Powder bed fusion processing of Ni-base superalloys - Defect formation and its mitigation

Powder bed fusion of Ni- and Ni-Fe-base superalloys is actively considered a promising manufacturing technology for critical components for the aerospace and industrial gas turbine industries. Such components often operate under harsh conditions, and hence, high demands are placed on both process and feedstock material to meet the strict safety and long-term reliability requirements. The aim of this thesis is to provide knowledge regarding the formation of damage-relevant defects in Ni- and Ni-Fe-base superalloys fabricated by powder bed fusion as well as how they can be mitigated.The first part of the thesis presents the connection between the surface oxidation of Alloy 718 powder for EBM, as a consequence of powder re-use, and the presence of oxide-related defects in the EBM fabricated material. The results indicate a clear connection between powder re-use and surface oxidation of the powder. Surface analysis of the progressively re-used powder by means of SEM, XPS and AES reveals significant growth of Al-rich oxide, which occurs via selective oxidation of Al due to the environment in the build chamber. Furthermore, the increased amount of oxide on the surface of the re-used powder results in an increased amount of oxide inclusions and lack of fusion defects in the EBM fabricated material. The morphology of the defects reveals that they originate from Al-rich oxide particulates on the surface of the re-used powder.The second part of the thesis presents a study on the cracking of IN-738LC fabricated by means of LPBF. Implementation of custom designed powder grades with varying content of B and Zr indicates that both elements have a strong negative effect on the susceptibility to grain boundary microcracking of the alloy during LPBF. The XPS, AES and APT analyses show the enrichment of B and Zr at the cracked grain boundaries. Moreover, a significant portion of both elements are found to be connected to oxide. Hence, it is suggested that the increased microcracking susceptibility of IN-738LC is connected to the embrittlement of high-angle grain boundaries due to the formation of B- and Zr-containing oxide. In addition, post-LPBF hot isostatic pressing (HIP) is evaluated as a concept for microcrack healing. A HIP strategy that suppresses formation of macrocracks during the HIP treatment is developed by tailoring the temperature and pressure profiles during the heating stage. However, when applying the developed HIP strategy to the material grade with high levels of B and Zr, brittleness-inducing secondary phase particles at the grain boundaries appear after HIP at 1210\ub0C, leading to a significant reduction of the impact toughness. Formation of the secondary phase is suppressed by lowering the HIP temperature to 1120\ub0C. Results from microscopy and Charpy impact testing suggest that significant healing of the microcracks is accomplished when applying the developed HIP strategy

SIMULATION OF DIFFERENT MOVEMENTS IN SPORTS VIA DIRECT DYNAMICS

Introduction: A standard method to determine loads on human joints during sports movements is inverse dynamics. This method is problematic for the analysis of movements with high accelerations, e.g., landings in sports, because the measurements used as input and modeling must be adapted with a high degree of accuracy. Even small inconsistencies affect result sensitivity and may lead to large errors in the values of the load. Therefore, a more appropriate method is the analysis of movements with models adapted optimally to the human body using direct dynamics. Methods: Using multi-linked wobbling mass models different gymnastics movements were calculated, partly including phases of landings. For this purpose we developed special algorithms for the torques of the joints and force-deformationrelations simulating the behavior of different surfaces, which were used as input. Results: The body movements were simulated and visualized. By varying the initial conditions and parameters of the model and the surface, the influence on the movement and the load on the joints was investigated. Conclusions: The simulation of movements in sports using direct dynamics offers many opportunities for theory and training practice. The condition of the human body and the environment can be varied in an arbitrary manner, the simulated movement can be visualized and the load on the joints can be determined simultaneously. Reference: Ruder, H., Ertl, T., Gruber, K., Günther, M. Hospach, F., Ruder, M., Subke, J., Widmayer, K. (1994). Kinematics and Dynamics for Computer Animation. In S. Coquillart, W. Straßer, P. Stucki (eds.). From Object Modelling to Advanced Visual Communication. Berlin, 76-117

How prior experience, cognitive skills and practice are related with eye-hand span and performance in video gaming

Research has shown that performance in visual domains depends on domain-specific cognitive and perceptual adaptations that result from extensive practice. However, less is known about processes and factors that underpin the acquisition of such adaptations. The present study investigated how prior experience, cognitive skills, task difficulty and practice effect eye-hand span (EHS) and performance in video gaming. Thirty-three participants played a platformer video game in a pre-test/practice/post-test experiment. Eye movements and keypresses were recorded. The results show that a short practice period improved performance but did not increase EHS. Instead, EHS was related to task difficulty. Furthermore, while EHS correlated with initial performance, this effect seemed to diminish after practice. Cognitive skills (concentration endurance, working memory, mental flexibility and executive functioning) predicted performance in some parts of the experiment. The study offers insights into the early development of visual adaptations and performance

Eye tracking in Educational Science: Theoretical frameworks and research agendas

Eye tracking is increasingly being used in Educational Science and so has the interest of the eye tracking community grown in this topic. In this paper we briefly introduce the discipline of Educational Science and why it might be interesting to couple it with eye tracking research. We then introduce three major research areas in Educational Science that have already successfully used eye tracking: First, eye tracking has been used to improve the instructional design of computer-based learning and testing environments, often using hyper- or multimedia. Second, eye tracking has shed light on expertise and its development in visual domains, such as chess or medicine. Third, eye tracking has recently been also used to promote visual expertise by means of eye movement modeling examples. We outline the main educational theories for these research areas and indicate where further eye tracking research is needed to expand them

Investigating visual expertise in sculpture: A methodological approach using eye tracking

Research on visual expertise has progressed significantly due to the availability of eye tracking tools. However, attempts to bring together research on expertise and eye tracking methodology provoke several challenges, because visual information processes should be studied in authentic and domain-specific environments. Among the barriers to designing appropriate research are the proper definition of levels of expertise, the tension between internal (experimental control) and external (authentic environments) validity, and the appropriate methodology to study eye movements in a three-dimensional environment. This exploratory study aims to address these challenges and to provide an adequate research setting by investigating visual expertise in sculpting. Eye movements and gaze patterns of 20 participants were investigated while looking at two sculptures in a museum. The participants were assigned to four different groups based on their level of expertise (laypersons, novices, semi-experts, experts). Using mobile eye tracking, the following parameters were measured: number of fixations, duration of fixation, dwell time in relevant areas, and revisits in relevant areas. Moreover, scan paths were analysed using the eyenalysis approach. Conclusions are drawn on both the nature of visual expertise in sculpting and the potential (and limitations) of empirical designs that aim to investigate expertise in authentic environments

Data collection for mixed method approaches in social network analysis

The aim of this chapter is to support researchers with their data collection and to provide an overview of how mixed method approaches can be applied in social network analysis. This topic is seldom discussed in the literature, although the selection and combination of formats and instruments for data collection is demanding and complex. This chapter provides an overview how different methods can be combined and integrated to address relational research questions from both qualitative and quantitative perspectives. The pros and cons of combining different methods are illustrated with recent examples of social network research. Finally, a guideline is suggested to support researchers with their data collection in a mixed methods social networks research design.<br/

MAP3D: An explorative approach for automatic mapping of real-world eye-tracking data on a virtual 3D model

Mobile eye tracking helps to investigate real-world settings, in which participants can move freely. This enhances the studies’ ecological validity but poses challenges for the analysis. Often, the 3D stimulus is reduced to a 2D image (reference view) and the fixations are manually mapped to this 2D image. This leads to a loss of information about the three-dimensionality of the stimulus. Using several reference images, from different perspectives, poses new problems, in particular concerning the mapping of fixations in the transition areas between two reference views. A newly developed approach (MAP3D) is presented that enables generating a 3D model and automatic mapping of fixations to this virtual 3D model of the stimulus. This avoids problems with the reduction to a 2D reference image and with transitions between images. The x, y and z coordinates of the fixations are available as a point cloud and as .csv output. First exploratory application and evaluation tests are promising: MAP3D offers innovative ways of post-hoc mapping fixation data on 3D stimuli with open-source software and thus provides cost-efficient new avenues for research

ELECTRON BEAM MELTING OF ALLOY 718 - POWDER RECYCLING AND ITS EFFECT ON DEFECT FORMATION

As for any production process, the performance of additively manufactured components is ultimately dependent on the quality of the feedstock material. Consequently, for critical components, the feedstock needs to be carefully controlled to assure a stable and reliable quality. At the same time, the materials efficiency of additive manufacturing is closely related to powder recycling, which may affect both physical and chemical properties of the powder. This is especially the case for electron beam melting (EBM) where the recycled powder may change significantly from exposure at the high temperature in the build chamber. The aim of this study is to investigate the connection between powder recycling, powder chemistry and presence of defects in EBM processed Alloy 718. For this purpose, recycled powder was studied with reference to its virgin counterpart to detect differences in surface morphology, surface chemical composition as well as bulk chemistry as a consequence of powder recycling. The amount of defects and their distribution in samples produced from virgin and recycled powder was studied by means of image analysis and oxygen measurements. Morphological analysis using scanning electron microscopy was performed to understand their origin and formation mechanism.The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. While the virgin powder is covered by a relatively thin and homogeneous oxide layer, the recycled powder has undergone transformation to a heterogeneous oxide layer rich in thermodynamically stable Al-rich oxide particulates. Significant growth of the Al-rich oxide occurs via selective oxidation of Al at the conditions in the build chamber, including both pick-up of oxygen from the process atmosphere and re-distribution of oxygen from less stable oxide products. The increasing amount of oxide is confirmed by an increase in total oxygen level with progressive recycling.Furthermore, a clear correlation between the powder oxygen level and the amount of oxide inclusions in the EBM fabricated samples was observed. Hot isostatic pressing can be used to reach a near-full densify in samples produced from virgin powder. The samples produced from recycled powder, however, have a higher amount of aluminium-rich oxide inclusions which remain after HIP treatment. A variety of oxide defects was observed, ranging from finely dispersed oxide particulates inside lack of fusion defects to large oxide agglomerates in the bulk metal. Based on their morphology, it is shown that most of them originate from aluminium-rich oxide particulates on the surface of the recycled powder. It is suggested that the quality of EBM processed Alloy 718 is at present dependent on the oxygen level in the powder in general and the surface chemistry of the powder in particular, which needs to be controlled to maintain a low amount of inclusion defects

Does knowledge help with the identification and control of a complex economics system?

Um Ansatzpunkte für instruktionale Förderungsmaßnahmen abzuleiten, wurde versucht, Probleme der Wissensanwendung bei einer komplexen ökonomischen Problemstellung zu identifizieren. Zu diesem Zweck wurde die Steuerung und Identifikation eines komplexen ökonomischen Systems durch Semiexperten (Mandl, Gruber & Renkl, 1992) und Novizen analysiert. Es wurden 18 Semiexperten (Graduierte Studenten der Wirtschaftswissenschaften) und 17 Novizen (Studenten der Psychologie und Pädagogik) untersucht. Sie explorierten und steuerten ein ökonomisches Planspiel, bei dem eine Fabrik zu leiten und deren Gewinn zu maximieren war. Die Ergebnisse zeigten, dass die Semiexperten Schwierigkeiten hatten, ihre im Vergleich zu den Novizen größere Wissensbasis zu nutzen: (1) Sie leiteten die Fabrik weniger erfolgreich als die Novizen, d.h. sie steuerten das System schlechter. (2) Sie konnten eine Explorationsphase nicht nutzen, um differenziertere und kohärentere mentale Modelle über das ökonomische System (Fabrik und entsprechender Markt) zu konstruieren als die Novizen. Erst nach der Systemsteuerung waren die mentalen Modelle der Semiexperten denjenigen der Novizen überlegen. Aus den Befunden werden pädagogische Schlussfolgerungen für die Förderung von Wissenserwerb und Wissensanwendung in komplexen Problemstellungen gezogen. (DIPF/Orig.)Difficulties of knowledge application in solving a complex economical problem were analyzed in order to get hints for the implementation of effective instructional support. For this purpose, intermediate experts and novices had to identify and control a computer-based economics simulation. Intermediate experts were graduate students of Economics (n = 18), novices were students of Psychology and Education (n = 17). The subjects had to explore a simulated jeans factory and, later on, to maximize the profit (system control). The results show that intermediate experts encountered difficulties in using their extensive knowledge base: (1) They were less successful than novices concerning the profit made in the economics computer simulation. (2) They were not able to use the exploration phase in order to construct more sophisticated and coherent mental models of the system than the novices. Not until the system control phase was finished the intermediates outperformed the novices with respect to the quality of their mental models. Conclusions about the educational consequences of the results are drawn. (DIPF/Orig.

Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718

The extent to which powder recycling can be permitted before risking a loss in performance of critical components is a major aspect for the viability of electron beam melting (EBM). In this study, the influence of powder oxidation during multi-cycle EBM processing on the formation of oxide-related defects in Alloy 718 is investigated. The amount of defects and their distribution in samples produced from virgin and re-used powder is studied by means of image analysis and oxygen measurements. Morphological analysis using scanning electron microscopy is performed to understand their origin and formation mechanism. The results indicate a clear correlation between the powder oxygen content and the amount of oxide inclusions present in the investigated samples. The inclusions consist of both molten and unmolten Al-rich oxide which originates from the surface of the recycled powder. Upon interaction with the electron beam, the oxide tends to cluster in the liquid metal and form critical sized defects. Hot isostatic pressing can be successfully used to densify samples produced from virgin powder. However, in the material fabricated from recycled powder, a considerable amount of damage relevant oxide inclusion defects remain after HIP treatment, especially in the contour region. It is suggested that the quality of EBM-processed Alloy 718 is at present dependent on the oxygen level in the powder in general, and on the surface chemistry of the power in particular, which needs to be controlled to maintain a low amount of inclusions