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      27
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    Data-based analysis of nanoporous metals: from microstructural features to structure-property relationships
    (2023)
    Richert, Claudia
    ;
    Huber, Norbert
    In this thesis, a comprehensive workflow for an FE beam modeling approach was developed, which allows to predict the macroscopic elastic and plastic mechanical behavior of nanoporous structures. An in-depth characterization of various geometrical and topological descriptors provided detailed insights about the comparability of experimental nanoporoug gold and computer-generated structures. The combination of the macroscopic mechanical predictions by FE simulations and the structural descriptors derived for the over one hundred computer-generated structures enabled to propose an improved scaling law.
    Publicationtype: Doctoral Thesis
      178  187
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    Damage tolerance behaviour of integral aircraft structures obtained by stationary shoulder friction stir welding
    (2021)
    Barbini, Alessandro
    ;
    Huber, Norbert
    Research focusing on new ways to achieve weight reduction without sacrificing the safety performances of an aircraft is the subject of a constant push in the aeronautical industry. The use of riveting to join skin and stringer is highly optimised but in order to reduce structural weight, aside from the continuous development of new materials, it is important to use the already available materials in an efficient way. For this reason, friction stir welding (FSW) was looked at for many years as a suitable replacement alternative for riveting but the focus of the research was mostly driven with a material science point of view. Therefore, the development of a new solid-state welding technology ensuring good material properties and, at the same time, being investigated as a structural joint is still a necessity. The present-study addresses the development of a new procedure for joining skin and stringers made of dissimilar aluminium alloys by stationary shoulder friction stir welding (SSFSW), which is to be applied in primary aircraft structures. Three different joint configurations were developed, and their performances were compared. At first, a fundamental analysis of the correlation between the two main process parameters, welding speed and rotational speed on heat development and joint formation, was conducted. The correlation between the generated microstructure and the mechanical properties of the three configurations was investigated on coupon-size specimens. In the following step, the fatigue behaviour in sub-component scale was studied in terms of fatigue life and crack initiation for two of the joint configurations. In the last phase of the work, 5-stringer panels were realised by SSFSW using the joint configuration, which showed the best properties in the performed analysis. These panels were investigated in the context of a damage-tolerant environment to understand fatigue crack growth and residual strength behaviour in comparison to conventional joining technologies. At the coupon level, the three joint configurations showed higher mechanical performances for different loading conditions with respect to a conventionally riveted skin and stringer. The interface between the skin and the stringer’s alloys proved to play the largest influence on the strength of the joints. The two welding parameters control the geometry of this by influencing the material flow and heat generation. The notch-skin double pass (NS-DP) configuration proved to have limitations in terms of process development and were excluded from the study for up-scaled components. The sub-components realised by SSFSW with the two remaining joint variants showed higher fatigue performances in hoop-directions with respect to riveted structures. The SSFSW technology leads to half the stress concentration of a riveted T-joint and an increase of 50% in the fatigue limit. In spite of a similar fatigue life, in the NS-SP joints, crack turning and unwanted delamination of the stringer was observed. In the longitudinal load direction, a higher fatigue life was recorded for the 3-Part joints in the high-cycle area with respect to the NS-SP variant. From the fatigue crack growth (FCG) tests of the 5-stringer panels, the optimum load redistribution between skin and stringers joined by the SSFSW led to a reduction of the FCG rate for short cracks away from the stringers. This structural behaviour improves the FCG with respect to riveted and adhesive bonded structures. An increase in lifespan over 62% for a crack length that reaches the two-bay width has been shown. The residual strength of the SSFSW panels was measured for a two-bay crack showing a significant improvement over conventional joining techniques and confirming the superior quality of structures realised by SSFSW in a damage-tolerant environment.
    Publicationtype: Doctoral Thesis
      358  870
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    Dissimilar titanium and titanium aluminide joints manufactured by laser beam welding and laser additive manufacturing
    (2021)
    Burkhardt, Irmela
    ;
    Huber, Norbert
    Motivated by the increasing demands for lightweight constructions, the aim is to obtain crack-free dissimilar Ti- and γ-TiAl-Ti-alloy joints by focusing on laser beam welding as the joining method. This was successfully achieved by preheating and the crack formation could be correlated to the microstructure-phase evolution. Through tensile tests the influence of the dissimilar interface on the fracture position and tensile properties could be revealed. Extending the thesis, the gained knowledge regarding the microstructure-phase evolution at the interface could be successfully transferred to the laser metal dep-ositions. Besides, the limited transferability concerning the crack formation was demonstrated. Hence, the fundamentals for crack-free γ-TiAl-Ti-alloy joints and depositions were established.
    Publicationtype: Doctoral Thesis
      349  1413
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    Microstructure evolution and stress corrosion cracking behavior in short-term thermomechanically processed Al-Cu-Li alloys
    (2019-08)
    Entringer, Jannik
    ;
    Huber, Norbert
    Increasing access to space leaves human footprints in orbits closer to the Earth. Today, space debris poses a major risk for in-service missions and on-ground casualties. Reducing the amount of debris is the only way to preserve key orbits. One strategy to avoid future debris is to maneuver spacecraft after mission into the atmosphere where the structure burns up. This strategy is taken into account at early design stages of modern spacecraft. Therefore, material choices must be made to ensure high structural demisability. Titanium, currently used in propellant tanks, is often a large portion, by mass, of the total structure and does not completely ablate during re-entry. Aluminum-copper-lithium alloys are being considered as a substitute for titanium in propellant tanks because they deliver comparable specific material properties at a higher demisability rate. Because the technology of aluminum-copper-lithium alloys in space structures is not as mature as that of titanium, new joining methods must be developed. Friction stir welding features the unique characteristic of joining in the solid state; thus, it is especially attractive for hard-to-weld alloys such as aluminum-copper-lithium. The development of space structures requires a fundamental knowledge of the material behavior, both during processing and during the subsequent exposure to the environment. In recent years, aluminum copper-lithium alloys have rarely been welded by bobbin tool friction stir welding. The underlying microstructure evolution has been described as complex, certain aspects are not fully understood and contradictory results have been reported. Current knowledge on the behavior of welded joints on this specific alloy under stress and exposed to corrosive environments is limited. Therefore, for the possible use in spacecraft structures, a scientific analysis of the stress corrosion behavior of welded joints is necessary. As part of the present work, two modern aluminum-copper lithium alloys were successfully joined by semi-stationary bobbin tool friction stir welding. Identical parameters allowed a detailed comparison of the process response. The welding process imposes a short time, thermomechanical exposure in the base material that leads to severe microstructure modification. Based on the chemical composition of the two alloys, the microstructural evolution is explained, and a precipitation sequence is proposed. Overaged strengthening precipitates and equilibrium phases of several types were identified to form during welding. These modifications led to a reduced mechanical performance of 78 % of the ultimate tensile strength. Stress corrosion analyses were performed on pre-, as- and post-welded samples and were correlated with the modified microstructure. Stress corrosion cracking phenomena were found to result from the short-time, thermomechanical effect induced by the welding process. The mechanism leading to stress corrosion lies in the modified microstructure, where coarse, precipitates accumulate at the grain boundaries. These particles are observed to promote local galvanic reactions, which promote dissolution and the consequent development of a cracking network under stress.
    Publicationtype: Doctoral Thesis
      429  1382
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    Damage tolerance of refill friction stir spot weld application for the aircraft industry
    (2019)
    Brzostek, Robson Cristiano
    ;
    Huber, Norbert
    Refill Friction Stir Spot Welding (Refill FSSW) is a solid-state process technology that is suitable for welding lightweight materials in similar or dissimilar overlapped configuration. It has proven to be a very promising new joining technique; especially, for high strength aluminum alloys, which has presented large advantages when compared to conventional welding processes. Currently, Refill FSSW is recognized as a potential alternative for riveted structure; it allows an increasing of the manufacturing cost effectiveness owing to sensible cost reduction and structural efficiency. The main aim of this work is to study the mechanical behavior and crack propagation in joints produced by Refill FSSSW. The study is focused on the application of the damage tolerance design philosophy in integral structures produced by Refill FSSSW in aluminum alloy AA2024-T3. Up to now the process development and the mechanical performance study has been mostly empirical. Thus, a transition to a science-based approach is highly necessary. The work presented here was conducted to stablish a relationship of experimental investigation and a set of numerical models that can be used for design optimization and fatigue crack growth analysis. Beforehand, the welded joints were assessed mechanically and metallurgically in order to investigate the mechanism and the optimization of the process parameters (rotation speed, welding time and plunge depth) in terms of quasi-static loading and fatigue loading. This investigation has assisted the development of the structural numerical models, where two structural models have been developed to study the design optimization. The first model covers the stress analysis, load transferred by friction, stress concentration and peak stress location; it was built considering the structural and cohesive approach. The second numerical model considers the embedded approach; it can be used for parametric studies with good accuracy. Then, the design optimization was developed considering the distances: number of spot welds rows, spot weld row spacing, spot weld pitch in row and distance of the spot weld from the sheet edge. The developments of the distances were performed considering its performance in quasi-static and fatigue loading. A fractography analysis at various fracture modes has been performed. This is necessary in order to understand and described the crack propagation according the fracture mechanics. Then, a numerical model has developed and calibrated in order to obtain stress intensity factors for the cracks described previously. The numerical model has been built with the eXtended Finite Element Method. Finally, the thesis deals with crack propagation and residual strength of Refill FSSW in thin panels for aircraft fuselage applications. Detailed experimental investigation has been carried out in panels FSSW in order to understand the crack propagation under different failure scenario. Moreover, the experimental results have been used to verify and calibrate the developed fatigue crack growth numerical model. The model has been used to simulate crack propagation in different joint configuration and initial cracks. The numerical model has been built with the eXtended Finite Element Method. The results have shown good agreement of the predict fatigue life with the experimental results. Then, both eXtended Finite Element Method models numerical models developed have been used for residual strength prediction of cracked unstiffened panels in terms of the stress intensity factor. The results obtained in the course of this study have shown the feasibility of Refill FSSW to produce high strength joints as well as the importance of the joint design, in which can be significantly improved by using the correct distances. The knowledge about the structural behavior and extent of crack propagation gained from the numerical models is valuable to understanding the influence of secondary bending on cracked panels and the development of residual strength diagrams.
    Publicationtype: Doctoral Thesis
      555  1153
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    On the fatigue behaviour and modelling of fatigue life for laser-welded Ti-6Al-4V
    (2019)
    Fomin, Fedor
    ;
    Huber, Norbert
    This aim of this work is to investigate the fatigue behaviour of laser-welded Ti-6Al-4V titanium alloy in the region of high cycle fatigue. The crack initiation mechanism and the influence of weld defects on fatigue performance were comprehensively studied. A set of post-weld treatments that can mitigate the effect of inherent weld imperfections and thereby extend the fatigue life were analysed. A robust and efficient analytical assessment model for internally-flawed materials has been developed based on a fracture mechanics approach. The developed methodology can be used for the reliability assessment of components with various types of defects, thus enabling a defect-tolerant design of welded structures.
    Publicationtype: Doctoral Thesis
      424  2956
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    Keyhole repair in precipitation hardening aluminum alloys using refill friction stir spot welding
    (2018)
    Reimann, Martin
    ;
    Huber, Norbert
    Aluminum alloys are widely used in transportation industries because of the increasing need to reduce the environmental impact. With advances in technology, the demand for complex parts and components that must be produced using several processing methods has increased. During fabrication and service, a wide range of defects can appear in aluminum components and structures, which could be repaired using a suitable through hole closure method. The search for a friction-based solid-state keyhole repair technique that fulfills the requirements for high-quality repair welds has become an important research topic because conventional fusion welding is difficult to apply in many aluminum alloys. However, many commonly available friction-based welding methods are complex and multistage processes that require specially designed equipment and are not suitable for sealing through holes. The development of an adequate keyhole repair process is thus actual necessity. The present study addresses the development of a suitable keyhole repair procedure of structural aluminum parts using the refill friction stir spot welding process (RFSSW). For this newly developed repair method, a plug made of a similar material is applied as a filler element into the keyhole and RFSSW is used to weld the plug to the surrounding workpiece. To cover a wide range of alloys and potential applications, the repair method was investigated in different precipitation hardening aluminum alloys as well as different keyhole diameters and workpiece thicknesses. A fundamental analysis of the process and resulting material properties considering the alloy-dependent metallurgical transformations was conducted. Moreover, a knowledge-based process analysis approach was chosen to study the behavior of the base material during high-shear-rate plastic deformation and exposure to typical thermal cycles, which are both associated with the conditions found during friction welding. The influence of the base material composition and properties on the energy input during friction welding was investigated and a comprehensive analysis of the friction condition and flow stress development was conducted. The developed keyhole repair process using RFSSW is a universal through-hole closure method with advantages such as defect-free welds, high weld efficiencies and superior surface appearance on both sides of the weld. Within the scope of the present work, processing conditions were defined that lead to defect free repair welds for all investigated materials and workpiece dimensions. The area of lowest strength was in all cases found outside of the weld spot, mainly in the heat affected zone. For all welded precipitation hardening aluminum alloys, metallurgical analysis revealed that the evolution of the strengthening precipitates during and after the weld primarily determines the final mechanical properties. Significant differences observed in the response of the base materials to the process were found to be caused by the alloy composition, specifically by the characteristics of the present precipitates. The fundamental process analysis revealed that in precipitation hardening aluminum alloys, the mechanical properties obtained under quasi-static testing conditions are not adequate to describe or predict the base material properties at the high strain rates and thermal cycles associated with friction welding operations. The alloy composition, initial temper condition and general precipitation evolution during the specific thermal cycles resulting from the friction welding operations were found to determine the material properties at the tested rates of deformation. The knowledge gained by this fundamental process analysis is key to enabling rapid process optimization by guiding the appropriate choice of process parameters for a given alloy.
    Publicationtype: Doctoral Thesis
      756  928
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    On the optimization of laser shock peening induced residual stresses
    (2018)
    Chupakhin, Sergey
    ;
    Huber, Norbert
    There is a strong economic motivation of the aircraft industry to explore novel residual stress-based approaches for the fatigue life extension, repair, and maintenance of the growing fleet of ageing aircrafts, although the effect of residual stresses is not taken into account by the established damage tolerance evaluation methods. Laser shock peening — the most promising life enhancement technique — has already demonstrated great success in regard to the mitigation of fatigue crack growth via deep compressive residual stresses. However, no comprehensive model exists which allows the prediction of generated residual stress fields depending on the laser peening parameters. Furthermore, the hole drilling method — a well-established technique for determining non-uniform residual stresses in metallic structures — is based on measuring strain relaxations at the material surface caused by the stress redistribution while drilling the hole. However, the hole drilling method assumes linear elastic material behavior and therefore, when measuring high residual stresses approaching the material yield strength, plastic deformation occurs, which in turn leads to errors in stress determination. In the light of these two points, the present work aims to optimize the laser shock peening process in regard to high residual stress profiles, their correct measurement by the hole drilling method and demonstration of the fatigue crack growth retardation through the laser peening treatment on the laboratory scale. First, the methodology for the correction of the residual stresses approaching the material yield strength when measuring by the hole drilling is established and experimentally validated. The correction methodology utilizes FE modelling and artificial neural networks. In contrast to the recent studies, the novelty of this methodology lies in the practical and elegant way to correct any non-uniform stress profile for a wide range of stress levels and material behaviors typically used in industrial applications. Therefore, this correction methodology can be applied in industry without changing the procedure of hole drilling measurement. Second, the laser shock peening process is optimized in regard to the generated residual stress profiles using design of experiments techniques. The strategy involves laser peening treatment with different parameters and subsequent measurement of induced residual stress profiles through hole drilling. The measured stress profiles are subjected to correction using the neural network methodology. After that the regression model is fitted into the experimental data in order to find the relationship between the laser peening parameters and the stress profiles’ shapes. In the final stage, it is experimentally demonstrated that the established regression model provides an accurate prediction of the residual stress profile when using defined laser peening parameters and vice versa. Third, the regression model obtained in the design of experiments study is used for generating the desired residual stresses in the C(T)50 AA2024-T3 specimens for the fatigue crack propagation test. Significant retardation of the fatigue crack propagation of specimens due to the presence of deep compressive residual stresses is experimentally demonstrated on the laboratory scale.
    Publicationtype: Doctoral Thesis
      1175  2340
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    Friction surfacing of titanium grade 1 and Ti-6Al-4V
    (2017)
    Fitseva, Viktoria
    ;
    Huber, Norbert
    The friction surfacing process is a relatively novel and promising surface modification technology, by which coatings can be deposited as a protection or surface repair method. The main feature of this process, compared with other technologies, is that the coatings are deposited in solid state phase. Therefore, the induced massive deformation alters the initial microstructure of the materials, resulting in a fully recrystallised microstructure of the coatings. Typically, the grain size of the coating is smaller than that of the raw material, which leads to improved mechanical properties. Since the process is a relatively new technology, several possible material combinations in similar or dissimilar configurations are unexplored. Titanium alloys are rather expensive. Therefore, new technologies are required to keep the production cost at an acceptable level and offer an additional cladding process that is environmental friendly. The aim of the current work was to deposit coatings from the titanium alloys by friction surfacing. Process development for Ti-6Al-4V alloy and Ti-Gr.1 as coating materials to be deposited by friction surfacing was carried out. At high temperatures titanium alloys exhibit complex deformation behaviour, particularly when passing through the alpha-beta phase transformation and in the beta phase state. Strain rate sensitivity and flow instabilities are characteristics of Ti-based materials that may hinder the deposition process. These difficulties were overcome by the selection of an adequate process control method and a systematic choice of process parameter combinations. A broad range of parameter sets for titanium depositions has been established, whereby two different acting rotational speed regimes were observed. The low rotational speed regime revealed variations in the process temperature, which influenced the material flow behaviour leading to flash generation at the coating. The microstructure in this regime consists of refined dynamically recrystallised grains. In contrast, the high rotational speed regime revealed a higher resulting temperature, which did not vary in this range. This constant temperature led to stable material flow behaviour and flash-free coatings were deposited. However, the high temperature influenced the grain size of the coatings resulting in coarse grains. Still, these differences in the grain size did not influence the fretting wear behaviour of the coatings. The investigation of fretting wear experiments exposed a similar behaviour of friction surfacing coatings and the base material. In micro tensile tests the coatings exhibited an increase in strength but a decrease in ductility, which is typical for dynamically recrystallised materials, which contain residual deformation. Therefore, it could be shown that friction surfacing can be considered as a repair method for titanium parts.
    Publicationtype: Doctoral Thesis
      561  3060