| 1. |
- Larsson, Erik, 1983, et al.
(författare)
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A Comparative Study of the Initial Corrosion of KCl and PbCl2 on a Low-Alloyed Steel
- 2017
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 87:5-6, s. 779-787
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Tidskriftsartikel (refereegranskat)abstract
- Heat exchange surfaces in biomass- and waste-fired boilers are exposed to corrosive species due to the considerable amounts of alkali chlorides and PbCl2 which are released during combustion. The corrosivity of alkali chlorides toward superheater alloys exposed at high temperature has been studied extensively. However, at lower material temperatures, i.e., at waterwall conditions, considerably less research has been performed. In order to investigate the effect of small amounts of KCl and PbCl2 during the initial stages of the corrosion attack, a Fe–2.25Cr–1Mo steel was exposed for 24 h in an atmospheres consisting of O2 + H2O + N2 at 400 °C. Both KCl and PbCl2 resulted in an increased corrosion rate compared to the reference. The aim of the present paper is to investigate the influence of KCl and PbCl2 on the initial oxidation of a Fe–2.25Cr–1Mo steel. The work involves a detailed microstructural investigation as well as thermodynamic equilibrium calculations.
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| 2. |
- Malakizadi, Amir, 1983, et al.
(författare)
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An FEM-based approach for tool wear estimation in machining
- 2016
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Ingår i: Wear. - : Elsevier BV. - 0043-1648. ; 368, s. 10-24
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Tidskriftsartikel (refereegranskat)abstract
- This study presents an FEM-based approach to predict the rate of flank wear evolution for uncoated cemented carbide tools in longitudinal turning processes. This novel approach combines the concept of experimental design and Response Surface Methodology (RSM) with Finite Element (FE) modelling of the cutting process, which allows for a fairly accurate tool wear prediction with a significantly lower computational cost compared to other available numerical methods.In the current approach, a series of three dimensional (3D) FE simulations were initially performed for different combinations of cutting data and tool flank geometries. The obtained results were used to establish the quadratic relation between the input variables and the responses required for tool wear prediction, such as interface temperature. Later, the flank wear rate equation was developed based on the relation between the volume loss due to wear and the dimensions of the worn tools. The results of the FE simulations were finally integrated with the established wear rate equation to estimate the flank wear evolution. The credibility of the presented approach was then assessed through estimation of the flank wear evolution rate for a wide range of cutting conditions. The predicted flank wear rates showed a good agreement with experimental measurements in most cases. The reasons for minor deviations from the experimental results were finally outlined.
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| 3. |
- Gruber, Hans, 1983, et al.
(författare)
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Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder
- 2019
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 50:9, s. 4410-4422
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Tidskriftsartikel (refereegranskat)abstract
- Process-induced degradation of the powder feedstock in additive manufacturing may have a negative influence on the final properties of built components. Consequently, it may lower the cost-effectiveness of powder bed additive manufacturing, which relies on recycling of the nonconsumed powder. This is especially the case for production of high-performance aero engine components where high material and process reliability is required. This study comprises a detailed investigation on the degradation of Alloy 718 powder during multicycle electron beam melting (EBM). The surface-sensitive analysis methods, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), were combined with scanning electron microscopy (SEM) to depict the differences in surface morphology, and surface composition of powder samples exposed to varying numbers of re-use cycles. The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. The virgin powder is covered mainly by a relatively thin and homogeneous oxide layer. The re-used powder, however, has undergone transformation to a heterogeneous oxide layer, rich in thermodynamically stable Al-rich oxide particulates, which started already during the first build cycle. Significant growth of the Al-rich oxide occurs via selective oxidation of Al under the conditions in the build chamber, including both pick-up of oxygen from the process atmosphere and redistribution of initial surface-bound oxygen from less-stable products like Ni-oxide and/or hydroxide.
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| 4. |
- Gruber, Hans, 1983, et al.
(författare)
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Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718
- 2020
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Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 51:5, s. 2430-2443
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Gruber, Hans, 1983, et al.
(författare)
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Effect of Powder Recycling on the Fracture Behavior of Electron Beam Melted Alloy 718
- 2018
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Ingår i: Powder Metallurgy Progress. - : de Gruyter. - 1335-8987 .- 1339-4533. ; 18:1, s. 40-48
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Understanding the effect of powder feedstock alterations during multicycle additive manufacturing on the quality of built components is crucial to meet the requirements on critical parts for aerospace engine applications. In this study, powder recycling of Alloy 718 during electron beam melting was studied to understand its influence on fracture behavior of Charpy impact test bars. High resolution scanning electron microscopy was employed for fracture surface analysis on test bars produced from virgin and recycled powder. For all investigated samples, an intergranular type of fracture, initiated by non-metallic phases and bonding defects, was typically observed in the regions close to or within the contour zone. The fracture mode in the bulk of the samples was mainly moderately ductile dimple fracture. The results show a clear correlation between powder degradation during multi-cycle powder reuse and the amount of damage relevant defects observed on the fracture surfaces. In particular, samples produced from recycled powder show a significant amount of aluminum-rich oxide defects, originating from aluminum-rich oxide particulates on the surface of the recycled powder. © 2018 H. Gruber et al., published by Sciendo.
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| 6. |
- Gruber, Hans, 1983
(författare)
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ELECTRON BEAM MELTING OF ALLOY 718 - POWDER RECYCLING AND ITS EFFECT ON DEFECT FORMATION
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Gruber, Hans, 1983
(författare)
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Powder bed fusion processing of Ni-base superalloys - Defect formation and its mitigation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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°C, leading to a significant reduction of the impact toughness. Formation of the secondary phase is suppressed by lowering the HIP temperature to 1120°C. Results from microscopy and Charpy impact testing suggest that significant healing of the microcracks is accomplished when applying the developed HIP strategy.
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| 8. |
- Gruber, Hans, 1983, et al.
(författare)
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The effect of boron and zirconium on the microcracking susceptibility of IN-738LC derivatives in laser powder bed fusion
- 2022
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Ingår i: Applied Surface Science. - : Elsevier BV. - 0169-4332. ; 573
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Tidskriftsartikel (refereegranskat)abstract
- The effect of boron (<0.01 to 0.03 wt%) and zirconium (<0.01 to 0.07 wt%) on the microcracking susceptibility of the γ’-strengthened Ni-base superalloy IN-738LC during laser powder bed fusion (LPBF) was studied using custom designed powder grades. It was found that both elements have a strong effect on the microcracking susceptibility, the microcracks are located at high angle grain boundaries based on EBSD measurements and crack density increases with the content of both elements. High crack density in the material with high boron and zirconium content corresponds to a large fraction of intergranular decohesion facets exhibiting a dendritic morphology on the fracture surface, typical for solidification cracking. Investigation of the fracture surface chemistry by X-ray photoelectron spectroscopy (XPS) indicates that considerable amounts of B and Zr are present in oxide state. Auger electron spectroscopy (AES) confirms that both elements are segregated to the intergranular decohesion facets on the fracture surface. Thin layers of B- and Zr-containing oxide on the microcrack surfaces were indicated by atom probe tomography (APT) as well. Hence, it is suggested that the cracking susceptibility of the studied alloying system is caused by formation of B- and Zr-containing oxide at high-angle grain boundaries during solidification.
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| 9. |
- Hryha, Eduard, 1980, et al.
(författare)
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Surface Oxide State on Metal Powder and its Changes during Additive Manufacturing: an Overview
- 2018
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Ingår i: Metallurgia Italiana. - 0026-0843. ; 110:3, s. 34-39
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Quality and usefulness of the powder for additive manufacturing (AM) are strongly determined by the surface composition of the powder. In addition, taking into account harsh conditions during AM process, significant changes in the powder surface chemical composition are taking place, limiting powder recyclability. Hence, knowledge concerning amount of oxides, their composition and spatial distribution on the powder surface determines further powder recycling. This communication summarizes possibilities of qualitative and quantitative analysis of powder surface chemistry by surface-sensitive chemical analyses using XPS and HR SEM coupled with EDX. The effect of alloy composition, AM process applied and powder handling on the surface composition of the powder are addressed. Results indicate significant enrichment in the thermodynamically stable surface oxides in case of high-alloyed powder for both, EBM and LS processes. A generic model for the oxide distribution, depending on the alloy composition and powder surface degradation during AM manufacturing, is proposed.
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| 10. |
- Karimi Neghlani, Paria, 1986-, et al.
(författare)
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Influence of build layout and orientation on microstructural characteristics of electron beam melted Alloy 718
- 2018
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Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer Science and Business Media LLC. - 0268-3768 .- 1433-3015. ; 99:S1, s. 2903-2913
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Tidskriftsartikel (refereegranskat)abstract
- Effects of build layout and orientation consisting of (a) height from the build plate (Z-axis), (b) distance between samples, and (c) location in the build plate (X-Y plane) on porosity, NbC fraction, and hardness in electron beam melted (EBM) Alloy 718 were studied. The as-built samples predominantly showed columnar structure with strong Ë‚001˃ crystallographic orientation parallel to the build direction, as well as NbC and ÎŽ-phase in inter-dendrites and grain boundaries. These microstructural characteristics were correlated with the thermal history, specifically cooling rate, resulted from the build layout and orientation parameters. The hardness and NbC fraction of the samples increased around 6% and 116%, respectively, as the height increased from 2 to 45 mm. Moreover, by increasing the height, formation of ÎŽ-phase was also enhanced associated with lower cooling rate in the samples built with a greater distance from the build plate. However, the porosity fraction was unaffected. Increasing the sample gap from 2 to 10 mm did not change the NbC fraction and hardness; however, the porosity fraction increased by 94%. The sample location in the build chamber influenced the porosity fraction, particularly in interior and exterior areas of the build plate. The hardness and NbC fraction were not dependent on the sample location in the build chamber. © 2018, The Author(s).
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