Subhayu Sen

Melt convection effects on the critical velocity of particle engulfment

Liquid convection ahead of the solidifying interface alters particle behavior in the vicinity of the interface. This effect has not been quantified to date. Relevant directional solidification experiments were conducted using samples of varying thicknesses, as well as normal and low-gravity experiments. A mixture of transparent biphenyl matrix and spherical glass particles, as well as one of succinonitrile matrix with polystyrene particles were used. Two experimental setups were used: a horizontal gradient heating facility (HGF) for horizontal solidification, and a Bridgman-type furnace (BF) for vertical solidification. The convection level during solidification in the HGF was varied by changing the distance between the glass slides containing the composite sample. The BF was used on ground and during parabolic flights, and thus the convection level was changed by alternating low-gravity and high-gravity solidified regions. It was found that the convection level and/or particle buoyancy significantly influences the critical velocity for particle engulfment. At higher natural convection during solidification the critical velocity increases by up to 40%. At very high convection levels engulfment may become impossible because particles fail to interact with the interface. A systematic analysis of some theoretical models was performed in an attempt to evaluate the present level of theoretical understanding of the problem. Methods of evaluating the surface energies required for model validation are also presented.

In Situ Observations of Interaction Between Particulate Agglomerates and an Advancing Planar Solid/Liquid Interface: Microgravity Experiments

Results are reported of directional solidification experiments on particulate agglomerate pushing and engulfment by a planar solid/liquid (s/l) interface. These experiments were conducted on the Space Shuttle Columbia during the United States Microgravity Payload 4 (USMP-4) Mission. It was found that the pushing to engulfment transition velocity, Vcr, for agglomerates depends not only on their effective size but also their orientation with respect to the s/l interface. The analytical model for predicting Vcr of a single particle was subsequently enhanced to predict Vcr, of the agglomerates by considering their shape factor and orientation.

Effect of melt convection at various gravity levels and orientations on the forces acting on a large spherical particle in the vicinity of a solidification interface

Numerical modeling was undertaken to analyze the influence of both radial and axial thermal gradients on convection patterns and velocities during solidification of pure Al and an Al-4 wt% Cu alloy. The objective of the numerical task was to predict the influence of convective velocity on an insoluble particle near a solid/liquid (s/l) interface. These predictions were then be used to define the minimum gravity level (g) required to investigate the fundamental physics of interactions between a particle and a s/l interface. This is an ongoing NASA funded flight experiment entitled particle engulfment and pushing by solidifying interfaces (PEP). Steady-state calculations were performed for different gravity levels and orientations with respect to the gravity vector. The furnace configuration used in this analysis is the quench module insert (QMI-1) proposed for the Material Science Research Facility (MSRF) on board the International Space Station (ISS). The general model of binary alloy solidification was based on the finite element code FIDAP. At a low g level of 10 -4 g 0 (g 0 = 9.8 m/s 2 ) maximum melt convection was obtained for an orientation of 90°. Calculations showed that even for this worst case orientation the dominant forces acting on the particle are the fundamental drag and interfacial forces.

Advances in the science and engineering of casting solidification : An MPMD symposium honoring Doru Michael Stefanescu

Advances in the science and engineering of casting solidification : An MPMD symposium honoring Doru Michael Stefanescu

Advances in the Science and Engineering of Casting Solidification

Many of the metal casting processes are still empirical in nature. Many others are deeply rooted in mathematics and therefore, suitable for modeling. Science of casting and solidification is a major technical asset for foundry operations and of extreme importance in understanding different length scales microstructural changes and evolution as well as developing new processes and materials. In his attempt to describe combinations of solidification theory, research results and industrial practice, Professor Doru Michael Stefanescu (ASM Fellow, 1997) has made tremendous contributions to the field. Many of his views on casting and solidification are valued as important impacts within professional environments such as TMS and ASM International. He has written many articles for the ASM Handbook series on subjects including basic metallurgy of cast iron, compacted graphite irons, solidification, thermodynamic properties of iron-base alloys, and computational modeling. He was also Volume Chair for Volume 15: Casting, of the 9 Edition Metals Handbook.

Real-time X-ray transmission microscopy for fundamental studies solidification: Al-Al2Au eutectic

High resolution real-time X-ray Transmission Microscopy, XTM, has been applied to obtain information fundamental to solidification of optically opaque metallic systems. We have previously reported the measurement of the solute profile in the liquid, phase growth, and detailed solid-liquid interfacial morphology of aluminum based alloys with exposure times less than 2 seconds. Recent advances in XTM furnace design have provided an increase in real-time magnification (during solidification) for the XTM from 40X to 160X. The increased magnification has enabled for the first time the XTM imaging of real-time growth of fibers and particles with diameters of 5 μm. We have previously applied this system to study the kinetics of formation and morphological evolution of secondary fibers and particles in Al-Bi monotectic alloys. In this paper we present the preliminary results of the first real-time observations of fiber morphology evolution in optically opaque bulk metal sample of Aluminum-Gold eutectic alloy. The...

Plasma Processing of Lunar Regolith Simulant for Diverse Applications

Versatile manufacturing technologies for extracting resources from the moon are needed to support future space missions. Of particular interest is the production of gases and metals from lunar resources for life support, propulsion, and in‐space fabrication. Deposits made from lunar regolith could yield highly emissive coatings and near net shaped parts for replacement or repair of critical components. Equally important is development of high fidelity lunar simulants for ground based validation of potential lunar surface operations. Described herein is an innovative plasma processing technique for in‐situ production of gases, metals, coatings, and deposits from lunar regolith, and synthesis of high fidelity lunar simulant from NASA issued lunar simulant JSC‐1. Initial plasma reduction trials of JSC‐1 lunar simulant have indicated production of metallic iron and magnesium. Evolution of carbon monoxide has been detected subsequent to reduction of the simulant using the plasma process. Plasma processing of t...

Molten Materials Transfer and Handling on the Lunar Surface

Electrolytic reduction processes as a means to provide pure elements for lunar resource utilization have the potential of removing all the oxygen from the lunar soil for use in life support and for propellant. Electrochemical reduction also provides a direct path for the production of pure metals and silicon which can be utilized for in situ manufacturing and power production. Some of the challenges encountered in the electrolytic reduction processes include the feeding of the electrolytic cell (the transfer of electrolyte containing lunar soil), the withdrawal of reactants and refined products such as the liquid iron‐silicon alloy with a number of impurities, and the spent regolith slag, produced in the hot electrolytic cell for the reduction of lunar regolith. The paper discusses some of the possible solutions to the challenges of handling molten materials on the lunar surface, as well as the path toward the construction and testing of a proof of concept facility.

Particle Engulfment and Pushing (PEP): Past Micro-Gravity Experiments and Future Experimental Plan on the International Space Station (ISS)

The interaction of an insoluble particle with a growing solid-liquid interface (SLI) has been a subject of investigation for the four decades. For a metallurgist or a material scientist understanding the fundamental physics of such an interaction is relevant for applications that include distribution of reinforcement particles in metal matrix composites, inclusion management in castings, and distribution of Y2Ba1Cu1O5 (211) precipitates (flux pinning sites) in Y1Ba2Cu3O7 (123) superconducting crystals. The same physics is also applicable to other areas including geological applications (frost heaving in soils) and preservation of biological cells. Experimentally this interaction can be quantified in terms of a critical growth velocity, Vcr, of the SLI below which particles are pushed ahead of the advancing interface, and above which the particles are engulfed. Past experimental evidence suggests that this Vcr is an inverse function of the particle radius, R. In order to isolate the fundamental physics that governs such a relationship it is necessary to minimize natural convection at the SLI that is inherent in ground based experiments. Hence for the purpose of producing benchmark data (Vcr vs. R) PEP is a natural candidate for micro-gravity experimentation. Accordingly, experiments with pure Al containing a dispersion of ZrO2 particles and an organic analogue, succinonitrile (SCN) containing polystyrene particles have been performed on the LMS and USMP-4 mission respectively. In this paper we will summarize the experimental data that was obtained during these two micro-gravity missions and show that the results differ compared to terrestrial experiments. We will also discuss the basic elements of our analytical and numerical model and present a comparison of the predictions of these models against micro-gravity experimental data. Finally. we will discuss our future experimental plan that includes the ISS glovebox and MSRRl.