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Defects in Electron Beam Melted Ti-6Al-4V: Fatigue Life Prediction Using Experimental Data and Extreme Value Statistics
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.ORCID iD: 0000-0003-3828-2149
Division 9654: Materials Engineering, GKN Aerospace, 461 38 Trollhättan, Sweden.
Division 9641: Solid Mechanics, GKN Aerospace, 461 38 Trollhättan, Sweden.
Division 9641: Solid Mechanics, GKN Aerospace, 461 38 Trollhättan, Sweden.
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2021 (English)In: Materials, E-ISSN 1996-1944, Vol. 14, no 3, article id 640Article in journal (Refereed) Published
Abstract [en]

Electron beam melting is a powder bed fusion (PBF) additive manufacturing (AM) method for metals offering opportunities for the reduction of material waste and freedom of design, but unfortunately also suffering from material defects from production. The stochastic nature of defect formation leads to a scatter in the fatigue performance of the material, preventing wider use of this production method for fatigue critical components. In this work, fatigue test data from electron beam melted Ti-6Al-4V specimens machined from as-built material are compared to deterministic fatigue crack growth calculations and probabilistically modeled fatigue life. X-ray computed tomography (XCT) data evaluated using extreme value statistics are used as the model input. Results show that the probabilistic model is able to provide a good conservative life estimate, as well as accurate predictive scatter bands. It is also shown that the use of XCT-data as the model input is feasible, requiring little investigated material volume for model calibration.

Place, publisher, year, edition, pages
MDPI, 2021. Vol. 14, no 3, article id 640
Keywords [en]
additive manufacturing, electron beam melting, Ti-6Al-4V, defects, fatigue life, fracture mechanics, fatigue crack propagation, probabilistic modeling
National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-81154DOI: 10.3390/ma14030640ISI: 000615396300001PubMedID: 33573246Scopus ID: 2-s2.0-85100309316OAI: oai:DiVA.org:ltu-81154DiVA, id: diva2:1476872
Projects
SUDDEN
Funder
Vinnova, 2017-04846
Note

Validerad;2021;Nivå 2;2021-02-16 (alebob);

Artikeln har tidigare förekommit som manuskript i avhandling

Available from: 2020-10-15 Created: 2020-10-15 Last updated: 2024-07-04Bibliographically approved
In thesis
1. Defects in E-PBF Ti-6Al-4V and their Effect on Fatigue Behaviour: Characteristics, Distribution and Impact on Life
Open this publication in new window or tab >>Defects in E-PBF Ti-6Al-4V and their Effect on Fatigue Behaviour: Characteristics, Distribution and Impact on Life
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Alternative title[sv]
Defekter i E-PBF Ti-6Al-4V och dess effekter på utmattningsegenskaper : Kännetecken, fördelning och livslängdspåverkan
Abstract [en]

Layer by layer manufacturing (additive manufacturing, AM) of metals is emerging as an alternative to conventional subtractive manufacturing with the goal of enabling near net-shape production of complex part geometries with reduced material waste and shorter lead times. Recently this field has experienced rapid growth through industrial adaptation but has simultaneously encountered challenges. One such challenge is the ability of AM metal to withstand loading conditions ranging from static loads to complex multiaxial thermo-mechanical fatigue loads. This makes fatigue performance of AM materials a key consideration for the implementation of AM in production. This is especially true for AM in the aerospace industry where safety standards are strict.

Defects in metal AM materials include rough surfaces, pores and lack-of-fusion (LOF) between build layers. These defects are detrimental to fatigue as they act as local stress concentrators that can give rise to cracks in the material.  Some defects can be avoided by careful build process optimization and/or post-processing but fully eliminating all defects is not possible. Because of this, a need arises for the capability to estimate the fatigue performance of AM produced critical components containing defects.

The aim of the thesis is to increase understanding regarding the connection between defect characteristics and the fatigue behaviour in AM produced Ti-6Al-4V. Defect distributions are statistically analysed for use in a simple fracture mechanical model for fatigue life prediction. Other study areas include the impact of post-production treatments such as chemical surface treatments and hot isostatic pressing (HIP) on defects and fatigue behaviour.

The thesis constitutes three scientific papers. The AM technique studied in these papers is Electron Beam Melting (EBM) in which an electron beam selectively melts pre-alloyed metal powder. In paper 1, defects were studied using X-ray computed tomography (XCT) and fatigue crack initiation was related to the observed defect distribution. In paper 2, XCT data was used to relate the surface morphology and roughness of post-production treated EBM material to the surface near defect distribution. The connection between this distribution and manufacturing parameter has also been explored. Paper 3 builds on and extends the work presented in paper 1 by including further fatigue testing as well as a method for predicting fatigue life using statistical analysis of the observed defect distribution.

The impact of a defect on the fatigue behaviour of the material was found to largely depend on its characteristics and position relative to the surface. Production and post-processing of the material was found to play a role in the severity of this impact. Finally, it was found that a probabilistic statistical analysis can be used to accurately predict the life of the studied material at the tested conditions.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2020
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Defects, Additive Manufacturing, Ti-6Al-4V, Probabilistic Modelling, Fatigue
National Category
Metallurgy and Metallic Materials
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-81155 (URN)978-91-7790-685-8 (ISBN)978-91-7790-686-5 (ISBN)
Presentation
2020-12-10, E632, Luleå, 09:30 (English)
Opponent
Supervisors
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SUDDEN
Available from: 2020-10-16 Created: 2020-10-15 Last updated: 2024-10-24Bibliographically approved
2. Defects and Surfaces and their Impact on Fatigue Behaviour of Powder Bed Fused Ti-6Al-4V: Characteristics and Modelling
Open this publication in new window or tab >>Defects and Surfaces and their Impact on Fatigue Behaviour of Powder Bed Fused Ti-6Al-4V: Characteristics and Modelling
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Additive manufacturing (AM), of metals is gaining popularity as an alternative to conventional manufacturing techniques such as casting and forging. Metal-AM allows for the production of complex part geometries with reduced material waste and shorter lead times. The aerospace industry has been quick to adopt this technology; however, the fatigue performance of  metal-AM is a critical consideration for ensuring safety.

One of the challenges of AM metal is limited knowledge in its ability to withstand various loading conditions, from static loads to complex multiaxial thermo-mechanical fatigue loads. Defects in AM materials, such as rough surfaces, pores, and lack-of-fusion between build layers, act as local stress concentrators and crack initiation sites in the material. Some defects can be reduced through careful build process optimization and post-processing treatments, but it is generally not considered possible to eliminate all defects. Therefore, it is necessary to estimate the fatigue performance of AM-produced critical components containing defects.

The aim of the thesis is to investigate the relationship between defect characteristics and fatigue behaviour in AM-produced metal. The AM-material studied is electron beam powder bed fusion (EB-PBF) produced Ti-6Al-4V. Defect distributions, both on the surface and further inside the material, are statistically analysed and a simple fracture mechanical model for predicting fatigue life is developed. Post-production treatments, such as machining, chemical surface treatments and hot isostatic pressing (HIP), are also examined to determine their impact on defects and fatigue behaviour.

The thesis consists of six scientific papers. In the first three papers (1-3), fatigue behaviour and material characteristics are studied using mechanical testing and materials characterisation techniques such as optical microscopy, scanning electron microscopy, interferometry, and X-ray computed tomography (XCT). Internal defects are documented using XCT and compared with fatigue crack initiations (paper 1). Surface roughness and morphology of post-production treated EB-PBF material are analysed using interferometry and microscopy, and its connection to the surface near distribution of internal defects is examined (paper 2). Material that has been surface treated and subjected to Hot Isostatic Pressing (HIP) was tested in four-point bending fatigue followed by a fractographic study (paper 3).

The final three papers (4-6) of the thesis aim to take the material characteristics investigated in the first three papers as input for a crack-propagation-based fracture mechanics model to predict fatigue life using statistical analysis of the observed surface quality and defect distribution. These papers include modelling based on information about internal defects, as studied in the first paper, applied in a tension-compression cyclic load case (paper 4);  an exploration of surface morphology and four-point fatigue testing combined with surface adjacent XCT to use as input for a surface-sensitive fatigue life model (paper 5); and an estimation of the impact of surface machining depth on the material's fatigue behaviour using the experience gained from all previous work (paper 6).

It was found that the severity of the impact of a defect on the fatigue behaviour of the material largely depends on its characteristics and position relative to the surface. Production and post-processing of the material also play a role in the severity of this impact. The thesis also concludes that probabilistic statistical analysis can be used to accurately predict the life of the studied material under the conditions tested for.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2023
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Defects, Additive Manufacturing, Ti-6Al-4V, Probabilistic Modelling, Fatigue, Extreme Value Statistics
National Category
Metallurgy and Metallic Materials
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-96282 (URN)978-91-8048-289-9 (ISBN)978-91-8048-290-5 (ISBN)
Public defence
2023-05-26, E632, Luleå tekniska universitet, Luleå, 08:30 (English)
Opponent
Supervisors
Projects
SUDDEN
Funder
Vinnova, 2017-04846
Available from: 2023-03-31 Created: 2023-03-31 Last updated: 2023-09-05Bibliographically approved

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Sandell, ViktorÅkerfeldt, PiaAntti, Marta-Lena

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