| 1. |
- Balachandramurthi, Arun Ramanathan, 1989-, et al.
(författare)
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Microstructure tailoring in Electron Beam Powder Bed Fusion Additive Manufacturing and its potential consequences
- 2019
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Ingår i: Results in Materials. - : Elsevier. - 2590-048X. ; 1
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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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| 2. |
- Balachandramurthi, Arun Ramanathan, 1989-
(författare)
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Towards understanding the fatigue behaviour of Alloy 718 manufactured by Powder Bed Fusion processes
- 2020
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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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| 3. |
- Cedergren, Stefan, 1982, et al.
(författare)
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Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V
- 2016
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Ingår i: The International Journal of Advanced Manufacturing Technology. - London : Springer. - 0268-3768 .- 1433-3015. ; 84:9-12, s. 2277-2291
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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.
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| 4. |
- Maimaitiyili, Tuerdi, et al.
(författare)
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Residual Lattice Strain and Phase Distribution in Ti-6Al-4V Produced by Electron Beam Melting
- 2019
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 12:4
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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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| 5. |
- Adegoke, Olutayo, 1979-, et al.
(författare)
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Influence of laser powder bed fusion process parameters on the microstructure and cracking susceptibility of nickel-based superalloy Alloy 247LC
- 2022
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Ingår i: Results in Materials. - : Elsevier. - 2590-048X. ; 13
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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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| 6. |
- Adegoke, Olutayo, 1979-, et al.
(författare)
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Influence of laser powder bed fusion process parameters on the microstructure of solution heat-treated nickel-based superalloy Alloy 247LC
- 2022
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Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 183
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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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| 7. |
- Adegoke, Olutayo, et al.
(författare)
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Influence of laser powder bed fusion process parameters on voids, cracks, and microhardness of nickel-based superalloy alloy 247LC
- 2020
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Ingår i: Materials. - 1996-1944. ; 13:17
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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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| 8. |
- Adegoke, Olutayo
(författare)
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Processability of Laser Powder Bed Fusion of Alloy 247LC : Influence of process parameters on microstructure and defects
- 2020
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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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| 9. |
- Adegoke, Olutayo, 1979-
(författare)
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Processability of Laser Powder Bed Fusion of Alloy 247LC-Influence of process parameters on microstructure and defects
- 2021
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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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| 10. |
- Adegoke, Olutayo, et al.
(författare)
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Review of laser powder bed fusion of gamma-prime-strengthened nickel-based superalloys
- 2020
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Ingår i: Metals. - : MDPI AG. - 2075-4701. ; 10:8
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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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