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Doru StefanescuAshland Designated Research ProfessorDr. Eng., University Politehnica Bucharest, 1973 Tel. (614) 292-5629 Office: 137 Fontana Labs
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Professor Stefanescu joined the MSE department in August 2005 after serving in the Metallurgical and Materials Engineering Department at the University of Alabama for 25 years. His last position was of Cudworth Professor of Engineering, Distinguished University Research Professor, and Director of the Solidification Laboratory.
Professor Stefanescu is an educator and a scientist. He believes that a university professor must both generate and transfer knowledge, and his activity has consistently reflected this philosophy. His scholastic record includes 339 publications numbering 28 invited papers, 29 books and chapters in books, 112 refereed journal publications, and 12 patents. Dr. Stefanescu has also delivered an additional 36 invited talks and seminars at universities, national and industrial laboratories in 12 countries. He has also been a Visiting Professor at the University of Wisconsin Madison (1980), Visiting Scholar at ƒcole de Mines de Nancy, France (1990), Invited Visiting Professor at Institute National Polytechnique de Toulouse (2002), and Visiting Scholar at Ohio State University (2003). He has advised 16 doctoral students and 34 master students. Professor Stefanescu is a co-editor of the International Journal of Cast Metals Research and key-reader for Metallurgical and Materials Transactions. He has served repeatedly as an external reviewer for the National Science Foundation, NASA, Caterpillar Inc., and for the Group of Metallic Materials, Univ. of Porto, Portugal.
Professor Stefanescu research interests include solidification science (e.g. effects of natural convection on nucleation and growth kinetics, particles behavior at the solid/liquid interface, computational modeling of microstructure evolution), solidification processing (e.g. manufacturing of metal-matrix composites and ceramic superconductors, computational modeling of shrinkage cavity, porosity, metal penetration defects in castings, and of mechanical properties), and new materials produced through solidification processing (e.g. particulate ceramics-intermetallic matrix composite materials, thin-wall spheroidal graphite cast iron). He is also a nationally and internationally recognized expert in metal casting technology.
His contributions in solving the difficult interdisciplinary problem of particles behavior at the liquid/solid interface has enjoyed significant publicity, following the experiment that he directed on the Life and Microgravity Science Mission of the shuttle Columbia in June 1996, and the experiment that he conducted on the Fourth United States Microgravity Payload (USMP-4) in November of 1997. The two astronauts designated to conduct the experiments were trained for two days in The Solidification Laboratory at The University of Alabama, under Dr. StefanescuÕs supervision.
Examples of recent research include the following:
A Quantitative Dendrite Growth Model and Analysis of Stability Concepts
While a number of cellular automaton (CA) based models for dendrite growth have been proposed, none so far have been validated, raising questions about their quantitative capabilities. All these models are mesh-dependent and cannot correctly describe the influence of crystallographic orientation on growth morphology.
An improved version of a previously developed CA-based model for dendrite growth controlled by solutal effects in the low Péclet number regime is described here. The model solves the solute and heat conservation equations subject to the boundary conditions at the interface which is tracked with a new virtual front tracking method. It contains an expression equivalent to the stability constant required in analytical models, termed stability parameter, which is not a constant. The process determines its value, changing with time and angular position during dendrite formation. The paper proposes solutions for the evaluation of local curvature, solid fraction, trapping rules and anisotropy of the mesh, which eliminates the mesh dependency of calculations.
Several tests were performed to demonstrate the mesh independence of the calculations using Fe-0.6wt%C and Al-4wt%Cu alloys. Computation results were validated in three ways. First, the simulated secondary dendrite arm spacing was compared with literature values for an Al-4.5wt%Cu alloy. Second, the predictions of the classic Lipton-Glicksman-Kurz analytical model for steady state tip variables, such as velocity, radius, and composition, were compared with simulated values as function of melt undercooling for Al-4wt%Cu alloy. In this validation it was found that the stability parameter approaches the experimentally and theoretically determined value of 0.02 of the stability constant. Finally, simulated results for succinonitrile-0.29wt% acetone alloy are compared with experimental data. Model calculations were found to be in very good agreement with both the analytical model and the experimental data. The model is used to simulate equiaxed (Fig. 1) and columnar (Fig. 2) growth of Fe-0.6wt%C and Al-4wt%Cu alloys offering insight into microstructure formation under these conditions.
Fig. 1 Simulation of equiaxed solidification of Al-4wt%Cu alloy showing grain boundary formation. From left to right: after 0.04, 0.08, 0.16, and 0.2 s.
Fig. 2 Simulation of a Fe-0.6wt%C alloy dendrite with q = 15o. Δs = 0.1 μm. From left to right: after 0.03, 0.05 and 0.07 s.
Thin Wall Ductile Iron
Today’s foundries are confronted with continuous demand to manufacture high-quality, cost-effective cast components. Those specializing in automotive castings have the added pressure of developing new processes and materials that reduce overall car weight to meet federally mandated fuel economy standards, without sacrificing performance. In order to meet these needs, automakers have increasingly turned to lighter weight materials, and castings continue to be a prime target. This is why some ferrous markets have been lost during the last 15 years to aluminum and other less dense materials in applications such as engine blocks and cylinder heads.
Is this market segment forever lost for the ferrous metal casters? A closer examination of the relative mechanical capabilities and cost of various materials shows that ferrous alloys are competitive with aluminum base alloys. Among these, ductile iron occupies a special position, as it is not only by far a cheaper material, but it is also comparable or superior to aluminum in most instances, based on the relative weight per unit of yield strength.
Under a research project funded by a consortium of foundries and foundry suppliers and by the U. S. Department of Energy it was demonstrated that mechanical properties of fully machined thin wall (2.5 to 6 mm thick, 4.6 to 12 °C/s cooling rate) ductile iron plates are equivalent or superior to regular section (12.7 mm) ductile iron (Fig. 3). The reasons for the lower values previously reported by other investigators are solidification anomalies and/or surface roughness. Indeed, the surface quality of the test plates greatly influences the level of mechanical properties.
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Fig. 3. Comparison of tensile properties of machined and non-machined vertical plates and ASTM minimum as-cast ductile iron properties. |
The design component of the project relied heavily on rapid prototyping and extensive computer modeling for test casting design (e.g. Fig. 4). While simulations are mathematical descriptions of the process physics, a significant number of assumptions are required to produce engineering results. Thus, simulations must be validated through experiments. In the past such work for mold filling has been done with model substances, as well as X-ray film recordings. However, actual mold filling occurs much faster than can be captured in detail with conventional film or video recording equipment. Accordingly, a high speed video camera was used to record the filling of resin-bonded molds with cast iron in real-time. Details of the dynamic filling were compared with simulation results. In addition the influence of filters on mold filling was assessed (e.g. Fig. 5). The possibility to use this technique to properly select the filter coefficient for computer simulations is also discussed.
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Fig. 4 Solidification simulation of cast iron poured in horizontal plates. |
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Fig. 5 Computer simulation (top) versus high speed video (bottom) of thin plate filling during pouring of cast iron.
