| 1. |
- Charles Murgau, Corinne
(author)
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Microstructure model for Ti-6Al-4V used in simulation of additive manufacturing
- 2016
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Doctoral thesis (other academic/artistic)abstract
- This thesis is devoted to microstructure modelling of Ti-6Al-4V. The microstructure and the mechanical properties of titanium alloys are highly dependent on the temperature history experienced by the material. The developed microstructure model accounts for thermaldriving forces and is applicable for general temperature histories. It has been applied to study wire feed additive manufacturing processes that induce repetitive heating and cooling cycles.The microstructure model adopts internal state variables to represent the microstructure through microstructure constituents' fractions in finite element simulation. This makes it possible to apply the model efficiently for large computational models of general thermomechanical processes. The model is calibrated and validated versus literature data. It is applied to Gas Tungsten Arc Welding -also known as Tungsten Inert Gas welding-wire feed additive manufacturing process.Four quantities are calculated in the model: the volume fraction of phase, consisting of Widmanstätten, grain boundary, and martensite. The phase transformations during cooling are modelled based on diffusional theory described by a Johnson-Mehl-Avrami-Kolmogorov formulation, except for diffusionless martensite formation where the Koistinen-Marburger equation is used. A parabolic growth rate equation is used for the to transformation upon heating. An added variable, structure size indicator of Widmanstätten, has also been implemented and calibrated. It is written in a simple Arrhenius format.The microstructure model is applied to in finite element simulation of wire feed additive manufacturing. Finally, coupling with a physically based constitutive model enables a comprehensive and predictive model of the properties that evolve during processing.
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| 2. |
- Azizoğlu, Yağız, 1986-
(author)
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Modeling of Cold Pilgering of Stainless Steel Tubes
- 2023
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Doctoral thesis (other academic/artistic)abstract
- Cold pilgering is a complex forming process used to produce seamless tubes in terms of modeling due to the complexity in kinematic of tools, friction condition and material behavior. The process development has mostly been based on simple formulas and costly full-scale tryouts. The aim in this study is to develop validated Finite element models of cold pilgering to support design of a robust process.A three-dimensional thermo-mechanical Finite element models of cold pilgering has been developed in the course of the work leading to this thesis. The commercial code MSC. Marcwas used in the simulations. General 3D models are needed to be able to capture asymmetric deformation in cold pilgering. Elastic deflections of tools and roll stand were included in the model via linear and nonlinear springs that were calibrated versus experiments. A temperature dependent Chaboche type plasticity model was employed in this simulation to mimic strain hardening and softening behavior under multidirectional loading. The model parameters were optimized using multi-directional compression and uni-directional tensile tests. Heat exchange between tools and lubricant was included in the simulation via heat convection films on the surfaces. The film parameters were calibrated using experimental data. Simulation predictions for hardening, rolling force, process temperature and geometry were compared with experiments for validation purposes. The predictions showed overall good agreement with validation experiments enabling the use of this model for understanding and improving the process.
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| 3. |
- Babu, Bijish, Tec. Lic. 1979-
(author)
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Mechanism-based flow stress model for Ti-6Al-4V : applicable for simulation of additive manufacturing and machining
- 2018
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 4. |
- Draxler, Joar
(author)
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Modeling and Simulation of Weld Hot Cracking
- 2019
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Doctoral thesis (other academic/artistic)abstract
- Several alloy systems are susceptible to weld hot cracking. Weld hot cracking occurs by fracture of liquid films, normally grain boundary liquid films, at the late stage of the solidification of the weld. The cracks can be small and therefore difficult to detect by nondestructive test methods. If hot cracks are not repaired, they can act as sites for initiation of fatigue and stress corrosion cracking, which in turn can lead to catastrophic failure in critical applications such as aerospace engines and nuclear power plants. Therefore, it is of highest importance to design weld processes so that hot cracking can be avoided. Here, numerical simulation can be a powerful tool for optimizing weld speed, heat input, weld path geometry, weld path sequences, weld fixturing, etc., such that the risk for hot cracking can be minimized. In this thesis, we propose a modeling approach for simulating weld hot cracking in sheet metals with low welding speeds and fully penetrating welds. These conditions are assumed to give rise to isolated grain boundary liquid films (GBLFs) whose crack susceptibility can be analyzed using one-dimensional models. The work is divided into four journal papers. The three first papers treat hot cracking that occurs in the fusion zone of the weld while the last paper treats hot cracking in the partially melted zone of the weld. The main content of the four papers are summarized below. In paper A, a pore-based crack criterion for hot cracking has been developed. This criterion states that cracking occurs in a GBLF if the liquid pressure in the film goes below a fracture pressure. The fracture pressure is determined from a pore model as the liquid pressure that is required to balance the surface tension of an axisymmetric pore in a liquid film located between two parallel plates at a given critical pore radius. The fracture pressure depends on the surface tension, the spacing between the parallel plates and the gas concentration in the liquid. In order to evaluate the above pore-based crack criterion in a GBLF the liquid pressure in the film most be known. In paper B, a one-dimensional GBLF pressure model for a columnar dendritic microstructure has been developed. This model is based on a combination of Poiseuille parallel plate flow and Darcy porous flow. Flow induced by mechanical straining of the GBLF is accounted for by a macroscopic mechanical strain field that is localized to the GBLF by a temperature dependent length scale. In paper C, a computational welding mechanics model for a Varestraint test is developed. The model is used to calibrate the crack criterion in paper A and the pressure model in paper B. It is then used to test the crack criterion in Varestraint tests with different augmented strains. Calculated crack locations, orientations, and widths are shown to correlate well to the experimental Varestraint tests. vii Finally, in paper D, a segregation model for predicting the thickness of eutectic bands has been developed. The thickness of eutectic bands affects the degree of liquation in partially melted zone, and therefore is an important factor for hot cracking in this region of the weld.
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