| 1. |
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| 2. |
- Goel, Sneha, 1993-, et al.
(författare)
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Residual stress determination by neutron diffraction in powder bed fusion-built Alloy 718: Influence of process parameters and post-treatment
- 2020
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Ingår i: Materials and Design. - : Elsevier BV. - 1873-4197 .- 0264-1275. ; 195
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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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| 3. |
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| 4. |
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| 5. |
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| 6. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Fatigue Crack Growth of Electron Beam Melted Ti-6Al-4V in High-Pressure Hydrogen
- 2020
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Ingår i: Materials. - : MDPI. - 1996-1944. ; 13:6
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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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| 7. |
- Alvi, Sajid, et al.
(författare)
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Tribological performance of Ti6Al4V at elevated temperatures fabricated by electron beam powder bed fusion
- 2021
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Ingår i: Tribology International. - : Elsevier. - 0301-679X .- 1879-2464. ; 153
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Tidskriftsartikel (refereegranskat)abstract
- Electron beam powder bed fusion (EBPBF) is a beneficial processing route to fabricate Ti6Al4V alloy for aerospace applications due to its relatively low lead time and the possibility of topology optimization. The dry sliding wear behavior of EBPBF-Ti6Al4V against steel and alumina counterballs from room temperature (RT) to 500 °C was investigated to evaluate the influence of EBPBF processing and microstructure on the wear properties for broadening the application criteria of this lightweight alloy. The wear tests revealed that the wear rate decreased with increasing temperature due to formation of stable oxide glaze layer. This study reveals elevated temperature sliding wear behavior, wear mechanisms and microstructural changes below the wear track of EBPBF Ti6Al4V alloy against steel and alumina counterbodies.
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| 8. |
- Andersson, Joel, 1981-, et al.
(författare)
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Welding of special alloys
- 2023
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Ingår i: Welding of Metallic Materials. - : Elsevier. - 9780323906708 - 9780323905527 ; , s. 279-316
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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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| 9. |
- Edin, Emil
(författare)
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Effect of Stress Relief Annealing: Part Distortion, Mechanical Properties, and Microstructure of Additively Manufactured Austenitic Stainless Steel
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing (AM) processes may introduce large residual stresses in the as-built part, in particular the laser powder bed fusion process (L-PBF). The residual stress state is an inherent consequence of the heterogeneous heating and subsequent cooling during the process. L-PBF has become renowned for its “free complexity” and rapid prototyping capabilities. However, it is vital to ensure shape stability after the component is removed from the build plate, which can be problematic due to the residual stress inducing nature of this manufacturing process. Residual stresses can be analyzed via many different characterization routes (e.g. X-ray and neutron diffraction, hole drilling, etc.), both quantitatively and qualitatively. From an industrial perspective, most of these techniques are either prohibitively expensive, complex or too slow to be implementable during the early prototyping stages of AM manufacturing.In this work a deformation based method employing a specific geometry, a so called “keyhole”-geometry, has been investigated to qualitatively evaluate the effect of different stress relief annealing routes with respect to macroscopic part deformation, mechanical properties and microstructure. Previous published work has focused on structures with open geometry, commonly referred to as bridge-like structures where the deformation required for analysis occurs during removal from the build plate. The proposed keyhole-geometry can be removed from the build plate without releasing the residual stresses required for subsequent measurement, which enables bulk manufacturing on single build plates, prior to removal and stress relief annealing. Two L-PBF manufactured austenitic stainless steel alloys were studied, 316L and 21-6-9. Tensile specimen blanks were manufactured and the subsequent heat treatments were carried out in pairs of keyhole and tensile blank. Both a contact (micrometer measurement), and a non-contact (optical profilometry) method were employed to measure the residual stress induced deformation in the keyholes. The annealing heat treatment matrix was iteratively expanded with input from the deformation analysis to find the lowest temperature at which approximately zero deformation remained after opening the structure via wire electrical discharge machining. The lowest allowable annealing temperature was sought after to minimize strength loss. After stress relief annealing at 900 ℃ for 1 hour, the 316L keyhole-geometry was considered shape stable. The lateral micrometer measurement yielded a length change of 1 µm, and a radius of 140 m (over the 22 mm top surface) was assigned from curve fitting the top surface height profiles. The complementary microstructural characterization revealed that this temperature corresponded to where the last remains of the cellular sub-grain structures disappears. Tensile testing showed that the specimen subjected to the 900 ℃ heat treatment had a marked reduction in yield stress (YS) compared to that of the as-built: 540 MPa → 402 MPa, whereas ultimate tensile strength (UTS) only reduced slightly: 595 MPa → 570 MPa. The ductility (4D elongation) was found to be ~13 % higher for the specimen heat treated at 900 ℃ than that of the as-built specimen, 76% and 67% respectively. For alloy 21-6-9 the residual stress induced deformation minimum (zero measurable deformation) was found after stress relief heat treatment at 850 ℃ for 1 hour. Slight changes in the microstructure were observable through light optical microscopy when comparing the different heat treatment temperatures. The characteristic sub-grain features associated with alloy 316L were not verified for alloy 21-6-9. Similar to the results for 316L, UTS was slightly lower for the tensile specimen subjected to the heat treatment temperature required for shape stability (850 ℃) compared to the as-built specimen: 810 MPa → 775 MPa. The measured ductility (4D elongation) was found to be approximately equal for the as-built (47%), and heat treated (48%) specimen. As-built material exhibited a YS of 640 MPa while the heat treated specimen had a YS of 540 MPa. For alloy 21-6-9, the lateral micrometer deformation measurements were compared with stress relaxation testing performed at 600 ℃, 700℃ and 800 ℃. Stress relaxation results were in good agreement with the results from the lateral deformation measurements. The study showed that for both steel alloys, the keyhole method could be successfully employed to rapidly find a suitable stress relief heat treatment route when shape stability is vital.
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| 10. |
- Edin, Emil, et al.
(författare)
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Stress relief heat treatment and mechanical properties of laser powder bed fusion built 21-6-9 stainless steel
- 2023
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Ingår i: Materials Science and Engineering A. - : Elsevier. - 0921-5093 .- 1873-4936. ; 868
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the effectiveness of residual stress relief annealing on a laser powder bed fusion (L-PBF) manufactured austenitic stainless steel, alloy 21-6-9 was investigated. Residual stress levels were gauged using geometrical distortion and relaxation testing results. In the investigated temperature interval (600–850 °C), shape stability was reached after subjecting the as-built material to an annealing temperature of 850 °C for 1 h. Microstructural characterization and tensile testing were also performed for each annealing temperature to evaluate the alloy's thermal stability and the resulting tensile properties. In the as-built state, a yield strength (YS) of 640 MPa, ultimate tensile strength (UTS) of 810 MPa and 4D elongation of 47% were measured. Annealing at 850 °C for 1 h had little measurable effect on ductility (48% 4D elongation) while still having a softening effect (UTS = 775 MPa, YS = 540 MPa). From the microstructural characterization, cell-like features were observed sporadically in the annealed condition and appeared stable up until 800 °C after which gradual dissolution began, with the last remnants disappearing after subjecting the material to 900 °C for 1 h.
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| 11. |
- Heidarzadeh, Akbar, et al.
(författare)
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Post-treatment of additively manufactured Fe-Cr-Ni stainless steels by high pressure torsion : TRIP effect
- 2021
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 811
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Tidskriftsartikel (refereegranskat)abstract
- High pressure torsion (HPT) at room temperature was used for post-treatment of additively manufactured Fe-Cr-Ni stainless steel with 12.9 wt. % Ni as a very strong austenite stabilizer. The results showed that HPT caused a considerable increase in nanohardness of the additively manufactured samples. In contrast with thermodynamic equilibrium-state modeling, a phase transformation from FCC to HCP structure occurred, leading to the formation of ε-martensite during HPT on high angle boundaries, low angle boundaries, and dislocation cells with no detection of deformation twins. It was demonstrated that the combination of additive manufacturing thanks to the high density of dislocations after solidification and HPT process expands the opportunities of both methods to control deformation mechanisms in stainless steels leading to different phase and microstructural features. Thus, the outcome of this study provides a fundamental basis to design advanced structural materials.
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| 12. |
- Karimi Neghlani, Paria, 1986-, et al.
(författare)
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Processing of high-performance materials by electron beam-powder bed fusion
- 2023. - 1.
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Ingår i: Additive Manufacturing of High-Performance Metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 103-181
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Bokkapitel (refereegranskat)abstract
- Electron beam-powder bed fusion (EB-PBF) is a process that uses a highly intense electron beam to melt metallic powders to create parts. In comparison to a conventional process, EB-PBF is more efficient at producing customized and specific parts in industries such as aerospace, space, and medical. Additionally, the EB-PBF process is used to manufacture highly complex parts for which other technologies would be prohibitively expensive or difficult to apply; increased geometric complexity does not necessarily imply increased cost. However, because the interaction of the electron beam with the powder and substrate material is complex, a high level of knowledge is required to master the skill of producing structurally sound components. This chapter discusses crucial features of the process parametermicrostructure-defect relationship that must be taken into Electron beam-powder bed fusion (EB-PBF) is a process that uses a highly intense electron beam to melt metallic powders to create parts. In comparison to a conventional process, EB-PBF is more efficient at producing customized and specific parts in industries such as aerospace, space, and medical. Additionally, the EB-PBF process is used to manufacture highly complex parts for which other technologies would be prohibitively expensive or difficult to apply; increased geometric complexity does not necessarily imply increased cost. However, because the interaction of the electron beam with the powder and substrate material is complex, a high level of knowledge is required to master the skill of producing structurally sound components. This chapter discusses crucial features of the process parametermicrostructure-defect relationship that must be taken into account in order to generate sufficiently sound builds of highperformance materials employing EB-PBF.
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| 13. |
- Mahade, Satyapal, et al.
(författare)
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Processing of high-performance materials by laser directed energy deposition with wire
- 2023. - 1.
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Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 260-305
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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.
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| 14. |
- 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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| 15. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Alpha texture variations in additive manufactured Ti-6Al-4V investigated with neutron diffraction
- 2018
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 23, s. 225-234
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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.
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| 16. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Defect characterization of electron beam melted Ti-6Al-4V and Alloy 718 with X-ray microtomography
- 2018
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Ingår i: Aeronautics and Aerospace Open Access Journal. - : MedCrave Group. - 2576-4500. ; 2:3, s. 139-145
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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.
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| 17. |
- Neikter, Magnus, et al.
(författare)
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Microstructural characterization and comparison of Ti-6Al-4V manufactured with different additive manufacturing processes
- 2018
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Ingår i: Materials Characterization. - : Elsevier. - 1044-5803 .- 1873-4189. ; 143:SI, s. 68-75
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the microstructures of Ti-6Al-4V manufactured by different additive manufacturing (AM) processes have been characterized and compared. The microstructural features that were characterized are the α lath thickness, grain boundary α (GB-α) thickness, prior β grain size and α colony size. In addition, the microhardnesses were also measured and compared. The microstructure of shaped metal deposited (SMD) Ti-6Al-4V material showed the smallest variations in α lath size, whereas the material manufactured with laser metal wire deposition-0 (LMwD-0) showed the largest variation. The prior β grain size was found to be smaller in material manufactured with powder bed fusion (PBF) as compared with corresponding material manufactured with the directed energy deposition (DED) processes. Parallel bands were only observed in materials manufactured with DED processes while being non-present in material manufactured with PBF processes.
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| 18. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Microstructural characterization of binary microstructure pattern in selective laser-melted Ti-6Al-4V
- 2019
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Ingår i: The International Journal of Advanced Manufacturing Technology. - : Springer. - 0268-3768 .- 1433-3015. ; 104:1-4, s. 1381-1391
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Tidskriftsartikel (refereegranskat)abstract
- Selective laser melting (SLM) is an additive manufacturing process that offers efficient manufacturing of complex parts with good mechanical properties. For SLM, process parameters and post-processing are of importance as they affect the microstructure and consequently the mechanical properties. A feature in the microstructure, which is formed in SLM due to the fast cooling rate, is a binary microstructure pattern (BMP). The BMP is found in the horizontal plane and is formed with various laser scan angles between adjacent layers. The easiest distinguishable strategy is 90°, which renders a shape similar to a chessboard. In this work, the BMP phenomenon was investigated in detail and a microstructural characterization was performed on the fine microstructure zone (FMZ) that separates the coarse microstructure zones (CMZ), by using light optical and scanning electron microscopes (SEM) that were equipped with electron backscattered (EBSD) and energy dispersive x-ray spectroscopy (EDS) detectors. Moreover, the effect of the process parameter hatch distance on the BMP was investigated and the overlapping between neighboring scan tracks in SLM was found to influence the size of the BMP, while the thickness of the FMZ remained constant. Different post-SLM heat treatments were performed and it was shown that the BMP retained unless the heat treatment temperature reached above the β transus temperature. EBSD and β grain reconstruction were performed as well to reveal the columnar β grain orientations. The result showed that each CMZ and FMZ originates from a respective parent β grains.
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| 19. |
- Neikter, Magnus, et al.
(författare)
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Microstructure and Defects in Additive Manufactured Titanium : a Comparison Between Microtomography and Optical Microscopy
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this work has been to compare two different analysing methods; x-ray microtomography and light optical microscopy, when it comes to defects and microstructure of additively manufactured Ti-6Al-4V. The results show that both techniques have their pros and cons:microtomography is the preferred choice for defect detection by analysing the full 3D sample volume, while light optical microscopy is better for analysing finer details in 2D.
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| 20. |
- Neikter, Magnus, 1988-
(författare)
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Microstructure and hydrogen embrittlement of additively manufactured Ti-6Al-4V
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work of this doctoral thesis has been performed between the 24th of August 2015 to the 6th of September 2019. The general purpose of this work has been to increase the knowledge of additively manufactured Ti-6Al-4V when it comes to microstructure, texture and hydrogen embrittlement. Several additive manufacturing (AM) processes have been addressed, the main focus, however, has been on the two processes electron beam melting (EBM) and selective laser melting (SLM). The work has been performed at Luleå University of Technology and at Monash Centre of Additive Manufacturing (MCAM), which is a part of Monash University, Melbourne Australia. GKN Aerospace Engine Systems has been involved in the work as a collaborative industrial partner. Where the main interaction and support has been between GKN in Trollhättan, Sweden. GKNs facility in Filton, United Kingdom, however, has also been involved in sample production. The texture measurements using neutron time of flight (TOF) diffraction were performed in Dubna, Russia at Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research, using the spektrometer kolitshestvennovo analiza tekstury (SKAT) instrument and in Los Alamos at Los Alamos Neutron Science Center, using the texture instrument high pressure preferred orientation (HIPPO). The interest of specifically additively manufactured Ti-6Al-4V originates from GKN Aerospace Engine Systems as they have been contracted to manufacture parts for the Ariane rockets. In aerospace, weight is of utter importance as reduced weight of the components reduce the fuel costs and makes it possible to transport increased loads. Therefore, titanium has been suggested to replace heavier metal alloys (such as nickel-based superalloys) due to titanium’s high specific strength and overall good properties. The work has been managed under the supervision of Professor Marta-Lena Antti, Professor Robert Pederson and Dr. Pia Åkerfeldt at Luleå University of Technology and by Professor Xinhua Wu and Professor Aijun Huang at MCAM. The project has been financed by three parties Space for Innovation and Growth (RIT), Nationellt Rymdtekniskt Forskningsprogram (NRFP) and the Graduate School of Space Technology. The thesis is composed of a literature review along with the appended papers that have been written throughout the duration of the project. The purpose of the literature review is to give the reader a broader knowledge and context about the theory behind the work conducted during the Ph.D. project. The thesis commences with a theoretical background about titanium, its crystal structures, microstructure, and AM. The chapter about AM is divided into three sub-chapters, explaining the two different AM sub-groups directed energy deposition (DED) and powder bed fusion (PBF). Then the chapter finalizes with explaining the benefits and drawbacks with AM. Once these subjects been explained there is a chapter explaining AM built Ti-6Al-4V. The theme of this chapter is the differences in the microstructure of AM built and conventional built Ti-6Al-4V, where phenomena like chessboard pattern and layers bands are discussed, followed by a comparative section of the mechanical properties. The chapter about AM built Ti-6Al-4V is followed by a chapter about texture where the two texture techniques electron backscattered diffraction (EBSD) and neutron (TOF) diffraction are explained. The literature review then continues with the chapters neutron tomography, hydrogen embrittlement, and fatigue crack growth. Then there is a chapter about the materials and experimental methods that have been used to perform all the experiments, then a summary of all the papers and conclusions. After the conclusions, the thesis finalizes with a chapter about possible future work and an outlook into the future of AM. In the end, the papers I to IV are appended.
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| 21. |
- Neikter, Magnus, 1988-
(författare)
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Microstructure and Texture of Additive Manufactured Ti-6Al-4V
- 2017
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing (AM) for metals is a manufacturing process that has increased a lot in popularity last few years as it has experienced significant improvements since its beginning, both when it comes to accuracy and deposition rates. There are many different AM processes where the energy sources and deposition methods varies. But the common denominator is their layer wise manufacturing process, melting layer on layer. AM has a great design freedom compared to conventional manufacturing, making it possible to design new structures with decreased weight and increased performance. A drawback is slow manufacturing speeds, making it more expensive. But when it comes to low lot sizes and complex structures AM is very competitive. So, for the aerospace and space industry AM is a good option as manufacturing cost is less of an issue and where saving weight is of great concern, both environmentally and economically. There are however many topics left to research before additive manufactured titanium can be widely adopted for critical components, such as microstructure and texture development and its correlation to mechanical properties. The aim of this work has been to investigate the microstructure and texture of various AM processes. Microstructural features such as prior β grains, grain boundary α (GB-α), α laths, α colonies have been characterized along with hardness measurements for 5 different AM processes. Some of these AM processes have also been investigated in the SKAT instrument in Dubna, Russia, to obtain their texture. These textures have then been compared with one another and correlated to previous microstructural investigations and mechanical properties. This is important knowledge as the microstructure and the texture sets the basis for the mechanical properties. In case there is a high texture, the material can have anisotropic mechanical behavior, which could be either wanted or unwanted for different applications. Some the findings are that α phase was found to increase in the prior β grain boundary for the AM processes with low cooling rates, while it was discontinuous and even non-present for the AM processes with high cooling rates. The prior β size are larger for the directed energy deposition (DED) processes than for the powder bed fusion (PBF) processes. Parallel bands were present for the DED process while being non-present for the PBF processes. Concerning the texture, it was found that LMwD had a higher texture than EBM and SLM. Texture inhomogeneity was also found for the LMwD process., where two parts of the same sample was investigated and the material closer to the surface had higher texture.
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| 22. |
- Neikter, Magnus, et al.
(författare)
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Microstructure characterisation of Ti-6Al-4V from different additive manufacturing processes
- 2017
-
Ingår i: IOP Conference Series. - : Institute of Physics (IOP). - 1757-8981 .- 1757-899X. ; 258, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- The focus of this work has been microstructure characterisation of Ti-6Al-4V manufactured by five different additive manufacturing (AM) processes. The microstructure features being characterised are the prior β size, grain boundary α and α lath thickness. It was found that material manufactured with powder bed fusion processes has smaller prior β grains than the material from directed energy deposition processes. The AM processes with fast cooling rate render in thinner α laths and also thinner, and in some cases discontinuous, grain boundary α. Furthermore, it has been observed that material manufactured with the directed energy deposition processes has parallel bands, except for one condition when the parameters were changed, while the powder bed fusion processes do not have any parallel bands.
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| 23. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Tailored ductility and strength for enhanced impact toughness of laser powder fusion built Alloy 718
- 2021
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Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 884
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Tidskriftsartikel (refereegranskat)abstract
- Impact toughness of Alloy 718 built via laser powder bed fusion (LPBF) in as-built and thermally post treated conditions were investigated. The effect of various stages in the thermal post-treatment, including stress relief, hot isostatic pressing, solution treatment, and aging on the microstructure, texture, ductility, hardness, and impact toughness were studied. The greatest impact toughness was found in the as-built Alloy 718 material, associated with the high ductility of the material. The ductility results were also inversely related to the hardness of the investigated materials. Where the material with the highest ductility had the lowest hardness etc. Anisotropy in the impact toughness behavior was present in the as-built and post-heat treated specimens, which was explained by the presence of texture in all of the investigated material. With the applied heat treatments, recrystallization occurred and the preferential crystal orientations were randomized, decreasing the texture. The thermal post-treating conditions rendered different types of microstructures, with various grain sizes. The carbide content remained the same for the three investigated thermal post-treating conditions.
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| 24. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Tensile properties of 21-6-9 austenitic stainless steel built using laser powder-bed fusion
- 2021
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Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 14:15
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Tidskriftsartikel (refereegranskat)abstract
- Alloy 21-6-9 is an austenitic stainless steel with high strength, thermal stability at high temperatures, and retained toughness at cryogenic temperatures. This type of steel has been used for aerospace applications for decades, using traditional manufacturing processes. However, limited research has been conducted on this alloy manufactured using laser powder-bed fusion (LPBF). Therefore, in this work, a design of experiment (DOE) was performed to obtain optimized process parameters with regard to low porosity. Once the optimized parameters were established, horizontal and vertical blanks were built to investigate the mechanical properties and potential anisotropic behavior. As this alloy is exposed to elevated temperatures in industrial applications, the effect of elevated temperatures (room temperature and 750◦C) on the tensile properties was investigated. In this work, it was shown that alloy 21-6-9 could be built successfully using LPBF, with good properties and a density of 99.7%, having an ultimate tensile strength of 825 MPa, with an elongation of 41%, and without any significant anisotropic behavior. © 2021 by the authors.
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| 25. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Tensile properties of laser powder bed fusion built JBK-75 austenitic stainless steel
- 2023
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Ingår i: Materials Science & Engineering. - : Elsevier Ltd. - 0921-5093 .- 1873-4936. ; 874
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Tidskriftsartikel (refereegranskat)abstract
- Laser powder bed fusion (PBF-LB) is an additive manufacturing (AM) process that has several advantages to conventional manufacturing, such as near net-shaping capabilities and reduced material wastage. To be able to manufacture a novel material, however, one needs to first optimize the process parameters, to decrease porosity content as low as possible. Therefore, in this work the process parameters of PBF-LB built JBK-75 austenitic stainless steel, and its influence on porosity, microstructure, and hardness have been investigated. The least amount of porosity was found by using 132 W laser power, 750 mm/s scan speed, layer thickness 30 μm, and 0.12 mm hatch distance. These process parameters were then used to manufacture material for tensile testing, to investigate the tensile properties of PBF-LB built JBK-75 and potential anisotropic behavior. Hot isostatic pressing (HIP) was also performed in two sets of samples, to investigate the effect of pore closure on the tensile properties. The ultimate tensile strength (UTS) for the un-HIPed specimens was 1180 (horizontally built) and 1110 (vertically built) MPa. For the HIPed specimens, it was 1160 (horizontally built) and 1100 (vertically built) MPa. The anisotropic presence was explained by the presence of texture, with a multiple of random distribution (MRD) up to 4.34 for the {001} planes, and defects.
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| 26. |
- Neikter, Magnus, 1988-, et al.
(författare)
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Texture of electron beam melted Ti-6Al-4V measured with neutron diffraction
- 2020
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Konferensbidrag (refereegranskat)abstract
- Texture in materials is important as it contributes to anisotropy in the bulk mechanical properties. Ti-6Al-4V built with the additive manufacturing process (AM) electron beam melting (EBM) has been found to have anisotropic mechanical properties. Therefore, this work has been performed to investigate the texture variations of EBM built Ti-6Al-4V with neutron time of flight (TOF). For the work, samples were produced with different build geometries off-set by 90 degrees. A cast sample was additionally analyzed to investigate the bulk texture of conventionally manufactured material. Microstructural characterization was performed and the cast material was found to have a coarse colony α microstructure, whereas the EBM built material had a finer basket weave microstructure. Overall, the texture of the EBM built material was found to be weak having an multiple of random distribution (MRD) index of ~1 for the α phase, whilst the cast material possessed more than twice the amount of preferred orientation i.e. MRD 2.51 for the α phase.
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| 27. |
- Pederson, Robert, 1973-, et al.
(författare)
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Metal additive manufacturing : Motivation, process portfolio, and application potential
- 2023. - 1
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Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 20-40
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Bokkapitel (refereegranskat)abstract
- The idea of adding material only where needed to manufacturesolid metallic high-performing components is intriguing andone of the main reasons for the great interest in additivemanufacturing (AM) around the world. Especially whensustainability comes into play, as in recent times more thanever, AM technology is most appropriate since it enables almostfull material utilization with minimal waste. From an economicstandpoint, this becomes particularly advantageous for moreexpensive materials such as superalloys and titanium alloys.However, the route of going from a CAD drawing of a part to anadditively manufactured final component that is qualified and inserial production involves numerous challenges. The intentionof this book is to shed light on and explain some of theassociated challenges beginning with the importance of thestarting material and how it is manufactured, i.e., wire orpowder, continuing into description of the conventional andPederson, R., Andersson, J., & Joshi, S. (2023). Additive manufacturing of high-performance metallic materials. Elsevier.Created from vast-ebooks on 2024-01-08 16:09:20. Copyright © 2023. Elsevier. All rights reserved.most commonly used AM processes, followed by postbuildtreatments and nondestructive evaluations, to eventuallyproduce the final part with mechanical performance consistentwith the application requirements. In the end, selected realindustry examples of AM parts for actual applications will bepresented
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| 28. |
- Raza, Tahira, 1972-, et al.
(författare)
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Processing of high-performancematerials by laser beam-powderbed fusion
- 2023. - 1.
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Ingår i: Additive Manufacturing of High-Performance Metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 182-229
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Bokkapitel (refereegranskat)abstract
- Processing of high-performance materials by laser beam powder bed fusion (LB-PBF) provides an alternative manufacturing route to, i.e., investment casting and is suitable for production of high-performance materials having complex geometry such as turbine blades. The main benefit of powder bed fusion in general is associated with the fact that increased geometrical complexity does not add any cost. However, the processability of the alloys of interest is closely linked to process parameters where highperformance materials belong to a special class of materials that need substantial attention to avoid problems, not at least with regard to different types of cracking. In this chapter, the relationship between process parameter-microstructure-defect relationship will be discussed and analyzed.
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| 29. |
- Saeidi, Kamran, et al.
(författare)
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316L stainless steel designed to withstand intermediate temperature
- 2017
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Ingår i: Materials & design. - : Elsevier BV. - 0264-1275 .- 1873-4197. ; 135, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- Austenitic stainless steel 316L was fabricated for withstanding elevated temperature by selective laser melting (SLM). Tensile tests at 800 degrees C were carried out on laser melted 316L with two different strain rates of 0.05 S-1 and 0.25 S-1. The laser melted 316L showed tensile strength of approximately 400 MPa at 800 degrees C, which was superior to conventional 316L. Analysis of fracture surface showed that the 316L fractured in mixed mode, ductile and brittle fracture, with an elongation of 18% at 800 degrees C. In order to understand the mechanical response, laser melted 316L was thermally treated at 800 degrees C for microstructure and phase stability. X-ray diffraction (XRD) and Electron back scattered diffraction (EBSD) of 316L treated at 800 degrees C disclosed a textured material with single austenitic phase. SEM and EBSD showed that the characteristic and inherent microstructure of laser melted 316L, consisting of elongated grains with high angle grain boundaries containing subgrains with a smaller misorientation, remained similar to as-built SLM 316L during hot tensile test at 800 degrees C. The stable austenite phase and its stable hierarchical microstructure at 800 degrees C led to the superior mechanical response of laser melted 316L.
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| 30. |
- Segerstark, Andreas, 1988-, et al.
(författare)
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Processing of high-performance materials by laser-directed energy deposition with powders
- 2023. - 1.
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Ingår i: Additive Manufacturing of High-Performance metallic Materials. - : Elsevier. - 9780323918855 - 9780323913829 ; , s. 230-259
-
Bokkapitel (refereegranskat)abstract
- Processing of high-performance materials by Directed Energy Deposition with Powders (L-DED-P) is frequently utilized in repair as well as remanufacturing apart from manufacturing. One benefit of the process is the low heat input in comparison to, i.e., L-DED with wire which is preferable regarding residual stresses and distortion. However, care must be taken to minimize defects that are at stake in forming if process parameters are not adequately adapted to the specific application. There is a strong correlation between the process parameters and metallurgical behavior which in turn give rise to potential defects and the final performance of the part to be produced. This chapter gives an overview of the processmicrostructure-defect relations that are of importance in L- DED-P processing.
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| 31. |
- Squillaci, Linda, et al.
(författare)
-
Extending powder particle size distribution of laser powder bed fusion Ti-6Al-4V : investigation of single tracks and multilayer experiments
- 2023
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper explores the effects of varying process parameters (i.e., laser power, laser scanning speed, hatch distance) on the characteristics of single tracks, triple tracks and cubes, in order to provide answers to Research Question 1. A full factorial DoE approach was adopted to produce the experiments. Data was extracted from different sources to find correlations between tracks and multilayer geometries. A digital microscope was used to obtain height profiles, whilst polished/etched cross sections cut parallel to the build direction were imaged using a LOM to obtain measurements of track height, width, melt pool depth, subsurface porosity and residual defect content in cubes. Track height was found to exceed the recoated value of 70μm for both single and triple tracks. The width of single tracks showed a clear upward trend when displayed against VED, showing a lateral expansion as energy input increased. It was also revealed that single tracks expand laterally as they grow above the substrate, indicating swelling. The melt pool depth showed a steady upward trend when plotted against LED, though less systematic than track width. A martensitic microstructure was detected, with hierarchical α’ needles growing at prescribed crystallographic directions within vertical prior-β grains. A large portion of spatter particles and unmelted powder granules were detected on the substrate and tracks, with many accumulating on the side of the tracks forming a denudation zone.
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| 32. |
- Squillaci, Linda
(författare)
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Investigating the effect of extending powder particle size distribution of Ti-6Al-4V produced by powder bed fusion laser beam process : Influence of process parameters on material integrity
- 2023
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on the topic of PBF-LB applied to titanium alloys. Of allalloys, an α + β is chosen, named Ti-6Al-4V. The selection of this particular alloy is driven by its current widespread use in many industrial applications where high strength coupled with low density are both desirable properties. For the last 50 years, parts made with this alloy have been cast or forged and then machined to achieve the final geometry. There is now an opportunity totransform this process chain by additive manufacturing, hence reducing material waste and achieving near net shape from powder feedstock.The process is summarised as follows: a laser selectively melts areas on a build plate where powder is pre-placed. Then a successive powder layer is spread and the process is repeated until completion. Upon removal of the part from the build plate, loose powder in the chamber is collected and recycled whenever possible. The design freedom provided by powder bed fusion methods enables production of intricate geometries and added functionality, despite the need for post-build consolidation and/or microstructural adjustments.Today’s fine and narrow powder cuts (e.g., 15-50μm) are designed to be coupled with low layer thicknesses (i.e., 30μm) to achieve smooth surfaces and high resolutions of small features e.g., internal cooling channels. However, costs associated with production of fine and narrow powder cuts are substantial as refinement of batches requires multiple sieving steps. In addition, resulting building times are considerably long (i.e., days), therefore a beneficial alternative could be that of exploring higher layer thicknesses together with wider and coarser powder cuts.The main idea of this work is to investigate the effects of employing a powder with a wider size distribution 15-90μm. The aim is to reduce the sievingrequired and consequently decrease the costs of developing and building parts made by PBF-LB.An extensive microstructural investigation is conducted on single tracks and cubes built with 27 different process parameter combinations, which also attempts to establish correlations between characteristics of tracks and responses measured in cubes. As a second step, the amount of residual porosity of asbuilt cubes is chosen as the discriminant for further mechanical testing of sub and super-β transus high-pressure heat treated material.
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| 33. |
- Thalavai Pandian, Karthikeyan, 1985-, et al.
(författare)
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Elevated-Temperature Tensile Properties of Low-Temperature HIP-Treated EBM-Built Ti-6Al-4V.
- 2022
-
Ingår i: Materials. - : MDPI. - 1996-1944. ; 15:10
-
Tidskriftsartikel (refereegranskat)abstract
- Evaluation of the high-temperature tensile properties of Ti-6Al-4V manufactured by electron beam melting (EBM) and subjected to a low-temperature hot isostatic pressing (HIP) treatment (800 °C) was performed in this study. The high-temperature tensile properties of as-built and standard HIP-treated (920 °C) materials were studied for comparison. Metallurgical characterization of the as-built, HIP-treated materials was carried out to understand the effect of temperature on the microstructure. As the HIP treatments were performed below the β-transus temperature (995 °C for Ti-6Al-4V), no significant difference was observed in β grain width between the as-built and HIP-treated samples. The standard HIP-treated material measured about 1.4×-1.7× wider α laths than those in the modified HIP (low-temperature HIP)-treated and as-built samples. The standard HIP-treated material showed about a 10-14% lower yield strength than other tested materials. At 350 °C, the yield strength decreased to about 65% compared to the room-temperature strength for all tested specimens. An increase in ductility was observed at 150 °C compared to that at room temperature, but the values decreased between 150 and 350 °C because of the activation of different slip systems.
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| 34. |
- Thalavai Pandian, Karthikeyan, 1985-, et al.
(författare)
-
Fatigue behavior of low-temperature hot isostatic pressed electron beam powder bed fusion manufactured Ti-6Al-4 V
- 2023
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 962
-
Tidskriftsartikel (refereegranskat)abstract
- Ti-6Al-4 V finds application in the fan and compressor modules of gas turbine engines due to its high specific strength. Ti-6Al-4 V components manufactured using one of the additive manufacturing (AM) techniques, the electron beam powder bed fusion (PBF-EB) process, has been an active area of research in the past decade. The fatigue life of such PBF-EB built Ti-6Al-4 V components is improved by hot isostatic pressing (HIP) treatment typically performed at about 920 ˚C. The HIP treatment at 920 ˚C results in coarsening of α laths and reduced static strength and therefore a low-temperature HIP treatment is performed at about 800 ˚C to limit the impact on static mechanical properties. In the present work, the low cycle fatigue and fatigue crack growth behavior of such a modified HIP (low-temperature HIP) treated material is assessed and compared with the respective data for the standard HIP-treated material. The modified HIP-treated material has fatigue performance comparable to the standard HIP-treated material. This work suggests that the modified HIP treatment improves the static mechanical properties without significantly impacting the fatigue performance. Also, fatigue life predictions were made from the measured defect size at the crack initiation site using a linear elastic fracture mechanics tool. The life predictions show good agreement with the experimental values for defects greater than the intrinsic crack length, where life is well predicted by large-crack growth methodology.
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| 35. |
- Thalavai Pandian, Karthikeyan, 1985-
(författare)
-
Microstructure and mechanical properties of Ti-6Al-4V manufactured by electron beam powder bed fusion
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ti-6Al-4V is an α+β titanium alloy that is widely used in aerospace engine applications due to its high specific strength. Typically, Ti-6Al-4V components are manufactured by conventional manufacturing processes such as castings or forgings. An alternative to conventional manufacturing processes is additive manufacturing (AM), which can be used to manufacture Ti-6Al-4V components, with increased geometrical complexity and potentially lower buy-to-fly ratio. AM material have been shown to have similar static mechanical properties as conventionally manufactured material, but the fatigue properties can be affected by the as-built surface quality and the defects in the AM material. Among several AM processes, the fatigue properties of electron beam powder bed fusion (PBF-EB) manufactured material have been primarily impacted by the as-built surface asperities. Therefore, for fatigue-critical applications, the as-built surfaces are typically removed by post-build treatment, making it challenging to manufacture net-shape geometries directly. Exploring ways to improve the surface quality of PBF-EB manufactured Ti-6Al-4V will reduce the post-finishing needs and increase sustainability. In the current thesis the effect of different contour settings on the as-built surface roughness and fatigue properties have been investigated.The results indicate that increasing the number of contours and melting the contours prior to the hatch positively affect the surface roughness. Among several roughness parameters, the parameter related to surface valley depths has a statistically significant influence on the fatigue life. Moreover, advanced characterization techniques such as x-ray computed tomography are needed to capture the hidden valleys open to the surface. Fatigue life prediction using these hidden valley depths resulted in life estimations comparable to experimental results.After eliminating the as-built surface asperities, defects in the bulk become critical for fatigue, typically addressed by post-build heat treatment such as hot isostatic pressing (HIP). HIP treatment of conventionally cast Ti-6Al-4V is typically performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has also viii been adapted for PBF-EB manufactured Ti-6Al-4V, which results in coarsening of α laths and reduction of yield strength compared to as-built material. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the as-built yield strength. However, the fatigue performance of the low-temperature HIP treatment needs to be evaluated which is performed in the current thesis. Even though the low-temperature HIP material had the lowest minimum life cycles to failure, the overall fatigue performance is comparable with that of the standard HIP material. Further, in aerospace engine applications, Ti-6Al-4V normally has a maximum operating temperature of 350 ˚C, therefore the elevated temperature tensile performance has also been investigated in this work. At 350 ˚C, the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility was observed at 150 ˚C compared to that at room temperature, but the ductility decreased between 150 – 350 ˚C because of the activation of different slip systems.
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| 36. |
- Thalavai Pandian, Karthikeyan, 1985-
(författare)
-
Microstructure and mechanical properties of low-temperature hot isostatic pressed Ti-6Al-4V manufactured by electron beam melting
- 2022
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility vi programvaran NASGRO där livsförutsägelserna visade god överensstämmelse med experimentella livscykler i de flesta fall. vii Abstract Title: Microstructure and mechanical properties of low-temperature hot isostatic pressed Ti-6Al-4V manufactured by electron beam melting Keywords: Additive manufacturing, high-temperature tensile properties, low cycle fatigue, neutron diffraction, fatigue crack growth ISBN: 978-91-89325-27-2 (Printed) 978-91-89325-26-5 (Electronic) Ti-6Al-4V is the most widely used α+β titanium alloy in aerospace engine applications due to its high specific strength. Typically, the alloy is manufactured as castings or forgings and then machined to final geometry. These conventional manufacturing processes do however generate a lot of waste material, whereas additive manufacturing (AM) can potentially produce a near-net-shape geometry directly from the feedstock. In the past decade, electron beam melting (EBM), one of the powder bed fusion techniques, has been widely researched to build Ti[1]6Al-4V components. Still, the as-built material can contain defects such as gas pores that require post-processing, such as hot isostatic pressing (HIP) to produce nearly fully dense components. HIP treatment of conventionally cast Ti-6Al-4V is normally performed at 920 ˚C, 100 MPa for 2 hours. This same HIP treatment has then been adapted also for EBM-manufactured Ti-6Al-4V, which however results in coarsening of α laths and reduction of yield strength. Therefore, finding a more appropriate HIP treatment for this new type of Ti-6Al-4V material, i.e. EBM manufactured, would be of great benefit for the industry. Lowering the HIP treatment temperature to 800 ˚C and increasing the pressure to 200 MPa has recently been proven to close the porosity to a high degree while sustaining the high yield strength. In this thesis, the high-temperature tensile properties of EBM-manufactured Ti[1]6Al-4V subjected to a low-temperature (800 ˚C) HIP treatment were evaluated and compared with standard HIP-treated (920 ˚C) materials. Metallurgical characterization of the as-built, HIP-treated materials have been carried out to understand the effect of temperature on the microstructures. The standard HIP[1]treated material measured about 1.4x - 1.7x wider α laths than those in the low[1]temperature HIP treated and as-built samples, respectively. The standard HIP[1]treated material showed about 10 - 14% lower yield strength than other HIP treated materials. At 350 ˚C the yield strength decreases to about 65% compared to the room temperature strength for all tested materials. An increase in ductility viii was observed at 150 ˚C compared to that at room temperature, but the ductility decreased between 150 - 350 ˚C because of activation of different slip systems. The low cycle fatigue (LCF) behavior of such a modified HIP (low-temperature HIP) material is assessed at two different strain levels and compared with the corresponding LCF properties for the standard HIP material. Even though the modified HIP material had lowest minimum life cycles to failure, the overall fatigue performance is comparable with that of the standard HIP material. Also, fatigue life predictions were made from the measured defect size at the crack initiation site using NASGRO. The calculated life predictions showed good agreement with the experimental values in most cases. In-situ neutron diffraction measurements on tensile test specimens were conducted, at both room temperature and at 350˚ C, for the standard and modified HIP-treated materials. The objective was to gain essential insights on how the crystal lattice strains relate to the macroscopic strengths in these specific microstructures. This investigation helped to understand the load partitioning between different slip planes and constituent phases in the microstructure at different temperatures.
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