Influence of natural and forced gravity conditions during directional columnar solidification

Abstract

In a recent study, the present authors reported an analysis of a transient process of directional solidification of TiAl alloys in the absence of convection [Battaglioli et al., 2017]. The adopted front tracking model, also coupled with indirect methods for predicting columnar to equiaxed transition (CET), showed how the development of different grain regions, namely axial columnar, radial columnar and equiaxed, depends on process parameters such as temperature distribution and applied cooling rates, as well as on properties such as the degree of inoculation of the melt and nucleation undercooling required for equiaxed growth. In this paper, the previous front tracking model is significantly developed, by including the solution of Navier-Stokes equations in order to predict thermal convection in the liquid region as well as in the columnar mush (treated as an isotropic porous medium). This improvement is introduced with the aim to investigate TiAl alloys solidification under different gravity conditions. Accordingly, the simulation setup employed in the study reproduces the one used in experimental campaigns carried out on the MAXUS 9 sounding rocket (microgravity) and on ESA's Large Diameter Centrifuge (hypergravity), within the framework of ESA's GRADECET (Gravity Dependence of CET in TiAl alloys) project. The ability of the model in predicting thermal convection is demonstrated by considering several case studies. Results show that the evolution of fluid flow patterns in the samples depends on the external forces considered, combined to the transient change of axial and radial temperature gradients that occur during the solidification process. A parametric study of the directional solidification process on a centrifuge is performed by changing the value of the centrifuge arm and rotation rate and results are compared to predictions derived from nondimensional considerations.

Columnar dendritic solidification of TiAl under diffusive and hypergravity conditions investigated by phase-field simulations

“…Thus, our results will be different compared to experimental 3D flow scenario but they demonstrate basic principles concerning the interaction between flow and tip operating state. The boundary conditions are not fully representative for the experimental case, especially the conditions at the top boundary does not represent a link to the far-field melt flow established at the macro-scale [32] of the whole melt volume. Keeping this limitation and the confinement to 2D in mind, one may not expect a quantitative match to experimental results.…”

Percolating Reaction–Diffusion Waves (PERWAVES)—Sounding rocket combustion experiments

“…In addition to PERWAVES, three other scientific experiments were flown on MAXUS-9: the EUGRAPHO experiment that studied the response of a single-cell algae to different gravity levels, XRMON-Diff2 that consisted of chemical diffusion experiments for semiconductor as well as for metallic material couples with in-situ X-ray monitoring, and GRADECET [43] that investigated gravity-dependent changes in the solidification process of metal alloys. The payload also included SUPERMAX, a drop capsule that was released for testing right after the separation of the rocket motor from the payload in order to test a supersonic parachute.…”

Numerical investigations on solute transport and freckle formation during directional solidification of nickle-based superalloy ingot

“…Directional solidification technology can control the grain orientation so that all grain boundaries are parallel to the height direction of the casting. The casting produced by this technology has longer fatigue life and higher creep resistance, thus becoming the main process in the production of alloy components for advanced manufacturing industries, such as aerospace, automobile and medical care [1]. However, the change of liquid density caused by thermal and solute concentration gradient usually cause the thermosolutal convection, which may lead to elongated solute enrichment channels [2].…”