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  • 1.
    Ekengren, Jens
    Avdelningen för maskin- och materialteknik, Karlstads universitet, Karlstad, Sverige.
    Large and rare: An extreme values approach to estimating the distribution of large defects in high-performance steels2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The presence of different types of defects is an important reality for manufacturers and users of engineering materials. Generally, the defects are either considered to be the unwanted products of impurities in the raw materials or to have been introduced during the manufacturing process. In high-quality steel materials, such as tool steel, the defects are usually non-metallic inclusions such as oxides or sulfides.

    Traditional methods for purity control during standard manufacturing practice are usually based on the light optical microscopy scanning of polished surfaces and some statistical evaluation of the results. Yet, as the steel manufacturing process has improved, large defects have become increasingly rare. A major disadvantage of the traditional quality control methods is that the accuracy decreases proportionally to the increased rarity of the largest defects unless large areas are examined.

    However, the use of very high cycle fatigue to 109 cycles has been shown to be a powerful method to locate the largest defects in steel samples. The distribution of the located defects may then be modelled using extreme value statistics.

    This work presents new methods for determining the volume distribution of large defects in high-quality steels, based on ultrasonic fatigue and the Generalized Extreme Value (GEV) distribution. The methods have been developed and verified by extensive experimental testing, including over 400 fatigue test specimens. Further, a method for reducing the distributions into one single ranking variable has been proposed, as well as a way to estimate an ideal endurance strength at different life lengths using the observed defects and endurance limits. The methods can not only be used to discriminate between different materials made by different process routes, but also to differentiate between different batches of the same material.

    It is also shown that all modes of the GEV are to be found in different steel materials, thereby challenging a common assumption that the Gumbel distribution, a special case of the GEV, is the appropriate distribution choice when determining the distribution of defects.

    The new methods have been compared to traditional quality control methods used in common practice (surface scanning using LOM/SEM and ultrasound C-scan), and suggest a greater number of large defects present in the steel than could otherwise be detected.

    List of papers
    1. Relating gigacycle fatigue to other methods in evaluating the inclusion distribution of a H13 tool steel
    Open this publication in new window or tab >>Relating gigacycle fatigue to other methods in evaluating the inclusion distribution of a H13 tool steel
    2007 (English)In: Fourth International Conference on Very High Cycle Fatigue (VHCF-4) / [ed] John E. Allison, J. Wayne Jones, James M. Larsen & Robert O. Ritchie, TMS (The Minerals, Metals & Materials Society) , 2007, p. 45-50Conference paper, Published paper (Refereed)
    Abstract [en]

    Inclusions play a crucial role for the fatigue properties of high strength steel, but to find the largest inclusions by microscopy measurements large areas have to be examined.In this study ultrasonic gigacycle staircase fatigue testing has been used to find large inclusions in an H13 tool steel. The inclusions have been examined in SEM and their size distribution modeled using methods from extreme value statistics. The inclusion distribution obtained from the fatigue crack surfaces is compared to distributions acquired by microscopy study of cross sections as well as ultrasound immersion tank measurements and to the corresponding staircase fatigue data via the Murakami √Area model.It is shown that the fatigue method more effectively finds large inclusions than the other methods. It is also shown that the correlation between predictions of inclusion sizes by the √Area model from stress levels and fatigue initiating inclusions is weak forthis material.

    Place, publisher, year, edition, pages
    TMS (The Minerals, Metals & Materials Society), 2007
    Keywords
    non-metallic inclusion, steel, gigacycle fatigue
    National Category
    Materials Engineering
    Research subject
    Materials Engineering
    Identifiers
    urn:nbn:se:oru:diva-39905 (URN)978-0-87339-704-9 (ISBN)
    Conference
    Fourth International Conference on Very High Cycle Fatigue (VHCF-4)
    Available from: 2009-02-05 Created: 2014-12-19 Last updated: 2017-10-17Bibliographically approved
    2. Detecting large inclusions in steels: evaluating methods
    Open this publication in new window or tab >>Detecting large inclusions in steels: evaluating methods
    2009 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 80, no 11, p. 854-858Article in journal (Refereed) Published
    Abstract [en]

    The distributions of large non-metallic inclusions in two steel grades have been investigated using light optical microscopy, scanning electron microscopy and ultrasonic fatigue testing in the gigacycle range. The different methods have inherently different capabilities for finding inclusions in different size ranges. A measure of the distribution of large inclusions is proposed as the size S at which half of the fatigue specimens are expected to contain at least one inclusion of size S or larger, corresponding to 50% failure probability. Values of S are obtained using the volume distribution estimated by the three methods. Extrapolation from microscopy measurements on surfaces agree with fatigue fractography results regarding density of large inclusions, as measured by the proposed ranking variable S.

    Place, publisher, year, edition, pages
    Weinheim, Germany: Wiley-VCH Verlagsgesellschaft, 2009
    Keywords
    Inclusion distribution, steel, very high cycle fatigue
    National Category
    Materials Engineering
    Research subject
    Materials Engineering
    Identifiers
    urn:nbn:se:oru:diva-39902 (URN)10.2374/SRI08SP156 (DOI)000272027100010 ()2-s2.0-73149120049 (Scopus ID)
    Note

    Vid tidpunkten för författarens licentiatseminarium, förelåg detta delarbete som manuskript. This paper was first published as a manuscript at time for the author's licentiate seminar.

    Available from: 2009-02-05 Created: 2014-12-19 Last updated: 2017-12-05Bibliographically approved
    3. Assessment of strength and inclusions of tool steels in very high cycle fatigue
    Open this publication in new window or tab >>Assessment of strength and inclusions of tool steels in very high cycle fatigue
    2009 (English)In: Proceedings of the 8th International Tooling Conference, Vol 1 / [ed] P. Beiss, C. Broeckmann, S. Franke, B. Keysselitz, Aachen: Verlag Mainz, Wissenschaftsverlag , 2009Conference paper, Published paper (Refereed)
    Abstract [en]

    Fatigue strength is an important material property for many tooling applications, particularly in high performance applications. The research in Very High Cycle Fatigue (VHCF) has demonstrated that the traditional fatigue limit may not be valid for many materials subjected to 107 or more load cycles. Presently, both materials data and mechanism knowledge is missing on VHCF applications, even though many components are run at these life lengths. The fatigue strength is commonly controlled by different defects initiating failure, as in well controlled laboratory experiments may be internal inclusions. In this paper VHCF experimental testing was accomplished by the use of ultrasonic fatigue testing run at 20 kHz allowing long life evaluation within reasonably short test time. Fatigue strength, failure mechanisms and inclusion content were accordingly assessed. Fatigue strength data on H13 tool steel are presented, as well as a statistical approach considering available defect distribution and load distribution in the critically stressed volume, important to both steel supplier and end-user.

    Place, publisher, year, edition, pages
    Aachen: Verlag Mainz, Wissenschaftsverlag, 2009
    Keywords
    very high cycle fatigue, tool steel, inclusion distribution
    National Category
    Metallurgy and Metallic Materials
    Identifiers
    urn:nbn:se:oru:diva-39903 (URN)9783810793058 (ISBN)
    Conference
    8th International Tooling Conference (TOOL 09), Tool steels - deciding factor in worldwide production, Aachen University, Aachen, Germany, June 2-4, 2009
    Available from: 2011-09-12 Created: 2014-12-19 Last updated: 2017-10-17Bibliographically approved
    4. Extreme value distributions of inclusions in six steels
    Open this publication in new window or tab >>Extreme value distributions of inclusions in six steels
    2012 (English)In: Extremes, ISSN 1386-1999, E-ISSN 1572-915X, Vol. 15, no 2, p. 257-265Article in journal (Refereed) Published
    Abstract [en]

    There is a prevailing assumption that the largest inclusions in steel volumes follows mode I of the Generalized Extreme Values (GEV) distribution. In this work, the GEV distributions of non-metallic inclusions in six different high performance steels, of different grades and processing routes, were investigated by means of fractography of inclusions causing failure in ultrasonic fatigue testing to one billion cycles and all three modes of the GEV were found for the different steel grades. Values of the shape parameter ξ of the GEV distribution as high as 0.51 (standard deviation 0.11) were found in one steel grade. Thus, the present results show that the assumption of GEV-I (Gumbel, LEVD) distribution has to be substantiated before being used to estimate the size of the largest inclusions.

    Keywords
    Extreme values; Non-metallic inclusions; Steel
    National Category
    Metallurgy and Metallic Materials
    Research subject
    Mechanical Engineering
    Identifiers
    urn:nbn:se:oru:diva-39899 (URN)10.1007/s10687-011-0139-5 (DOI)000303585200006 ()2-s2.0-84860624529 (Scopus ID)
    Available from: 2011-09-09 Created: 2014-12-19 Last updated: 2017-12-05Bibliographically approved
    5. Influence of life length on estimated defect distribution in a low defect steel material
    Open this publication in new window or tab >>Influence of life length on estimated defect distribution in a low defect steel material
    2011 (English)In: Proceedings of the Fifth International Conference on Very High Cycle Fatigue / [ed] C. Berger and H.-J. Christ, Berlin: Deutscher Verband für Materialforschung und -prüfung , 2011, p. 177-182Conference paper, Published paper (Refereed)
    Abstract [en]

    This paper reports on results of very high cycle fatigue tests on a steel with a relatively low number of defects per unit volume. Two series of newly designed dog-bone specimens, with low stress gradients, were tested at two constant stress levels, aiming at providing fatigue life lengths of 106–108 cycles and 107–109 cycles, respectively. Fatigue-initiating defects on the fracture surfaces were measured and the corresponding size distributions were analysed to investigate the influence of the elevated stress level needed to achieve shorter life lengths. The presented results indicate a difference between the distribution of initiating defects in the long and shorter life length test series. The main conclusion is that fixed stress level fatigue testing to life lengths around 107 cycles may be a suitable method to estimate the content of large defects in low defect steel materials, provided a suitable test specimen geometry with low stress gradients is used.

    Place, publisher, year, edition, pages
    Berlin: Deutscher Verband für Materialforschung und -prüfung, 2011
    Keywords
    Defect volume distribution, Extreme value statistics, Tool steel
    National Category
    Metallurgy and Metallic Materials
    Identifiers
    urn:nbn:se:oru:diva-39900 (URN)978-3-9814516-0-3 (ISBN)
    Conference
    Fifth International Conference on Very High Cycle Fatigue, Berlin, Germany, June 28-30, 2011
    Available from: 2011-09-09 Created: 2014-12-19 Last updated: 2017-10-17Bibliographically approved
    6. Estimating the volume distribution of large defects using Generalized Extreme Values
    Open this publication in new window or tab >>Estimating the volume distribution of large defects using Generalized Extreme Values
    2011 (English)In: Extremes, ISSN 1386-1999, E-ISSN 1572-915XArticle in journal (Other academic) Submitted
    Abstract [en]

    The presence of defects, such as oxides and other non-metallic inclusions, is an important factor in determining the properties of steels. Due to improvements in the manufacturing of high-quality steels, the amount of large defects has decreased and therefore it has become increasingly difficult to accurately determine their distribution using conventional methods. Previously, a method for estimating the distribution of large defects using a conversion from the Gumbel distribution has been presented. However, it has been shown that the Gumbel distribution is not always appropriate for modelling the sizes of the largest defects and that the Generalized Extreme Values (GEV) distribution should be used instead. In this work a more general method for the estimation of the total volume distribution of large defects isproposed, showing how the volume distribution may be calculated from the estimated parameters for the GEV distribution. The new method is applied to the results of a series of specimens made from high-quality tool steel tested in ultrasonic resonance fatigue. Possible methods for obtaining the confidence limits of thevolume distribution are also discussed.

    National Category
    Materials Engineering Metallurgy and Metallic Materials
    Identifiers
    urn:nbn:se:oru:diva-39901 (URN)
    Available from: 2011-09-14 Created: 2014-12-19 Last updated: 2017-12-05Bibliographically approved
  • 2.
    Ekengren, Jens
    et al.
    Karlstads universitet.
    Bergström, Jens
    Karlstads universitet.
    Detecting large inclusions in steels: evaluating methodsManuscript (preprint) (Other academic)
  • 3.
    Ekengren, Jens
    et al.
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Bergström, Jens
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Estimating the volume distribution of large defects using Generalized Extreme Values2011In: Extremes, ISSN 1386-1999, E-ISSN 1572-915XArticle in journal (Other academic)
    Abstract [en]

    The presence of defects, such as oxides and other non-metallic inclusions, is an important factor in determining the properties of steels. Due to improvements in the manufacturing of high-quality steels, the amount of large defects has decreased and therefore it has become increasingly difficult to accurately determine their distribution using conventional methods. Previously, a method for estimating the distribution of large defects using a conversion from the Gumbel distribution has been presented. However, it has been shown that the Gumbel distribution is not always appropriate for modelling the sizes of the largest defects and that the Generalized Extreme Values (GEV) distribution should be used instead. In this work a more general method for the estimation of the total volume distribution of large defects isproposed, showing how the volume distribution may be calculated from the estimated parameters for the GEV distribution. The new method is applied to the results of a series of specimens made from high-quality tool steel tested in ultrasonic resonance fatigue. Possible methods for obtaining the confidence limits of thevolume distribution are also discussed.

  • 4.
    Ekengren, Jens
    et al.
    Karlstads universitet.
    Bergström, Jens
    Karlstads universitet.
    Extreme value distributions of inclusions in six steelsManuscript (preprint) (Other academic)
  • 5.
    Ekengren, Jens
    et al.
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Bergström, Jens
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Extreme value distributions of inclusions in six steels2012In: Extremes, ISSN 1386-1999, E-ISSN 1572-915X, Vol. 15, no 2, p. 257-265Article in journal (Refereed)
    Abstract [en]

    There is a prevailing assumption that the largest inclusions in steel volumes follows mode I of the Generalized Extreme Values (GEV) distribution. In this work, the GEV distributions of non-metallic inclusions in six different high performance steels, of different grades and processing routes, were investigated by means of fractography of inclusions causing failure in ultrasonic fatigue testing to one billion cycles and all three modes of the GEV were found for the different steel grades. Values of the shape parameter ξ of the GEV distribution as high as 0.51 (standard deviation 0.11) were found in one steel grade. Thus, the present results show that the assumption of GEV-I (Gumbel, LEVD) distribution has to be substantiated before being used to estimate the size of the largest inclusions.

  • 6.
    Ekengren, Jens
    et al.
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Bergström, Jens
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Influence of life length on estimated defect distribution in a low defect steel material2011In: Proceedings of the Fifth International Conference on Very High Cycle Fatigue / [ed] C. Berger and H.-J. Christ, Berlin: Deutscher Verband für Materialforschung und -prüfung , 2011, p. 177-182Conference paper (Refereed)
    Abstract [en]

    This paper reports on results of very high cycle fatigue tests on a steel with a relatively low number of defects per unit volume. Two series of newly designed dog-bone specimens, with low stress gradients, were tested at two constant stress levels, aiming at providing fatigue life lengths of 106–108 cycles and 107–109 cycles, respectively. Fatigue-initiating defects on the fracture surfaces were measured and the corresponding size distributions were analysed to investigate the influence of the elevated stress level needed to achieve shorter life lengths. The presented results indicate a difference between the distribution of initiating defects in the long and shorter life length test series. The main conclusion is that fixed stress level fatigue testing to life lengths around 107 cycles may be a suitable method to estimate the content of large defects in low defect steel materials, provided a suitable test specimen geometry with low stress gradients is used.

  • 7.
    Ekengren, Jens
    et al.
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Kazymyrovych, Vitaliy
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Bergström, Jens
    Department of Mechanical and Materials Engineering, Karlstad University, Karlstad, Sweden.
    Assessment of strength and inclusions of tool steels in very high cycle fatigue2009In: Proceedings of the 8th International Tooling Conference, Vol 1 / [ed] P. Beiss, C. Broeckmann, S. Franke, B. Keysselitz, Aachen: Verlag Mainz, Wissenschaftsverlag , 2009Conference paper (Refereed)
    Abstract [en]

    Fatigue strength is an important material property for many tooling applications, particularly in high performance applications. The research in Very High Cycle Fatigue (VHCF) has demonstrated that the traditional fatigue limit may not be valid for many materials subjected to 107 or more load cycles. Presently, both materials data and mechanism knowledge is missing on VHCF applications, even though many components are run at these life lengths. The fatigue strength is commonly controlled by different defects initiating failure, as in well controlled laboratory experiments may be internal inclusions. In this paper VHCF experimental testing was accomplished by the use of ultrasonic fatigue testing run at 20 kHz allowing long life evaluation within reasonably short test time. Fatigue strength, failure mechanisms and inclusion content were accordingly assessed. Fatigue strength data on H13 tool steel are presented, as well as a statistical approach considering available defect distribution and load distribution in the critically stressed volume, important to both steel supplier and end-user.

  • 8.
    El-Amine, Karim
    et al.
    Örebro University, School of Science and Technology. Suzuki Garphyttan AB, Garphyttan, Sweden.
    Larsson, Joakim
    Örebro University, School of Science and Technology. Department of Mechanical Engineering.
    Pejryd, Lars
    Örebro University, School of Science and Technology. Department of Mechanical Engineering.
    Experimental comparison of roller die and conventional wire drawing2018In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 257, p. 7-14Article in journal (Refereed)
    Abstract [en]

    In this work, the application of roller dies as an alternative to conventional dies in the wire drawing process – claimed in earlier works to offer many advantageous improvements – is evaluated. To this end, experimental procedures using a single stage drawing machine were conducted, drawing low and medium carbon steel wires using both roller dies and conventional dies. The two reduction processes were compared with respect to drawing force and the resulting wire temperature. Also the mechanical properties of the drawn wires were investigated. The obtained results have not shown any large differences between the two processes, and the main improvement using the roller die method was a better wire surface. This advantage was counteracted with downsides including higher wire temperature and lower strain at fracture.

    Moreover, a formula to theoretically calculate drawing force for the conventional drawing process was modified and used to calculate drawing force for the two reduction steps in the roller die cassette. The results showed that the proportion of inhomogeneous deformation was much higher in the rolling process as compared to the conventional process.

    In view of the outcomes in this work, the alternative of drawing wire through roller dies was not considered to be offering more improvements with respect to the conventional drawing method.

  • 9. Hadad, Rabie
    et al.
    Lindgren, Patrik
    Jonsson, Mikael
    Lundberg, Sven-Erik
    Örebro University, Department of Technology.
    A Theoretical / Empirical study of void closure in heavy plate hot rolling of tool steel2009In: Metallurgy and New Materials Researches / Cercetari metalurgice, si de noi materiale, ISSN 1221-5503, Vol. 17, no 1, p. 9-26Article in journal (Other academic)
    Abstract [en]

    By the introduction of TOOLOX®, SSAB in Oxelösund has turned into a new market segment, including heavier sizes and dramatically increased requirements on inner quality of the plate. To be able to determine the minimum reduction from slab to finished plate with acceptable inner quality, a study was made, to evaluate the minimum reduction for a given requirement of inner quality. By experimental rolling in the heavy plate mill in Oxelösund it has been proven that more than 40 % total reduction is necessary for total closure of voids under the geometrical deformation conditions in that mill. The total reduction is more important than the geometrical rolling conditions, and heavy pass reductions are favourable for void closure. The closure of internal defects can be improved by cooling the surface of the slabs.

  • 10.
    Huang, Shuo
    et al.
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Holmstrom, Erik
    Sandvik Coromant R&D, Stockholm, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden.
    Vitos, Levente
    Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden; Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Uppsala, Sweden; Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Budapest, Hungary.
    Mapping the magnetic transition temperatures for medium- and high-entropy alloys2018In: Intermetallics (Barking), ISSN 0966-9795, E-ISSN 1879-0216, Vol. 95, p. 80-84Article in journal (Refereed)
    Abstract [en]

    Tailorable magnetic state near room temperature is very promising for several technological, including magnetocaloric applications. Here using first-principle alloy theory, we determine the Curie temperature (T-C) of a number of equiatomic medium- and high-entropy alloys with solid solution phases. All calculations are performed at the computed lattice parameters, which are in line with the available experimental data. Theory predicts a large crystal structure dependence of T-C, which explains the experimental observations under specified conditions. The sensitivity of the magnetic state to the crystal lattice is reflected by the magnetic exchange interactions entering the Heisenberg Hamiltonian. The analysis of the effect of composition on T-C allows researchers to explore chemistry-dependent trends and design new multi-component alloys with pre-assigned magnetic properties.

  • 11.
    Jarl, Magnus
    et al.
    Örebro University, Department of Technology.
    Axelsson, Fredrik
    Wear of dies, measured and calculated by the process control system in a drawing machine2009In: Wire Journal International, ISSN 0277-4275, Vol. 42, no 10, p. 80-83Article in journal (Refereed)
  • 12.
    Jarl, Magnus
    et al.
    Örebro University, Department of Technology.
    Överstam, Henrik
    Örebro University, Department of Technology.
    Simulation of temperatures in the wiredrawing process2008In: Wire Journal International, ISSN 0277-4275, Vol. 41, no 2, p. 150-155Article in journal (Refereed)
  • 13.
    Pervishko, Anastasiia A.
    et al.
    ITMO University, Saint Petersburg, Russia.
    Baglai, Mikhail I.
    ITMO University, Saint Petersburg, Russia; Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Yudin, Dmitry
    ITMO University, Saint Petersburg, Russia.
    Another view on Gilbert damping in two-dimensional ferromagnets2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 17148Article in journal (Refereed)
    Abstract [en]

    A keen interest towards technological implications of spin-orbit driven magnetization dynamics requests a proper theoretical description, especially in the context of a microscopic framework, to be developed. Indeed, magnetization dynamics is so far approached within Landau-Lifshitz-Gilbert equation which characterizes torques on magnetization on purely phenomenological grounds. Particularly, spin-orbit coupling does not respect spin conservation, leading thus to angular momentum transfer to lattice and damping as a result. This mechanism is accounted by the Gilbert damping torque which describes relaxation of the magnetization to equilibrium. In this study we work out a microscopic Kubo-Streda formula for the components of the Gilbert damping tensor and apply the elaborated formalism to a two-dimensional Rashba ferromagnet in the weak disorder limit. We show that an exact analytical expression corresponding to the Gilbert damping parameter manifests linear dependence on the scattering rate and retains the constant value up to room temperature when no vibrational degrees of freedom are present in the system. We argue that the methodology developed in this paper can be safely applied to bilayers made of non- and ferromagnetic metals, e.g., CoPt.

  • 14.
    Sadeghimeresht, Esmaeil
    et al.
    Department of Engineering Science, University West, Trollhättan, Sweden.
    Karimi, Paria
    Department of Engineering Science, University West, Trollhättan, Sweden.
    Zhang, Pimin
    Department of Management and Engineering, Linköping University, Linköping, Sweden.
    Peng, Ru
    Department of Management and Engineering, Linköping University, Linköping, Sweden.
    Andersson, Joel
    Department of Engineering Science, University West, Trollhättan, Sweden.
    Pejryd, Lars
    Örebro University, School of Science and Technology.
    Joshi, Shrikant
    Department of Engineering Science, University West, Trollhättan, Sweden.
    Isothermal Oxidation Behavior of EBM-Additive Manufactured Alloy 7182018In: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications / [ed] Ott, E.; Liu, X.; Andersson, J.; Bi, Z.; Bockenstedt, K.; Dempster, I.; Groh, J.; Heck, K.; Jablonski, P.; Kaplan, M:, Nagahama, D.; Sudbrack, C, Springer, 2018, p. 219-240Conference paper (Refereed)
    Abstract [en]

    Oxidation of Alloy 718 manufactured by electron beam melting (EBM) process has been undertaken in ambient air at 650, 700, and 800 degrees C for up to 168 h. At 800 degrees C, a continuous external chromia oxide enriched in (Cr, Ti, Mn, Ni) and an internal oxide that was branched structure of alumina formed, whereas at 650 and 700 degrees C, a continuous, thin and protective chromia layer was detected. The oxidation kinetics of the exposed EBM Alloy 718 followed the parabolic rate law with an effective activation energy of similar to 248 +/- 22 kJ/mol in good agreement with values in the literature for conventionally processed chromia-forming Ni-based superalloys. The oxide scale formed on the surface perpendicular to the build direction was slightly thicker, and more adherent compared to the scale formed on the surface along the build direction, attributed to the varied grain texture in the two directions of the EBM-manufactured specimens. The increased oxygen diffusion and high Cr depletion found on the surface along the build direction were attributed to the fine grains and formation of vacancies/voids along this grain orientation.

  • 15.
    Surreddi, K. B.
    et al.
    Materials Science, Dalarna University, Falun, Sweden.
    Oikonomou, C.
    Uddeholms AB, Hagfors, Sweden.
    Karlsson, Patrik
    Örebro University, School of Science and Technology.
    Olsson, M.
    Materials Science, Dalarna University, Falun, Sweden.
    Pejryd, Lars
    Örebro University, School of Science and Technology.
    In-situ micro-tensile testing of additive manufactured maraging steels in the SEM: Influence of build orientation, thickness and roughness on the resulting mechanical properties2018In: La Metallurgia Italiana, ISSN 0026-0843, no 3, p. 27-33Article in journal (Refereed)
    Abstract [en]

    Selective laser melting (SLM) is frequently used additive manufacturing technique capable of producing various complex parts including thin-wall sections. However the surface roughness is a limiting factor in thin sections produced by SLM process when strength is the main criterion. In this study, the influence of build orientation, thickness and roughness on the resulting mechanical properties of as-built test samples was investigated. Various thin sheets of EN 1.2709 maraging steel built in horizontal and vertical orientations produced by SLM were investigated using in-situ micro-tensile testing in a scanning electron microscope. The mechanical strength and deformation mechanisms were analyzed and explained based on thickness and build orientation. Increased ductility was observed in thicker samples as well as in the horizontal build samples. The results illustrate the potential of the in-situ test technique and aspects important to consider in design guidelines for thin AM structures.

  • 16.
    Thonig, Danny
    et al.
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Kvashnin, Yaroslav
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Eriksson, Olle
    Örebro University, School of Science and Technology. Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Pereiro, Manuel
    Department of Physics and Astronomy, Materials Theory, Uppsala University, Uppsala, Sweden.
    Nonlocal Gilbert damping tensor within the torque-torque correlation model2018In: Physical Review Materials, ISSN 2475-9953, Vol. 2, no 1, article id 013801Article in journal (Refereed)
    Abstract [en]

    An essential property of magnetic devices is the relaxation rate in magnetic switching, which depends strongly on the damping in the magnetization dynamics. It was recently measured that damping depends on the magnetic texture and, consequently, is a nonlocal quantity. The damping enters the Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping parameter a, which does not, in a straightforward formulation, account for nonlocality. Efforts were spent recently to obtain Gilbert damping from first principles for magnons of wave vector q. However, to the best of our knowledge, there is no report about real-space nonlocal Gilbert damping aij. Here, a torque-torque correlation model based on a tight-binding approach is applied to the bulk elemental itinerant magnets and it predicts significant off-site Gilbert damping contributions, which could be also negative. Supported by atomistic magnetization dynamics simulations, we reveal the importance of the nonlocal Gilbert damping in atomistic magnetization dynamics. This study gives a deeper understanding of the dynamics of the magnetic moments and dissipation processes in real magnetic materials. Ways of manipulating nonlocal damping are explored, either by temperature, materials doping, or strain.

  • 17.
    Överstam, Henrik
    et al.
    Örebro University, Department of Technology.
    Lundberg, Sven-Erik
    Örebro University, Department of Technology.
    A new approach to roll force and torque modelling in wire rod and bar mills2008In: Der Kalibreur, ISSN 0022-796X, Vol. 69, p. 61-72Article in journal (Refereed)
1 - 17 of 17
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