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Mechanism-based flow stress model for Ti-6Al-4V: applicable for simulation of additive manufacturing and machining
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea MEFOS AB.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ti-6Al-4V has remarkable properties such as high specific mechanical properties (viz. stiffness, strength, toughness, fatigue resistance), corrosion resistance, biocompatibility etc. These properties make it attractive for applications in aerospace, chemical industry, energy production, surgical implants, etc. Many of these applications have to satisfy high requirements on mechanical properties, which are directly affected by the microstructure. Therefore, it is essential to understand as well as to model the microstructure evolution during manufacturing as well as in-service. Furthermore, this alloy has a narrow temperature and strain rate window of workability.

This work was initiated as part of a project aimed at performing finite element simulations of a manufacturing process chain involving hot forming, welding, machining, additive manufacturing and heat treatment of Ti-6Al-4V components within the aerospace industry. Manufacturing process chain simulations can compute the cumulative effect of the various processes by following the material state through the whole chain and give a realistic prediction of the final component. Capacity to describe material behavior in a wide range of temperatures and strain rates is crucial for this task.

A material model based on the dominant deformation mechanisms of the alloy is assumed to have a more extensive range of validity compared to an empirical relationship. Explicit dislocation dynamics based models are not practically feasible for manufacturing process simulation, and therefore the concept of dislocation density, (length of dislocations per unit volume) developed by (Kocks1966; Bergström, 1970) is followed here. This mean field approach provides a representation of the average behavior of a large number of dislocations, grains, etc. Conrad (1981) studied the influence of various factors like solutes, interstitials, strain, strain rate, temperature, etc., on the strength and ductility of titanium systems and proposed a binary additive relationship for its yield strength. The first component relates to long-range interactions and second short-range relates to lattice resistance for dislocation motion. For high strain rate deformation, this short-range term is extended to include the effects of a viscous drag given by phonon and electron drag (Lesuer et al. 2001).  Immobilisation of dislocation by pile-ups gives hardening and remobilization/annihilation by dislocation glide and climb gives restoration. Globularization is also considered to restore the material. The material model is calibrated using isothermal compression tests at a wide range of temperatures and strain rates. Compression tests performed using Gleeble thermo-mechanical simulator is used at low-strain rates and split-Hopkins pressure bar is used at high strain rates for calibration.

During additive manufacturing depending on the temperature, heating/cooling rates, Ti-6Al-4V undergoes allotropic phase transformation. This transformation results in a variety of textures that can give different mechanical properties.  Based on the texture (Semiatin et al., 1999b; Seetharaman and Semiatin, 2002; Thomas et al., 2005) identified few microstructural features that are relevant to the mechanical properties. The three separate alpha phase fractions; Widmanstatten,  grain boundary, Martensite, and the beta-phase fraction are included in the current model. However, since the strengthening contributions of these individual alpha phases are not known, a linear rule of mixtures for the total alpha-beta composition is developed. This model is calibrated using continuous cooling tests performed by Malinov et al. 2001 with differential scanning calorimeter at varying cooling rates.  

This mechanism-based model is formulated in such a way that it can be implemented in any standard finite element software. In the current work, this is implemented as subroutines within MSC Marc and used for simulation of hot-forming and additive manufacturing. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords [en]
Material modeling, Ti-6Al-4V, dislocation density, vacancy concentration, additive manufacturing, hot forming, heat treatment
National Category
Applied Mechanics Other Materials Engineering
Research subject
Material Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-70906ISBN: 978-91-7790-207-2 (print)ISBN: 978-91-7790-208-9 (electronic)OAI: oai:DiVA.org:ltu-70906DiVA, id: diva2:1249616
Public defence
2018-11-13, E231, E-Building, LTU, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2018-09-25 Created: 2018-09-19 Last updated: 2018-11-21Bibliographically approved
List of papers
1. Simulation of manufacturing chain of a titanium aerospace component with experimental validation
Open this publication in new window or tab >>Simulation of manufacturing chain of a titanium aerospace component with experimental validation
Show others...
2012 (English)In: Finite elements in analysis and design (Print), ISSN 0168-874X, E-ISSN 1872-6925, Vol. 51, p. 10-21Article in journal (Refereed) Published
Abstract [en]

Manufacturing of advanced components like aeroengine parts is performed in a global network. Different manufacturers deliver individual components to the engine and even different manufacturing steps for a given component may be performed at different companies. Furthermore, quality is of utmost importance in this context. Simulations are increasingly used to assure the latter. The current paper describes the simulation of a chain of manufacturing processes for an aeroengine component. Different partners have performed the simulations of the different steps using a variety of finite element codes. The results are discussed in the paper and particularly the lessons learned regarding the modelling process.

Keywords
Engineering mechanics - Solid mechanics, Engineering mechanics - Mechanical manufacturing, Teknisk mekanik - Fastkroppsmekanik, Teknisk mekanik - Mekanisk tillverkningsteknik
National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-6176 (URN)10.1016/j.finel.2011.10.002 (DOI)000299011300002 ()2-s2.0-83155163890 (Scopus ID)460af32e-0ce9-4d49-91f3-e87964dbab96 (Local ID)460af32e-0ce9-4d49-91f3-e87964dbab96 (Archive number)460af32e-0ce9-4d49-91f3-e87964dbab96 (OAI)
Note
Validerad; 2012; 20111215 (ysko)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-09-19Bibliographically approved
2. Dislocation density based model for plastic deformation and globularization of Ti-6Al-4V
Open this publication in new window or tab >>Dislocation density based model for plastic deformation and globularization of Ti-6Al-4V
2013 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 50, p. 94-108Article in journal (Refereed) Published
Abstract [en]

Although Ti-6Al-4V has numerous salient properties, its usage for certain applications is limited due to the challenges faced during manufacturing. Understanding the dominant deformation mechanisms and numerically modeling the process is the key to overcoming this hurdle. This paper investigates plastic deformation of the alloy at strain rates from 0.001s−1 to 1s−1 and temperatures between 20° C and 1100° C. Pertinent deformation mechanisms of the material when subjected to thermo-mechanical processing are discussed. A physically founded constitutive model based on the evolution of immobile dislocation density and excess vacancy concentration is developed. Parameters of the model are obtained by calibration using isothermal compression tests. This model is capable of describing plastic flow of the alloy in a wide range of temperature and strain rates by including the dominant deformation mechanisms like dislocation pile-up, dislocation glide, thermally activated dislocation climb, globularization, etc. The phenomena of flow softening and stress relaxation, crucial for the simulation of hot forming and heat treatment of Ti-6Al-4V, can also be accurately reproduced using this model.

National Category
Other Materials Engineering
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-12601 (URN)10.1016/j.ijplas.2013.04.003 (DOI)000323872600006 ()2-s2.0-84882237464 (Scopus ID)bc2aa2ca-2f53-4501-a934-cc05dbc761d0 (Local ID)bc2aa2ca-2f53-4501-a934-cc05dbc761d0 (Archive number)bc2aa2ca-2f53-4501-a934-cc05dbc761d0 (OAI)
Note
Validerad; 2013; 20130425 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-09-19Bibliographically approved
3. Physically Based Constitutive Model of Ti-6Al-4V for Arbitrary Phase Composition
Open this publication in new window or tab >>Physically Based Constitutive Model of Ti-6Al-4V for Arbitrary Phase Composition
2018 (English)In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154Article in journal (Refereed) Submitted
Abstract [en]

The principal challenge in producing aerospace components using Ti-6Al-4V alloy is to employ the optimum process window of deformation rate and temperature to achieve desired material properties. Qualitatively understanding the microstructure-property relationship is not enough to accomplish this goal. Developing advanced material models to be used in manufacturing process simulation is the key to compute and optimize the process iteratively. The focus in this work is on physically based flow stress models coupled with microstructure evolution models. Such a model can be used to simulate processes involving complex and cyclic thermo-mechanical loading.

Keywords
Finite Element Method, Dislocation density, Vacancy concentration, Ti-6Al-4V, Alpha, Beta
National Category
Applied Mechanics
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-70904 (URN)
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-11-23
4. Dislocation density based plasticity model extended to high strain rate deformation of Ti-6Al-4V
Open this publication in new window or tab >>Dislocation density based plasticity model extended to high strain rate deformation of Ti-6Al-4V
(English)Manuscript (preprint) (Other academic)
Abstract [en]

One of the main challenges in the simulation of machining is accurately describing the material behavior during severe plastic deformation at strain rates ranging six orders of magnitude and temperature between room temperature to nearly melting temperature. High strain rate measurements are performed using Split-Hopkinson Pressure Bar (SHPB) technique at a range of temperatures. The temperature change during deformation is included by computing the plastic work converted to heat energy. A physics-based material model published earlier (Babu and Lindgren, 2013) is extended in this paper to include the high strain rate mechanisms of phonon and electron drag. Characterization of the microstructure is performed using Electron Backscatter Diffraction (EBSD), and a novel method is proposed in this work to quantify the extent of globularization which is compared with model predictions.

Keywords
Dislocation density, Vacancy concentration, Ti-6Al-4V, Phonon-drag, Electron-drag, Finite Element (FE), Split-Hopkinson Pressure Bar (SHPB), Electron Backscatter Diffraction (EBSD)
National Category
Applied Mechanics
Research subject
Material Mechanics
Identifiers
urn:nbn:se:ltu:diva-70903 (URN)
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2019-08-27
5. Simulation of additive manufacturing of Ti-6Al-4V using a coupled physics-based flow stress and microstructure model
Open this publication in new window or tab >>Simulation of additive manufacturing of Ti-6Al-4V using a coupled physics-based flow stress and microstructure model
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Simulating the additive manufacturing process of Ti-6Al-4V is very complex owing to the microstructural changes and allotropic transformation occurring during its thermo-mechanical processing. The alpha-phase with a hexagonal close pack structure is present in three different forms; Widmanstatten, grain boundary, and Martensite. A metallurgical model that computes the formation and dissolution of each of these phases is used in this work. Furthermore, a physically based flow-stress model coupled with the metallurgical model is applied in the simulation of direct energy deposition additive manufacturing case.

Keywords
Dislocation density, Vacancy concentration, Ti-6Al-4V, Additive Manufacturing, Direct Energy Deposition
National Category
Applied Mechanics
Identifiers
urn:nbn:se:ltu:diva-70901 (URN)
Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2019-08-27

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