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Publications (9 of 9) Show all publications
Kumara, C., Segerstark, A., Hanning, F., Dixit, N., Joshi, S. V., Moverare, J. & Nylén, P. (2019). Microstructure modelling of laser metal powder directed energy deposition of alloy 718. Additive Manufacturing, 25, 357-364
Open this publication in new window or tab >>Microstructure modelling of laser metal powder directed energy deposition of alloy 718
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2019 (English)In: Additive Manufacturing, ISSN 2214-8604, Vol. 25, p. 357-364Article in journal (Refereed) Published
Abstract [en]

A multi-component and multi-phase-field modelling approach, combined with transformation kinetics modelling, was used to model microstructure evolution during laser metal powder directed energy deposition of Alloy 718 and subsequent heat treatments. Experimental temperature measurements were utilised to predict microstructural evolution during successive addition of layers. Segregation of alloying elements as well as formation of Laves and δ phase was specifically modelled. The predicted elemental concentrations were then used in transformation kinetics to estimate changes in Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams for Alloy 718. Modelling results showed good agreement with experimentally observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool, both for improving process understanding and for process development including subsequent heat treatment.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Phase-field, DED, Heat treatment, Thermal cycle, Modelling
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13195 (URN)10.1016/j.addma.2018.11.024 (DOI)000456378800034 ()2-s2.0-85057193791 (Scopus ID)
Funder
Knowledge Foundation
Note

Funders: European Regional Development Fund for project 3Dprint

Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2019-04-03Bibliographically approved
Kumara, C., Deng, D., Hanning, F., Raanes, M., Moverare, J. & Nylén, P. (2019). Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling. Metallurgical and Materials Transactions. A, 50A(5), 527-2537
Open this publication in new window or tab >>Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling
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2019 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 50A, no 5, p. 527-2537Article in journal (Refereed) Published
Abstract [en]

Electron beam melting (EBM) is a powder bed additive manufacturing process where a powder material is melted selectively in a layer-by-layer approach using an electron beam. EBM has some unique features during the manufacture of components with high-performance superalloys that are commonly used in gas turbines such as Alloy 718. EBM has a high deposition rate due to its high beam energy and speed, comparatively low residual stresses, and limited problems with oxidation. However, due to the layer-by-layer melting approach and high powder bed temperature, the as-built EBM Alloy 718 exhibits a microstructural gradient starting from the top of the sample. In this study, we conducted modeling to obtain a deeper understanding of microstructural development during EBM and the homogenization that occurs during manufacturing with Alloy 718. A multicomponent phase-field modeling approach was combined with transformation kinetic modeling to predict the microstructural gradient and the results were compared with experimental observations. In particular, we investigated the segregation of elements during solidification and the subsequent "in situ" homogenization heat treatment at the elevated powder bed temperature. The predicted elemental composition was then used for thermodynamic modeling to predict the changes in the continuous cooling transformation and time-temperature transformation diagrams for Alloy 718, which helped to explain the observed phase evolution within the microstructure. The results indicate that the proposed approach can be employed as a valuable tool for understanding processes and for process development, including post-heat treatments. © 2019, The Author(s).

Keywords
3D printers; Deposition rates; Electron beam melting; Electron beams; Forecasting; Gas turbines; Microstructural evolution; Solid solutions; Temperature, Additive manufacturing process; Continuous cooling transformation; Elemental compositions; Layer-by-layer approaches; Microstructural development; Microstructural gradients; Transformation diagrams; Transformation kinetics, Heat treatment
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13756 (URN)10.1007/s11661-019-05163-7 (DOI)000463991300038 ()2-s2.0-85062604965 (Scopus ID)
Funder
Knowledge FoundationEuropean Regional Development Fund (ERDF)
Available from: 2019-05-10 Created: 2019-05-10 Last updated: 2019-07-25Bibliographically approved
Kumara, C. (2018). Microstructure Modelling of Additive Manufacturing of Alloy 718. (Licentiate dissertation). Trollhättan: University West
Open this publication in new window or tab >>Microstructure Modelling of Additive Manufacturing of Alloy 718
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In recent years, additive manufacturing (AM) of Alloy 718 has received increasing interest in the field of manufacturing engineering owing to its attractive features compared to those of conventional manufacturing methods. The ability to produce complicated geometries, low cost of retooling, and control of the microstructure are some of the advantages of the AM process over traditional manufacturing methods. Nevertheless, during the building process, the build material undergoes complex thermal conditions owing to the inherent nature of the process. This results in phase transformation from liquid to solid and solid state. Thus, it creates microstructural gradients in the built objects, and as a result,heterogeneous material properties. The manufacturing process, including the following heat treatment that is used to minimise the heterogeneity, will cause the additively manufactured material to behave differently when compared to components produced by conventional manufacturing methods. Therefore, understanding the microstructure formation during the building and subsequent post-heat treatment is important, which is the objective of this work. Alloy 718 is a nickel-iron based super alloy that is widely used in the aerospace industry and in the gas turbine power plants for making components subjected tohigh temperatures. Good weldability, good mechanical properties at high temperatures, and high corrosion resistance make this alloy particularly suitablefor these applications. Nevertheless, the manufacturing of Alloy 718 components through traditional manufacturing methods is time-consuming and expensive. For example, machining of Alloy 718 to obtain the desired shape is difficult and resource-consuming, owing to significant material waste. Therefore, the application of novel non-conventional processing methods, such as AM, seems to be a promising technique for manufacturing near-net-shape complex components.In this work, microstructure modelling was carried out by using multiphase-field modelling to model the microstructure evolution in electron beam melting (EBM) and laser metal powder directed energy deposition (LMPDED) of Alloy 718 and x subsequent heat treatments. The thermal conditions that are generated during the building process were used as input to the models to predict the as-built microstructure. This as-built microstructure was then used as an input for the heat treatment simulations to predict the microstructural evolution during heat treatments. The results showed smaller dendrite arm spacing (one order of magnitude smaller than the casting material) in these additive manufactured microstructures, which creates a shorter diffusion length for the elements compared to the cast material. In EBM Alloy 718, this caused the material to have a faster homogenisation during in-situ heat treatment that resulting from the elevated powder bed temperature (> 1000 °C). In addition, the compositional segregation that occurs during solidification was shown to alter the local thermodynamic and kinetic properties of the alloy. This was observed in the predicted TTT and CCT diagrams using the JMat Pro software based on the predicted local segregated compositions from the multiphase-field models. In the LMPDED Alloy 718 samples, this resulted in the formation of δ phase in the interdendritic region during the solution heat treatment. Moreover, this resulted in different-size precipitation of γ'/γ'' in the inter-dendritic region and in the dendrite core. Themicro structure modelling predictions agreed well with the experimental observations. The proposed methodology used in this thesis work can be an appropriate tool to understand how the thermal conditions in AM affect themicro structure formation during the building process and how these as-built microstructures behave under different heat treatments.

Place, publisher, year, edition, pages
Trollhättan: University West, 2018. p. 52
Series
Licentiate Thesis: University West ; 23
Keywords
Phase-Field Modelling; Laser metal powder directed energy deposition; Electron Beam Melting; Heat Treatment; Superalloy
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology; ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13197 (URN)978-91-88847-10-2 (ISBN)978-91-88847-09-6 (ISBN)
Presentation
2018-12-12, 10:00 (English)
Supervisors
Available from: 2018-12-12 Created: 2018-12-12
Kumara, C., Deng, D., Moverare, J. & Nylén, P. (2018). Modelling of anisotropic elastic properties in alloy 718 built by electron beam melting. Materials Science and Technology, 34(5), 529-537
Open this publication in new window or tab >>Modelling of anisotropic elastic properties in alloy 718 built by electron beam melting
2018 (English)In: Materials Science and Technology, ISSN 0267-0836, E-ISSN 1743-2847, Vol. 34, no 5, p. 529-537Article in journal (Refereed) Published
Abstract [en]

Owing to the inherent nature of the process, typically material produced via electron beam melting (EBM) has a columnar microstructure. As a result of that, the material will have anisotropic mechanical properties. In this work, anisotropic elastic properties of EBM built Alloy 718 samples at room temperature were investigated by using experiments and modelling work. Electron backscatter diffraction data from the sample microstructure was used to predict the Young’s modulus. The results showed that the model developed in the finite element software OOF2 was able to capture the anisotropy in the Young’s modulus. The samples showed transversely isotropic elastic properties having lowest Young’s modulus along build direction. In addition to that, complete transversely isotropic stiffness tensor of the sample was also calculated. © 2018 Institute of Materials, Minerals and Mining.

Keywords
Elastic constants; Elasticity; Electron beam melting; Electron beams; Finite element method; Melting; Microstructure; Models, Alloy 718; Anisotropic elastic properties; Anisotropic mechanical properties; Columnar microstructures; EBSD; Electron back scatter diffraction; Finite element software; Transversely isotropic, Anisotropy
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12134 (URN)10.1080/02670836.2018.1426258 (DOI)000428303200005 ()2-s2.0-85041234684 (Scopus ID)
Funder
Knowledge Foundation, SUMAN-Next
Note

Published online: 28 Jan 2018

Funders:  European Regional Development Fund, 3Dprint

Available from: 2018-02-20 Created: 2018-02-20 Last updated: 2019-05-28Bibliographically approved
Hosseini, V., Karlsson, L., Hurtig, K., Choquet, I., Engelberg, D., Roy, M. J. & Kumara, C. (2017). A novel arc heat treatment technique for producing graded microstructures through controlled temperature gradients. Materials & design, 121(May), 11-23
Open this publication in new window or tab >>A novel arc heat treatment technique for producing graded microstructures through controlled temperature gradients
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2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 121, no May, p. 11-23Article in journal (Refereed) Published
Abstract [en]

This paper introduces a novel arc heat treatment technique to produce samples with graded microstructures through the application of controlled temperature gradients. Steady state temperature distributions within the sample can be achieved and maintained, for times ranging from a few seconds to several hours. The technique reduces the number of samples needed to characterize the response of a material to thermal treatments, and can consequently be used as a physical simulator for materials processing. The technique is suitable for conventional heat treatment analogues, welding simulations, multi-step heat treatments, and heat treatments with controlled heating and cooling rates. To demonstrate this technique, a super duplex stainless steel was treated with a stationary TIG arc, to confirm the relationship between generated steady-state temperature fields, microstructure development, hardness, and sensitization to corrosion. Metallographic imaging and hardness mapping provided information about graded microstructures, confirming the formation of secondary phases and microstructure sensitization in the temperature range 850–950 °C. Modelling of temperature distributions and thermodynamic calculations of phase stabilities were used to simulate microstructure development and associated welding cycles.

Keywords
Stationary arc, Heat treatment, Graded microstructure, Super duplex stainless steels, Physical simulation, Welding
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-10760 (URN)10.1016/j.matdes.2017.02.042 (DOI)000399625000002 ()2-s2.0-85013031461 (Scopus ID)
Funder
Knowledge Foundation
Note

Funders: EPSRC (EP/L01680X/1) through the Materials for Demanding Environments Centre for Doctoral Training.

Available from: 2017-02-28 Created: 2017-02-28 Last updated: 2019-05-23Bibliographically approved
Gupta, M. K., Kumara, C. & Nylén, P. (2017). Bilayer Suspension Plasma-Sprayed Thermal Barrier Coatings with Enhanced Thermal Cyclic Lifetime: Experiments and Modeling. Journal of thermal spray technology (Print), 26(6), 1038-1051
Open this publication in new window or tab >>Bilayer Suspension Plasma-Sprayed Thermal Barrier Coatings with Enhanced Thermal Cyclic Lifetime: Experiments and Modeling
2017 (English)In: Journal of thermal spray technology (Print), ISSN 1059-9630, E-ISSN 1544-1016, Vol. 26, no 6, p. 1038-1051Article in journal (Refereed) Published
Abstract [en]

Suspension plasma spraying (SPS) has been shown as a promising process to produce porous columnar strain tolerant coatings for thermal barrier coatings (TBCs) in gas turbine engines. However, the highly porous structure is vulnerable to crack propagation, especially near the topcoat-bondcoat interface where high stresses are generated due to thermal cycling. A topcoat layer with high toughness near the topcoat-bondcoat interface could be beneficial to enhance thermal cyclic lifetime of SPS TBCs. In this work, a bilayer coating system consisting of first a dense layer near the topcoat-bondcoat interface followed by a porous columnar layer was fabricated by SPS using Yttria-stabilised zirconia suspension. The objective of this work was to investigate if the bilayer topcoat architecture could enhance the thermal cyclic lifetime of SPS TBCs through experiments and to understand the effect of the column gaps/vertical cracks and the dense layer on the generated stresses in the TBC during thermal cyclic loading through finite element modeling. The experimental results show that the bilayer TBC had significantly higher lifetime than the single-layer TBC. The modeling results show that the dense layer and vertical cracks are beneficial as they reduce the thermally induced stresses which thus increase the lifetime.

Keywords
bilayer coating system; finite element modeling; lifetime; suspension plasma spraying; stresses; thermal barrier coatings
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-11411 (URN)10.1007/s11666-017-0595-9 (DOI)000407335200005 ()2-s2.0-85022002318 (Scopus ID)
Available from: 2017-09-19 Created: 2017-09-19 Last updated: 2019-05-23Bibliographically approved
Ganvir, A., Kumara, C., Gupta, M. K. & Nylen, P. (2017). Thermal Conductivity in Suspension Sprayed Thermal Barrier Coatings: Modeling and Experiments. Paper presented at ITSC 2016, International Thermal Spray Conference, Shanghai, P.R. China, May 10-12, 2016. Journal of thermal spray technology (Print), 26(1-2), 71-82
Open this publication in new window or tab >>Thermal Conductivity in Suspension Sprayed Thermal Barrier Coatings: Modeling and Experiments
2017 (English)In: Journal of thermal spray technology (Print), ISSN 1059-9630, E-ISSN 1544-1016, Vol. 26, no 1-2, p. 71-82Article in journal (Refereed) Published
Abstract [en]

Axial suspension plasma spraying (ASPS) can generate microstructures with higher porosity and pores in the size range from submicron to nanometer. ASPS thermal barrier coatings (TBC) have already shown a great potential to produce low thermal conductivity coatings for gas turbine applications. It is important to understand the fundamental relationships between microstructural defects in ASPS coatings such as crystallite boundaries, porosity etc. and thermal conductivity. Object-oriented finite element (OOF) analysis has been shown as an effective tool for evaluating thermal conductivity of conventional TBCs as this method is capable of incorporating the inherent microstructure in the model. The objective of this work was to analyze the thermal conductivity of ASPS TBCs using experimental techniques and also to evaluate a procedure where OOF can be used to predict and analyze the thermal conductivity for these coatings. Verification of the model was done by comparing modeling results with the experimental thermal conductivity. The results showed that the varied scaled porosity has a significant influence on the thermal conductivity. Smaller crystallites and higher overall porosity content resulted in lower thermal conductivity. It was shown that OOF could be a powerful tool to predict and rank thermal conductivity of ASPS TBCs.

Keywords
finite element modeling, microstructure, OOF2, suspension plasma spraying, thermal barrier coatings, thermal conductivity
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-10417 (URN)10.1007/s11666-016-0503-8 (DOI)000392060300007 ()2-s2.0-85003888950 (Scopus ID)
Conference
ITSC 2016, International Thermal Spray Conference, Shanghai, P.R. China, May 10-12, 2016
Available from: 2016-12-28 Created: 2016-12-28 Last updated: 2019-01-04Bibliographically approved
Kumara, C., Gupta, M. K. & Nylén, P. (2016). Effect of columnar cracks and dense layer in suspension plasma sprayed thermal barrier coatings on the coating stresses under thermal shock loadings: A modelling approach. In: The 7th International Swedish Production Symposium, SPS16, Conference Proceedings: 25th – 27th of October 2016. Paper presented at 7th International Swedish Production Symposium, SPS16, Lund, Sweden, October 25–27, 2016 (pp. 1-8). Lund: Swedish Production Academy
Open this publication in new window or tab >>Effect of columnar cracks and dense layer in suspension plasma sprayed thermal barrier coatings on the coating stresses under thermal shock loadings: A modelling approach
2016 (English)In: The 7th International Swedish Production Symposium, SPS16, Conference Proceedings: 25th – 27th of October 2016, Lund: Swedish Production Academy , 2016, p. 1-8Conference paper, Published paper (Refereed)
Abstract [en]

Suspension Plasma Spraying (SPS) has been shown as a promising process to produce porous columnar strain tolerant coatings for thermal barrier coating applications in gas turbine engines. However, the highly porous structure is vulnerable to crack propagation, especially near the top coat-bond coat interface, where high stresses are generated due to thermal shock conditions that occur during engine operation. These stresses directly influence the coating lifetime. A topcoat layer with high toughness near the top coat-bond coat interface could thus be beneficial to withstand the stresses. In this work, a bilayer coating system was fabricated by SPS using yttria stabilised zirconia. The bilayer coating consisted of a dense topcoat layer near the top coat-bond coat interface which could provide the necessary toughness followed by a porous columnar structured layer which contains columnar cracks which could provide the necessary strain tolerance. The objective of this work is to study the effect of the columnar/vertical crack features on the generated stresses in the thermal barrier coating during thermal cyclic loading. A finite element modelling approach has been used for this purpose. The results show that the bilayer coating structure reduces the thermally induced stresses and could thus increase the lifetime.

Place, publisher, year, edition, pages
Lund: Swedish Production Academy, 2016
Keywords
Thermal barrier coatings, Suspension Plasma Spraying, bilayer coating system, finite element modelling, stresses, lifetime
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-10244 (URN)
Conference
7th International Swedish Production Symposium, SPS16, Lund, Sweden, October 25–27, 2016
Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2019-01-04Bibliographically approved
Ganvir, A., Kumara, C., Gupta, M. K. & Nylén, P. (2016). Thermal conductivity in suspension sprayed thermal barrier coatings: Modelling and experiments. In: Proceedings of the International Thermal Spray Conference: . Paper presented at Conference of International Thermal Spray Conference and Exposition, ITSC 2016 ; 10 May 2016 Through 12 May 2016 (pp. 368-374). ASM International, 1
Open this publication in new window or tab >>Thermal conductivity in suspension sprayed thermal barrier coatings: Modelling and experiments
2016 (English)In: Proceedings of the International Thermal Spray Conference, ASM International, 2016, Vol. 1, p. 368-374Conference paper, Published paper (Refereed)
Abstract [en]

Axial Suspension Plasma spraying (ASPS) can generate microstructures with higher porosity and pores in the size range from submicron to nanometre. ASPS Thermal Barrier Coatings (TBCs) have already shown a great potential to produce low thermal conductivity coatings for gas turbine applications. It is important to understand the fundamental relationship between microstructural defects in the coating such as grain boundaries, porosity etc. and thermal conductivity. Object Oriented Finite element analysis (OOF) has been shown to be an effective tool for evaluating thermal conductivity for conventional TBCs as this method is capable of incorporating the inherent microstructure as an input to the model. The objective of this work was to analyse the thermal conductivity of ASPS TBCs using experimental techniques and also to evaluate a procedure where OOF can be used to predict and analyse the thermal conductivity for these coatings. Verification of the model was done using experimental thermal conductivity. Results showed that the varied scaled porosity has a significant influence on the thermal conductivity. Smaller grains, higher overall porosity content and lower columnar density resulted in lower thermal conductivity. It was shown that OOF could be a powerful tool to predict and rank thermal conductivity of ASPS TBCs.

Place, publisher, year, edition, pages
ASM International, 2016
Keywords
Coatings; Finite element method; Gas turbines; Grain boundaries; Microstructure; Plasma spraying; Porosity; Sprayed coatings; Thermal barrier coatings; Thermal conductivity of gases, Experimental techniques; Gas turbine applications; Low thermal conductivity; Microstructural defects; Modelling and experiments; Object-oriented finite elements; Suspension plasma spraying; Thermal barrier coating (TBCs), Thermal conductivity
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-11921 (URN)000392060300007 ()2-s2.0-85034765075 (Scopus ID)
Conference
Conference of International Thermal Spray Conference and Exposition, ITSC 2016 ; 10 May 2016 Through 12 May 2016
Available from: 2017-12-13 Created: 2017-12-13 Last updated: 2019-02-22Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-4087-6467

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