Laserstrahlumformen von Aluminiumwerkstoffen - Beeinflussung der Mikrostruktur und der mechanischen Eigenschaften

Das Laserstrahlumformen als flexibles Formgebungsverfahren in Kombination mit dem Leichtbauwerkstoff Aluminium stellt eine besondere Herausforderung an die Ingenieurwissenschaften dar. Die Untersuchung und Beschreibung der Auswirkungen einer kurzzeitigen Erwärmung auf das Bauteil und dessen mechanische Eigenschaften ermöglichen eine am Werkstoff orientierte Auslegung des Verfahrens und erweitert damit das industrielle Einsatzpotential des Laserstrahlumformens für den Rapid-Prototyping Bereich sowie die Kleinserienfertigung.Laser beam forming as a flexible shaping process in combination with the lightweight aluminum material represents a particular challenge for the engineering sciences. The investigation and description of the effects of short-term heating on the component and its mechanical properties enable the process to be designed based on the material, thereby expanding the industrial potential laser beam forming for rapid prototyping and small series production

Laserstrahlumformen von Aluminiumwerkstoffen - Beeinflussung der Mikrostruktur und der mechanischen Eigenschaften

Laser beam forming as a flexible shaping process in combination with the lightweight aluminum material represents a particular challenge for the engineering sciences. The investigation and description of the effects of short-term heating on the component and its mechanical properties enable the process to be designed based on the material, thereby expanding the industrial potential laser beam forming for rapid prototyping and small series production.Das Laserstrahlumformen als flexibles Formgebungsverfahren in Kombination mit dem Leichtbauwerkstoff Aluminium stellt eine besondere Herausforderung an die Ingenieurwissenschaften dar. Die Untersuchung und Beschreibung der Auswirkungen einer kurzzeitigen Erwärmung auf das Bauteil und dessen mechanische Eigenschaften ermöglichen eine am Werkstoff orientierte Auslegung des Verfahrens und erweitert damit das industrielle Einsatzpotential des Laserstrahlumformens für den Rapid-Prototyping Bereich sowie die Kleinserienfertigung

Analysis of stress pins for the local prestressing of cold forging tools

Abstract The trend towards lightweight design leads to an increasing demand for sophisticated part geometries with high functional integration. In order to use the advantages of cold forging regarding the time- and resource-efficient production of high-quality parts, high local stresses causing fatigue failure in geometrically complex tools have to be controlled. The objective of this manuscript is to analyse the use of stress pins for a local influence on the stress state within forging, especially in non-circular symmetrical cold forging dies. For this purpose, a closed-die forging process for elliptical parts is designed and analysed regarding the die stresses. Distinct areas with local compressive and tensile stresses occur in the process. To counteract the tensile stresses critical for fatigue failure, the effect of stress pins pressed into the die creating a local prestress is analysed. Around the pins, compressive radial and tensile tangential stresses occur. While large pin diameters, interferences and close positioning to the tensile area lead to an increasing prestressing effect, too high values of these parameters cause a detrimental superposition of tensile process and pin stresses. If used correctly, there is high potential to improve the stress state and tool life especially for locally stressed complex tools

Analysis of the influence of surface modifications on the fatigue behavior of hot work tool steel components

Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS

Improvement of the drawing ratio of the anisotropic material behaviour under near plane strain conditions for DP600 characterized in elliptic hydraulic bulge test

Abstract The plane strain condition is one of the most frequent reasons for failure in a deep drawing process. State of the art material models do not consider this strain state in FE simulations. Since there is only the notched tensile test to determine the first principal stress, an improved testing setup is needed to characterize the first and second principal stresses up to high deformations to determine the material behaviour under plane strain conditions. Within this contribution, an innovative testing setup is used, which induces a near plane strain regime in the specimen with an elliptical hydraulic bulge test. Experiments are carried out in rolling and transversal direction for DP600. Based on the experiments, stress and stain based material characteristics are evaluated. For validation of the setup, square cups are deep drawn, which have a plane strain area at the drawing radius which causes cracking under this strain state. Due to the highly directional dependent material behaviour, the drawing ratio can be increased by considering the anisotropic material behaviour by cutting the blank in an optimized position according to the rolling direction

Characterization of the Tribological Behavior of Different Tool Coatings and Dry Lubricant for High‐Strength Aluminum Alloys at Elevated Temperatures

The use of high‐strength aluminum components in automotive manufacturing offers the opportunity to reduce vehicle weight significantly and provide new lightweight potentials. In the past, the so‐called hot forming and quench process (HFQ) successfully demonstrates the potential for the production of complex‐shaped components made out of age‐hardenable high‐strength aluminum alloys. Currently, no method permits wear‐free quench forming without the use of lubricants. To fulfill the increasing ecological and economic requirements, it is necessary to identify wear‐reducing techniques to promote this forming technology in the future. This contribution investigates the interaction of lubricant and tool coatings on the tribological performance during quench forming of the high‐strength aluminum alloy AA7075 at elevated temperatures. For this purpose, the tribological behavior is investigated using both, flat strip drawing tests and deep drawing operations. Subsequently, the component quality is compared and discussed. The results demonstrate that tool coatings are effective for the production of high‐strength components in the HFQ process with minimal or even no lubrication and thus provide ecological as well as economic advantages. High friction and wear limit the thermal‐assisted forming of high‐strength aluminum alloys. Tool coatings and lubricants enable failure‐free forming at elevated temperatures. Diamond‐like carbon (DLC) tool coatings exhibit beneficial tribological properties. Strip drawing tests as well as forming experiments illustrate that deep drawing operations under dry conditions are possible at elevated temperatures. image © 2023 WILEY‐VCH GmbH Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

Investigation of the Microstructural Evolution during Hot Stamping of a Carburized Complex Phase Steel by Laser-Ultrasonics

Prior carburization of semi-finished steel sheets is a new process variant in hot stamping to manufacture parts with tailored properties. Compared to conventional hot stamping processes, a complex phase typed steel alloy is used instead of 22MnB5. Yet recent investigations focused on final mechanical properties rather than microstructural mechanisms cause an increase in strength. Thus, the influence of additional carburization on the microstructural evolution during hot stamping of a complex phase steel CP-W®800 is investigated within this work. The phase transformation behavior, as well as the grain growth during austenitization, is evaluated by in-situ measurements employing a laser-ultrasound sensor. The results are correlated with additional hardness measurements in as-quenched condition and supplementary micrographs. The experiments reveal that the carburization process significantly improves the hardenability of the CP-W®800. However, even at quenching rates of 70 K/s no fully martensitic microstructure was achievable. Still, the resulting hardness of the carburized samples might exceed the fully martensitic hardness of 22MnB5 derived from literature. Furthermore, the carburization process has no adverse effect on the fine grain stability of the complex phase steel. This makes it more robust in terms of grain size than the conventional hot stamping steel 22MnB5

Numerical and experimental investigations for distortion-reduced laser heat treatment of aluminum

In the field of mobility, increased safety and emission requirements lead to steadily rising demands on materials used and their performance. Over the last decades, 5000 and 6000 series aluminum alloys have become more and more attractive as lightweight material due to their beneficial weight to strength ratio. The 7000 series offers extended lightweight potential due to its high strength. Until now, this class of alloys has not been widely used in mass production due to its limited corrosion resistance and poor forming behavior. By using so-called Tailor Heat Treated Blanks, it is possible to set increased forming limits of previously locally heat treated components. The reason for the enhanced formability is the local softening, with the resulting improved material flow and the reduced critical forming stresses of the sheet metal before the forming operation. Despite these advantages, the use of previously heat treated materials has been very limited so far. For example, the distortion that occurs during local heat treatment reduces geometrical accuracy and thus automated handling. Therefore, the focus of this thesis is the investigation of tailored heat treatment strategies, permitting a distortion-reduced local short-term heat treatment. For this purpose, the distortion behavior is represented and quantified both numerically and experimentally. The generated knowledge is then transferred to a large volume component and characterized

Determination of the Mechanical Properties of Hot Stamped Parts from Numerical Simulations

AbstractHot stamping is a well-established process in car manufacturing today. However, the resulting mechanical properties of a hot stamped part and its behaviour during a crash are still open questions. The usual procedure includes destructive experiments to determine the mechanical properties resulting from the forming and quenching process. The gained information is then used for crash simulation. Using images from micrographs to determine the proportion of bainite and martensite resulting from the hot stamping process has proved to be difficult, as these structures are fairly similar and hard to distinguish.Sophisticated numerical simulations of the hot stamping process are available. The hardness resulting from the hot stamping process can be predicted fairly well from these process simulations. However, information like the tensile strength that is more relevant for the crash behaviour cannot be predicted that easily. It is not yet state of the art to map the results from the hot stamping simulation directly into the crash simulation. The approach to be presented in detail in this contribution uses the forming speed and the quenching velocity to predict the relevant mechanical properties of the hot stamped parts. Both input parameters, the forming speed and the quenching velocity, can be derived from the numerical hot stamping simulation. By means of experiments using a thermomechanical test system Gleeble well defined process parameters were used. Micro tensile test specimens were manufactured out of the Gleeble specimens to eliminate the effect of the Gaussian temperature profile created during the Gleeble experiments. Afterwards, tensile tests were carried out to derive a response surface for 22MnB5. The validated results allow the determination of the tensile strength of hot stamped parts from the numerical simulation of the hot stamping process with good accuracy

Fundamental Investigations to Evaluate the Influence of Notching Processes on a Subsequent Cyclic Bending Process for the Production of Wire Cores

The production of wire cores by notch rolling and cyclic bending promises an ecologically and economically efficient manufacturing option for steel fibers. The paper at hand evaluates the influence of wire strips on cyclic bending by applying rolled wire strips of DP600 sheet metal (t0 = 0.8 mm) and a new cyclic bending testing tool. Analysis of material separation with varying parameters, rolling gap d and bending angle β, proves the interdependency of both process step, but indicates reduced adjustability of the notch rolling process. To enable better adjustability of the wire strip’s characteristics and analysis of their effects, wire strip production in the laboratory by notch stamping instead of rolling is aspired. The prior interaction analysis states the web height b, the notch angle α, and the hardening distribution as relevant wire strip’s characteristics to be replicated. Based on experimental analysis, an equivalent of notch rolling by notch stamping is deduced by considering the web height b identical for stamping and rolling, by adjusting the tool’s notch angle αt based on an equation considering geometric evaluations of α, and by taking advantage of the asymmetric hardening distribution of the outer notch which is comparable to rolled wire strip