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

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

Investigation on blasted tool surfaces as a measure for material flow control in sheet-bulk metal forming

Highly integrated and closely tolerated functional components can be produced by sheet-bulk metal forming which is the application of bulk forming operations on sheet metal. These processes are characterized by a successive and/or simultaneous occurrence of different load conditions which reduce the geometrical accuracy of the parts. One challenge within sheet-bulk metal forming is the identification of methods to control the material flow to improve the product quality. A suitable approach is the local modification of the tribological conditions. Within this study, requirements regarding the needed adaption of the tribological system for a specific process were defined by numerical investigations. The results reveal that a local increase of the friction leads to an improved geometrical accuracy. Based on these results, abrasive blasting as a method to modify the tool surface and thus influencing the tribological behaviour was investigated. For the determination of the tribological mechanism of blasted tool surfaces, the influence of different blasting media as well as blasting pressures on the surface integrity and the friction were determined. Additionally, the functional stability of a modification was investigated. Finally, the correlations between surface properties and friction conditions were used to derive the mechanisms of blasted tool surfaces

Robustness Analysis of Pin Joining

The trend towards lightweight design, driven by increasingly stringent emission targets, poses challenges to conventional joining processes due to the different mechanical properties of the joining partners used to manufacture multi-material systems. For this reason, new versatile joining processes are in demand for joining dissimilar materials. In this regard, pin joining with cold extruded pin structures is a relatively new, two-stage joining process for joining materials such as high-strength steel and aluminium as well as steel and fibre-reinforced plastic to multi-material systems, without the need for auxiliary elements. Due to the novelty of the process, there are currently only a few studies on the robustness of this joining process available. Thus, limited statements on the stability of the joining process considering uncertain process conditions, such as varying material properties or friction values, can be provided. Motivated by this, the presented work investigates the influence of different uncertain process parameters on the pin extrusion as well as on the joining process itself, carrying out a systematic robustness analysis. Therefore, the methodical approach covers the complete process chain of pin joining, including the load-bearing capacity of the joint by means of numerical simulation and data-driven methods. Thereby, a deeper understanding of the pin joining process is generated and the versatility of the novel joining process is increased. Additionally, the provision of manufacturing recommendations for the forming of pin joints leads to a significant decrease in the failure probability caused by ploughing or buckling effects

Functional Analysis of Components Manufactured by a Sheet-Bulk Metal Forming Process

Due to rising demands regarding the functionality and load-bearing capacity of functional components such as synchronizer rings in gear systems, conventional forming operations are reaching their limits with respect to formability and efficiency. One way to meet these challenges is the application of the innovative process class of sheet-bulk metal forming (SBMF). By applying bulk forming operations to sheet metal, the advantages of both process classes can be combined, thus realizing an optimized part weight and an adapted load-bearing capacity. Different approaches to manufacturing relevant part geometries were presented and evaluated regarding the process properties and applicability. In this contribution, a self-learning engineering workbench was used to provide geometry-based data regarding a novel component geometry with circumferential involute gearing manufactured in an SBMF process combination of deep drawing and upsetting. Within the comprehensive investigations, the mechanical and geometrical properties of the part were analyzed. Moreover, the manufactured components were compared regarding the increased fatigue strength in cyclic load tests. With the gained experimental and numerical data, the workbench was used for the first time to generate the desired component as a CAD model, as well as to derive design guidelines referring to the investigated properties and fatigue behavior

Evaluating material failure of AHSS using acoustic emission analysis

Driven by high energy prices and strict legal requirements on CO2 emissions, high-strength sheet steel materials are increasingly gaining importance in the automotive industry regarding electric vehicles and their battery range. Simulation-based design of forming processes can contribute to exploiting their high potential for lightweight design. However, previous studies show that numerical simulation with conventional forming limit curves does not always provide adequate prediction quality. Failure models that take the stress state into account represent an alternative prediction method for the shear-dominated failure, that frequently occur in high-strength steels during forming. The failure behaviour of the sheet materials can be determined by different specimen geometries for a wide range of stress states and by using an optical measurement system to record the local strain on the surface of the specimen at the location of failure. However, for many high-strength steels, critical damage or failure initiation already occurs inside the specimen. Therefore, a method is needed that allows detection of failure initiation at an early stage before the crack becomes visible on the surface of the specimen. One possible method is the use of acoustic emission analysis. By coupling it with an imaging technique, the critical strains leading to failure initiation inside the specimen can be determined. In the presented paper, butterfly tests are performed for a wide range of stress states and measured with an optical as well as an acoustical measurement system. The tests are analysed regarding the failure initiation using a mechanical, optical as well as acoustical evaluation method and compared with each other

Determination of Forming Limits in Sheet Metal Forming Using Deep Learning

The forming limit curve (FLC) is used to model the onset of sheet metal instability during forming processes e.g., in the area of finite element analysis, and is usually determined by evaluation of strain distributions, derived with optical measurement systems during Nakajima tests. Current methods comprise of the standardized DIN EN ISO 12004-2 or time-dependent approaches that heuristically limit the evaluation area to a fraction of the available information and show weaknesses in the context of brittle materials without a pronounced necking phase. To address these limitations, supervised and unsupervised pattern recognition methods were introduced recently. However, these approaches are still dependent on prior knowledge, time, and localization information. This study overcomes these limitations by adopting a Siamese convolutional neural network (CNN), as a feature extractor. Suitable features are automatically learned using the extreme cases of the homogeneous and inhomogeneous forming phase in a supervised setup. Using robust Student’s t mixture models, the learned features are clustered into three distributions in an unsupervised manner that cover the complete forming process. Due to the location and time independency of the method, the knowledge learned from formed specimen up until fracture can be transferred on to other forming processes that were prematurely stopped and assessed using metallographic examinations, enabling probabilistic cluster membership assignments for each frame of the forming sequence. The generalization of the method to unseen materials is evaluated in multiple experiments, and additionally tested on an aluminum alloy AA5182, which is characterized by Portevin-LE Chatlier effects

Numerical analyses of the influence of a counter punch during deep drawing

In the automotive sector, the demand for high crash safety and lightweight construction has led to an increased use of steels with higher strengths. However, the rising number of varying materials with different strengths and ductilities lead to an increasing complexity in productionmaking it more challenging to ensure robust processes. Therefore, the focus of current researches still lays on the further development and extension of forming processes to enable high productivity and reliable production. A powerful tool for an efficient optimisation and extension of forming processes is the Finite Element Method (FEM), which offers time-and cost saving potentials in the design phase. In deep drawing, the use of a counter punch offers the possibility oextending the process limits. By superimposing compressive stresses on the workpiece, the initiation of cracks can be delayed, thus higher drawing ratios can be achieved. The aim of this research is therefore the numerical investigation of a deep drawing process with a counter punch to analyse the influence on the crack initiation and identify optimisation potentials for the processFor this cause, the applied force as well as the position and geometry of the counter punch are varied and the influence on fracture initiation is evaluated. It is found that the applied force on the counter punch is the major influencing factor for crack initiation. Furthermore, it was concluded that the contact between the counter punch and the workpiece should be applied as soon as the bottom of the cup is shaped. A further improvement can be achieved if the counter punch is geometrically adapted to the bottom of the workpiece

Development of polygon forming processes for aerospace engineering

The focus of this research lays on the further development of the Polygon Forming Technology, which is already successfully used for cold forming components in the aerospace industry. One example is the fuselage shell of the Airbus Beluga XL. According to the current state of the art it is possible to incrementally form large cylindrical or conical fuselage components by Polygon Forming. With the use of so-called infills, the Polygon Forming process can also be used to form components with pockets milled in the initial plane state. The limits of this technology exclude the creation of spherical geometries, such as those used in the front or rear fuselage sections of aircrafts. Presently, such components are produced by more complex stretch forming processes, which result in a considerable amount of scrap. In this work, a tool is developed to replicate the Polygon Forming process on experimental scale at the Institute of Forming Technology and Machines (IFUM) for materials commonly used in aerospace engineering. In addition, a downscaled pre-test tool is developed to investigate different tool geometries for incremental spherical forming inexpensive and easy according to the method of rapid prototyping

Fringe Projection Profilometry in Production Metrology: A Multi-Scale Comparison in Sheet-Bulk Metal Forming

Fringe projection profilometry in combination with other optical measuring technologies has established itself over the last decades as an essential complement to conventional, tactile measuring devices. The non-contact, holistic reconstruction of complex geometries within fractions of a second in conjunction with the lightweight and transportable sensor design open up many fields of application in production metrology. Furthermore, triangulation-based measuring principles feature good scalability, which has led to 3D scanners for various scale ranges. Innovative and modern production processes, such as sheet-bulk metal forming, thus, utilize fringe projection profilometry in many respects to monitor the process, quantify possible wear and improve production technology. Therefore, it is essential to identify the appropriate 3D scanner for each application and to properly evaluate the acquired data. Through precise knowledge of the measurement volume and the relative uncertainty with respect to the specimen and scanner position, adapted measurement strategies and integrated production concepts can be realized. Although there are extensive industrial standards and guidelines for the quantification of sensor performance, evaluation and tolerancing is mainly global and can, therefore, neither provide assistance in the correct, application-specific positioning and alignment of the sensor nor reflect the local characteristics within the measuring volume. Therefore, this article compares fringe projection systems across various scale ranges by positioning and scanning a calibrated sphere in a high resolution grid