mechanisms and derive suitable wear coefficients. The findings allow the development of an improved numerical approach for the prediction of the alternating die wear effects in Tailored Forming processes in the future.

Hot stamping of manganese–boron steels is widely used in automotive manufacturing to produce ultra-high-strength components with tensile strengths exceeding 1500 MPa . Conventional industrial heating relies on gas-fired roller hearth furnaces, which require 5 – 10 min to reach austenitization and exhibit low energy efficiency . Resistance heating offers a compact and energy-efficient alternative, enabling heating rates above 100 K/s and full austenitization within seconds. However, rapid heating of uncoated steels leads to severe oxidation, and established coating systems such as AlSi are not designed for diffusion-controlled bonding within such short times . This study demonstrates that resistance heating in an XHV-adequate atmosphere – consisting of nitrogen and monosilane – suppresses oxidation while simultaneously enabling adhesion of a pre-laminated aluminum foil to the steel substrate. For coating preparation, 22MnB5 sheets were roughened by corundum blasting, cleaned, and laminated with an aluminum foil using a flat-die pressing tool. The pre-coated blanks were heated in a self-developed resistance-heating chamber, in which the oxygen concentration was reduced to an XHV-adequate level. Several heating profiles were investigated to determine suitable process windows for coating formation. The results show that resistance heating achieves austenitization within a few seconds, reducing heating times by more than an order of magnitude compared to furnace heating. The XHV-adequate atmosphere reliably prevents scale formation, enabling completely oxidation-free surfaces during rapid heating. Under these conditions, the laminated aluminum foil bonds uniformly to the substrate, forming a continuous coating layer. Metallographic cross-sections and SEM analyses confirm the formation of Al–Fe intermetallic phases at the interface, demonstrating robust metallurgical bonding suitable for subsequent hot stamping operations. Overall, the combination of resistance rapid heating and an XHV-adequate atmosphere provides a highly energy-efficient process route for hot stamping while offering an opportunity to integrate aluminum-based protective coatings directly into the heating step. This approach addresses the limitations of current furnace-based heating and coating technologies and opens a promising pathway toward more flexible, sustainable, and functionally integrated hot-stamping process chains.

Hybrid joints made of steel and aluminium alloy produced by rotary friction welding enable load-adapted lightweight components. However, a major challenge is the inhomogeneous radial temperature distribution caused by different relative velocities between the specimen centre and edge during rotation. This effect leads to local insufficient bonding and reduces the overall joint strength, especially in the centre, where low relative rotation speeds occur. Previous studies mainly addressed preheating before the friction phase, whereas superimposed heating during the upsetting phase has not been investigated so far. To achieve temperature equalisation along the cross-section during rotary friction welding, a modified KUKA Genius plus machine equipped with joule heating was used to introduce an electric current during the upsetting phase. Experiments were conducted on EN AW-6082 (AA-6082) joined to 20MnCr5 (AISI 5120H). A three-step variation of current intensity (10, 24 and 36 A/mm 2 ), alongside a reference without current, was investigated. Temperatures were monitored using type K thermocouples, confirming temperature equalisation. Mechanical performance was assessed by uniaxial tensile tests, while hardness measurements and metallographic analyses characterised the influence of superimposed heating on the interfacial microstructure. Joint strength improves up to 17% with increasing current, even under otherwise unsuitable welding parameters that would normally result in insufficient bond strength. This improvement is linked to a uniform temperature distribution and enhanced material flow, resulting in a defect-free specimen centre.

In hot forming processes and during heat treatment, scale layer formation occurs on the surface by temperatures over 570 °C. The structure of the scale layer depends on many different parameters, such as temperature, time, atmosphere and the alloying elements of the material. The aim of this work is to characterise the scale layer forming on steels with different carbon and chromium contents. Focus is on phase analysis using EDX and correlation of the iron oxide phases with hardness using nanoindentation. In addition to the well-known iron oxide phases and the three-layer-structure a fourth layer, called subscale, was identified, which primarily consists of chromium and silicon oxides. As this layer is the interface with the base material, it could influence the adhesion of the scale layer. The hardness measurements show high values in the area of the subscale, which indicates brittle fracture behaviour. The thickness of the subscale and the primary elements it contains depend on the alloying elements of the base material. For an unalloyed steel, the subscale thickness was reduced, compared to the low-alloy steels.

Adaptive thermal management is a prerequisite for multi-stage tailored forming of hybrid steel-aluminium blocks, as each section of material must remain within its forming temperature window and the joining zone must be protected from excessive thermal stress. This study defines a control-oriented process space for a combined induction heating and dual-fluid spray cooling concept developed in the Collaborative Research Center (SFB) 1153 “Tailored Forming.” A three-phase test program is applied: Phase A quantifies and evaluates the influence of air pressure p air , water pressure p w , and nozzle distance d on the cooling performance and the formation of an axial gradient using standardized regression coefficients. In phases B and C, a reference setting is applied to rotationally friction-welded 20MnCr5/EN AW-6082 blocks in a cold-start and preheated state, which are representative of multi-stage forming processes. The results show that p air and p w dominate both the cooling capacity and the formation of gradients, while d plays a subordinate role in the range investigated. The relationships remain qualitatively consistent for hybrid blanks and preheated conditions when the heating program is adapted to the aluminium and joining zone boundaries. The derived actuator ranking forms the basis for closed-loop temperature control in volatile, multi-stage tailored forming chains.

Determining suitable lubrication settings is crucial for designing an effective hot forging process. Currently, this determination is predominantly carried out through trial and error at the beginning of production, with subsequent adjustments based on the operator's expertise. To reduce manual labor, costs, and especially resource consumption, it is advantageous to enable the selection of appropriate lubrication settings in advance. This supports the industry in achieving its carbon–neutral objectives while enhancing process stability. This paper presents simplified experimental methods to evaluate the tribological properties of hot forging processes; the ring compression test was chosen to determine the influence of the lubrication layer thickness on the tribological system. This study was structured in two stages: first, a design space exploration of spraying parameters; second, an evaluation of the friction factor for various lubrication layer thicknesses.

Titanium alloys are frequently manufactured using isothermal forging approach, which is complex in both process and equipment technology. Inert gas is required during forging to prevent oxidation of the titanium. By using a steel casing of AISI 316L around the titanium billet (Ti-6Al-4 V), a quasi-isothermal process is possible without inert gas, whilst sustaining the protection against oxidation and simultaneously reducing cooling. Due to the process-dependant deformation of the steel casing, the simple die design through inversion of the desired forging part geometry no longer applies. To design a die for different casing thicknesses, with a defined titanium forging part, the finite element method was applied. The variation of the initial steel wall thickness and the ram velocities are expected to influence the cooling behaviour during the process and thus formability. This was analysed using the finite-element method. Forging experiments were executed, investigating the aforementioned parameter variations and to evaluate the agreement between the simulation and the experiments. The resulting material distribution of the titanium parts was measured optically. The material distribution of the titanium corresponded well with the simulation results. Independent of the forming speed, thinner wall thicknesses result in cracks in the casing, which can be prevented by using a thicker casing.

Rotary friction welding (RFW) is a solid-state process used to join similar and dissimilar materials, such as steel and aluminium. In RFW, interface temperature development and its distribution are essential factors influencing material bonding. It governs bond strength, the formation of intermetallic phases (IMPs) and the evolution of the heat-affected zone (HAZ). Thus, precise prediction of temperature distribution is vital for the reliable design and optimization of the RFW process, as well as for the prediction and control of IMP formation. This work presents an experimental investigation of the thermo-mechanical behaviour of EN AW-6082 and 20MnCr5 during RFW in addition to a corresponding novel numerical modelling framework. A systematic parameter study was conducted to evaluate the influence of the friction pressure, friction time, forging pressure, forging time and rotational speed on the peak temperature, sledge path and flash formation. In-situ temperature measurements were performed using thermocouples (TC) embedded in the steel component, while axial force, displacement and rotational speed were recorded. The results demonstrate that, within the investigated parameter ranges, the rotational speed is the dominant factor governing frictional heat generation and the peak temperature, while the friction pressure primarily influences the sledge path. In parallel, a 2D axisymmetric finite-element model with a user-defined subroutine was developed to compute the heat flux based on process parameters and contact conditions, providing a transparent and extensible numerical framework for RFW. The experimental findings establish a robust basis for the calibration and future validation of the numerical model.