Numerical modelling of directional solidification in metal alloys
Citation:
BATTAGLIOLI, SARA, Numerical modelling of directional solidification in metal alloys, Trinity College Dublin.School of Engineering.MECHANICAL AND MANUFACTURING ENGINEERING, 2018Download Item:
Abstract:
Manufacturing routes for metal alloy components often involve solidification processes. In
order to obtain a product that meets given requirements in terms of mechanical properties,
the ability to predict the grain structure development during solidification is crucial. In particular,grains with two different morphologies can form, i.e. columnar or equiaxed grains.
When both types of grain structures develop during the process, the final product exhibits
a columnar to equiaxed transition (CET), which is usually to be avoided since it introduces abrupt changes in the mechanical properties. In order to investigate the influence of process parameters on grain structure evolution, directional solidification processes such as Bridgman and power-down solidification are widely used in research, since they allow to control nominal temperature gradients and cooling rates in the solidifying samples. The overarching objective of this thesis is to provide a methodology for the numerical investigation of directional solidification in metal alloys. In particular, this study is carried on in the framework of ESA?s GRADECET (Gravity Dependence of CET in Peritectic TiAl Alloys) project, with the aim to investigate the influence of process parameters and gravity conditions (micro, terrestrial, hyper-gravity) on grain structure evolution and CET. In order to achieve this goal, three ever more advanced models are presented. Firstly, a 2D axisymmetric enthalpy method is developed.
This model allows to predict the transient evolution of temperature and solid fraction
during Bridgman solidification in absence of convection. Secondly, the model is improved by the inclusion of a front tracking algorithm. Such improvement allows to model the growth of the columnar grain region by the means of a series of markers connected by linear segments, representing the envelope of the columnar grain tips. Furthermore, the front tracking model does not assume equilibrium solidification, therefore it allows to identify a region of undercooled liquid ahead of the front. This is a fully liquid region below the alloy?s liquidus temperature, where equiaxed grains might nucleate and grow. Hence, these features allow to predict the development of columnar grains with different orientation (axial columnar and radial columnar). Furthermore, the analysis of the thermal conditions in the undercooled liquid region during the solidification process enables to evaluate the likelihood of CET occurrence. Finally, the front tracking model is further expanded by including the treatment of thermal convection arising under terrestrial and hypergravity conditions. Throughout the thesis, the three models are used for simulating different experimental scenarios, and to perform several parametric studies. These simulations corroborate the ability of the models in reproducing realistic results. Furthermore, they enable to gain information on hard to observe phenomena, such as transient evolution of temperature and fluid flow in the bulk of the samples, or
transient growth of different grain morphologies, providing a significant contribution to the analysis and understanding of the directional solidification process.
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European Space Agency
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http://people.tcd.ie/battaglsDescription:
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Author: BATTAGLIOLI, SARA
Advisor:
Robinson, AnthonyPublisher:
Trinity College Dublin. School of Engineering. Discipline of Mechanical & Manuf. EngType of material:
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