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Microstructure and mechanical properties of Ti-6Al-4V manufactured by electron beam powder bed fusion
University West, Department of Engineering Science, Division of Subtractive and Additive Manufacturing. (KAMPT)
2024 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Mikrostruktur och mekaniska egenskaper hos Ti-6Al-4V tillverkad genom elektronstrålepulverbädd metoden (Swedish)
Abstract [en]

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.

Abstract [sv]

Ti-6Al-4V är en α+β titanlegering som används flitigt i flygmotortillämpningar på grund av sin höga specifika hållfasthet. Typiskt tillverkas Ti-6Al-4V-komponenter genom konventionella tillverkningsprocesser såsom gjutning eller smidning. Ett alternativ till konventionella tillverkningsprocesser är additiv tillverkning (AM, från engelskans Additiv Manufacturing), som kan användas för att tillverka Ti-6Al-4V komponenter med ökad geometrisk komplexitet och potentiellt lägre ”buy-to-fly ratio”. AM-material har visat sig ha liknande statiska mekaniska egenskaper som konventionellt tillverkade material, men utmattningsegenskaperna kan påverkas av ytkvalitét och defekter.

I likhet med flera andra AM-processer påverkas utmattningsegenskaperna hos material tillverkat med elektronstrålepulverbäddsteknik (PBF-EB) i första hand av skillnader i ytfinheten. För utmattningskritiska applikationer, bearbetas därför de byggda råytorna vanligtvis genom olika ytförbättringsmetoder, vilket gör det utmanande att tillverka komplexa geometrier utan efterbearbetning. Att utforska sätt att förbättra ytkvalitéten hos PBF-EB-tillverkad Ti-6Al-4V kommer att minska behovet av efterbearbetning och öka hållfastheten.

I denna avhandling har effekten av olika PBF-EB konturparameterinställningar på ytans ytfinhet och utmattningsegenskaper undersökts. Resultaten indikerar att en ökning av antalet konturer och smältning av konturerna före smältning av bulken, påverkar ytjämnheten positivt. Bland flera ytfinhetsparametrar har parametern relaterad till dalarnas djup en signifikant inverkan på utmattningslivslängden. För att fånga de dolda dalarna som är öppna mot ytan behövs mera avancerade ytkarakteriseringsmetoder så som röntgen tomografi. Prediktering av utmattningslivslängden då dessa dolda dalar tas hänsyn till resulterade i livlängdssuppskattningar som var jämförbara med experimentella resultat.

Då ojämnheter på ytan är eliminerade blir defekter i bulken kritiska för utmattningshållfastheten. Vanligtvis åtgärdas detta genom värmebehandling efter tillverkning, såsom het isostatisk pressning (HIP). HIP-behandling av konventionellt gjuten Ti-6Al-4V utförs vanligtvis vid 920 ˚C, 100 MPa under 2 timmar. Samma HIP-behandling har även använts för PBF-EB tillverkad Ti-6Al-4V, vilket resulterar i förgrovning av α-lamellerna och minskning av sträckgränsen vi jämfört med som byggt materiell. Att sänka HIP-behandlingstemperaturen till 800 ˚C och öka trycket till 200 MPa har nyligen visat sig stänga porositeten i hög grad samtidigt som materialet bibehåller sträckgränsen. Utmattningsprestandan för material med lågtemperatur-HIP-behandlingen har utvärderats i detta arbete. Även om det lågtemperatur-HIP-behandlade materialet hade de lägsta minimumlivslängderna till brott, är utmattningsprestandan jämförbar med den för standard HIP-behandlat material.

I flygmotorapplikationer har Ti-6Al-4V normalt en maximal driftstemperatur på 350 ˚C, därför har draghållfastheten vid denna temperatur undersökts i detta arbete. Vid 350 ˚C minskar sträckgränsen till cirka 65 % jämfört med hållfastheten vid rumstemperatur. En ökning av duktiliteten observerades vid 150 ˚C jämfört med den vid rumstemperatur, men duktiliteten minskade vid temperaturer mellan 150 – 350 ˚C på grund av aktiveringen av olika glidsystem.

Place, publisher, year, edition, pages
Trollhättan: University West , 2024. , p. 67
Series
PhD Thesis: University West ; 66
Keywords [en]
Additive manufacturing, Electron beam melting, Surface topography, Hot isostatic pressing, Fatigue life
Keywords [sv]
Additiv tillverkning, Elektronstrålesmältning, Yttopografi, Het isostatisk pressning, Utmattningsegenskaper
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
URN: urn:nbn:se:hv:diva-21508ISBN: 978-91-89325-84-5 (print)ISBN: 978-91-89325-83-8 (electronic)OAI: oai:DiVA.org:hv-21508DiVA, id: diva2:1853791
Public defence
2024-05-30, F313, Gustava Melins gata, Trollhättan, 09:00 (English)
Opponent
Supervisors
Note

Paper C, D and E are to be submitted and not included in this digital thesis.

Available from: 2024-05-06 Created: 2024-04-23 Last updated: 2024-04-23
List of papers
1. Elevated-Temperature Tensile Properties of Low-Temperature HIP-Treated EBM-Built Ti-6Al-4V.
Open this publication in new window or tab >>Elevated-Temperature Tensile Properties of Low-Temperature HIP-Treated EBM-Built Ti-6Al-4V.
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2022 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 15, no 10, article id 3624Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
MDPI, 2022
Keywords
Ti-6Al-4V, additive manufacturing, electron beam melting, elevated temperature tensile strength, high-temperature mechanical properties, hot isostatic pressing
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-18442 (URN)10.3390/ma15103624 (DOI)000801377700001 ()35629650 (PubMedID)2-s2.0-85130891800 (Scopus ID)
Funder
Vinnova, 2019-02741
Note

This research was funded by VINNOVA, through the “Nationella flygtekniska forskningsprogrammet7” (NFFP) (project #: 2019-02741).

Available from: 2022-10-31 Created: 2022-10-31 Last updated: 2024-04-23
2. Fatigue behavior of low-temperature hot isostatic pressed electron beam powder bed fusion manufactured Ti-6Al-4 V
Open this publication in new window or tab >>Fatigue behavior of low-temperature hot isostatic pressed electron beam powder bed fusion manufactured Ti-6Al-4 V
Show others...
2023 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, Vol. 962, article id 171086Article in journal (Refereed) Published
Abstract [en]

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. 

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Additive manufacturing Electron beam melting Hot isostatic pressing Low cycle fatigue Fatigue crack growth Ti-6Al-4 V
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-20691 (URN)10.1016/j.jallcom.2023.171086 (DOI)001031870200001 ()2-s2.0-85163861805 (Scopus ID)
Funder
Vinnova, 2019-02741
Note

CC BY 4.0

Available from: 2023-09-06 Created: 2023-09-06 Last updated: 2024-04-23Bibliographically approved

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Thalavai Pandian, Karthikeyan

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