| 1. |
- Goel, Sneha, 1993-, et al.
(författare)
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Effect of post-treatments under hot isostatic pressure on microstructural characteristics of EBM-built Alloy 718
- 2019
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Ingår i: Additive Manufacturing. - : Elsevier BV. - 2214-8604 .- 2214-7810. ; 28, s. 727-737
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Tidskriftsartikel (refereegranskat)abstract
- Electron beam melting (EBM) has emerged as an important additive manufacturing technique. In this study, Alloy 718 produced by EBM was investigated in as-built and post-treated conditions for microstructural characteristics and hardness. The post-treatments investigated were hot isostatic pressing (HIP) and combined HIP + heat treatment (HIP + HT) carried out as a single cycle inside the HIP vessel. Both the post-treatments resulted in significant decrease in defects inevitably present in the as-built material. The columnar grain structure of the as-built material was found to be maintained after post-treatment, with some sporadic localized grain coarsening noted. Although HIP led to complete dissolution of δ and γ′′ phase, stable NbC and TiN (occasionally present) particles were observed in the post-treated specimens. Significant precipitation of γ′′ phase was observed after HIP + HT, which was attributed to the two-step aging heat treatment carried out during HIP + HT. The presence of γ′′ phase or otherwise was correlated to the hardness of the material. While the HIP treatment resulted in drop in hardness, HIP + HT led to ‘recovery’ of the hardness to values exceeding those exhibited by the as-built material.
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| 2. |
- Goel, Sneha, 1993-, et al.
(författare)
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The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718
- 2018
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Ingår i: Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives. - Cham : Springer. - 2367-1696 .- 2367-1181. - 9783319894799 - 9783319894805 ; , s. 115-129
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Konferensbidrag (refereegranskat)abstract
- Additive manufacturing (AM) of Ni-based superalloys such as Alloy 718 may obviate the need for difficult machining and welding operations associated with geometrically intricate parts, thus potentially expanding design possibilities and facilitating cost-effective manufacture of complex components. However, processing AM builds completely free from defects, which may impair mechanical properties such as fatigue and ductility, is challenging. Anisotropic properties, microstructural heterogeneities and local formation of undesired phases are additional concerns that have motivated post-treatment of AM builds. This work investigates the microstructural changes associated with post-treatment of Alloy 718 specimens produced by Electron Beam Melting (EBM) for as-built microstructures at 3 build heights: near base plate, in the middle of build and near the top of the build. Two different post-treatment conditions, hot isostatic pressing (HIP) alone and a combined HIP with solutionising and two-step aging were examined and compared to the results for the as-built condition. The influence of various post-treatments on minor phase distributions (δ, γ″, carbides), overall porosity, longitudinal grain widths and Vickers microhardness was considered. The HIP treatment led to significant reduction in overall porosity and dissolution of δ phase, which led to appreciable grain growth for both post-treatment conditions. The variation in hardness noted as a function of build height for the as-built specimens was eliminated after post-treatment. Overall, the hardness was found to decrease after HIP and increase after the full HIP, solutionising and aging treatment, which was attributed to dissolution of γ″ during HIP and its re-precipitation in subsequent heat treatment steps.
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| 3. |
- Hosseini, Seyed, 1981, et al.
(författare)
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Characterization of the Surface Integrity induced by Hard Turning of Bainitic and Martensitic AISI 52100 Steel
- 2012
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Ingår i: Procedia CIRP. - : Elsevier BV. - 2212-8271. ; 1:1, s. 494 - 499
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Konferensbidrag (refereegranskat)abstract
- Depending on the process parameters and the tool condition, hard turned surfaces can consist of a “white” and a “dark” etching layer having other mechanical properties compared to the bulk material. X-ray diffraction measurements revealed that tensileresidual stresses accompanied with higher volume fraction of retained austenite are present in the thermally induced white layer. While compressive residual stresses and decreased retained austenite content was found in the plastically created white layer. The surface temperature was estimated to be ~1200 C during white layer formation by hard turning.
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| 4. |
- Åkerfeldt, Pia, et al.
(författare)
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The effect of crystallographic orientation on solid metal induced embrittlement of Ti-6Al-1Mo-1V in contact with copper
- 2013
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Ingår i: IOP Conference Series: Materials Science and Engineering. - : IOP Publishing Ltd. - 1757-8981 .- 1757-899X. ; 48:1, s. 012011-
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Konferensbidrag (refereegranskat)abstract
- Solid metal induced embrittlement (SMIE) occurs when a metal experiences tensilestress and is in contact with another metal with lower melting temperature. SMIE is believed tobe a combined action of surface self-diffusion of the embrittling species to the crack tip andadsorption of the embrittling species at the crack tip, which weakens the crack tip region. In thepresent study, both SMIE of the near alpha alloy Ti-8Al-1Mo-1V in contact with copper and itsinfluence by crystallographic orientation have been studied. U-bend specimens coated withcopper were heat treated at 480°C for 8 hours. One of the cracks was examined in detail usingelectron backscatter diffraction technique. A preferable crack path was found along high anglegrain boundaries with grains oriented close to [0001] in the crack direction; this indicates thatthere is a connection between the SMIE crack characteristics and the crystallographicorientation.
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| 5. |
- da Silva, Melina, 1978, et al.
(författare)
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A comparison of thermal stability in nanocrystalline Ni- and Co-based materials
- 2005
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Ingår i: International Journal of Materials Research. - : Walter de Gruyter GmbH. - 1862-5282 .- 2195-8556. ; 96:09, s. 1009-1014
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Tidskriftsartikel (refereegranskat)abstract
- In this paper the microstructural development upon annealing of nanocrystalline Ni- and Co-based electrodeposits is described. New investigations on Ni, Ni - Fe, and Co-P are compared with previous results on Ni, Co, and Ni -P in terms of microstructural changes and stabilizing mechanisms. The conclusions are: pure nanocrystalline Ni and Co are stabilized by impurities in the grain boundaries. In the case of Co, also an allotropic phase transformation influences the occurrence of abnormal grain growth. Alloying and/or adding solutes is found to increase thermal stability. While in Ni-20 at.% Fe the ordering transformation is expected to be the reason for stabilization, in strongly segregating systems (Ni -P and Co-P) the stabilizing effect is the decrease in grain boundary energy due to solute segregation. After precipitation, Zener pinning still hinders grain boundary migration, but not sufficient to stabilize the nanocrystalline structure.
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| 6. |
- Goel, Sneha, 1993-, et al.
(författare)
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Can Appropriate Thermal Post-Treatment Make Defect Content in as-Built Electron Beam Additively Manufactured Alloy 718 Irrelevant?
- 2020
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Ingår i: Materials. - : MDPI AG. - 1996-1944. ; 13:3
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Tidskriftsartikel (refereegranskat)abstract
- Electron beam melting (EBM) is gaining rapid popularity for production of complex customized parts. For strategic applications involving materials like superalloys (e.g., Alloy 718), post-treatments including hot isostatic pressing (HIPing) to eliminate defects, and solutionizing and aging to achieve the desired phase constitution are often practiced. The present study specifically explores the ability of the combination of the above post-treatments to render the as-built defect content in EBM Alloy 718 irrelevant. Results show that HIPing can reduce defect content from as high as 17% in as-built samples (intentionally generated employing increased processing speeds in this illustrative proof-of-concept study) to <0.3%, with the small amount of remnant defects being mainly associated with oxide inclusions. The subsequent solution and aging treatments are also found to yield virtually identical phase distribution and hardness values in samples with vastly varying as-built defect contents. This can have considerable implications in contributing to minimizing elaborate process optimization efforts as well as slightly enhancing production speeds to promote industrialization of EBM for applications that demand the above post-treatments.
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| 7. |
- Goel, Sneha, et al.
(författare)
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Role of HIPing and Heat Treatment on Properties of Alloy 718 Fabricated by Electron Beam Melting
- 2019
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Ingår i: Euro PM 2019 Congress and Exhibition.
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Konferensbidrag (refereegranskat)abstract
- Although additive manufacturing (AM) of difficult-to-machine materials can open new design possibilities for production of parts with complex geometries, AM builds are characterized by presence of detrimental defects like shrinkage/ gas porosity and lack-of-fusion. Microstructural inhomogeneity, anisotropy in properties and formation of undesirable phases are added concerns. Motivated by the need for investigation of appropriate thermal post-treatments to eliminate defects, and to achieve desired microstructural and mechanical properties, the influence of hot isostatic pressing (HIPing) and heat treatment on electron beam melted (EBM) Alloy 718 specimens built at different heights from the base-plate has been studied. While HIPing led to orders of magnitude reduction in defect content, conventional heat treatment without applied pressure was found to have no effect. Thermal post-treatment was found to eliminate inherent variability in phase constitution and hardness of as-built specimens. HIPing at high temperature was accompanied by grain coarsening, suggesting modification of the HIPing protocol.
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| 8. |
- Klement, Uta, 1962, et al.
(författare)
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Thermal stability of white layers intended as process-induced functional coatings
- 2020
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Ingår i: 12th Asia-Pacific Microscopy Conference. ; , s. 189-199
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Konferensbidrag (refereegranskat)abstract
- Hard turning and grinding are finishing processes for the manufacture of precision mechanical components. However, a major concern with respect to service performance (e.g. in bearing races receiving high contact stresses) is white layer (WL) formation on the component surface. WLs are microstructure alterations which are generally considered detrimental to fatigue life and wear resistance. But WLs can also be regarded as process-induced functional surfaces which result in improved wear resistance and produce compressive residual stresses which may significantly increase the component’s fatigue life. In fact, it was not clear if a favourable surface integrity can be produced in a controlled way using a set of selected process parameters until recently a predictive phenomenological model was presented by Hosseini [1]. The investigations on AISI 52100 have shown that different types of WL can be created that possess significantly different characteristics when one of the two driving forces, excessive heat or plastic deformation, can be isolated. The white layers would then be differentiated into thermally-induced WL (T-WL) and mechanically induced WL (M-WL). Due to their nanocrystalline microstructure (Fig. 1), no or severely reduced retained austenite content, compressive residual stresses and lack of dark layer, M-WLs are expected to have advantageous properties. For them to also have industrial relevance as process-induced functional coatings, the WL would have to endure elevated temperatures as well as external dynamic/static loading as most applications would induce them. However, the intrinsic instability of the nanostructure in the M-WLs may compromise the gain in properties (compressive stresses, hardness, etc.) by the occurrence of grain growth during exposure to elevated temperatures. Thermal stability is therefore a fundamental materials issue for these process-induces functional coatings and the investigation is the prime task in this study, although the applied loads (dynamic/static) will also play an important role in destabilizing the microstructure. T-WL and M-WLs were investigated in detail by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). For determining the thermal stability, both types of WL were also investigated after heat treatments at 235, 335 and 435°C using OM and SEM. Furthermore, the hardness of the structures was evaluated at different temperatures. The microstructural characterization indicated a clear decomposition of the WLs at elevated temperatures, transforming from not being able to resolve any features at all at room temperature and 235°C, to being able to detect features present in the structure after annealing at 335 and 435°C, respectively (compare Fig. 2). At the highest temperature, the primary (Fe, Cr)3C-carbides are more visible, whereas at the lowest temperature even such carbides were difficult to detect using SEM as they are embedded in the WL. Given, the major differences between T-WL and M-WL, the grain growth process will also be significantly different. In T-WLs, Hosseini et al. [2] reported about 13 vol.% of retained austenite, while in the case of M-WLs, no austenite could be detected. That means that during tempering, in T-WLs the austenite will decompose into ferrite and cementite, whereas in the M-WLs, with increasing tempering temperature, alloy partitioning of e.g. Cr will allow for transformation from the as obtained non-stoichiometric carbides to stoichiometric carbides. Cr, Mn and Mo partitioning inside carbides at temperatures of about 450°C was also reported by Clarke et al. [3]. XRD and TEM analyses are used for a better understanding of the thermal stability of such structures and for determination of the microstructure constituents. The microstructural changes in the WLs are also accompanied by a decrease in hardness with increasing annealing temperature. Our investigation revealed that the M-WL as compared to T-WL possess a ~50 HV higher hardness at 235°C and 335°C. However, at 435°C, the differences are equalized between the WLs. Summary and conclusion: · It can clearly be shown that with increasing temperature, a larger portion of the WLs decompose. This occurs independent of if the WLs are mechanically or thermally induced. · The hardness measurements showed that the M-WL had initially a higher hardness than the T-WL near the machined surface. However, with increasing annealing temperature, the hardness values between them equalized and reached the same hardness. · Both M-WL and T-WL seem to be stable up to at least 235°C. This is promising for a potential application of M-WLs as functional coatings as temperatures above 200°C are not expected in intended applications.
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| 9. |
- Tolvanen, Sakari, 1984, et al.
(författare)
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Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron
- 2018
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer Science and Business Media LLC. - 1073-5623 .- 1543-1940. ; 49A:5, s. 1683-1691
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Tidskriftsartikel (refereegranskat)abstract
- Addition of small amounts of boron to cast Ti-6Al-4V alloy has shown to render a finer microstructure and improved mechanical properties. For such an improved alloy to be widely applicable for large aerospace structural components, successful welding of such castings is essential. In the present work, the microstructure and porosity of laser welds in a standard grade cast Ti-6Al-4V alloy as well as two modified alloy versions with different boron concentrations have been investigated. Prior-β grain reconstruction revealed the prior-β grain structure in the weld zones. In fusion zones of the welds, boron was found to refine the grain size significantly and rendered narrow elongated grains. TiB particles in the prior-β grain boundaries in the cast base material restricted grain growth in the heat-affected zone. The TiB particles that existed in the as cast alloys decreased in size in the fusion zones of welds. The hardness in the weld zones was higher than in the base material and boron did not have a significant effect on hardness of the weld zones. The fusion zones were smaller in the boron-modified alloys as compared with Ti-6Al-4V without boron. Computed tomography X-ray investigations of the laser welds showed that pores in the FZ of the boron modified alloys were confined to the lower part of the welds, suggesting that boron addition influences melt pool flow. © 2018 The Author(s)
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| 10. |
- Goel, Sneha, 1993-, et al.
(författare)
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Microstructure evolution and mechanical response-based shortening of thermal post-treatment for electron beam melting (EBM) produced Alloy 718
- 2021
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Ingår i: Materials Science and Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 820
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Tidskriftsartikel (refereegranskat)abstract
- Electron beam melting (EBM) produced Alloy 718 was subjected to thermal post-treatment involving hot isostatic pressing (HIPing) and heat treatment (HT). Subjecting the material to HIPing at 1120 °C led to significant densification. Study of microstructure evolution during HT (comprising of solution treatment and aging) showed possibility of significantly shortening the HT duration, particularly the time for two-step aging from the standard (8 h + 8 h) long cycle to possibly a shortened (4 h + 1 h) cycle. Another approach for shortening the post-treatment cycle by integrating the HIPing with HT inside the HIP vessel was also successfully implemented. The above observations were further substantiated by tensile response of the material subjected to the varied post-treatment cycles; out of all the post-treatments steps, tensile behaviour was observed to be mainly affected by the aging treatment. Further prospects for shortening the post-treatment protocol are also described, such as shortening of HIPing duration for the typical 4 h to 1 h cycle as well as possible elimination of solution treatment step from the entire post-treatment protocol specifically when prior HIPing is performed. Heat treatment with prior HIPing was found to be crucial for improving fatigue life, because subjecting EBM Alloy 718 to only HT, irrespective of the short or standard long protocol, rendered inferior fatigue response.
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