6th Decennial International Conference on Solidification Processing
25th-28th July 2017, Beaumont Estate, Old Windsor, UK
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PROFESSOR WILFRED KURZ;
50 Year’s Solidification Research: The Dendrite Growth Problem between Brighton and Old Windsor
W. Kurz - EPFL, Ecole Polytechnique Fédérale de Lausanne, Switzerland
This review presents the progress of research on dendritic growth modelling between the first and the sixth decennial Solidification Conference. During these 50 years appeared two most important concepts that have changed our modelling capabilities of solidification microstructures: (i) Marginal Stability (including the follow-up theory of Microscopic Solvability) and (ii) Phase-Field Theory. Both of these theories were published in 1978, a major turning point of the field. Other tools were developed in the last decades with the aim of multi-scale modelling, such as Phase-Field Crystal, Cellular Automata, Mesoscopic Envelope, and Dendritic Needle Network techniques. Examples of applications of the different models for the simulation of dendritic solidification microstructures are also presented.
PROFESSOR ROBERT F. SINGER;
Solidification processing of superalloys
Professor Robert F. Singer - University of Erlangen-Nuernberg Lehrstuhl WTM, Martensstr. 5, 91058 Erlangen, Germany and Neue Materialien Fürth GmbH, Dr.-Mack-Straße 81, 90762 Fürth, Germany
Recent progress in superalloy research at the University of Erlangen will be reviewed. Important issues include directional solidification/single crystal casting and rapid solidification/additive manufacturing.
In single crystal casting we are focusing on Fluidized Carbon Bed Cooling (FCBC). The high potential of FCBC is based on a high heat flux. More importantly, the buoyancy effect allows for creation of a Dynamic Baffle. One challenge lies in the transfer of bed debris into the mold heater and the mold sprue. Using a counter pressure concept and an advanced chill ring design, such problems could be solved. Baffle and bed materials in our approach are based on carbon to avoid contamination of the melt with ceramic particles. A 10 kg-pilot plant was installed to demonstrate our new FCBC concept. For a small 5 blade mold cluster a 40% reduction of dendrite arm spacing compared to the conventional HRS process has been demonstrated.
Additive Manufacturing by selective laser or electron beam melting (SLM and SEBM) is very attractive because of the possibility to realize complex geometries that cannot be manufactured in conventional ways. Due to rapid solidification, segregation free microstructures can be achieved. Drawbacks include high cost and high energy consumption. In order to reduce cost, we developed a concept that allows to double the build tank size compared to the state of the art.
Another challenge is to control the generation of defects, such as porosity from entrapped gas or solidification or stress
relief cracking. As will be shown, with proper process control mechanical properties of wrought material can be achieved.
By adoption of the melt pool geometry grain structures can be switched from columnar grain to equiaxed grain in the same
PROFESSOR NACK J. KIM;
Twin-roll Casting of Mg Alloys
Professor Nack J. Kim - Pohang University of Science and Technology (POSTECH), Pohang, Korea
Lightweighting of automobiles is possible through the application of materials such as advanced high strength steels and low density alloys including Al and Mg alloys. Mg alloys having a density about one-fourth of steel and two-thirds of Al can offer greater weight reduction over other materials, provided that Mg alloys have comparable properties to those of steels and Al alloys.
The use of large amounts of Mg alloys in automobiles has been frequently predicted, but their actual application, particularly of sheet products, is quite limited, due to their several shortcomings such as high cost, poor mechanical properties including formability, etc. It has been recently shown that twin-roll casting (TRC) can produce low cost, high quality Mg alloy sheets that have comparable or better mechanical properties compared to those of conventional Mg alloy sheets produced by ingot casting.
One of the main advantages of TRC is that it provides much faster solidification rates, 102 to 103 K/s, than the conventional ingot casting, resulting in the microstructure having reduced segregation and refined microstructural features. It also enables the utilization of alloying elements that have limited solid solubilities in Mg. Such elements are usually avoided in ingot casting since at slow solidification rate they form coarse deleterious intermetallic particles. On the other hand, those intermetallic particles can be dispersed in a fine scale in the case of TRC Mg alloys.
In this presentation, the recent progress in the development of wrought Mg alloys by TRC will be reviewed with particular emphasis on the fundamental aspects such as microstructure and texture evolution and deformation behavior. Also presented will be some examples showing the applications of TRC Mg alloys in automobiles and the author’s own perspective, which would guide future research aimed at improving the properties and broadening the structural applications of wrought Mg alloys.
DR PHILIPPE JARRY;
Solidification Structure Selection as a guideline to the optimization of Al-alloys DC Casting
Mr Philippe Jarry - C-TEC Constellium Technology Center
The scientific and engineering approach of DC Casting has been profoundly renewed in the past twenty years thanks to both numerical modelling of the process and significant advances in solidification science. Much deeper understanding of defect formation and structure selection mechanisms has been acquired.
The presentation will try to illustrate how science and engineering have cross-fertilized each other in the field of Al alloys solidification. Whereas science is about finding behaviour laws in an open-loop scheme between inputs and outputs, industry is always operating in a closed-loop scheme, as downstream specifications should guide the optimal choice of inputs. Namely, end product specifications and recycling constraints should guide the choice of upstream process parameters based on the knowledge of transfer functions, that is in our case, knowledge of the solidification structure heredity in the fabrication schedule downstream of the casting stage.
This knowledge is obviously not sufficient as one must also understand what degrees of freedom are available at the casting stage, and this is where solidification science must be used. Several illustrations of solidification structure selection will be given relative to various types of Al alloys solidified by DC Casting. Structure selection is a general term which encompasses nuclei selection, phase selection and growth morphology selection, three mechanisms which stand at the heart of Al alloys design for casting. In turn, understanding how such selection mechanisms work can also be source of inspiration for the development of new technological processes which might make the activation of the desired selection mechanisms possible.
Intertwining between science and engineering is key to both industrial and engineering science progress, and understanding how it occurred in the past for Al alloy casting may help fostering progress in the future: a few perspectives will be carefully suggested.
PROFESSOR CHRISTIAN BERNHARD;
Increasing quality demands on continuously cast steel - new challenges for solidification research?
Professor Christian Bernhard - Montanuniversitaet Leoben, 8700 Leoben, Austria
The worldwide challenging economic situation in steel industry caused a global trend towards a commercially favourable, higher share of advanced steel grades produced via continuous casting. Increasing quality demands initiated a dynamic realization of new caster concepts (e.g. vertical caster for large blooms or semi-continuous caster), new engineering solutions (e.g. smart secondary cooling systems) and new maintenance strategies.
Besides these conventional technological developments, process automation evolved rapidly during the last decade with focus on productivity and quality control. For these software solutions, the time-saving but nevertheless accurate handling of phase transformation kinetics of steel close to solidus temperature plays the central role. Reliable results require the availability of proper thermodynamic data, of thermo-physical properties and of thermal boundary conditions. The linkage between phase transformation kinetics and prediction models for product quality is also essential. With respect to the solidification of steel, considerable deficits in all of these fields seem still to exist.
The presentation will give a brief overview on quality features of continuously cast, advanced steel grades and the origin of defects related to process parameters and steel composition. Using specific examples, common strategies for quality prediction will be discussed. A critical analysis will show quite clearly that most available data is restricted to common alloying concepts. For more complex steel compositions or newly developed steel grades (e.g. high manganese steel) not even the available thermodynamic data seems reliable. Thus, ongoing research efforts appear to be urgently needed for the further development of continuous casting related to the reliable production of advanced steel grades.
PROFESSOR JOHN CAMPBELL;
Fifty Years of Casting R&D
Professor John Campbell - Emeritus Professor of Casting Technology, Department of Metallurgy and Materials, University of Birmingham, UK
Two years ago, the author was invited to review the past 60 years of casting research. The request this year to review the past 50 years has therefore presented an interesting challenge. This paper therefore concentrates on the major updates to those features of the 60 year review which have occurred during these past two years. During this period, the bifilm concept has continued to be a focus of interest. It has now been shown to be theoretically necessary for fracture, and has been observed directly for the first time in cast irons, confirming its predicted role as controlling the graphite morphology of grey irons. It is for the first time predicted to control high temperature creep of metals. The apparently innocuous bifilm as a by-product of casting requires our attention and respect; it seems it can not only the control structures and properties of castings, but appears to hold a fundamental place in metallurgy.
PROFESSOR MICHEL RAPPAZ;
Microstructure modelling beyond phase field
Professor Michel Rappaz - Ecole Polytechnique Fédérale de Lausanne, Institute of Materials
MXD 130, Station 12, 1015 Lausanne, Switzerland
With its introduction in the solidification field in the mid-90’s, the phase-field method was thought initially to be THE solution to all modelling issues related to microstructure formation. Yet, after more than two decades, the limitations of this technique have to be recognized. Although it provides a powerful support to experimental investigations by bringing enlightening results, phase field is limited indeed by several constraints: fine mesh size imposed by the thickness and curvature of diffuse solid-liquid interfaces, and thus small time step associated with explicit scheme. While strategies have been implemented to overcome them (adaptative meshes, implicit schemes, GPU’s and parallel implementation), the 3D computation domains are still limited in size.
These limitations have left room for other methods to be developed or improved over the years in order to model solidification at a larger scale typically called “mesoscale”. Among those, one can mention Cellular Automata coupled with Finite Elements (CAFE method) and analogous methods, the Dendrite Needle Network method (DNN), phase field-type mesoscopic models, the level-set method or multi-phase analytical approaches.
After showing a few examples for which phase field has been a key element for the understanding of microstructure formation in solidification, the present contribution will briefly present the most important mesoscale approaches. Emphasis will be put on those for which comparison with experiments or validation on more accurate solutions has been made. Their specificities and limitations, which must be kept in mind when addressing a given solidification problem, will be recalled and future trends and needs in this matter will be outlined.
PROFESSOR MATHIS PLAPP;
Phase-field simulations of coupled eutectic growth
Professor Mathis Plapp - Laboratoire de Physique de la Matière Condensée, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau, France
The solidification of eutectic alloys yields, for a broad range of compositions, materials that are made of two distinct solid phases. These composites form by coupled growth, that is, crystallization of both solids takes place simultaneously at a macroscopically flat growth front. The patterns formed in the solid are a ‘frozen-in’ trace of the dynamical phenomena that take place at the solid-liquid interfaces. In recent years, quantitative three-dimensional phase-field simulations and their comparison to in situ directional solidification experiments have yielded new insights into several aspects of this pattern formation process, several of which will be reviewed here. The stability of lamellar and rod patterns in binary alloys has been investigated, the transition between these patterns has been examined, and other stable steady-state patterns have been found. Furthermore, the influence of the temperature field on pattern dynamics has been highlighted. Finally, anisotropy of the solid-solid interphase boundaries has been incorporated in the model, and its influence on lamellar patterns has been studied. As an outlook, several recent results on patterns in ternary eutectic alloys will also be presented.
PROFESSOR LINDSEY GREER;
Formation and Properties of Metallic Glasses
A. Lindsay Greer – University of Cambridge
Metallic glasses are formed when metallic melts are cooled into the solid state without the intervention of crystallization. For solidification processing, this raises some issues that are quite distinct from those in conventional casting. In understanding the glass, the properties of its precursor liquid are important over a wide temperature range down to the glass transition (at 50–70% of the absolute liquidus temperature). Over this range, the viscosity of the liquid varies over some 12 orders of magnitude, and the thermodynamic driving force for crystallization does not continue to increase linearly with supercooling. The interplay of these effects allows for casting, viscous-flow shaping (e.g. blow-moulding) and ultra-fine surface patterning of metallic glasses, opening up a range of applications. We review glass-forming compositions and note that these are now considered to include some pure metals . While a glass has no microstructure, a given composition is nevertheless capable of being formed in different states, most readily characterized by their energy, and varying from very high (‘rejuvenated’) to very low (‘relaxed’ and even ‘ultrastable’). The range of energy in a given composition can be nearly as large as the heat of fusion, and is much wider than the range of energies that can be readily attained in conventional crystalline alloys . We review the structures that can be attained in the glassy state, noting that these may be inhomogeneous, and may have induced anisotropy. Finally we review some of the properties of metallic glasses and relate these to their solidification processing. A particular focus will be the ongoing quest for improved plasticity in metallic glasses .
1. A. L. Greer, Nature Mater., 14 (2015) 542.
2. Y. H. Sun et al., Nature Rev. Mater. 1 (2016) 16039.
3. S. V. Ketov et al., Nature 524 (2015) 200.
PROFESSOR LÁSZLÓ GRÁNÁSY;
Hydrodynamic theory of freezing: Nucleation and polycrystalline growth
Professor László Gránásy - Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, PO Box 49, Hungary
Structural aspects of crystal nucleation in undercooled liquids are explored using a nonlinear hydrodynamic theory of crystallization proposed recently [G.I. T oth et al., J. Phys.: Condens. Matter 26, 055001 (2014)], which is based on combining fluctuating hydrodynamics with the phase-field crystal theory.
We show that in this hydrodynamic approach not only homogeneous and heterogeneous nucleation processes are accessible, but also growth front nucleation, which leads to the formation of new (differently oriented) grains at the solid-liquid front in highly undercooled systems. Formation of dislocations at the solid-liquid interface and interference of density waves ahead of the crystallization front are responsible for the appearance of the new orientations.
PROFESSOR INGO STEINBACH;
Why Solidification? Why Phase-Field?
Prof Ingo Steinbach - Ruhr-University Bochum, Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Universitaetsstrasse 150, Bochum, Germany
‘‘Solidification’’ is a branch of pattern formation in theoretical physics. ‘‘Phasefield’’ is an applied tool in engineering. This strange combination of basic and applied research is reviewed against its historical background: a story of failure and success. The main achievements in both fields are highlighted. The talk will also present new material since publication in 2013 .
 Why solidification? Why phase-field?, JOM - The Journal of The Minerals, Metals & Materials Society (TMS), Springerlink.com, 65, 1096-1102, (2013)
Professor Zhongyun Fan;
A New Framework for Understanding Heterogeneous Nucleation and Grain Refinement
Z Fan - BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, UK.
Heterogeneous nucleation occurs during solidification process of all metallic materials and plays a critical role in determining the solidification microstructures. However, our current knowledge of heterogeneous nucleation is very limited. Except the experimental difficulties in studying heterogeneous nucleation, an important contributing factor is the classical nucleation theory (CNT) itself. Although CNT is mathematically elegant and theoretically sound, it’s a closed and non-inviting system. Based on the recent research progress in BCAST, in this talk I present a new framework for understanding heterogeneous nucleation and its implications to grain refinement. It covers
• The concept of pre-nucleation
• Epitaxial nucleation process
• Structural templating
• Compositional templating
• Manipulation of the lattice misfit through interfacial adsorption
• Competition for heterogeneous nucleation between different types of substrates
• Explosive nucleation vs. progressive grain initiation.
Professor MARTIN GLICKSMAN;
Capillary Perturbations: Interface Fields Affecting Solidification
M.Glicksman - Florida Institute of Technology, College of Engineering, Melbourne, Florida, USA
Solid-liquid interfaces support capillary-mediated perturbation fields that add or withdraw small amounts of thermal energy. Perturbation fields modulate interface motion and stimulate pattern formation. Capillary fields depend primarily on an interface’s curvature distribution and influence energy/mass balances at spatial scales at or above 100 nm. The presence of capillary fields was established using multiphase-field simulations, allowing measurement of their detailed intensity distributions on equilibrated grain boundary grooves. Perturbations resident on constrained microstructures were precisely measured as “residuals” of the interface thermo-potential, by subtracting the applied thermal gradient field from precisely measured interface potentials. Source fields exposed on grain boundary grooves entail persistent energy withdrawal along their interfaces, a result confirming independent predictions based on sharp-interface thermodynamics. Phase-field simulations, without any modification, provide quantitative support for the active presence of 4th-order perturbations on curved solid-liquid interfaces. This study is broadly directed to the question of whether natural autogenous fields can stimulate formation of solidification microstructures, which might allow developing novel processing methods to improve microstructure-level control in alloy castings.
Professor Baicheng Liu;
Numerical Simulation of Macrosegregation of Large Steel Ingot with Multicomponent and Multiphase Model
Baicheng LIU - School of Materials Science and Engineering, Tsinghua University, CHINA
The solidification process of large steel ingots, over hundred tons in weight, is very slow, always taking several days or even ten more days. The solid-liquid relative motion time is longer during solidification and the macrosegregation patterns are more significant. It is still a great challenge to predict macrosegregation in practice with numerical simulation. A three dimensional multicomponent and multiphase solidification model has been developed to predict macrosegregation of large steel ingots. The macroscopic transport of mass, momentum, species and energy of liquid phase, solid phase and air phase is coupled with microscopic descriptions of grain nucleation and growth. Besides, equiaxed grain sedimentation and columnar to equiaxed transition (CET) have been taken into consideration. Interfacial solute constraint relationships are derived to close the model by solving the solidification paths of multicomponent alloy. Numerical simulation of macrosegregation in Fe-C-Cr-Mn, Fe-C-Si-Mo and Fe-C-Cr-Mo quaternary alloy system has been carried out for three different types of steel ingots of 170-ton, 231-ton and 535-ton respectively. The distribution of C-Cr-Mn elements in the riser and bottom of the 170-ton ingot and the distribution of C-Si-Mo elements in the bottom of the 231-ton ingot have been characterized by spectrometer. The multicomponent and multiphase solidification model was validated by using these experimental data. Then, the model was used to predict macrosegregation of the 535-ton steel ingot and detailed analysis of macrosegregation formation process of solute elements was performed, providing meaningful insights in macrosegregation prediction of large steel ingot.
Professor Deiter Herlach;
Design of Metastable Materials: Experimental Results and Modelling of Non-equilibrium Solidification
D.M. Herlach - DLR - Germany
Solidification of metastable materials require large undercooling of the melt prior to solidification. Large undercoolings of metallic alloys are achieved by the application of levitation techniques which provide the extra benefit of a freely suspended drop directly accessible for in situ measurements of crystallization by combining the levitation with appropriate diagnostic means such as high speed video camera technique to measure dendrite growth kinetics and X-ray diffraction using synchrotron radiation to investigate phase selection. Post mortem analysis of as solidified samples gives access to microstructure selection maps with undercooling as experimental parameter. Comparative experiments on Earth and in Space allow for estimations of the influence of forced convection on dendrite growth kinetics. Solidification of highly undercooled melts occurs far away from local equilibrium. We apply sharp interface modelling to analyse non-equilibrium solidification. It includes effects beyond thermal equilibrium like interface undercooling, solute and disorder trapping.