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Modelling of Forming and Welding in Alloy 718
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea IVF AB. (Solid Mechanics)ORCID iD: 0000-0002-1432-444X
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The reduction of fuel consumption and carbon dioxide emissions are currently a key factor for the aviation industry due to major concerns about climate change and more restrictive environmental laws. One way to reduce both fuel consumption and CO2 emissions is by significantly decreasing the vehicle’s weight while increasing engine's efficiency. In order to meet these requirements, the European aero engine industry is continuously focusing on alternative manufacturing methods for load carrying structures in advanced materials, such as titanium and nickel-based superalloys. Alternatively to traditional large-scale single castings, new manufacturing methods involve sheet metal parts, small castings and forgings assembled by welding. These new manufacturing methods allow more flexible designs in which each part is made of the most suitable material state, leading to several advantages such as reduction of product cost and weight while increasing engine's efficiency. Nickel-based superalloys are widely used in the aero engine industry, typically constituting up to 50% of the total weight of the aircraft engine. Due to their excellent material properties at high temperatures in severe corrosive environments, these superalloys are employed most extensively in the hot sections of gas turbine engines for both military and civil aircrafts with running temperatures up to 650°C.

In this thesis, a manufacturing process chain including forming and welding in the nickelbased superalloy 718 is studied. The main focus in the work lies on determining the thermomechanical properties, modelling and simulation of cold forming, study forming limits based on Nakazima tests for forming limit curves (FLC) and applying a damage and failure criterion. The work also comprises a brief study on hot forming. Finally, modelling of a subsequent welding procedure is included where residual stresses from the forming simulation are used to predict shape distortions due to the welding procedure. The results are compared with experimental observations.

The cold forming procedure of a double-curved component made of alloy 718 is studied using FE-analyses and forming tests. The same geometry was used to produce a hot forming tool. During forming tests at room temperature, micro cracks and open cracks were observed in the draw bead regions, not indicated when formability is assessed using a forming limit curve (FLC). Standard material models such as von Mises or Barlat Yld2000-2D were not capable of accurately predict the behaviour of the material after the point of diffuse necking, making the prediction of damage and failure during forming a challenge. The GISSMO damage model was therefore calibrated and used to predict material failure in forming of alloy 718. Tensile, plane strain, shear and biaxial tests at room temperature are performed up to fracture and continuously evaluated using Digital Image Correlation (DIC) by ARAMIS™. In this work, the GISSMO damage model is coupled with the anisotropic Barlat Yld2000-2D material model for forming simulations in alloy 718 at room temperature using LS-DYNA. Numerical predictions are able to accurately predict failure on the same regions as observed during the experimental forming tests. Comparisons of the distribution of damage on one of the draw beads between simulations and damage measurements by acoustic emission indicate that higher damage values correspond to bigger micro cracks. Numerical FE-predictions of the cold forming and subsequent welding procedure shows that the welding procedure further increases the shape distortions. This was found to agree with experimental observations.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords [en]
Forming, welding, Alloy 718, damage, GISSMO, failure
National Category
Applied Mechanics Metallurgy and Metallic Materials
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-62901ISBN: 978-91-7583-870-0 (print)ISBN: 978-91-7583-871-7 (electronic)OAI: oai:DiVA.org:ltu-62901DiVA, id: diva2:1087064
Presentation
2017-06-16, E231, Luleå, 13:00 (English)
Supervisors
Projects
Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator
Funder
VINNOVA, 2013-01173Available from: 2017-04-06 Created: 2017-04-05 Last updated: 2017-11-24Bibliographically approved
List of papers
1. Calibration of a damage and fracture model for alloy 718
Open this publication in new window or tab >>Calibration of a damage and fracture model for alloy 718
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2016 (English)In: Advancements in Theoretical and Applied Plasticity: Proceedings of PLASTICITY ’16: The Twenty Second International Symposium on Plasticity and its Current Applications / [ed] Akhtar S. Khan, Fulton, Maryland 20759-0591, USA: NEAT PRESS , 2016, p. 223-225Conference paper, Published paper (Refereed)
Abstract [en]

Nickel-based superalloys are primarily used in the hot sections of aircraft engines because they can maintain their mechanical properties and chemical stabilities at high temperatures under severe corrosive environments i.e. for a long time. In order to simulate forming procedures in Alloy 718 sheets, the GISSMO damage and failure model is coupled with both isotropic von Mises and anisotropic Barlat YLD2000 material models in the finite element code LS-DYNA. In this study, the calibration of the GISSMO model for forming simulations at room temperature is discussed. The calibration requires failure strains for different stress states as a function of triaxiality, which are obtained by testing six different specimen geometries up to fracture. Numerical predictions will be compared with experimental observations from forming tests.

Place, publisher, year, edition, pages
Fulton, Maryland 20759-0591, USA: NEAT PRESS, 2016
Keywords
damage, fracture, nickel-based superalloys, Alloy 718, forming, GISSMO
National Category
Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62818 (URN)978-0-9911654-7-6 (ISBN)
Conference
22nd International Symposium on Plasticity and Its Current Applications, (Plasticity 2016), Kona, Hawaii, January 3-9, 2016
Projects
NFFP6 - Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator
Funder
VINNOVA, 2013-01173
Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2019-10-04Bibliographically approved
2. Damage and fracture during sheet-metal forming of alloy 718
Open this publication in new window or tab >>Damage and fracture during sheet-metal forming of alloy 718
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2020 (English)In: International Journal of Material Forming, ISSN 1960-6206, E-ISSN 1960-6214, Vol. 13, p. 15-28Article in journal (Refereed) Published
Abstract [en]

Forming nickel-based superalloy aero-engine components is a challenging process, largely because of the risk of high degree of springback and issues with formability. In the forming tests conducted on alloy 718 at room temperature, open fractures are observed in the drawbead regions, which are not predicted while evaluating the formability using the traditional forming-limit diagram(FLD). This highlights the importance of an accurate prediction of failure during forming as, in some cases, may severely influence the springback and thereby the accuracy of the predicted shape distortions, leading the final shape of the formed component out of tolerance. In this study, the generalised incremental stress-state dependent damage model (GISSMO) is coupled with the isotropic von Mises and the anisotropic Barlat Yld2000-2D yield criteria to predict the material failure in the forming simulations conducted on alloy 718 using LS-DYNA. Their effect on the predicted effective plastic strains and shape deviations is discussed. The failure and instability strains needed to calibrate the GISSMO are directly obtained from digital image correlation (DIC) measurements in four different specimen geometries i.e. tensile, plane strain, shear, and biaxial. The damage distribution over the drawbeads is measured using a non-linear acoustic technique for validation purposes. The numerical simulations accurately predict failure at the same regions as those observed in the experimental forming tests. The expected distribution of the damage over the drawbeads is in accordance with the experimental measurements. The results highlight the potential of considering DIC to calibrate the GISSMO in combination with an anisotropic material model for forming simulations in alloy 718.

Place, publisher, year, edition, pages
Springer, 2020
Keywords
alloy 718, damage, fracture, GISSMO, non-linear acoustic technique, optimisation
National Category
Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62903 (URN)10.1007/s12289-018-01461-4 (DOI)
Projects
Virtual process chain for superalloy sheet metal aero engine structures - Validation and demonstrator (NFFP6)
Funder
Vinnova, 2013-01173
Note

Validerad;2020;Nivå 2;2020-01-27 (johcin)

Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2020-01-27Bibliographically approved
3. Prediction of shape distortions during forming and welding in alloy 718
Open this publication in new window or tab >>Prediction of shape distortions during forming and welding in alloy 718
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The finite-element method (FEM) has considerably contributed to the development of more advanced manufacturing methods for metal structures. The prediction of the final shape of a component is of great interest to the manufacturing industry. In addition to its inherent difficulties, the presence of various types of processes in the manufacturing chain may dramatically increase the level of demand. Therefore, including all steps of the manufacturing process in the simulations is key to being successful. This has been done for a long time in the stamping industry, which involves sequences of forming, trimming, and springback. However, more complex manufacturing procedures, that include assembling of formed parts with forgings and castings via welding, have been modeled with simplifications, resulting in a reduced prediction accuracy. This hinders the compensation of accumulated shape distortions based on the simulation results. One such example is the fabrication of aero-engine structures, in which the history from the forming procedure has not been considered in subsequent welding and heat treatment analyses. In the present study, a double-shaped part manufactured from alloy 718 is formed at 20 °C and laser-welded using the bead-on-plate procedure. The coupling of different manufacturing analyses, including cold forming, trimming, result mapping, welding, cooling, and springback, is achieved using LS-DYNA. Additionally, the effect of adding the GISSMO damage model in the forming simulation is studied. The results of the forming analysis are used as inputs for the material model *MAT_CWM in the welding simulation. The anisotropic thermomechanical properties of alloy 718 are determined at temperatures up to 1000 °C. Encouraging agreement is found between the model predictions and the results of forming and welding tests. The findings underscore the importance of including the material history and accurate process conditions along the manufacturing chain to both the prediction accuracy of shape distortions, and to the potential of the industry.

Keywords
forming, welding, alloy 718, springback, nickel-base superalloys, superalloys, GISSMO, anisotropy
National Category
Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-63348 (URN)
Projects
Virtual process chain for superalloy sheet metal aero engine structures - Validation and demonstrator (NFFP6)
Funder
Vinnova, 2013-01173
Available from: 2017-05-14 Created: 2017-05-14 Last updated: 2019-10-04

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