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1.
  • Granqvist, Pehr, et al. (författare)
  • Disorganized attachment in infancy : a review of the phenomenon and its implications for clinicians and policy-makers
  • 2017
  • Ingår i: Attachment & Human Development. - : Informa UK Limited. - 1461-6734 .- 1469-2988. ; 19:6, s. 534-558
  • Forskningsöversikt (refereegranskat)abstract
    • Disorganized/Disoriented (D) attachment has seen widespread interest from policy makers, practitioners, and clinicians in recent years. However, some of this interest seems to have been based on some false assumptions that (1) attachment measures can be used as definitive assessments of the individual in forensic/child protection settings and that disorganized attachment (2) reliably indicates child maltreatment, (3) is a strong predictor of pathology, and (4) represents a fixed or static trait of the child, impervious to development or help. This paper summarizes the evidence showing that these four assumptions are false and misleading. The paper reviews what is known about disorganized infant attachment and clarifies the implications of the classification for clinical and welfare practice with children. In particular, the difference between disorganized attachment and attachment disorder is examined, and a strong case is made for the value of attachment theory for supportive work with families and for the development and evaluation of evidence-based caregiving interventions.
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2.
  • Maimaitiyili, Tuerdi, et al. (författare)
  • Residual Lattice Strain and Phase Distribution in Ti-6Al-4V Produced by Electron Beam Melting
  • 2019
  • Ingår i: Materials. - : MDPI. - 1996-1944. ; 12:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Residual stress/strain and microstructure used in additively manufactured material are strongly dependent on process parameter combination. With the aim to better understand and correlate process parameters used in electron beam melting (EBM) of Ti-6Al-4V with resulting phase distributions and residual stress/strains, extensive experimental work has been performed. A large number of polycrystalline Ti-6Al-4V specimens were produced with different optimized EBM process parameter combinations. These specimens were post-sequentially studied by using high-energy X-ray and neutron diffraction. In addition, visible light microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) studies were performed and linked to the other findings. Results show that the influence of scan speed and offset focus on resulting residual strain in a fully dense sample was not significant. In contrast to some previous literature, a uniform α- and β-Ti phase distribution was found in all investigated specimens. Furthermore, no strong strain variations along the build direction with respect to the deposition were found. The magnitude of strain in α and β phase show some variations both in the build plane and along the build direction, which seemed to correlate with the size of the primary β grains. However, no relation was found between measured residual strains in α and β phase. Large primary β grains and texture appear to have a strong effect on X-ray based stress results with relatively small beam size, therefore it is suggested to use a large beam for representative bulk measurements and also to consider the prior β grain size in experimental planning, as well as for mathematical modelling.
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3.
  • Adegoke, Olutayo, 1979-, et al. (författare)
  • Influence of laser powder bed fusion process parameters on the microstructure and cracking susceptibility of nickel-based superalloy Alloy 247LC
  • 2022
  • Ingår i: Results in Materials. - : Elsevier. - 2590-048X. ; 13
  • Tidskriftsartikel (refereegranskat)abstract
    • Microstructures of material conditions of nickel-based superalloy Alloy 247LC fabricated using laser powder bed fusion (L-PBF) were investigated. Experiments designed in a prior study revealed the L-PBF process parameters for which the material conditions displayed a reduced susceptibility to cracking. Certain process parameters produced material conditions with an increased susceptibility to cracking. In this study, the material conditions were investigated in detail to reveal their microstructure and to determine the cause of cracking. The reason for the transition between a reduced to an increased susceptibility to cracking was examined. The results revealed solidification cracking occurred at high-angle grain boundaries. Solidification cracking may have been promoted at high-angle grain boundaries because of the undercooling contribution of the grain boundary energy. Furthermore, Si segregation was observed in the cracks. Thus, the presence of Si most likely promoted solidification cracking. It was observed that a high crack density, which occurred in the high energy density material condition, was associated with a large average grain size. The fact that certain combination of process parameters produced microstructures with a low susceptibility to cracking, indicates that reliable Alloy 247LC material may be printed using L-PBF by employing improved process parameters. © 2022
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4.
  • Adegoke, Olutayo, 1979-, et al. (författare)
  • Influence of laser powder bed fusion process parameters on the microstructure of solution heat-treated nickel-based superalloy Alloy 247LC
  • 2022
  • Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 183
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, Alloy 247LC samples were built with different laser powder bed fusion (L-PBF) process parameters. The samples were then subjected to solution heat treatment at 1260 °C for 2 h. The grain size of all the samples increased significantly after the heat treatment. The relationship between the process parameters and grain size of the samples was investigated by performing a design of experiment analysis. The results indicated that the laser power was the most significant process parameter that influenced the grain height and aspect ratio. The laser power also significantly influenced the grain width. The as-built and as-built + heat-treated samples with high, medium, and low energy densities were characterized using a field emission gun scanning electron microscope equipped with an electron backscatter diffraction detector. The micrographs revealed that the cells present in the as-built samples disappeared after the heat treatment. Isolated cases of twinning were observed in the grains of the as-built + heat-treated samples. The disappearance of cells, increase in the grain size, and appearance of twins suggested that recrystallization occurred in the alloy after the heat treatment. The occurrence of recrystallization was confirmed by analyzing the grain orientation spread of the alloy, which was lower and more predominantly <1° in the as-built + heat-treated conditions than in the as-built conditions. The microhardness of the as-built + heat-treated samples were high which was plausible because γ’ precipitates were observed in the samples. However, the L-PBF process parameters had a very low correlation with the microhardness of the as-built + heat-treated samples.
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5.
  • Adegoke, Olutayo, et al. (författare)
  • Influence of laser powder bed fusion process parameters on voids, cracks, and microhardness of nickel-based superalloy alloy 247LC
  • 2020
  • Ingår i: Materials. - 1996-1944. ; 13:17
  • Tidskriftsartikel (refereegranskat)abstract
    • The manufacturing of parts from nickel-based superalloy Alloy 247LC by laser powder bed fusion (L-PBF) is challenging, primarily owing to the alloy’s susceptibility to cracks. Apart from the cracks, voids created during the L-PBF process should also be minimized to produce dense parts. In this study, samples of Alloy 247LC were manufactured by L-PBF, several of which could be produced with voids and crack density close to zero. A statistical design of experiments was used to evaluate the influence of the process parameters, namely laser power, scanning speed, and hatch distance (inherent to the volumetric energy density) on void formation, crack density, and microhardness of the samples. The window of process parameters, in which minimum voids and/or cracks were present, was predicted. It was shown that the void content increased steeply at a volumetric energy density threshold below 81 J/mm3. The crack density, on the other hand, increased steeply at a volumetric energy density threshold above 163 J/mm3. The microhardness displayed a relatively low value in three samples which displayed the lowest volumetric energy density and highest void content. It was also observed that two samples, which displayed the highest volumetric energy density and crack density, demonstrated a relatively high microhardness; which could be a vital evidence in future investigations to determine the fundamental mechanism of cracking. The laser power was concluded to be the strongest and statistically most significant process parameter that influenced void formation and microhardness. The interaction of laser power and hatch distance was the strongest and most significant factor that influenced the crack density. © 2020 by the authors.
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6.
  • Adegoke, Olutayo, et al. (författare)
  • Laser beam powder bed fusion and post processing of alloy 247LC
  • 2019
  • Ingår i: MS and T 2019 - Materials Science and Technology. - : Materials Science and Technology. - 9780873397704 ; , s. 27-34
  • Konferensbidrag (refereegranskat)abstract
    • Alloy 247LC is sensitive to cracking during laser beam powder bed fusion (PBF-LB) manufacturing. Post processing is thus required to close cracks and achieve desired properties. In this study, samples of Alloy 247LC were manufactured by PBF-LB and subsequently post processed by hot isostatic pressing (HIP), HIP + solution and ageing heat treatments. The microstructure was characterized. Results showed cracks in the as-built condition. Cracks were not detected after HIP. Bright microconstituents were observed in the region between the cells, mainly, because of the partitioning of Hf and Ta into the intercellular region, where they presumably form carbides. What is assumed to be oxides were prominent in the microstructure. Thermodynamic calculations showed rapid formation of ?’ precipitates in the alloy, due to the high total concentration of Al and Ta and this was linked to the high hardness values in the as-built condition. © 2019 MS&T19®
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7.
  • Adegoke, Olutayo (författare)
  • Processability of Laser Powder Bed Fusion of Alloy 247LC : Influence of process parameters on microstructure and defects
  • 2020
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis is about laser powder bed fusion (L-PBF) of the nickel-based superalloy: Alloy 247LC. Alloy 247LC is used mainly in gas turbine blades and processing the blades with L-PBF confers performance advantage over the blades manufactured with conventional methods. This is mainly because L-PBF is more suitable, than conventional methods, for manufacturing the complex cooling holes in the blades. The research was motivated by the need for academia and industry to gain knowledge about the processability of the alloy using L-PBF. The knowledge is essential in order to eventually solve the problem of cracking which is a major problem when manufacturing the alloy. In addition, dense parts with low void content should be manufactured and the parts should meet the required performance. Thus, the thesis answered some of the important questions related to process parameter-microstructure-defect relationships.The thesis presented an introduction in chapter 1. A literature review was made in chapter 2 to 4. In chapter 2, the topic of additive manufacturing was introduced followed by an overview of laser powder bed fusion. Chapter 3 focused on superalloys. Here, a review was made from the broader perspective of superalloys but was eventually narrowed down to the characteristics of nickelbased superalloys and finally Alloy 247LC. Chapter 4 reviewed the main research on L-PBF of Alloy 247LC. The methodology applied in the thesis was discussed in chapter 5. The thesis applied statistical design of experiments to show the influence of process parameters on the defects and microstructure, so a detail description of the method was warranted. This was given at the beginning of chapter 5 and followed by the description of the L-PBF manufacturing and the characterization methods. The main results and discussions, in chapter 6, included a preliminary investigation on how the process parameters influenced the amount of discontinuity in single track samples. This was followed by the results and discussions on the investigation of voids, cracks and microhardness in cube samples (detail presentation was given in the attached paper B). Finally, the thesis presented results of the microstructure obtainable in L-PBF manufactured Alloy 247LC. The initial results of the microstructure investigation were presented in paper A.
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8.
  • Adegoke, Olutayo, 1979- (författare)
  • Processability of Laser Powder Bed Fusion of Alloy 247LC-Influence of process parameters on microstructure and defects
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis is about laser powder bed fusion (L-PBF) of the nickel-basedsuperalloy Alloy 247LC. Alloy 247LC is mainly used in gas turbine blades and processing the blades with L-PBF may confer performance advantage over the blades manufactured with conventional methods. This is mainly because L-PBFis more suitable, than conventional methods, for manufacturing the complex cooling holes in the blades. The research was motivated by the need for academia and industry to gain knowledge about the processability of the alloy using L-PBF. The knowledge is essential to eventually solve the problem of cracking encountered when processing the alloy. In addition, dense parts with low void content should be processed and the microstructure and properties should meett he required performance. Heat-treatment is usually performed to acquire final properties, so it is also of interest to study this aspect. Thus, the thesis answered some of the important questions related to process parameter-microstructure- property relationships.
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9.
  • Adegoke, Olutayo, et al. (författare)
  • Review of laser powder bed fusion of gamma-prime-strengthened nickel-based superalloys
  • 2020
  • Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 10:8
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper reviews state of the art laser powder bed fusion (L-PBF) manufacturing of γ′ nickel-based superalloys. L-PBF resembles welding; therefore, weld-cracking mechanisms, such as solidification, liquation, strain age, and ductility-dip cracking, may occur during L-PBF manufacturing. Spherical pores and lack-of-fusion voids are other defects that may occur in γ′-strengthened nickel-based superalloys manufactured with L-PBF. There is a correlation between defect formation and the process parameters used in the L-PBF process. Prerequisites for solidification cracking include nonequilibrium solidification due to segregating elements, the presence of liquid film between cells, a wide critical temperature range, and the presence of thermal or residual stress. These prerequisites are present in L-PBF processes. The phases found in L-PBF-manufactured γ′-strengthened superalloys closely resemble those of the equivalent cast materials, where γ, γ′, and γ/γ′ eutectic and carbides are typically present in the microstructure. Additionally, the sizes of the γ′ particles are small in as-built L-PBF materials because of the high cooling rate. Furthermore, the creep performance of L-PBF-manufactured materials is inferior to that of cast material because of the presence of defects and the small grain size in the L-PBF materials; however, some vertically built L-PBF materials have demonstrated creep properties that are close to those of cast materials.© 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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10.
  • Adegoke, Olutayo, et al. (författare)
  • Scanning electron microscopy and atom probe tomography characterization of laser powder bed fusion precipitation strengthening nickel-based superalloy
  • 2023
  • Ingår i: Micron. - 0968-4328 .- 1878-4291. ; 171
  • Tidskriftsartikel (refereegranskat)abstract
    • Atom probe tomography (APT) was utilized to supplement scanning electron microscopy (SEM) characterizationof a precipitation strengthening nickel-based superalloy, Alloy 247LC, processed by laser powder bed fusion (LPBF). It was observed that the material in the as-built condition had a relatively high strength. Using both SEMand APT, it was concluded that the high strength was not attributed to the typical precipitation strengtheningeffect of γ’. In the absence of γ’ it could be reasonably inferred that the numerous black dots observed in thecells/grains with SEM were dislocations and as such should be contributing significantly to the strengthening.Thus, the current investigation demonstrated that relatively high strengthening can be attained in L-PBF even inthe absence of precipitated γ’. Even though γ’ was not precipitated, the APT analysis displayed a nanometer scalepartitioning of Cr that could be contributing to the strengthening. After heat-treatment, γ’ was precipitated and itdemonstrated the expected high strengthening behavior. Al, Ta and Ti partitioned to γ’. The strong partitioningof Ta in γ’ is indicative that the element, together with Al and Ti, was contributing to the strain-age crackingoccurring during heat-treatment. Cr, Mo and Co partitioned to the matrix γ phase. Hf, Ta, Ti and W were found inthe carbides corroborating previous reports that they are MC. 
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11.
  • Andersson, Joel, 1981-, et al. (författare)
  • Welding of special alloys
  • 2023
  • Ingår i: Welding of Metallic Materials. - : Elsevier. - 9780323906708 - 9780323905527 ; , s. 279-316
  • Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
    • Specialty alloys are a broad group of materials providing superior properties to common materials and are therefore used for more demanding applications. Specialty alloys require sophisticated manufacturing routes, e.g., vacuum metallurgy, to account for all the alloying elements needed to finalize the specific alloy for its intended purpose. The alloys of Duplex stainless steels, superalloys, and Titanium alloys are examples of so-called specialty alloys where aerospace, chemical, and petrochemical industries are just a few areas mentioned where these specialty alloys are frequently used. Duplex stainless steel, had superior mechanical properties and corrosion resistance, making them a sustainable choice for a wide variety of applications i.e., petrochemical industries. The superalloys, and especially the precipitation hardening ones belong to a unique plethora of alloys commonly used in aerospace as well as land-based gas turbines which possess superb mechanical performance at elevated temperatures. However, the superalloys are unfortunately very challenging to process, not at least regarding weld cracking. With their high specific strength and corrosion resistance, titanium alloys are favorable for numerous applications. However, they react readily with oxygen at elevated temperatures and therefore inert atmosphere must be used during welding. 
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12.
  • Anti, Marta-Lena, et al. (författare)
  • Effect of silicon on creep properties of titanium 6Al-2Sn-4Zr-2Mo alloy
  • 2020
  • Ingår i: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 321
  • Tidskriftsartikel (refereegranskat)abstract
    • The alloy Ti-6Al-2Sn-4Zr-2Mo is a titanium alloy for elevated temperatures often used in aerospace applications. Minor additions of silicon have proven to improve the creep resistance of this alloy. In this work, three different amounts of silicon (0.015, 0.07 and 0.162 wt% Si) were added to cast Ti-6242 and creep tests were performed at different temperatures and loads. Creep resistance increased significantly with silicon addition by means of silicide precipitation hindering dislocations movement. Silicon rich nanoparticles in the microstructure were detected and their effect on creep resistance was investigated. The instruments used in this study were light optical microscope (LOM) and scanning electron microscopy (SEM).
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13.
  • Balachandramurthi, Arun Ramanathan, 1989-, et al. (författare)
  • Anisotropic fatigue properties of Alloy 718 manufactured by Electron Beam Powder Bed Fusion
  • 2020
  • Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 141
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, Alloy 718 specimens manufactured by Electron Beam Powder Bed Fusion process are subjected to two different post-treatments to have different microstructural features. Low cycle fatigue testing has been performed both parallel and transverse to the build direction. EB-PBF Alloy 718 exhibits anisotropic fatigue behaviour; the fatigue life is better along the parallel direction compared to the transverse direction. The anisotropy in fatigue life is related to the anisotropy in the Young’s modulus. The pseudo-elastic stress vs. fatigue life approach is presented as a potential solution to handle anisotropy in fatigue life assessment of additively manufactured engineering components. © 2020 The Authors
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14.
  • Balachandramurthi, Arun Ramanathan (författare)
  • Fatigue Properties of Additively Manufactured Alloy 718
  • 2018
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive Manufacturing (AM), commonly known as 3D Printing, is a disruptive modern manufacturing process, in which parts are manufactured in a layer-wise fashion. Among the metal AM processes, Powder Bed Fusion (PBF) technology has opened up a design space that was not formerly accessible with conventional manufacturing processes. It is, now, possible to manufacture complex geometries, such as topology-optimized structures, lattice structures and intricate internal channels, with relative ease. PBF is comprised of Electron Beam Melting (EBM) and Selective Laser Melting (SLM) processes.Though AM processes offer several advantages, the suitability of these processes to replace conventional manufacturing processes must be studied in detail; for instance, the capability to produce components of consistent quality. Therefore, understanding the relationship between the AM process together with the post treatment used and the resulting microstructure and its influence on the mechanical properties is crucial, to enable manufacturing of high-performance components. In this regard, for AM built Alloy 718, only a limited amount of work has been performed compared to conventional processes such as casting and forging. The aim of this work, therefore, is to understand how the fatigue properties of EBM and SLM built Alloy 718, subjected to different thermal post-treatments, is affected by the microstructure. In addition, the effect of as-built surface roughness is also studied.Defects can have a detrimental effect on fatigue life. Numerous factors such as the defect type, size, shape, location, distribution and nature determine the effect of defects on properties. Hot Isostatic Pressing (HIP) improves fatigue life as it leads to closure of most defects. Presence of oxides in the defects, however, hinders complete closure by HIP. Machining the as-built surface improves fatiguelife; however, for EBM manufactured material, the extent of improvement is dependent on the amount of material removed. The as-built surface roughness, which has numerous crack initiation sites, leads to lower scatter in fatigue life. In both SLM and EBM manufactured material, fatigue crack propagation is transgranular. Crack propagation is affected by grain size and texture of the material.
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15.
  • Balachandramurthi, Arun Ramanathan, et al. (författare)
  • Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718
  • 2018
  • Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 735, s. 463-474
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life. © 2018 Elsevier B.V.
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16.
  • Balachandramurthi, Arun Ramanathan, 1989-, et al. (författare)
  • Microstructural influence on fatigue crack propagation during high cycle fatigue testing of additively manufactured Alloy 718
  • 2019
  • Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 149, s. 82-94
  • Tidskriftsartikel (refereegranskat)abstract
    • A study of the microstructure of additively manufactured Alloy 718 was performed in order to better understand the parameters that have an influence on the fatigue properties of the material. The specimens were manufactured using two powder bed fusion techniques – Electron Beam Melting (EBM) and Selective Laser Melting (SLM). Four point bending fatigue tests were performed at room temperature with a stress ratio of R = 0.1 and 20 Hz frequency, on material that was either in hot isostatically pressed (HIP) and solution treated and aged (STA) condition or in STA condition without a prior HIP treatment. The grains in the SLM material in the HIP + STA condition have grown considerably both in the hatch and the contour regions; EBM material, in contrast, shows grain growth only in the contour region. Fractographic analysis of the specimens in HIP + STA condition showed a faceted appearance while the specimens in STA condition showed a more planar crack appearance. The crack propagation occurred in a transgranular mode and it was found that precipitatessuch as NbC, TiN or δ-phase, when present, did not affect the crack path. The areas with larger grains corresponded to the faceted appearance of the fracture surface. This could be attributed to the plastic zone ahead of the crack tip being confined within one grain, in case of the larger grains, which promotes single shear crack growth mode
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17.
  • Balachandramurthi, Arun Ramanathan, 1989-, et al. (författare)
  • Microstructure tailoring in Electron Beam Powder Bed Fusion Additive Manufacturing and its potential consequences
  • 2019
  • Ingår i: Results in Materials. - : Elsevier. - 2590-048X. ; 1
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron Beam Powder Bed Fusion process for Alloy 718 was investigated, in the sense of microstructural evolution with varying process conditions. The existence of a geometric relationship between the melt front and the processing parameters was observed. By understanding and capitalizing on this relationship, it was possible to obtain columnar, equiaxed or bimodal microstructure.
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18.
  • Balachandramurthi, Arun Ramanathan, 1989-, et al. (författare)
  • On the microstructure of laser beam powder bed fusion alloy 718 and its influence on the low cycle fatigue behaviour
  • 2020
  • Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 13:22
  • Tidskriftsartikel (refereegranskat)abstract
    • Additive manufacturing of Alloy 718 has become a popular subject of research in recent years. Understanding the process-microstructure-property relationship of additively manufactured Alloy 718 is crucial for maturing the technology to manufacture critical components. Fatigue behaviour is a key mechanical property that is required in applications such as gas turbines. Therefore, in the present work, low cycle fatigue behaviour of Alloy 718 manufactured by laser beam powder bed fusion process has been investigated. The material was tested in as-built condition as well as after two different thermal post-treatments. Three orientations with respect to the building direction were tested to evaluate the anisotropy. Testing was performed at room temperature under controlled amplitudes of strain. It was found that defects, inclusions, strengthening precipitates, and Young’s modulus influence the fatigue behaviour under strain-controlled conditions. The strengthening precipitates affected the deformation mechanism as well as the cycle-dependent hardening/softening behaviour. The defects and the inclusions had a detrimental effect on fatigue life. The presence of Laves phase in LB-PBF Alloy 718 did not have a detrimental effect on fatigue life. Young’s modulus was anisotropic and it contributed to the anisotropy in strain-life relationship. Pseudo-elastic stress vs. fatigue life approach could be used to handle the modulus-induced anisotropy in the strain-life relationship. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
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19.
  • Balachandramurthi, Arun Ramanathan, et al. (författare)
  • On the microstructure of laser beam powder bed fusion alloy 718 and its influence on the low cycle fatigue behaviour
  • 2020
  • Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 13:22, s. 1-21
  • Tidskriftsartikel (refereegranskat)abstract
    • Additive manufacturing of Alloy 718 has become a popular subject of research in recent years. Understanding the process-microstructure-property relationship of additively manufactured Alloy 718 is crucial for maturing the technology to manufacture critical components. Fatigue behaviour is a key mechanical property that is required in applications such as gas turbines. Therefore, in the present work, low cycle fatigue behaviour of Alloy 718 manufactured by laser beam powder bed fusion process has been investigated. The material was tested in as-built condition as well as after two different thermal post-treatments. Three orientations with respect to the building direction were tested to evaluate the anisotropy. Testing was performed at room temperature under controlled amplitudes of strain. It was found that defects, inclusions, strengthening precipitates, and Young’s modulus influence the fatigue behaviour under strain-controlled conditions. The strengthening precipitates affected the deformation mechanism as well as the cycle-dependent hardening/softening behaviour. The defects and the inclusions had a detrimental effect on fatigue life. The presence of Laves phase in LB-PBF Alloy 718 did not have a detrimental effect on fatigue life. Young’s modulus was anisotropic and it contributed to the anisotropy in strain-life relationship. Pseudo-elastic stress vs. fatigue life approach could be used to handle the modulus-induced anisotropy in the strain-life relationship.
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20.
  • Balachandramurthi, Arun Ramanathan, 1989- (författare)
  • Towards understanding the fatigue behaviour of Alloy 718 manufactured by Powder Bed Fusion processes
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive Manufacturing (AM) is a disruptive modern manufacturing process in which parts are manufactured in a layer-wise fashion. Among the metal AM processes, Powder Bed Fusion (PBF) technology — comprised of Electron Beam Powder Bed Fusion (EB-PBF) and Laser Beam Powder Bed Fusion (LB-PBF) —has opened up a design space that was formerly unavailable with conventionalmanufacturing processes. PBF processes offer several advantages; however, thesuitability of these processes to replace the conventional processes must be investigatedin detail. Therefore, understanding the AM process – post-processing –microstructure – property relationships is crucial for the manufacturing of high performance components. In this regard, only limited work has been done towards understanding the fatigue behaviour of PBF Alloy 718. The aim of this work, therefore, is to understand how the fatigue behaviour of PBF Alloy 718 is affected by its microstructure. Besides, the influence of the rough as-built surface is also investigated. In general, the <100> fibre texture along the build direction that resulted from PBF processing of Alloy 718 led to anisotropy in Young's modulus. Consequently,the fatigue performance under controlled amplitudes of strain was anisotropic such that the low-modulus direction had longer fatigue life and vice versa. This texture-induced elasticity-dependent anisotropic strain-life behaviour couldbe normalized by the pseudo-elastic stress vs fatigue life approach.Inclusions and defects had a detrimental effect on fatigue performance. Numerousfactors, such as their geometry, volume fraction, and distribution, determinedthe effect on fatigue behaviour. Hot Isostatic Pressing (HIP) eliminated most defect sand led to an improvement in fatigue performance. However, HIP did not alter the inclusions, which acted as crack initiation sites and reduced fatigue life. The rough as-built surface, which had numerous notch-like crack initiation sites, deteriorated fatigue performance; however, it lowered the scatter in fatigue life. Machining off the as-built surface improved fatigue life but increased the scatter.
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21.
  • Balachandramurthi Ramanathan, Arun, et al. (författare)
  • Additive Manufacturing of Alloy 718 via Electron Beam Melting : Effect of Post-Treatment on the Microstructure and the Mechanical Properties.
  • 2018
  • Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 12:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Alloy 718 finds application in gas turbine engine components, such as turbine disks, compressor blades and so forth, due to its excellent mechanical and corrosion properties at elevated temperatures. Electron beam melting (EBM) is a recent addition to the list of additive manufacturing processes and has shown the capability to produce components with unique microstructural features. In this work, Alloy 718 specimens were manufactured using the EBM process with a single batch of virgin plasma atomized powder. One set of as-built specimens was subjected to solution treatment and ageing (STA); another set of as-built specimens was subjected to hot isostatic pressing (HIP), followed by STA (and referred to as HIP+STA). Microstructural analysis of as-built specimens, STA specimens and HIP+STA specimens was carried out using optical microscopy and scanning electron microscopy. Typical columnar microstructure, which is a characteristic of the EBM manufactured alloy, was observed. Hardness evaluation of the as-built, STA and HIP+STA specimens showed that the post-treatments led to an increase in hardness in the range of ~50 HV1. Tensile properties of the three material conditions (as-built, STA and HIP+STA) were evaluated. Post-treatments lead to an increase in the yield strength (YS) and the ultimate tensile strength (UTS). HIP+STA led to improved elongation compared to STA due to the closure of defects but YS and UTS were comparable for the two post-treatment conditions. Fractographic analysis of the tensile tested specimens showed that the closure of shrinkage porosity and the partial healing of lack of fusion (LoF) defects were responsible for improved properties. Fatigue properties were evaluated in both STA and HIP+STA conditions. In addition, three surface conditions were also investigated, namely the 'raw' as-built surface, the machined surface with the contour region and the machined surface without the contour region. Machining off the contour region completely together with HIP+STA led to significant improvement in fatigue performance.
  •  
22.
  • Čapek, J., et al. (författare)
  • Influence of laser powder bed fusion scanning pattern on residual stress and microstructure of alloy 718
  • 2022
  • Ingår i: Materials & design. - : Elsevier Ltd. - 0264-1275 .- 1873-4197. ; 221
  • Tidskriftsartikel (refereegranskat)abstract
    • A comprehensive investigation is undertaken on the effect of laser scanning pattern on the microstructure of cylindrical samples made of Alloy 718 processed by Laser Powder Bed Fusion. It is observed that the common alternate direction scanning of the laser results in a more homogeneous microstructure than the less common concentric line scans where significant microstructural heterogeneities are seen between the edges and the center of the sample. The investigation focuses on the precipitation, crystallographic texture, grain size, grain morphology and residual stresses utilizing synchrotron X-ray diffraction, neutron diffraction and electron microscopy. The heterogeneous microstructure of the sample processed with the concentric laser pattern influences the chemical composition of the matrix, which alters the reference “strain free” interplanar spacing used for evaluating the residual strain. The investigation underlines the significance of the processing parameters on the homogeneity of the microstructure and the effect of the chemical variations on the determination of residual stresses in materials such as Alloy 718, where strong local chemical variations occur because of different types and extent of precipitation. © 2022
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23.
  • Čapek, J., et al. (författare)
  • The Effect of γ″ and δ Phase Precipitation on the Mechanical Properties of Inconel 718 Manufactured by Selective Laser Melting : An In Situ Neutron Diffraction and Acoustic Emission Study
  • 2021
  • Ingår i: JOM. - : Springer. - 1047-4838 .- 1543-1851. ; 73:1, s. 223-232
  • Tidskriftsartikel (refereegranskat)abstract
    • The deformation behavior of additively manufactured Alloy 718 in as-built condition and after annealing was studied in situ under tensile loading along the build direction. Pre-characterization by synchrotron X-ray diffraction and electron microscopy revealed a significant amount of γ″ precipitates in the as-built samples, whereas the γ″ phase was entirely consumed and needle-like δ precipitates appeared in the annealed sample. In situ neutron diffraction (ND) and acoustic emission (AE) enabled indirect observation of the role of the precipitates on the mechanical behavior. ND provided information on the load accommodation in the matrix, while AE detected a strong signal from the interaction of dislocations with the δ-phase precipitates during deformation of the annealed samples. The results imply that in the annealed samples the matrix sheds the load to the precipitates, while in the as-built material the matrix bares a significant load. © 2020, The Author(s).
  •  
24.
  • Cedergren, Stefan, 1982, et al. (författare)
  • Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V
  • 2016
  • Ingår i: The International Journal of Advanced Manufacturing Technology. - London : Springer. - 0268-3768 .- 1433-3015. ; 84:9-12, s. 2277-2291
  • Tidskriftsartikel (refereegranskat)abstract
    • Titanium alloys are known to produce shear-localized chips during machining, resulting in cyclic variations in cutting forces which in turn could cause severe problems with vibrations. However, at low cutting speeds and feed rates, continuous chips are formed, with an increase in both parameters favoring the transition to shear-localized chips. This transition is affected by work material microstructure, where a coarse microstructure gives anisotropic effects, e.g., when the size of alpha colonies is on the same order of magnitude as the primary cutting zone. The change in chip morphology with an increase in cutting parameters will then be dependent on the orientation of alpha colonies within the cutting zone. The microstructure of work material can show large variations depending on product form, e.g., cast, wrought, or sheet material, thus affecting whether the chip formation is isotropic or anisotropic. Other sources of variations also exist that can be found within the same component, such as segregation of alloying elements and differences in thermo-mechanical history during processing due to geometry. In this study, the interaction between work material microstructure, process parameters, and the machining system’s structural characteristics is studied. The aim is to further increase the knowledge about vibrations during machining of titanium and the role of microstructure and machining system properties. Different microstructures were produced by adding boron to cast Ti-6Al-4V material, where the resulting colony sizes gave both isotropic and anisotropic chip formation within the chosen cutting data range. The machining systems dynamic properties were varied by using different tool overhangs, thereby simulating different configurations of natural frequencies and stiffness. The results show the influence of both microstructure and machining system’s structural characteristics on the dynamic response of the system for different process parameters. This information can be used to increase robustness of machining operations taking into consideration this three-way relationship.
  •  
25.
  • Cedergren, Stefan, et al. (författare)
  • Machinability of cast titanium alloy Ti-6Al-4V with addition of boron
  • 2020
  • Ingår i: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 321
  • Tidskriftsartikel (refereegranskat)abstract
    • Cast titanium products generally have coarser microstructures compared to wrought, which explains their limited mechanical properties. However, instead of additional thermo-mechanical processing steps, as in the case of wrought components, the microstructure of castings can be refined by additions of boron. This enhances yield strength, tensile strength and ductility. In order to investigate the influence on machining of this microstructural refinement, cutting tests were performed in three different Ti-6Al-4V castings, having 0, 0.06 and 0.11 wt.% boron. Five machinability criteria were studied; cutting force, chip breakability, burr formation, surface roughness and tool wear. The results show anisotropic deformation behavior in chips when alpha colony size is on the same order of magnitude as the primary deformation zone, or larger. There was little or no influence on cutting forces and burr formation, however chip breakability and tool life was reduced when boron was added. Surface roughness showed a more complex behavior where 0.06 wt.% boron resulted in rougher surfaces compared to the other compositions at low feed rate.
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26.
  • Charles Murgau, Corinne, 1979-, et al. (författare)
  • A model for Ti-6Al-4V microstructure evolution for arbitrary temperature changes
  • 2012
  • Ingår i: Modelling and Simulation in Materials Science and Engineering. - : IOP Publishing. - 0965-0393 .- 1361-651X. ; 20:5, s. 055006-
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a microstructure model for the titanium alloy Ti-6Al-4V designed to be used in coupled thermo-metallurgical-mechanical simulations of, e.g., welding processes. The microstructure evolution is increasingly taken into consideration in analyses of manufacturing processes since it directly affects the mechanical properties. Thermally driven phase evolutions are accounted for in the model. A state variable approach is adopted to represent the microstructure with the objective to integrate the microstructure changes with a thermomechanical model of manufacturing process simulation such as welding. The model is calibrated using the literature data and also validated against a cyclic temperature history during multi-pass welding.
  •  
27.
  • Charles Murgau, Corinne (författare)
  • Microstructure model for Ti-6Al-4V used in simulation of additive manufacturing
  • 2016
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis is devoted to microstructure modelling of Ti-6Al-4V. The microstructure and the mechanical properties of titanium alloys are highly dependent on the temperature history experienced by the material. The developed microstructure model accounts for thermaldriving forces and is applicable for general temperature histories. It has been applied to study wire feed additive manufacturing processes that induce repetitive heating and cooling cycles.The microstructure model adopts internal state variables to represent the microstructure through microstructure constituents' fractions in finite element simulation. This makes it possible to apply the model efficiently for large computational models of general thermomechanical processes. The model is calibrated and validated versus literature data. It is applied to Gas Tungsten Arc Welding -also known as Tungsten Inert Gas welding-wire feed additive manufacturing process.Four quantities are calculated in the model: the volume fraction of phase, consisting of Widmanstätten, grain boundary, and martensite. The phase transformations during cooling are modelled based on diffusional theory described by a Johnson-Mehl-Avrami-Kolmogorov formulation, except for diffusionless martensite formation where the Koistinen-Marburger equation is used. A parabolic growth rate equation is used for the to transformation upon heating. An added variable, structure size indicator of Widmanstätten, has also been implemented and calibrated. It is written in a simple Arrhenius format.The microstructure model is applied to in finite element simulation of wire feed additive manufacturing. Finally, coupling with a physically based constitutive model enables a comprehensive and predictive model of the properties that evolve during processing.
  •  
28.
  • Charles Murgau, Corinne, et al. (författare)
  • Temperature and Microstructure Evolution in Gas Tungsten Arc Welding Wire Feed Additive Manufacturing of Ti-6Al-4V
  • 2019
  • Ingår i: Materials. - : MDPI. - 1996-1944. ; 12:21
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present study, the gas tungsten arc welding wire feed additive manufacturing process is simulated and its final microstructure predicted by microstructural modelling, which is validated by microstructural characterization. The Finite Element Method is used to solve the temperature field and microstructural evolution during a gas tungsten arc welding wire feed additive manufacturing process. The microstructure of titanium alloy Ti-6Al-4V is computed based on the temperature evolution in a density-based approach and coupled to a model that predicts the thickness of the α lath morphology. The work presented herein includes the first coupling of the process simulation and microstructural modelling, which have been studied separately in previous work by the authors. In addition, the results from simulations are presented and validated with qualitative and quantitative microstructural analyses. The coupling of the process simulation and microstructural modeling indicate promising results, since the microstructural analysis shows good agreement with the predicted alpha lath size.
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29.
  • Davies, P., et al. (författare)
  • The hierarchy of microstructure parameters affecting the tensile ductility in centrifugally cast and forged Ti-834 alloy during high temperature exposure in air
  • 2016
  • Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 117, s. 51-67
  • Tidskriftsartikel (refereegranskat)abstract
    • Ductility regression is the main concern in using Ti-834 titanium alloy at temperatures above 500 °C for aerospace applications. The reduction of ductility in titanium alloys at high temperatures is strongly correlated to the exposure time. In the current study the effect of prolonged exposure at 500 °C on the tensile ductility of two differently processed Ti-834 alloys was investigated. In order to simulate actual Ti-834 processing routes, forged and centrifugally cast materials were used. The tensile tests were conducted on various specimens exposed at 500 °C for 100, 200 and 500 h to observe microstructure feature changes. Moreover, the effect of microstructure, microtexture, α-case, α2 and silicide precipitate coarsening during high temperature exposure was studied thoroughly. The cast alloy was found to have a minimum ductility and failed at 1.8% strain after exposure at 500 °C/500 h when the α-case layer was retained during testing, whilst, the ductility of the forged alloy was unaffected. The effects of individual microstructural parameters on the ductility regression in Ti-834 alloy were quantified. The results showed that 7.1% strain differences between the cast and forged alloy are related to microstructural variations including; morphology, lath widths, grain size and shape, grain orientations and microtexture. A total of 9.6% strain loss was observed in centrifugally cast Ti-834 after aging at 500°C/500 h and quantified as follow; 3.6% due to α-case formation during high temperature exposure, 0.2% due to α2-precipitates coarsening, 4.4% due to further silicide formation and coarsening, 1.4% due to additional microstructure changes during high temperature exposure. Furthermore, silicide coarsening on α/β phase boundaries caused large void formation around the precipitates. A theoretical model supported by experimental observations for silicide precipitation in fully colony and duplex microstructures was established. The element partitioning during exposure caused Al and Ti depletion in the vicinity of the β phase in the lamellae, i.e., αs area. This resulted in lowering the strength of the alloy and facilitated the formation of Ti3(SiZr)2 precipitates. The Al depletion and nano-scale partitioning observed at the αs/β boundaries resulted in easy crack initiation and promoted propagation in the centrifugally cast colony microstructure and reduced the basal slip τcrss. Furthermore, silicides were not formed in αp (high Al, Ti and low Zr areas) in the forged duplex microstructure that promoted superior mechanical performance and ductility over the cast alloy.Graphical abstract
  •  
30.
  • Fargas, G., et al. (författare)
  • Influence of cyclic thermal treatments on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo alloy
  • 2018
  • Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 145, s. 218-224
  • Tidskriftsartikel (refereegranskat)abstract
    • Ti-6Al-2Sn-4Zr-2Mo is one of the most common titanium alloys for aerospace industry. This alloy experiences oxidation phenomenon at elevated temperatures. In the present study, cyclic thermal treatments were performed in air at 500, 593 and 700 °C, up to 500 cycles, in order to determine the oxidation kinetics and to analyze the oxide scale and alpha-case formation. Moreover, results were compared to those achieved under isothermal conditions to elucidate differences between both thermal conditions. In this sense, metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Field Emission Scanning Electron Microscopy and Focus Ions Beam, were used to analyze surface oxidation evolution. Results pointed out that cyclic treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed the formation of not only rutile, as isothermal treatments, but also anatase. Thickness of the oxide scale was higher for cyclic conditions, while alpha case did not exceed values reached by isothermal treatments and even became lower at 500 °C.
  •  
31.
  • Fargas, Gemma, et al. (författare)
  • Oxidation behavior of TI-6Al-4V alloy exposed to isothermal and cyclic thermal treatments
  • 2017
  • Ingår i: Proceedings of the conference METAL 2017. - Ostrava : TANGER Ltd.. - 9788087294796 ; , s. 1573-1579
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • One of the most common titanium alloys for aerospace industry is Ti-6Al-4V (usually designed as Ti-64) which is used for manufacturing aero-engine components, such as fan discs, compressor discs, blades andstators. The maximum service temperature for this alloy is limited partly because of degradation of mechanical properties at elevated temperatures (above 480 ºC). During the first stage of oxidation the oxidescale is protective, whereas after prolonged oxidation time it loses its protective nature and favours higher diffusion of oxygen through the oxide. In the present study, cyclic thermal treatments were performed in air at 500 and 700 ºC, up to 500 hours, and compared with similar studies carried out on isothermal oxidation conditions. The evolution of the surface oxidation was analyzed by metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Scanning Electron Microscopy and Focus Ions Beam. The results point out that the cyclic thermal treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed not only the presence of rutile, at 700 ºC, but also anatase and TiOx at 500 ºC for both isothermal and cyclic thermal treatments. At 700 ºC, thermal stress caused by cyclic thermal treatments promoted the fracture of the oxide after the first 20 hours.
  •  
32.
  • Gaddam, Raghuveer, et al. (författare)
  • Fatigue crack growth behaviour of forged Ti-6Al-4V in gaseous hydrogen
  • 2014
  • Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X .- 1879-0496. ; 78, s. 378-383
  • Tidskriftsartikel (refereegranskat)abstract
    • Fatigue crack growth (FCG) tests were performed to evaluate the fatigue behaviour of forged Ti-6Al-4V in air and high-pressure gaseous hydrogen (15 MPa) at room temperature. The results indicate that the effect of gaseous hydrogen is dependent on the stress intensity factor (ΔK). The FCG rate was unaffected by hydrogen below a critical stress intensity, ΔK* ≈ 20 MPa√m. Above ΔK*, the FCG rate fluctuated and subsequently accelerated at higher ΔK values. The observed behaviour is attributed to the change in the fracture processes. A hypothesis is proposed that describes the FCG behaviour in gaseous hydrogen.
  •  
33.
  • Gaddam, Raghuveer, et al. (författare)
  • Influence of alpha–case layer on the low cycle fatigue properties of Ti–6Al–2Sn–4Zr–2Mo alloy
  • 2014
  • Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 599, s. 51-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Strain–controlled low cycle fatigue properties of Ti–6Al–2Sn–4Zr–2Mo with different thickness of alpha–case layers were investigated. Results show that at strain amplitudes 0.3 and 0.4%, the fatigue life of the alloy is reduced for the specimens with alpha–case layer compared to the ones without any alpha–case. It was noted that with a 2 μm thick alpha–case layer the low cycle fatigue life is reduced about 50% at the higher strain amplitude. The degrading effect of the alpha–case layer on fatigue life increased with increasing thickness. The alpha–case layer at the surface is enriched with oxygen making the surface harder and brittle, which results in easier crack initiation and thus decrease in fatigue life.
  •  
34.
  • Gaddam, Raghuveer, et al. (författare)
  • Influence of high-pressure gaseous hydrogen on the low-cycle fatigue and fatigue crack growth properties of a cast titanium alloy
  • 2014
  • Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 612, s. 354-362
  • Tidskriftsartikel (refereegranskat)abstract
    • In the present study, the effect of gaseous hydrogen on the fatigue properties of a commonly used aerospace titanium alloy (Ti–6Al–4V) was studied. The low-cycle fatigue and fatigue crack growth properties were investigated at room temperature in ambient air and 15 MPa gaseous hydrogen. Results showed that the low-cycle fatigue life was significantly reduced in hydrogen, and the detrimental effect was larger at higher strain amplitudes. The fatigue crack growth rate in hydrogen remained unaffected below a critical stress intensity ΔKn E 17 MPa√m, while beyond this value, the fatigue crack growth rate fluctuated and increased with increasing ΔK. Fractography analysis clearly showed that gaseous hydrogen mainly affected the fatigue crack growth rate. On the fracture surfaces, striations were noted over the entire crack growth region in air, whereas in hydrogen striations were noted at stress intensities lower than ΔKn. Above ΔKn, secondary cracks and brittle flat surfaces with features similar to crack arrest marks were mostly observed in hydrogen. Microstructural analysis along the crack growth direction showed that the crack followed a transgranular path in air, i.e. through α colonies. In hydrogen, the crack also grew along the prior β grain boundaries and at α/β interface within the α colonies. Thereby, the detrimental effect of hydrogen in cast titanium alloy was attributed to a change in the fracture process during crack propagation.
  •  
35.
  • Gaddam, Raghuveer, et al. (författare)
  • Influence of hydrogen environment on fatigue crack growth in forged Ti-6Al-4V : fractographic analysis
  • 2013
  • Ingår i: 7th EEIGM International Conference on Advanced Materials Research. - : IOP Publishing Ltd. ; 48, s. 012010-
  • Konferensbidrag (refereegranskat)abstract
    • In this study, the influence of hydrogen environment (15 MPa) on the fatigue crack growth in forged Ti-6A1-4V at room temperature is investigated. It is observed that at 21 < ΔK > 25 MPa√m, there exists a change of fatigue crack growth rate (FCGR) in hydrogen environment, and it is accelerated at ΔK > 25MPa√m. FCGR in hydrogen environment is dependent on the stress intensity levels (ΔK). Detailed fractographic analysis of the fracture surfaces were performed at different ΔK using high-resolution scanning electron microscope (HR-SEM). Fatigue striations were observed in air and hydrogen at ΔK < 21MPa√m. At ΔK > 21MPa√m, secondary cracks were observed in hydrogen environment. The differences in appearances of fracture surfaces in air and hydrogen are discussed.
  •  
36.
  • Gaddam, Raghuveer, et al. (författare)
  • Influence of hydrogen environment on the mechanical properties of cast and electron beam melted Ti-6Al-4V
  • 2012
  • Ingår i: Ti-2011. - Beijing : Social Sciences Academic Press (China). - 9787030338945 ; , s. 1885-1888
  • Konferensbidrag (refereegranskat)abstract
    • In order to save weight in a certain engine application the possibility of replacing the currently used material with cast Ti-6Al-4V is investigated here. The working environment for this particular engine part is pure hydrogen gas at high pressure. Therefore selected mechanical properties such as tensile and low cycle fatigue (LCF) in air and hydrogen atmosphere have been studied for cast Ti-6Al-4V. In addition to cast Ti-6Al-4Vt the corresponding mechanical properties of a more recently developed additive manufacturing method, electron beam melting (EBM), is also investigated in hydrogen and compared with cast Ti-6Al-4V. Cast Ti-6Al-4V showed lower yield strength and lower ultimate tensile strength in hydrogen compared with air. However, no significant change in the ductility was observed. The LCF was significantly reduced in the hydrogen atmosphere, mostly at high strain range (π 2%). The EBM Ti-6Al-4V in hydrogen showed higher yield strength, higher ultimate strength and higher ductility as well as improved fatigue life compared with cast Ti-6Al-4V under the same test conditions. Microstructural and fractographic characterization were also performed and the results are included.
  •  
37.
  • Gaddam, Raghuveer, et al. (författare)
  • Oxidation and alpha–case formation in Ti–6Al–2Sn–4Zr–2Mo alloy
  • 2015
  • Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803 .- 1873-4189. ; 99, s. 166-174
  • Tidskriftsartikel (refereegranskat)abstract
    • Isothermal heat treatments in ambient air were performed on wrought Ti–6Al–2Sn–4Zr–2Mo (Ti–6242) material at 500, 593 and 700 °C for times up to 500 hours. In presence of oxygen at elevated temperatures simultaneous reactions occurred in Ti–6242 alloy, which resulted in formation of an oxide scale and a layer with higher oxygen concentration (termed as alpha–case). Total weight gain analysis showed that there was a transition in the oxidation kinetics. At 500 °C, the oxidation kinetics obeyed cubic relationship up to 200 hours and thereafter changed to parabolic at prolonged exposure times. At 593 °C, it followed parabolic relationship. After heat treatment at 700 °C, the oxidation obeyed parabolic relationship up to 200 hours and thereafter changed to linear at prolonged exposure times. The observed transition is believed to be due to the differences observed in the oxide scale. The activation energy for parabolic oxidation was estimated to be 157 kJ/mol. In addition, alpha–case layer was evaluated using optical microscope, electron probe micro analyser and microhardness tester. The thickness of the alpha–case layer was found to be a function of temperature and time, increasing proportionally, and following parabolic relationship. The activation energy for formation of alpha–case layer was estimated to be 153 kJ/mol.
  •  
38.
  • Gaddam, Raghuveer, et al. (författare)
  • Study of alpha case depth in Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V
  • 2013
  • Ingår i: 7th EEIGM International Conference on Advanced Materials Research. - : IOP Publishing Ltd.
  • Konferensbidrag (refereegranskat)abstract
    • Titanium alloys, mostly Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) and Ti-6Al-4V (Ti-64) are used in aero engine applications, because they possess high specific strength. The future concept in designing aircraft engines results in higher pressure, which increases the efficiency of aircraft engines by achieving high thrust and lowering the fuel consumption. Nevertheless, higher pressure in the engine means increase of service temperature. These conditions enforce new requirements on the materials used for manufacturing the engine components (compressors). Ti-6242 is mostly used in compressors where the service temperature is in the range of 400-450°C. It is well known that titanium alloys above 480°C for longer service time have tendency to form a hard and brittle oxygen stabilized surface layer (α-case). This layer has impact on the mechanical properties of the surface, by lowering the tensile ductility and the fatigue resistance. Factors that contribute for growth of α-case are: presence of oxygen, exposure time, temperature and pressure. In order to extend the service temperature of titanium alloys, it is required to understand the formation of α-case at high temperatures for long exposure times. In the present study, isothermal oxidation experiments in air were performed on forged Ti-6242 alloy at 500°C and 593°C up to 500 hours. Similar studies were also performed on Ti-64 sheet at 593°C and 700°C. Alpha case depths for both alloys were quantified using metallography techniques and compared.
  •  
39.
  • Goel, Sneha, 1993-, et al. (författare)
  • Residual stress determination by neutron diffraction in powder bed fusion-built Alloy 718: Influence of process parameters and post-treatment
  • 2020
  • Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275. ; 195
  • Tidskriftsartikel (refereegranskat)abstract
    • Alloy 718 is a nickel-based superalloy that is widely used as a structural material for high-temperature applications. One concern that arises when Alloy 718 is manufactured using powder bed fusion (PBF) is that residual stresses appear due to the high thermal gradients. These residual stresses can be detrimental as they can degrade mechanical properties and distort components. In this work, residual stresses in PBF built Alloy 718, using both electron and laser energy sources, were measured by neutron diffraction. The effects of process parameters and thermal post-treatments were studied. The results show that thermal post-treatments effectively reduce the residual stresses present in the material. Moreover, the material built with laser based PBF showed a higher residual stress compared to the material built with electron-beam based PBF. The scanning strategy with the lower amount of residual stresses in case of laser based PBF was the chessboard strategy compared to the bi-directional raster strategy. In addition, the influence of measured and calculated lattice spacing (d0) on the evaluated residual stresses was investigated.
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40.
  • Hällgren, Sebastian, 1971- (författare)
  • Some aspects on designing for metal Powder Bed Fusion
  • 2017
  • Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Additive Manufacturing (AM) using the Powder Bed Fusion (PBF) is a relatively new manufacturing method that is capable of creating shapes that was previously practically impossible to manufacture. Many think it will revolutionize how manufacturing will be done in the future. This thesis is about some aspects of when and how to Design for Additive Manufacturing (DfAM) when using the PBF method in metal materials. Designing complex shapes is neither easy nor always needed, so when to design for AM is a question with different answers depending on industry or product. The cost versus performance is an important metric in making that selection. How to design for AM can be divided into how to improve performance and how to improve additive manufacturability where how to improve performance once depends on product, company and customer needs. Using advanced part shaping techniques like using Lattices or Topology Optimization (TO) to lower part mass may increase customer value in addition to lowering part cost due to faster part builds and less powder and energy use. Improving PBF manufacturability is then warranted for parts that reach series production, where determining an optimal build direction is key as it affects many properties of PBF parts. Complex shapes which are designed for optimal performance are usually more sensitive to defects which might reduce the expected performance of the part. Non Destructive Evaluation (NDE) might be needed to certify a part for dimensional accuracy and internal defects prior use. The licentiate thesis covers some aspects of both when to DfAM and how to DfAM of products destined for series production. It uses design by Lattices and Topology Optimization to reduce mass and looks at the effect on part cost and mass. It also shows effects on geometry translation accuracies from design to AM caused by differences in geometric definitions. Finally it shows the effect on how different NDE methods are capable of detecting defects in additively manufactured parts.
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41.
  • Isoaho, Jerry, et al. (författare)
  • Applications of additive manufacturing : Selected case studies and future prospects
  • 2023. - 1
  • Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 676-716
  • Bokkapitel (refereegranskat)abstract
    • From an industrial standpoint, cost is of one of the most important drivers for utilizing new technologies such as additive manufacturing (AM). Other important drivers for why AM can be an advantageous technology for component manufacturing is decreased manufacturing lead time, rapid demonstration capability, freedom of design/geometry, advancing technology development, and not least sustainability in terms of both material utilization and improved part/system performance. In this chapter, six selected “case studies” are compiled, in which AM techniques have been used to manufacture components for actual applications. In some case studies, a comparison between the additive manufacturing route and the corresponding conventional manufacturing route is also included.
  •  
42.
  •  
43.
  • Lindgren, Lars-Erik, et al. (författare)
  • Simulation of additive manufacturing using coupled constitutive and microstructure models
  • 2016
  • Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 12:Part B, s. 144-158
  • Tidskriftsartikel (refereegranskat)abstract
    • The paper describes the application of modeling approaches used in Computational Welding Mechanics (CWM) applicable for simulating Additive Manufacturing (AM). It focuses on the approximation of the behavior in the process zone and the behavior of the solid material, particularly in the context of changing microstructure. Two examples are shown, one for the precipitation hardening Alloy 718 and one for Ti-6Al-4V. The latter alloy is subject to phase changes due to the thermal cycling
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44.
  • Lundbäck, Andreas, et al. (författare)
  • Modeling and Experimental Measurement with Synchrotron Radiation of Residual Stresses in Laser Metal Deposited Ti-6Al-4V
  • 2016
  • Ingår i: Proceedings of the 13th World Conference on Titanium. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. - 9781119293668 - 9781119296126 ; , s. 1279-1282
  • Konferensbidrag (refereegranskat)abstract
    • There are many challenges in producing aerospace components by additive manufacturing (AM). One of them is to keep the residual stresses and deformations to a minimum. Another one is to achieve the desired material properties in the final component. A computer model can be of great assistance when trying to reduce the negative effects of the manufacturing process. In this work a finite element model is used to predict the thermo-mechanical response during the AM-process. This work features a physically based plasticity model coupled with a microstructure evolution model for the titanium alloy Ti -6Al-4V. Residual stresses in AM components were measured non-destructively using high-energy synchrotron X-ray diffraction on beam line ID15A at the ESRF, Grenoble. The results are compared with FE model predictions of residual stresses. During the process, temperatures and deformations was continuously measured. The measured and computed thermal history agrees well. The result with respect to the deformations agrees well qualitatively. Meaning that the change in deformation in each sequence is well predicted but there is a systematic error that is summing so that the quantitative agreement is lost.
  •  
45.
  •  
46.
  • Lundbäck, Andreas, et al. (författare)
  • Modelling and Simulation of Metal Deposition on a Ti-6al-4v Plate
  • 2015
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • There are many challenges in producing aerospace components by metal deposition (MD). One of them is to keep the residual stresses and deformations to a minimum. Anotherone is to achieve the desired material properties in the final component. A computer model can be of great assistance when trying to reduce the negative effects of the manufacturing process. In this work a finite element model is used to predict the thermo-mechanical response during the MD-process. This work features a pysically based plasticity model coupled with a microstructure evolution model for the titanium alloy Ti-6Al-4V. A thermally driven microstructure model is used to derive the evolution of the non-equilibrium compositions of α-phases and β-phase. Addition of material is done by activation of elements. The method is taking large deformations into consideration and adjusts the shape and position of the activated elements. This is particularilly important when adding material onto thin and flexible structures. The FE-model can be used to evaluate the effect of different welding sequenses. Validation of the model is performed by comparing measured deformations, strains, residual stresses and temperatures with the computed result. The deformations, strains and temepratures are measured during the process. The deformations are measured with a LVDT-gauge at one location. The strains are measured with a strain gauge at the same location as the deformations. The temperature is measured at five locations, close to the weld and with an increasing distance of one millimeter between each thermo couple. The residual stresses in MD component were measured non-destructively using high-energy synchrotron X-ray diffraction on beam line ID15A at the ESRF, Grenoble.
  •  
47.
  • Mahade, Satyapal, et al. (författare)
  • Processing of high-performance materials by laser directed energy deposition with wire
  • 2023. - 1.
  • Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 260-305
  • Bokkapitel (refereegranskat)abstract
    • Processing of metallic materials by Laser Directed Energy Deposition (LDED), with Wire (w) as the feedstock, enables the manufacturing of high precision, near-net shape components that require minimal postmachining, without compromising the performance. L-DEDw has also shown the capability to add intricate features on large structures, which makes it an attractive fabrication technique for aerospace application. The key merits of wire as the feedstock when compared to powder include; higher deposition rates, low porosity in the deposited material, excellent surface finish, and, ∼ 100% utilization of the feedstock. However, despite the attractive merits, the difference in solidification rates during L-DEDw processing when compared to other fabrication routes could induce high residual stresses, which can be detrimental to the integrity of cracksensitive alloys. Additionally, there exists an inherent challenge during L-DEDw fabrication, where controlling the process variables to ensure stable deposition conditions becomes essential to achieve repeatable, and desired results. The recent advancements in the area of monitoring and control systems, and their integration with L-DEDw processing, have enabled to overcome the processing instability related challenges. Furthermore, different L-DEDw processing strategies for alleviating residual stresses (tensile) accumulation in the deposits are discussed, which could enable defectfree, high-performance component fabrication. Although the utilization of L-DEDw for processing diverse alloy systems has been explored in the literature, the current chapter's scope is restricted to L-DEDw processing of Nickel-based and Titanium-based alloys, which are often utilized in the aeroengine. This work aims to provide a holistic perspective and shed light on the state-of-the-art, recent developments, sustainability aspects and future directions for L-DEDw processed, highperformance Ni-based and Ti-based alloys.
  •  
48.
  • Neikter, Magnus, 1988-, et al. (författare)
  • Alpha texture variations in additive manufactured Ti-6Al-4V investigated with neutron diffraction
  • 2018
  • Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 23, s. 225-234
  • Tidskriftsartikel (refereegranskat)abstract
    • Variation of texture in Ti-6Al-4V samples produced by three different additive manufacturing (AM) processes has been studied by neutron time-of-flight (TOF) diffraction. The investigated AM processes were electron beam melting (EBM), selective laser melting (SLM) and laser metal wire deposition (LMwD). Additionally, for the LMwD material separate measurements were done on samples from the top and bottom pieces in order to detect potential texture variations between areas close to and distant from the supporting substrate in the manufacturing process. Electron backscattered diffraction (EBSD) was also performed on material parallel and perpendicular to the build direction to characterize the microstructure. Understanding the context of texture for AM processes is of significant relevance as texture can be linked to anisotropic mechanical behavior. It was found that LMwD had the strongest texture while the two powder bed fusion (PBF) processes EBM and SLM displayed comparatively weaker texture. The texture of EBM and SLM was of the same order of magnitude. These results correlate well with previous microstructural studies. Additionally, texture variations were found in the LMwD sample, where the part closest to the substrate featured stronger texture than the corresponding top part. The crystal direction of the α phase with the strongest texture component was [112¯3]. © 2018 Elsevier B.V.
  •  
49.
  • Neikter, Magnus, 1988-, et al. (författare)
  • Defect characterization of electron beam melted Ti-6Al-4V and Alloy 718 with X-ray microtomography
  • 2018
  • Ingår i: Aeronautics and Aerospace Open Access Journal. - : MedCrave Group. - 2576-4500. ; 2:3, s. 139-145
  • Tidskriftsartikel (refereegranskat)abstract
    • Electron beam melting (EBM) is emerging as a promising manufacturing process where metallic components are manufactured from three-dimensional (3D) computer aided design models by melting layers onto layers. There are several advantages with this manufacturing process such as near net shaping, reduced lead times and the possibility to decrease weight by topology optimization, aspects that are of interest for the aerospace industry. In this work two alloys, Ti-6Al-4V and Alloy 718, widely used within the aerospace industry were investigated with X-ray microtomography (XMT), to characterize defects such as lack of fusion (LOF) and inclusions. It was furthermore possible to view the macrostructure with XMT, which was compared to macrostructure images obtained by light optical microscopy (LOM). XMT proved to be a useful tool for defect characterization and both LOF and un-melted powder could be found in the two investigated samples. In the EBM built Ti-6Al-4V sample high density inclusions, believed to be composed of tungsten, were found. One of the high-density inclusions was found to be hollow, which indicate that the inclusion stems from the powder manufacturing process and not related with the EBM process. By performing defect analyses with the XMT software it was also possible to quantify the amount of LOF and un-melted powder in vol%. From the XMT-data meshes were produced so that finite element method (FEM) simulations could be performed. From these FEM simulations the significant impact of defects on the material properties was evident, as the defects led to high stress concentrations. It could moreover, with FEM, be shown that the as-built surface roughness of EBM material is of importance as high surface roughness led to increased stress concentrations.
  •  
50.
  • Neikter, Magnus, 1988-, et al. (författare)
  • Fatigue Crack Growth of Electron Beam Melted Ti-6Al-4V in High-Pressure Hydrogen
  • 2020
  • Ingår i: Materials. - : MDPI. - 1996-1944. ; 13:6
  • Tidskriftsartikel (refereegranskat)abstract
    • Titanium-based alloys are susceptible to hydrogen embrittlement (HE), a phenomenon that deteriorates fatigue properties. Ti-6Al-4V is the most widely used titanium alloy and the effect of hydrogen embrittlement on fatigue crack growth (FCG) was investigated by carrying out crack propagation tests in air and high-pressure H2 environment. The FCG test in hydrogen environment resulted in a drastic increase in crack growth rate at a certain Δ K, with crack propagation rates up to 13 times higher than those observed in air. Possible reasons for such behavior were discussed in this paper. The relationship between FCG results in high-pressure H2 environment and microstructure was investigated by comparison with already published results of cast and forged Ti-6Al-4V. Coarser microstructure was found to be more sensitive to HE. Moreover, the electron beam melting (EBM) materials experienced a crack growth acceleration in-between that of cast and wrought Ti-6Al-4V.
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