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  • 1.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V at low strain rates2007In: Ti-2007 : science and technology: proceedings of the 11th World Conference on Titanium (JIMIC 5), held at Kyoto International Conference Center, Kyoto, Japan, 3 - 7 June 2007 / [ed] M. Niinomi, Kyoto: Japan Institute of Metals , 2007, p. 311-314Conference paper (Refereed)
  • 2.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V: including recovery and recrystallisation2007In: Computational plasticity: Fundamentals and Applications / [ed] Eugenio Onate; Roger Owen; Benjamin Suarez, International Center for Numerical Methods in Engineering (CIMNE), 2007, p. 631-634Conference paper (Refereed)
  • 3.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density model for plastic behaviour of Ti-6-42006In: WCCM VII: 7th World Congress on Computational Mechanics ; Los Angeles, California, USA ; July 16 - 22, 2006, MAdison, Wis: Omnipress , 2006Conference paper (Other academic)
  • 4.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea MEFOS AB.
    Mechanism-based flow stress model for Ti-6Al-4V: applicable for simulation of additive manufacturing and machining2018Doctoral 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. 

  • 5.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically based model for plasticity and creep of Ti-6Al-4V2008Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Albeit 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 modelling the process is the key to overcome this hurdle. This work investigates plastic deformation of the alloy at strain rates from 0.001/s to 1/s and temperatures between 20 and 1100 Celsius. Pertinent deformation mechanisms of the material when subjected to thermo-mechanical processing is discussed. A physically based 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. Model is compared with relaxation test data to demonstrate its validity.

  • 6.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Charles, Corinne
    Department of Industrial Production, Högskolan Väst, Trollhättan.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically Based Constitutive Model of Ti-6Al-4V for Arbitrary Phase Composition2018In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154Article in journal (Refereed)
    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.

  • 7.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based model for plastic deformation and globularization of Ti-6Al-4V2013In: International journal of plasticity, ISSN 0749-6419, E-ISSN 1879-2154, Vol. 50, p. 94-108Article in journal (Refereed)
    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.

  • 8.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based constitutive model for Ti-6Al-4V used in simulation of metal deposition2007In: Svenska Mekanikdagar 2007: Program och abstracts / [ed] Niklas Davidsson; Elianne Wassvik, Luleå: Luleå tekniska universitet, 2007, p. 84-Conference paper (Other academic)
  • 9. Babu, Bijish
    et al.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Physically based constitutive model for Ti-6Al-4V used in the simulation of manufacturing chain2009In: Computational Plasticity X: fundamentals and applications ; proceedings of the X International Conference on Computational Plasticity - fundamentals and applications held in Barcelona, Spain, 02 - 04 September 2009 / [ed] E. Onate; D.R.J. Owen; B. Suarez, International Center for Numerical Methods in Engineering (CIMNE), 2009Conference paper (Refereed)
    Abstract [en]

    Simulations of manufacturing process chain involving forming, welding and heat treatment are complex because of the varying length and time scales and the range of temperatures which trigger the different associated deformation mechanisms. This paper demonstrates the use of a physically based constitutive model in simulation of a manufacturing chain.

  • 10.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. Swerea MEFOS.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simulation of additive manufacturing of Ti-6Al-4V using a coupled physics-based flow stress and microstructure modelManuscript (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.

  • 11.
    Babu, Bijish
    et al.
    Swerim AB, Heating and Metalworking, Luleå, Sweden.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simulation of Ti-6Al-4V Additive Manufacturing Using Coupled Physically Based Flow Stress and Metallurgical Model2019In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 23, article id 3844Article in journal (Refereed)
    Abstract [en]

    Simulating the additive manufacturing process of Ti-6Al-4V is very complex due to the microstructural changes and allotropic transformation occurring during its thermomechanical processing. The α -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 was used here. Furthermore, a physically based flow-stress model coupled with the metallurgical model was applied in the simulation of an additive manufacturing case using the directed energy-deposition method. The result from the metallurgical model explicitly affects the mechanical properties in the flow-stress model. Validation of the thermal and mechanical model was performed by comparing the simulation results with measurements available in the literature, which showed good agreement

  • 12.
    Babu, Bijish
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Svoboda, Ales
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Ghassemali, Ehsan
    School of Engineering, Jönköping University..
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Dislocation density based plasticity model extended to high strain rate deformation of Ti-6Al-4VManuscript (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.

  • 13.
    Lindgren, Lars-Erik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Charles, Corinne
    Högskolan Väst.
    Wedberg, Dan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Simulation of manufacturing chains and use of coupled microstructure and constitutive models2010In: Finite Plasticity and Visco-plasticity of Conventional and Emerging Materials: the 16th International Symposium on Plasticity & its Current Applications ; January 3 - 8, 2010, Marriott Resort, St. Kitts / [ed] Akhtar S Khan; Babak Farrokh, Fulton, Md.: NEAT PRESS , 2010Conference paper (Other academic)
  • 14.
    Lundbäck, Andreas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Charles, Corinne
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Simulating a chain of manufacturing processes for prediction of component properties2011In: XXth International Symposium on Air Breathing Engines 2011: (ISABE 2011) : Gothenburg, Sweden 12-16 Swptember 2011, Red Hook, NY: Curran Associates, Inc., 2011Conference paper (Refereed)
    Abstract [en]

    An integrated design of material and process is necessary when designing a component where the effect of the manufacturing route on its performance must be accounted for. This is particularly the case for welded components even when post weld heat treatment is performed. The paper describes developments done at Luleå University of Technology in cooperation with Volvo Aero in the Swedish National Programme for Aeronautical Research (NFFP) and in different European projects. The paper focuses on two particular issues of importance. The first is of more administrative character, the transfer of data between different finite element models used in each of the manufacturing steps. The other aspect is the extremely important issue of material modeling.Material models for simulation of a chain of manufacturing processes include additional complications besides large variations in strain rates and temperatures. These complications are caused by the changing microstructure that may occur. The authors expect that physically based models can have a larger range of applicability than engineering type of models. Physical based models are formulated by considering the underlying physics of the deformation whereas engineering type of models are more of a curve-fitting nature. The physical based models may also have a natural coupling to models of the microstructure evolution. However, the models must still be tractable for large-scale computations. Thus, they should be of the internal state variable type with relatively few additional parameters and equations to solve at the integration point level of finite elements. The paper describes a basic dislocation density model used in modelling different manufacturing processes and how it can be coupled to microstructure models. It is based on dislocation glide as the dominating mechanism for the plastic deformation. This may be models for phase changes, like in Ti6-4, or precipitate growth/dissolution as in Alloy 718. The coupled models will not only make it possible to describe the material behavior more correct over the process cycles but also predict the obtained microstructure. It is expected that future research may couple this information with defect predictions in order to contribute to life assessment. The paper includes some example of manufacturing simulations and also an example of simulation of a chain of manufacturing processes.

  • 15.
    Tersing, Henrik
    et al.
    Volvo Aero Corporation.
    Lorentzon, John
    Volvo Aero Corporation.
    Francois, Arnaud
    Cenaero.
    Lundbäck, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Babu, Bijish
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Barboza, Josué
    Cenaero.
    Bäcker, Vladimir
    Laboratory for Machine Tools and Production Engineering of RWTH Aachen University.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Simulation of manufacturing chain of a titanium aerospace component with experimental validation2012In: Finite elements in analysis and design (Print), ISSN 0168-874X, E-ISSN 1872-6925, Vol. 51, p. 10-21Article in journal (Refereed)
    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.

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