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Processability of Laser Powder Bed Fusion of Alloy 247LC: Influence of process parameters on microstructure and defects
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (PTW)
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Trollhättan: University West , 2020. , p. 59
Series
Licentiate Thesis: University West ; 31
Keywords [en]
laser powder bed fusion; Alloy 247LC; additive manufacturing; nickel-based superalloys; processability; cracks; voids.
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-16114ISBN: 978-91-88847-71-3 (print)ISBN: 978-91-88847-70-6 (electronic)OAI: oai:DiVA.org:hv-16114DiVA, id: diva2:1509628
Supervisors
Available from: 2020-12-14 Created: 2020-12-14 Last updated: 2021-11-30Bibliographically approved
List of papers
1. Laser beam powder bed fusion and post processing of alloy 247LC
Open this publication in new window or tab >>Laser beam powder bed fusion and post processing of alloy 247LC
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2019 (English)In: MS and T 2019 - Materials Science and Technology, Materials Science and Technology , 2019, p. 27-34Conference paper, Published paper (Refereed)
Abstract [en]

Alloy 247LC is sensitive to cracking during laser beam powder bed fusion (PBF-LB) manufacturing. Post processing is thus required to close cracks and achieve desired properties. In this study, samples of Alloy 247LC were manufactured by PBF-LB and subsequently post processed by hot isostatic pressing (HIP), HIP + solution and ageing heat treatments. The microstructure was characterized. Results showed cracks in the as-built condition. Cracks were not detected after HIP. Bright microconstituents were observed in the region between the cells, mainly, because of the partitioning of Hf and Ta into the intercellular region, where they presumably form carbides. What is assumed to be oxides were prominent in the microstructure. Thermodynamic calculations showed rapid formation of ?’ precipitates in the alloy, due to the high total concentration of Al and Ta and this was linked to the high hardness values in the as-built condition. © 2019 MS&T19®

Place, publisher, year, edition, pages
Materials Science and Technology, 2019
Keywords
Carbides; Cracks; Hot isostatic pressing; Microstructure; Tantalum, High hardness; Intercellular regions; Micro-constituents; Phases; Post processing; Powder bed; Thermodynamic calculations, Laser beams
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14914 (URN)2-s2.0-85075366814 (Scopus ID)9780873397704 (ISBN)
Conference
Materials Science and Technology 2019, MS and T 2019; Oregon Convention CenterPortland; United States; 29 September 2019 through 3 October 2019
Note

10.7449/2019/MST_2019_27_34

Available from: 2020-01-29 Created: 2020-01-29 Last updated: 2021-11-18Bibliographically approved
2. Influence of laser powder bed fusion process parameters on voids, cracks, and microhardness of nickel-based superalloy alloy 247LC
Open this publication in new window or tab >>Influence of laser powder bed fusion process parameters on voids, cracks, and microhardness of nickel-based superalloy alloy 247LC
2020 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 13, no 17, article id 3770Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
Design of experiments; Hatches; Microhardness; Superalloys, Fundamental mechanisms; Fusion process; Nickel- based superalloys; Process parameters; Scanning speed; Statistical design of experiments; Void formation; Volumetric energy densities, Nickel alloys
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hv:diva-15831 (URN)10.3390/MA13173770 (DOI)000571624500001 ()2-s2.0-85090499027 (Scopus ID)
Funder
Knowledge Foundation, 20160281
Available from: 2020-09-19 Created: 2020-09-19 Last updated: 2021-11-18
3. Review of laser powder bed fusion of gamma-prime-strengthened nickel-based superalloys
Open this publication in new window or tab >>Review of laser powder bed fusion of gamma-prime-strengthened nickel-based superalloys
2020 (English)In: Metals, ISSN 2075-4701, Vol. 10, no 8, article id 996Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
MDPI AG, 2020
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:hv:diva-15747 (URN)10.3390/met10080996 (DOI)000564737100001 ()2-s2.0-85088689740 (Scopus ID)
Funder
Vinnova, 2016-05175
Available from: 2020-08-26 Created: 2020-08-26 Last updated: 2021-11-18

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