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History Reduction by Lumping for Time-Efficient Simulation of Additive Manufacturing
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0002-2592-9073
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0002-0053-5537
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.ORCID iD: 0000-0002-2544-9168
2020 (English)In: Metals, ISSN 2075-4701, Vol. 10, no 1, article id 58Article in journal (Refereed) Published
Abstract [en]

Additive manufacturing is the process by which material is added layer by layer. In most cases, many layers are added, and the passes are lengthy relative to their thicknesses and widths. This makes finite element simulations of the process computationally demanding owing to the short time steps and large number of elements. The classical lumping approach in computational welding mechanics, popular in the 80s, is therefore, of renewed interest and is evaluated in this work. The method of lumping means that welds are merged. This allows fewer time steps and a coarser mesh. It was found that the computation time can be reduced considerably, with retained accuracy for the resulting temperatures and deformations. The residual stresses become, to a certain degree, smaller. The simulations were validated against a directed energy deposition (DED) experiment with alloy 625.

Place, publisher, year, edition, pages
MDPI, 2020. Vol. 10, no 1, article id 58
Keywords [en]
finite element, thermo-mechanical analysis, additive manufacturing, alloy 625
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-77958DOI: 10.3390/met10010058ISI: 000516827800058Scopus ID: 2-s2.0-85077642374OAI: oai:DiVA.org:ltu-77958DiVA, id: diva2:1411615
Funder
Swedish Foundation for Strategic Research , GMT14-0048
Note

Validerad;2020;Nivå 2;2020-04-01 (alebob)

Available from: 2020-03-04 Created: 2020-03-04 Last updated: 2022-02-02Bibliographically approved
In thesis
1. History Reduction Techniques for Simulation of Additive Manufacturing and Physically based Material Modeling
Open this publication in new window or tab >>History Reduction Techniques for Simulation of Additive Manufacturing and Physically based Material Modeling
2020 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, finite element (FE) simulations of additive manufacturing (AM) and physically based material modeling are presented. AM is a process where the component is built layer-wise. The material undergoes repeated heating and cooling cycles when layers are added, which may result in undesired deformation and residual stress in the built component. The choice of process parameters and scan strategy affect the resulting residual stress. Simulations can be used to support the experimental determination of process parameters and scan strategy. AM processes often comprise many added layers, and the passes are lengthy relative to their thicknesses and widths. This makes the FE simulations computationally expensive, with many elements and time steps. In this work, AM processes have been simulated with the FE-method using a lumping technique. This technique allows fewer time steps and a coarser mesh. Thermal behavior, deformation, and residual stresses have been simulated and compared with experiments. The simulations show that, by using the lumping technique, the computational effort can be reduced significantly with retained accuracy for the resulting temperature and deformations. The residual stresses become somewhat smaller. Alloy 625 is a nickel-based superalloy used in high-temperature applications owing to the hightemperature strength. The material is difficult to manufacture by conventional machining owing to excessive tool wear and low material removal rates. Thus alloy 625 is a material appropriate for the AM technology with its near-net shape potential. An existing, physically based flow stress model has been further developed to fit the mechanisms typical for alloy 625. This model gives an accurate mechanical behavior and capture viscoplasticity, creep, and relaxation. The physically based model has been calibrated versus compression tests and validated with a stress relaxation test performed in a Gleeble 3800 machine. The predicted relaxation was in good agreement with the measured relaxation. The usage of this kind of material model is expected to improve the prediction of the material behavior during the AM process and, thereby, the overall prediction of the AM process.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2020
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-78579 (URN)978-91-7790-591-2 (ISBN)978-91-7790-592-9 (ISBN)
Presentation
2020-06-16, E231, Luleå Tekniska Universitet, 97187, Luleå, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , GMT14-0048
Available from: 2020-04-21 Created: 2020-04-21 Last updated: 2020-05-19Bibliographically approved
2. Simulation of additive manufacturing using a mechanism based plasticity model
Open this publication in new window or tab >>Simulation of additive manufacturing using a mechanism based plasticity model
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents finite element (FE) simulations of additive manufacturing (AM) and physically based material modeling of alloy 625 and alloy 718. In recent years, there has been an increasing interest in AM and there has been a dramatic increase in publications in the field. AM can be beneficial compared to conventional manufacturing methods in many applications. The method offers short component lead times and large design freedom with the possibility to create complex components. Alloy 625 and alloy 718 are nickel-based superalloys used in high-temperature applications owing to their high-temperature strength. The materials are difficult to manufacture by conventional machining due to rapid tool wear and low material removal rates. Thus, the alloys are appropriate for the AM technology with its near-net shape potential.Owing to the rapid heating and solidification in the AM process, residual stresses are induced in the component. This is a well-known problem and causes distortion of the samples when removing them from the build plate. The residual stresses may also deteriorate the fatigue properties. It is important for the manufacturer to understand how the choice of process parameters and scanning strategy affect the residual stresses to minimize those and improve the quality of the components. Simulation can be used as a tool while developing the process parameters and support the experimental efforts. FEM is generally the preferred method for simulation of deformations and residual stresses in AM. The simulation technique used when modeling AM has its origin from welding simulations that was performed already since the beginning of 1970. However, it is not possible in practice to simulate an AM process in the traditional way due to a large number of elements and time increments to be calculated. This is especially true for the laser-based powder bed fusion (PBF-LB) process where the process of a full-scale part may comprise many thousands of added layers, and the passes are lengthy relative to their thicknesses and widths.The aim of this thesis work is to develop FE simulation techniques that reduce the computational effort when modeling residual stresses in AM processes to enable simu-lation of full-scale parts. This has been done with thermo-mechanical FE-models using different lumping techniques e.g., lumping of layers and lumping of hatches. Lumping of layers and hatches means that several physical layers, or several physical hatches, are merged and added in one modeled layer or hatch respectively. Lumping allows fewer time steps and a coarser mesh which reduces the computational effort. An existing mechanism based flow stress model has been developed to fit the mechanisms typical for alloy 625 and alloy 718 and implemented in the FE model. Also, synchrotron X-ray diffraction was performed to measure the residual stress for comparison with the models. The stress was extracted from the diffraction data using the full Debye ring fitting method.In this work, using the lumping techniques described above, it was possible to simu-late AM processes with up to physical 1500 layers. For different process parameter sets and scan strategies, thermal behavior, deformation and residual stresses have been mod-eled and compared with experiments. Using the lumping of layer technique resulted in modeled residual stresses showing the same trend as measured stresses from synchrotron X-ray diffraction for two different process parameter sets. Utilizing lumping of hatches, the resulting deflection in a part was modeled successfully for different scanning strate-gies. In the modeling, the larger deflection was seen for the samples printed with the scanning direction parallel to the long-side which was also shown experimentally.The results in this work shows that the presented lumping approaches are promising when it comes to modeling of the deformations and residual stresses in AM. Using lumping approaches, it is also possible to simulate different scanning strategies for processes of larger parts. The description of the mechanical behavior of the material is improved, using the mechanism based material model, compared to when the flow stress was modeled with tabulated data, since it takes mechanisms as viscoplasticity and stress relaxation into account. The mechanism based model includes microstructural information as grain size and solutes and can thus more easily be combined with a microstructure model. The combination of the mechanism based material model and the use of lumping techniques is thus an advance in the development of predictive models of the AM process.

Place, publisher, year, edition, pages
Luleå University of Technology, 2022
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Residual stress, Material model, Alloy 625, Alloy 718, deformations, Finite Element Method, synchrotron X-ray diffraction
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-89095 (URN)978-91-8048-020-8 (ISBN)978-91-8048-021-5 (ISBN)
Public defence
2022-03-31, E632, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , GMT14-0048
Available from: 2022-02-02 Created: 2022-02-02 Last updated: 2022-03-10Bibliographically approved

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Malmelöv, AndreasLundbäck, AndreasLindgren, Lars-Erik

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