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According to the current state of the art, wear conditions of forging dies are assessed visually in the dismantled state, as there is no measuring procedure available for inline wear measurement of hot forging dies. This paper introduces a handling concept for automated loading and in-situ tool inspection for a hot forging process. Based on an industrial robot and quickchange systems, the process integration of hightemperature grippers and an endoscopic 3D-measurement sensor for the in-situ inspection of hot forging dies is carried out. By adapting the measuring method of fringe projection to an endoscopic design, the measuring system can be navigated into the difficult-to-access geometry of the die and take high-precision 3D-measurements inside. The ambient air heated by the forming process creates an inhomogeneous refractive index field around the measuring system and the hot die, so that the straight beam path required for optical measuring systems cannot be ensured. This can lead to strong deviations in the reconstructed point clouds and the functional geometries derived from them. By means of a compressed air actuator, the measuring system can be protected from the hot tool and dirt as well as the effect of inhomogeneous refractive index can be reduced. With this approach the in-situ wear measurement at highly stressed regions using the example of the mandrel radius and the flash radius will be demonstrated. These functional elements are of particular relevance, as the thermalstress is high and large material flow takes place. For the wear measurement, the functional elements of the tool are examined in detail by fitting geometrical features into the reconstructed point clouds and determining the deviations from a reference geometry. In addition, the measurement data is validated with the aid of a commercially available state-of-the-art measurement system.

Im Rahmen dieser Studie wurde ein drucküberlagerter Hohl-Vorwärts-Fließpressprozess für reibgeschweißte hybride Halbzeuge aus Stahl und Aluminium entwickelt und experimentell durchgeführt. Der Gegendruck wurde variiert, um den Einfluss auf die Festigkeit und das Schädigungsverhalten des Verbundes zu untersuchen. Es konnte eine Verbesserung der erzielten Verbundfestigkeiten und eine Eliminierung oberflächennaher Delaminationen durch die Drucküberlagerung erzielt werden. This study has undertaken to develop and experimentally carry out a pressure superimposed hollow forward extrusion process for friction-welded hybrid semi-finished products of steel and aluminium was. The counter pressure was varied to investigate the influence on strength and damage behaviour of the bond. This helped to improve the bond strengths and eliminate near-surface delamination due to pressure superposition.

The nitriding of forging tools is an industrially established standard used to increase the hardness of the tool surface layer and reduce wear. However, this modification of the tool surface layer, as well as the microstructural changes that occur during this operation due to the thermo-mechanical load, cannot be considered during wear calculations with the widely used Archard wear model in the context of FE simulations. Based on previous work, this study further develops two tempering tests for the investigation of the hardness evolution of two nitride profiles based on H11 tool steel. Here, significant tempering effects could be observed depending on temperature, mechanical stress superposition and time. The results are used for setting up a new material model that is implemented in an existing numerical wear model. The validation is carried out in two laboratory forging test series. The evaluation shows that the hardness development in terms of tempering effects of a nitrided forging tool can be numerically predicted, especially for high forging cycles. However, due to the unexpected occurrence of adhesion effects, only limited applicability of the wear prediction then carried out is achieved.

Advanced High Strength Steels (AHSS) are widely used in today's automotive structures for lightweight design purposes. FE simulation is commonly used for the design of forming processes in automotive industry. Therefore, besides the description of the plastic flow behaviour, also the definition of forming limits in order to efficiently exploit the forming potential of a material is required. AHSS are prone for crack appearances without prior indication by thinning, like exemplary shear fracture on tight radii and edge-fracture, which can not be predicted by conventional Forming Limit Curve (FLC). Stress based damage models are able to do this. However, the parameterisation of such models has not yet been standardised. In this study a butterfly specimen geometry, which was developed at the Institute for Forming Technology and Machines (IFUM), was used for a stress state dependent fracture characterisation. The fracture behaviour of two AHSS, CP800 and DP1000, at varied stress states between pure shear and uniaxial loading was characterised by an experimental-numerical approach. For variation of the stress state, the specimen orientation relative to the force direction of the uniaxial testing machine was orientated at different angles. In this way, the relevant displacement until fracture initiation was determined experimentally. Subsequently, the experimental tests have been numerically reproduced giving information about the strain and stress evolution in the crack impact area of the specimen for the experimentally identified fracture initiation. With the help of this testing procedure, two different stress-based damage models, Modified Mohr-Coulomb (MMC) and CrachFEM, were parameterised and compared.

Diffusion treatments offer possibilities to enhance the performance and the service lifetime of hot forging tools. In combination with coating after nitriding, the surface layer hardness can be further increased. Within the scope of this study, a surface layer hardness above 2,000 HV0.005 was determined for borided or DLC (diamond-like carbon) coated surface layers. An increased surface layer hardness improves the abrasive wear resistance of forging dies. Furthermore, the plastic deformation of thermally softened forging die areas can be reduced. Beside these desirable effects, the ductility of diffusion treated or coated near surface layers is reduced and thermomechanical cracks are promoted. Therefore, additional approaches were developed to improve the thermomechanical crack behaviour of forging dies. Patterned plasmanitriding by the use of coverages to prevent areas from nitrogen diffusion, new combination processes of plasmanitrocarburizing (PNC) followed by plasmanitriding (PN) and the innovative boriding were investigated on different abstraction levels. A system of several testing rigs was set up to enable the abstraction of the thermal shock conditions in different stages. The patterned nitriding, boriding and combination plasma process (PN + PNC) were evaluated in a series of industrial field tests to derive recommendations for suitable tool treatments.

The aim of this work is to generate an advantageous compressive residual stress distribution in the surface area of hot-formed components by intelligent process control with tailored cooling. Adapted cooling is achieved by partial or temporal instationary exposure of the specimens to a water–air spray. In this way, macroscopic effects such as local plastification caused by inhomogeneous strains due to thermal and transformation-induced loads can be controlled in order to finally customise the surface-near residual stress distribution. Applications for hot-formed components often require special microstructural properties, which guarantee a certain hardness or ductility. For this reason, the scientific challenge of this work is to generate different residual stress distributions on components surfaces, while the geometric as well as microstructural properties of AISI 52100 alloy stay the same. The changes in the residual stresses should therefore not result from the mentioned changed component properties, but solely from the targeted process control. Within the scope of preliminary experimental studies, tensile residual stresses in a martensitic microstructure were determined on reference components, which had undergone a simple cooling in water (from the forming heat), or low compressive stresses in pearlitic microstructures were determined after simple cooling in atmospheric air. Numerical studies are used to design two tailored cooling strategies capable of generating compressive stresses in the same components. The developed processes with tailored cooling are experimentally realised, and their properties are compared to those of components manufactured involving simple cooling. Based on the numerical and experimental analyses, this work demonstrates that it is possible to influence and even invert the sign of the residual stresses within a component by controlling the macroscopic effects mentioned above.

Due to their higher weight-specific and high-temperature strength, iron-aluminium alloys have a high potential to replace steel in various applications. The good availability of the two materials, the excellent recyclability, lower density with increasing aluminium content and the high corrosion resistance in sulphide- and sulphur-rich environments are further advantages. However, with increasing aluminium content, ductility of FeAl alloys decreases due to hydrogen embrittlement at room temperature. As a result, iron-aluminium alloys have been excluded from potential applications, particularly structural ones. Investigations on powder metallurgical produced iron-aluminium alloys show that fine-grained microstructures can lead to significant improvement in ductility. Assuming equal grain diameters, higher toughness is expected in case of metallurgical ingot production followed by hot forming. The present work deals with the mechanical properties of fine-grained microstructure in iron-rich iron-aluminium alloys, pre-processed through Equal Channel Angular Pressing. In order to characterize the mechanical properties, compression tests with the alloys Fe9AI, Fe28AI and Fe38AI are carried out at different temperatures. The flow curves determined are then compared with those from as-cast state. In addition, deformation capacity is examined optically on slopes of external cracks. In conclusion, the results are discussed based on the microstructure.

Hot forging is a complex process involving the mutual influence of numerous thermo-mechanical-metallurgical material phenomena. In particular, the strains of transformation-induced plasticity (TRIP) have a significant influence on the distortions and residual stresses of the components. The TRIP strains refer to the anisotropic strains depending on the orientation and significance of the stress conditions during cooling superimposed to the phase transformation. With the use of numerical models, the impact of this effect can be investigated in order to ensure the production of high quality components. However, an experimental determination of the characteristic values of TRIP is challenging, which is why only few corresponding data are available in the literature. Therefore, this paper presents an experimental and numerical methodology as well as the results of studies on the interaction between stresses and phase transformations in the materials AISI 4140 and AISI 52100. The investigations of the TRIP strains are carried out using hollow specimens, which are thermo-mechanically treated in the physical forming simulator Gleeble 3800-GTC. The specimens are austenitised, quenched to test temperature and held there while diffusion controlled phase transformation takes place. The extent of TRIP as a result of different superimposed tensile or compressive loads is determined by means of dilatometry. In addition, the extent of TRIP for diffusionless martensitic phase transformations was investigated by continuous cooling tests under tensile and compressive loads. It was found that the transformation plasticity varies depending on the material, the phase type, the temperature and the tensile or compressive stresses. Subsequently, simulations of the physical experiments using the FE software Simufact.Forming verified the determined phase specific values of TRIP.

As one of the oldest shaping manufacturing processes, forging and especially hot forging is characterized by extreme loads on the tool. The thermal load in particular is able to cause constant changes in the hardness of the surface layer, which in turn has a decisive influence on the numerical estimation of wear. Thus, also during numerical wear, modeling hardness changes need to be taken into account. Within the scope of this paper, a new implementation of a numerical wear model is presented, which, in addition to dynamic hardness models for the base material, can also take into account the properties of a nitride wear protection layer as a function of the wear depth. After a functional representation, the new model is applied to the wear calculation of a multi-stage industrial hot forging process. The applicability of the new implementation is validated by the evaluation of the occurring hardness, wear depths and the locally associated removal of the wear protection layer. Consecutively, a tool life calculation module based on the calculated wear depth is implemented and demonstrated. In general, a good agreement of the results is achieved, making the model suitable for detailed 2D as well as large 3D Finite Element calculations.

In hot forging processes, geometry of the formed workpieces deviate from the desired target geometry, due to complex interactions between tools and billets which result in inhomogeneous temperature and stress fields. The resulting deviation can only be mapped insufficiently by using numerical simulation which makes it difficult to be considered when designing the tool. Therefore, the development of forging tools requires an iterative adaptation process through a large number of revisions in the tool geometry, which escalates the resulting costs. To compensate the deviations and reduce the number of tool revisions, a holistic view of the influencing factors on the geometrical deviation is necessary. In order to address this issue, a hot forging process was developed, whose geometry is prone to high deviations, and a stress-based compensation model was applied. For this, forging experiments were carried out and a comparison was made between the actual geometry and the desired one by means of 3D coordinate measurements. The compensation methodology, which directly takes into account the complex 3D stress states during forming, allows to determine a compensating tool geometry. This opened up the possibility of validating the simulation results and testing a compensation model while eliminating deterministic deviations in hot forging processes.

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultra-high-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints.

In metal forming, the arising residual stresses influence the material behaviour during manufacturing as well as the performance of the final component. In the past, the focus of forming process design was on minimising or eliminating residual stresses. However, residual stresses can also serve to improve the properties of the components through targeted use, for example with regard to distortions or wear behaviour. For this purpose, knowledge of the interactions between the process parameters of the hot forming process and the resulting residual stresses in the final component is required. In this work, the influences of the process parameters are analysed by means of a reference process of hot forming. In this process, cylindrical specimens with eccentric holes are hot-formed, which leads to an inhomogeneous stress distribution in the material as it occurs in an industrial hot forming process. In the reference process, forming temperature, cooling strategy, forming speed, degree of deformation and steel alloys are varied. It is observed that both, process parameters and material properties, have a significant influence on the resulting residual stresses. Mainly responsible for these phenomena are microstructural effects in the material. As a result of forming at temperatures between 1000 °C and 1200 °C, static and dynamic recrystallisation processes occur, which affect the austenite grain size. The austenite grain size as well as the cooling strategy have a significant influence on the microstructure transformation behaviour, which has a decisive effect on the resulting residual stresses. In addition, the cooling strategy determines whether a diffusion-free phase transformation or a diffusion-controlled phase transformation occurs. At high cooling rates, diffusion-free transformation of the austenitic into the martensitic phase takes place, which leads to severe stresses in the crystal lattice. During diffusion-controlled phase transformation, which occurs during air cooling, comparatively lower residual stresses in the range of zero can be observed.

Due to high thermo-mechanical loads, tools used in hot forming operations need a high resistance to different damage phenomena, such as deformation, cracking and abrasion. They are exposed to cyclic thermo-mechanical stress conditions, which leads to tool failure and subsequent tool replacement during cost-intensive production interruptions. To increase wear resistance, forging tools can be produced in the metastable austenite area. Forming of steel below the recrystallisation temperature, also known as “ausforming”, offers the possibility to increase strength without affecting ductile properties. This is due to grain refinement during forming. In this study, the thermo-mechanical treatment ausforming will be used to form the final contour of forging dies. For this purpose, an analogy study was performed where a cup-preform is ausformed, which represents the inner contour of a highly mechanically loaded forging die. It is investigated to what extent a fine-grained microstructure generated in the last forming stage can be achieved and how it influences the tool’s performance. The hot-working tool steel X37CrMoV5-1 (AISI H11) was used as workpiece material. To achieve optimal properties, process routes with tempering temperatures from 300 °C to 500 °C and global true plastic strains of φ = 0.25 and φ = 0.45 were examined. The results were evaluated by pulsation tests, metallographic analysis and hardness measurements of the formed parts. Optimal ausforming parameters were derived to produce a high performance forging die.

Tools are of strategic importance for industrial manufacturing processes. Their behaviour has a great influence on the productivity of the process and the quality of the product. A material saving and efficient technique for processing metallic workpieces is cold forging. One major challenge of this production method is the handling of high contact normal stresses in the tool contact, which can lead to severe tool wear. To investigate promising approaches for understanding wear modelling and wear reduction a demonstrator process based on the first stage of a total five-staged cold forging process for the manufacturing of a bolt anchor is considered in the scope of this research. This work aims at the further development of a numerical wear calculation based on an adapted Archard model in order to be able to realistically predict the tool wear in cold forging processes. Therefore, the material characterization of the used workpiece material as well as an investigation of the worn tool dies takes place to validate a numerical wear calculation model. Furthermore, this research addresses a reduction in wear by identifying critical areas and changing the inlet geometry of the investigated demonstrator tool die. This way, conclusions can be drawn about the wear sensitivity during numerical process design, and particularly critical areas can be geometrically modified in terms of the design. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

The present work deals with adjusting a fine-grained microstructure in iron-rich iron-aluminium alloys using the ECAP-process (Equal Channel Angular Pressing). Due to the limited formability of Fe-Al alloys with increased aluminium content, high forming temperatures and low forming speeds are required. Therefore, tool temperatures above 1100 &deg, C are permanently needed to prevent cooling of the work pieces, which makes the design of the ECAP-process challenging. For the investigation, the Fe-Al work pieces were heated to the respective hot forming temperature in a chamber furnace and then formed in the ECAP tool at a constant punch speed of 5 mm/s. Besides the chemical composition (Fe9Al, Fe28Al and Fe38Al (at.%&mdash, Al)), the influences of a subsequent heat treatment and the holding time on the microstructure development were investigated. For this purpose, the average grain size of the microstructure was measured using the AGI (Average Grain Intercept) method and correlated with the aforementioned parameters. The results show that no significant grain refinement could be achieved with the parameters used, which is largely due to the high forming temperature significantly promoting grain growth. The holding times in the examined area do not have any influence on the grain refinement.

An indicator for the quality of products that are formed in an embossing process is the contour of the product. Highest contour quality is achieved, if the shape of the forming tool is perfectly imprinted into the material. However, elastic forces and tribological effects, which occur during the forming process, impair the shape-setting progress. In order to improve the embossing process, an electromagnetic actuation unit is presented in this paper, which applies horizontal vibrations into the press ram during the forming process. The actuation unit consists of two strong linear solenoids, which generate oscillating force with variable levels and frequencies. The vibrations improve the shape setting during the embossing process. The concept, design, and the function of the actuators are presented in this paper. Experimental studies on an embossing process in a production environment were performed, to demonstrate the features of the novel system.

Residual stresses resulting from hot-forming processes represent an important aspect of a component’s performance and service life. Considering the whole process chain of hot forming, the integrated heat treatment provided by a defined temperature profile during cooling offers a great potential for the targeted adjustment of the desired residual stress state. Finite element (FE) simulation is a powerful tool for virtual process design aimed at generating a beneficial residual stress profile. The validation of these FE models is typically carried out on the basis of individual surface points, as these are accessible through methods like X-ray diffraction, hole-drilling, or the nanoindentation method. However, especially in bulk forming components, it is important to evaluate the quality of the model based on residual stress data from the volume. For these reasons, in this paper, an FE model which was already validated by near surface X-ray diffraction analyses was used to explain the development of residual stresses in a reference hot forming process for different cooling scenarios. Subsequently, the reference process scenarios were experimentally performed, and the resulting residual stress distributions in the cross-section of the bulk specimens were determined by means of the contour method. These data were used to further validate the numerical simulation of the hot forming process, wherein a good agreement between the contour method and process simulation was observed.

ZusammenfassungZiel dieser Arbeit ist die Einstellung eines vorteilhaften Druckeigenspannungsprofils in warmumgeformten Bauteilen durch intelligente Prozessführung mit angepasster Abkühlung aus der Schmiedewärme. Die Machbarkeit und das Potenzial werden an einem Warmumformprozess, bei dem zylindrische Proben mit exzentrischer Bohrung bei 1000 °C umgeformt und anschließend aus der Schmiedewärme im Wasser abgekühlt werden, aufgezeigt. Vorige Arbeiten zeigen, dass sich Zugeigenspannungen in den derartig umgeformten Proben aus dem Material 1.3505 einstellen. Mittels der vorgestellten mehrskaligen FE-Modelle, wird in dieser Arbeit eine alternative Prozessvariante analysiert, mit der vorteilhafte Druckeigenspannungen anstelle von Zugeigenspannungen durch eine angepasste Abkühlung aus der Umformwärme in den Proben erzeugt werden können. Die angepasste Kühlung wird durch eine partielle Beaufschlagung der Proben mit einem Wasser-Luft-Spray erreicht. Auf diese Weise kann die lokale Plastifizierung durch inhomogene Verzerrungen aufgrund thermischer und umwandlungsinduzierter Effekte beeinflusst werden, um letztlich das Eigenspannungsprofil individuell zu gestalten. Die wissenschaftliche Herausforderung dieser Arbeit besteht darin, unterschiedliche Eigenspannungen in der Oberfläche der Proben zu erzeugen, während die geometrischen und mikrostrukturellen Eigenschaften gleichbleiben. Es wird nachgewiesen, dass eine Beeinflussung der Eigenspannungen und sogar die Umkehr des Spannungsvorzeichens allein durch eine geschickte Prozessführung beim Abkühlen möglich ist.

From a metal forming perspective, warm forming technologies can be used to reclaim the shape and functionality of worn engineering steel parts such as case-hardened geared components. Since forming temperatures are beneath the material-transformation temperature, no loss in hardness is expected and a new heat treatment can be omitted. In order to determine a process window for reclaiming geared components, upsetting tests were carried out on case-hardened workpieces made of AISI 5115.

Thermomechanically treated (TMT) materials are characterized by a fine-grained microstructure, which leads to extraordinary mechanical properties. In this study, the alloyed tempering steel 42CrMo4 (AISI 4137) is used to set up a two-step TMT upsetting process with intermediate cooling. A water-air based cooling system was used to adjust different phase configurations between the two steps by varying the target temperature and cooling rate. Standardized test specimens for Charpy impact tests and tensile tests as well as for metallographic analyses are cut out of the formed parts. Tensile tests showed that the yield strength can be enhanced up to 1188 MPa while the elongation at break is about 12% without any additional heat treatment by forming the material after rapid cooling. This fulfills the demands of the standard in quenched and tempered state. This process route allows for local-load tailored part design and manufacturing by adjusting the forming conditions conveniently.

The increasing use of high-strength materials in sheet metal processing results in challenges for cutting presses. The required cutting force increases with the strength of the material. After material separation, a sudden release of the energy stored in the machine leads to intense system vibrations and noise emission. Noise protection and reduction solutions such as machine cabins or active vibration damping systems are expensive, require much space or frequent maintenance. Due to this, forming machines must be acoustically optimized. In this article, selected structure measures for the machine-side noise reduction are derived and evaluated in the context of simulation studies using an acoustic machine model.

Hot forming processes, especially open die forging, are often used for production of high-performance, large-scale objects. The main benefits compared to, e.g. shape cutting methods, include lower material use and higher stress resistance. Inline process control by 3d geometry measurement is an important part of a cost-effective component production. However, there are no automated control systems commercially available for open die forging, which results in a limited precision of the final component geometry. The main challenges for a control system in said conditions are imposed by the temperature influence of the hot object on the measurement systems as well as limited actuator accuracy for the precise handling of hot, heavy objects. Additionally, the tools used in open die forging are kept simple for financial reasons. Comparable tools for, e.g., drop forging, need to be exclusively made for each new object form and therefore cannot be used for a cost-efficient production of low-quantity components. In this paper, we present a production concept in order to control a hot forming method for large scale, low quantity components. The approach combines an adaptable high-resolution 3d geometry measurement system and an incremental open die forging press for cost- and time-efficient production. Forming simulations will need to be conducted prior to the process to gain access to a large database of possible forming steps to reach the desired final geometry. The control system itself compares the measured geometry and temperature to the simulated ones. Occurring deviations are analysed and a sequence of forming steps is calculated from the database. The necessary forging forces and strokes of the actuating system are extracted from the chosen forming sequence and linked back into the system to achieve maximum precision.

Lubrication is essential in metal forming processes. As an example, it has a significant effect on the extent of homogeneity of forming and the resulting component surface. Thus, an adequate lubrication technique is required for a consistent part quality. In this contribution a new lubrication approach for cold bulk metal forming is presented. By exploiting the process-related porosity of powder metallurgical (PM) components as a lubricant storage, the resulting pressure of a forming process forces the stored oil to leak out lubricating the process. As a result, advantages like a decrease of process time because of the omission of the additional lubrication step, and an increased deformability due to compensation of lubricating film break are expected. Besides that, the porosity of the PM component is reduced leading to improved mechanical properties. Experimental investigations with cylindrical PM components were carried out by compression tests in dependence of initial porosity, impregnation time and lubricant. It was found that the required maximum load is reduced by up to 39 % by applying the new lubrication method. In addition, relative densities up to 99 % are reached after deformation.

Shearing of bulk material is a cost efficient method of producing billets for subsequent forging operations. The procedure of rod shearing is characterised by high billet output quantities without metal removal. This technique is widely used in industrial processing of steel materials. Aluminium billets however, are mostly produced by sawing, causing higher manufacturing costs due to comparably low output. The aim of this paper is an enhancement of sheared aluminium billet quality in order to fulfil industrial standards required for further forging operations. To enhance the quality of sheared aluminium billets, the shearing process parameter clearance (distance between the blades) was varied to create billets of four different aluminium alloys, as EN AW 5754 (UNS A95754), EN AW 5083 (UNS A95083), EN AW 6082 (UNS A96082) and EN AW 7075 (UNS A97075) in different conditions. The quality of sheared billets is assessed by analysing inclination angle, waviness and surface defects of the sheared parts with 3D-measurement. In the investigation, it can be shown that the tensile strength of the investigated alloys influences the billet surfaces. High tensile strength results in positive billet quality. Further, small clearances show best results concerning inclination angles, while higher clearances result in decreased waviness.

Overmoulding of thermoplastic composites combines the steps of thermoforming and injection moulding in an integrated manufacturing process. The combination of continuous fibre-reinforced thermoplastics with overmoulded polymer enables the manufacturing of highly functionally integrated structures with excellent mechanical properties. When performed as a one-shot process, an economically efficient manufacturing of geometrical complex lightweight parts within short cycle times is possible. However, a major challenge in the part and process design of overmoulded thermoplastic composites (OTC) is the assurance of sufficient bond strength between the composite and the overmoulded polymers. Within the framework of a simulation-based approach, this study aims to develop a methodology for predicting the bond strength in OTC using simulation data and a numerical model formulation of the bonding mechanisms. Therefore, a modelling approach for the determination of the bond strength depending on different process parameters is presented. In order to validate the bond strength model, specimens are manufactured with different process settings and mechanical tests are carried out. Overall, the results of the numerical computation are in good agreement with the experimentally determined bond strength. The proposed modelling approach enables the prediction of the local bond strength in OTC, considering the interface conditions and the processing history.

Thermomechanical treatment (TMT) describes the effect of thermal and mechanical conditions on the microstructure of materials during processing and offers possible integration in the forging process. TMT materials exhibit a fine-grained microstructure, leading to excellent mechanical properties. In this study, a two-step TMT upsetting process with intermediate cooling is used to demonstrate possibilities for a process-integrated treatment and corresponding properties. A water&ndash, air-based cooling system was designed to adjust different phase configurations by varying the target temperature and cooling rate. Four different thermal processing routes and four combinations of applied plastic strains are investigated in standardized mechanical tests and metallographic analyses. The applied TMT results in a finely structured bainitic microstructure of the investigated tempering steel AISI 4140 (42CrMo4) with different characteristics depending on the forming conditions. It can be shown that the demands of the standard (DIN EN ISO 683) in a quenched and tempered state can be fulfilled by means of appropriate forming conditions. The yield strength can be enhanced up to 1174 MPa while elongation at break is about 12.6% and absorbed impact energy reaches 58.5 J without additional heat treatment when the material is formed after rapid cooling.

A possible alternative to the established press hardening steel Usibor 1500 (22MnB5) are hot formed martensitic chromium steels. The combination of strength and ductility of the martensitic chromium steels can be adjusted to an extreme level. By this means conventional lightweight solutions can be enhanced and this leads to a more economical use of resources. Furthermore, the extreme strength combined with the higher ductility provides an extreme crash safety of the produced car body parts. Forming limit curves (FLC) are used to identify the forming capacity of the sheet material, so the production process can be designed to the limiting range. Conventional FLC recorded at room temperature do not adequately describe the forming capacity for hot forming as it strongly depends on temperature. Therefore, in this paper an experimental-numerical method for determining quasi-isothermal FLC at high forming temperatures is introduced, applied to the steel AISI 420C (X46Cr13) at 900 °C and compared to literature values of Usibor 1500.

The newly developed forging process chain is based on a thermomechanically treated base material with an optimum microstructure at the edge of the nano-scale. It is associated with maximum properties in terms of high strength and excellent toughness. This offers the possibility of a novel and extraordinarily energy-efficient process chain of manufacturing of highly loaded and safety relevant parts in all kinds of machinery. Therefore, it is necessary to shape the thermomechanically treated base material, with its superior properties, without a loss to the near net shape part. Hot and warm forging processes are known as appropriate and efficient for the forming of high strength materials while also ensuring the materials’ formability is not overstressed through strain hardening. Within this paper it could be shown that the original microstructures were maintained, and in some cases improved. Therefore, a targeted heating strategy and a warm forging process were designed. The characterisation and numerical modelling of the novel material is presented and the effects of heating and forming on the microstructure of the pre-treated base material are shown in microsections. The influence on the resulting mechanical properties such as the significant increase of roughly 150 J (+400 %) in the notch impact energy (compared with forged parts from untreated material) are detailed described. Additionally, the influence of forming parameters on the microstructure such as effective plastic strain and forming temperatures are discussed.

The importance of saving resources and reducing CO2 emissions is continuously increasing in the current and future development of products. In addition, the ever-increasing guidelines lead to the need to increase the efficiency of manufacturing and recycling processes, so that current production adheres to these guidelines. To that end, the product recycling of steel components is a realistic approach to save a high amount of energy, while also ascertaining environmental protection.

Hot forging is an effective production method producing safety relevant parts with excellent mechanical properties. The economic efficiency directly depends on the occurring wear of the tools, which limits service lifetime. Several approaches of the presenting research group aim at minimizing the wear caused by interacting mechanical and thermal loads by using enhanced nitriding technology. Thus, by modifying the surface zone layer it is possible to create a resistance against thermal softening provoking plastic deformation and pronounced abrasive wear. As a disadvantage, intensely nitrided surfaces may possibly include the risk of increased crack sensitivity and therefore feature the chipping of material at the treated surface. Recent projects (evaluated in several industrial applications) show the high technological potential of adapted treatments: A first approach evaluated localized treatments by preventing areas from nitrogen diffusion with applied pastes or other coverages. Now, further ideas are to use this principle to structure the surface with differently designed patterns generating smaller ductile zones beneath nitrided ones. The selection of suitable designs is subject to certain geo-metrical requirements though. The intention of this approach is to prevent the formation and propagation of cracks under thermal shock conditions. Analytical characterization methods for crack sensitivity of surface zone layers and an accurate system of testing rigs for thermal shock conditions verified the treatment concepts. Additionally, serial forging tests using adapted testing geometries and finally, tests in the industrial production field were performed. Besides stabilizing the service lifetime and decreasing specific wear mechanisms caused by thermal influences, the crack behavior was influenced positively. This leads to a higher efficiency of the industrial production process and enables higher output in forging campaigns of industrial partners.

Interacting high mechanical, tribological, chemical and thermal cyclic load components of hot forging processes are process-related and acting geometrically dependent. To enhance the cost effectiveness, there is a big interest in reducing the occurring wear which is a result of these complex load regimes during processing. The presented work identifies the different main wear mechanisms in forging dies and the subsequent abstraction in the configuration of different model tests. Thus, an accentuation of the predominant load component was possible in order to develop adapted plasma nitriding processes partially combined with PVD or PEVD hard coatings to reduce effectively the specific wear like plastic deformation, cracking and abrasive or adhesive wear respectively. The change of the primary surface shape of forging tools was found to be essential for the development of the wear progress. This was described by using the Abbott-Firestone graph finding the core roughness Sk to be a suitable describing parameter. Thus, conditioning methods like abrasive finishing or severe shot peening represent tool enhancements for a further wear reduction. The definition of distinct tool topographies in addition with a stabilization of the initial state of the tool surface achieved with hard coatings and/or additional diffusion treatment is a very promising approach to enhance the wear resistance of forming tools.