(sponsored through the Cast Metal Coalition, CMC, by DOE and numerous steel foundries within the Steel Founders' Society of America, SFSA)
An overview of the DOE metalcasting program can be found here: DOE
Project Fact Sheet from DOE
The Cast Metal Coalition (CMC) home page can be found here: CMC
The Steel Founders Society of America (SFSA) home page can be found here: SFSA
Solidification is a crucial stage in steel casting production. During solidification, contractions or distortions of the steel, known as “dimensional changes,” can cause the final product to vary significantly from the original pattern. Cracks in the casting that form during the late stages of solidification, called “hot tears,” occur when contractions can no longer be accommodated by residual liquid flow or solid displacement. Dimensional changes and hot tears are major problems. These occurrences are difficult to anticipate and correct using traditional foundry engineering methods. While dimensional changes are accommodated using pattern allowances, the desired dimensions are often inaccurate. Castings that form hot tears must then be scrapped or weld repaired, expending unnecessary energy. Correcting either of these problems requires a tedious trial-and-error process that may not necessarily yield accurate results.
Research conducted by the University of Iowa has shown that large dimensional changes are not possible without liquid flowing into or out of a casting section. Hot tears occur along the grain boundaries during the terminal stages of solidification when the stresses developed across adjacent grains exceed the strength of the almost completely solidified steel. Thus, hot tears are initiated in the residual liquid, which are ruptured by contraction stresses. Some successes have been reported in predicting the final dimensions of and residual stresses in castings using stress analysis. However, these models cannot always predict results, in part due to their inability to account for liquid flow. Recently, the researchers developed a model that predicts feeding flow and porosity formation during solidification, but that model assumes the solidified steel to be rigid and immobile. Hence, the occurrence of hot tears cannot be predicted. This project will combine these recently developed stress analysis and feeding flow/porosity models to form a reasonable starting point for a comprehensive, physics-based model of dimensional changes and hot tearing.
The goal of this project is to develop a model to predict dimensional changes and hot tears during solidification of steel castings. The results of this model would be a reduction in scrapped castings and rework/repair due to dimensional changes or hot tears, and also an increase in mold yield due to reduction in the use of padding and improved placement of risers, leading to better riser efficiency.
The objectives of this research are:
• Develop and implement a model into an existing casting simulation code to predict dimensional changes and hot tears during solidification of steel castings,
• Perform a casting experiment to test and validate the model,
• Apply the simulation model to a production casting in a case study illustrating the use of the model in foundry practice.
(sponsored through the Cast Metal Coalition, CMC, by DOE and numerous steel foundries within the Steel Founders' Society of America, SFSA)
An overview of the DOE metalcasting program can be found here: DOE
Project Fact Sheet from DOE
The Cast Metal Coalition (CMC) home page can be found here: CMC
The Steel Founders Society of America (SFSA) home page can be found here: SFSA
Heat treatment and associated processing, such as quenching, are critical during high strength steel casting production. These processes must be managed closely to prevent thermal and residual stresses that may result in distortion, cracking (particularly after machining), rework, and weld repair. The risk of casting distortion limits aggressive quenching that can be beneficial to the process and yield an improved outcome. As a result of these distortions, adjustments must be made to the casting or pattern design or tie bars added. Straightening castings after heat treatments can be both time-consuming and expensive. Residual stresses may reduce a casting’s overall service performance, possibly resulting in catastrophic failure. Stress relieving may help, but expends additional energy in the process. Casting software is very limited in predicting distortions during heat treatment, so corrective measures most often involve a tedious trial and error procedure.
According to researchers from the University of Iowa, the casting process must be considered when attempting to predict the final dimensions and residual stresses that may develop after heat treatment. These researchers have investigated the stresses and distortions that develop during solidification and cooling, before the heat treatment stage begins. Extending modeling capabilities to the heat treatment processes, the researchers will examine microstructural and property changes in the steel, in addition to heat transfer and stresses that develop during the heating and quenching processes. The proposed project will develop and verify a model for predicting the distortions and residual stresses that occur during heat treatment of steel castings.
The goal of this project is to develop a model for predicting the distortions and residual stresses that develop during heat treatment of steel castings. Foundry engineers will then be able to reduce trial and error processing, and economically use heat treatment resources.
The objectives of this research are:
• develop and implement a model into an existing casting simulation code to predict distortion and residual stress development during heat treatment of steel castings,
• perform a casting/heat treatment experiment to test and validate the model,
• apply the simulation model to a production casting in a case study illustrating the use of the model in foundry practice.
The research focuses on the following issues:
• Selection of the dendrite tip operating state and the dendrite sidebranching behavior in the presence of melt convection for free growth into a supercooled melt of a pure substance: this research will primarily rely on our recently developed ability to perform phase-field simulations with convection in three dimensions; results will be compared to theories and experiments.
• Growth of multiple equiaxed dendrites inside a unit cell with/without flow: this research is aimed at improving models of solidifying mushy zones and understanding the thermal/solutal interactions between dendrites; it will again rely on the phase-field method, incorporating a model for multiple grains.
• Free growth of binary alloy dendrites in the presence of coupled heat and solute transport: issues to be addressed are the selection of the dendrite tip operating state, sidebranching behavior, effect of the large disparity in the thermal and solutal diffusion lengths, and influence of convection; the research will involve development of a phase-field model and performing benchmark experiments using the existing dendrite growth setup built under the EDSE/DASE projects at the University of Iowa.
Under the present acceptance standards for steel castings, it is entirely possible for a small shrinkage defect to be passed whereas the larger centerline shrinkage causes a rejection of the casting. A large level of discontinuities located in the center of a casting section may not affect its mechanical properties or fatigue performance, while a small discontinuity near a surface may have a significant effect on fatigue life. Consequently, design engineers uses large safety factors, over-specify the casting making it heavier, less casting friendly, expensive and more time consuming to produce, or they reject the use of steel castings altogether for more expensive fabrications that often have longer lead-times.
Simulation tools have been developed and are now being applied to military and commercial steel castings. Starting with the liquid steel and the mold, the location and amount of shrinkage discontinuities formed during the casting process are predicted, and then included in the analysis to evaluate the load-carrying capacity and fatigue durability of cast components. The ability to design steel castings with non-uniform mechanical properties and examine the tradeoffs between manufacturing costs and structural performance will reduce design process iterations. Applications will allow a discontinuity where it can be tolerated while ensuring soundness where needed.
Click here to view a recent project presentation (only works with Microsoft Internet Explorer)
The objective of this research project is to advance fundamental understanding of microporosity formation during solidification of alloys. The single most important feature of the research is that it focuses solely on the microscopic scale (on the order of microns) where nucleation, interfaces, growth morphologies, and micro-flows can be directly observed. Previous studies of microporosity formation have only been concerned with larger length scales, and have not resolved the actual solid and pore microstructures. The present study will provide valuable knowledge of pore nucleation and growth rates, microscopic flow and solute fields in the solid-liquid-gas system present during pore formation, and the interactions between the pores and the evolving microstructure. This information can then be used to develop improved averaged descriptions of microporosity formation for use in macro-scale casting simulations. It also allows for the prediction of the exact characteristics of the pores in a fully solid microstructure as a function of the alloy composition and processing conditions.
The research will be accomplished through the combined use of novel modeling, numerical simulation, and experimental techniques. A phase-field method will be developed to model the complex phase transformation and transport phenomena present, including multi-component thermodynamics, curvature effects, and convection. Modern numerical techniques, such as deforming finite element meshes and highly efficient parallel solvers, will be employed to solve the model equations. The experiments will use transparent model alloys inside a Hele-Shaw cell under a high-resolution microscope. In the experiments, the initial gas content and the solidification conditions will be carefully controlled. The model predictions will be validated by the experimental measurements.
Figure: SCN-acetone dendrites growing around an artificially introduced bubble. Picture taken by Ahmet Guner.
The present work finds application in nearly every metal casting process. The problem of microporosity is of renewed interest because of the dramatically increased use of castings in the automotive and aerospace industries. Only the detailed prediction of microporosity can aid in its prevention and in assessing its influence on the strength and fatigue life of cast components. The transfer of the knowledge obtained through the proposed project will take place through continued collaboration with the casting industry, the development of improved microporosity models for use in casting simulation software, and the education of students. From a more fundamental point of view, the project will advance a largely uncharted area of research that is concerned with micro-scale, multi-component, multi-phase systems with phase change. Such systems are important not only in metal casting, but also in other materials processing operations, in nature, and in living organisms.
Figure: Porosity in an Al-10%Cu alloy.
(sponsored through the Cast Metal Coalition, CMC, by DOE and numerous steel foundries within the Steel Founders' Society of America, SFSA)
An overview of the DOE metalcasting program can be found here: DOE
Project Fact Sheet from DOE
The Cast Metal Coalition (CMC) home page can be found here: CMC
The Steel Founders Society of America (SFSA) home page can be found here: SFSA
The University of Iowa, in collaboration with numerous foundries, is conducting a research project on yield improvement and defect reduction in steel casting. A survey among American steel foundries conducted in 1997 revealed that the present yield in the industry is only about 55%, implying that almost two tons of steel must be melted for every ton of casting produced. The additional metal is primarily present in so-called risers and used to prevent holes or voids from forming inside the casting due to shrinkage occuring when the liquid steel becomes solid. The survey indicated that a 10% increase in yield translates into an energy savings of 1.8 trillion BTUs per year for melting alone, which corresponds to a yearly cost savings of about $40 million. One emphasis in the project is to re-examine the engineering rules that have been used in the industry for more than 30 years to determine riser sizes and locations. Extensive three-dimensional computer casting simulations showed that the present rules are overly conservative and result in poor yields. New rules, which can increase yield by up to 20%, are currently being developed and made available to foundries. Another task in the project is to develop accurate methods that allow those foundries that are already using computer simulation to predict the exact casting soundness level and design risers without going through extensive casting trials in the foundry. Such simulations are especially important for complicated casting shapes where simple rules would not be applicable. More than five foundries are currently participating in casting experiments and case studies, which are needed to verify the new methods. Yet another project task is the development of novel yield improvement techniques promoting directional solidification through a variety of active heating/cooling schemes. It is envisioned that the techniques will allow certain castings to be produced with a yield that is at least 25% higher than the current level.
An image of a cast steel component analyzed in this project is here:
The new feeding and risering rules are described in the following report: Feeding and Risering Guidelines for Steel Castings, SFSA, 2001.
This project investigates the formation of porosity during casting of steel and other alloys. The model will be implemented in the Magmasoft commercial casting simulation software.
Co-Investigators:
I. Steinbach (Access, Aachen, Germany), A. Karma (NEU), H.C. deGroh III (Nasa Glenn)
This is a NASA-sponsored project to define a microgravity flight experiment in materials science. Please look at the NASA Space Research home page to learn more about such experiments. This page is also a good place to look research announcements, task books, etc. We plan to have more detailed information on this project in the near future. For now, the following overview is provided:
Objectives:
The objectives of the research are to determine the microstructural
evolution and thermo-solutal interactions during equiaxed dendritic growth of dendrites of the alloy succinonitrile-acetone (SCN-ACE). This alloy is a transparent model substance that solidifies like metallic alloys but allows for visual observations during growth. The benchmark data acquired in the microgravity experiment will be used to test and develop equiaxed dendritic solidification
models.
Significance and Background:
The research will establish a more firm scientific basis for the
modeling and simulation of microstructure evolution in metal casting processes
in order to optimize material properties. The planned experiment extends
previous microgravity experiments on the growth of a single dendrite
of a pure substance (i.e., the IDGE experiment of Prof. Glicksman at RPI) to equiaxed dendritic growth of alloy dendrites.
[Please consult the RPI-NASA
IDGE Home Page for further information on the IDGE.]
Co-Investigators:
I. Steinbach (Access, Aachen, Germany), A. Karma (NEU), H.C. deGroh III (Nasa Glenn)
This is a NASA-sponsored project to define a microgravity flight experiment in materials science. Please look at the NASA Space Research home page to learn more about such experiments. This page is also a good place to look research announcements, task books, etc. We plan to have more detailed information on this project in the near future. For now, the following overview is provided:
Objectives:
The objectives of the research are to determine the microstructural
evolution of and thermal interactions between several equiaxed crystals
growing dendritically in an undercooled melt of a pure substance, and to
use this benchmark data to test and develop equiaxed dendritic solidification
models.
Significance and Background:
The research will establish a more firm scientific basis for the
modeling and simulation of microstructure evolution in metal casting processes
in order to optimize material properties. The planned experiment extends
previous microgravity experiments on the growth of a single, isolated dendrite
tip (i.e., the IDGE experiment of Prof. Glicksman at RPI) to multiple crystals
and transient growth - a situation more close to actual casting conditions.
[Please consult the RPI-NASA
IDGE Home Page for further information on the IDGE.]
Some pictures of mesoscopic simulations (by co-I I.Steinbach):
Please also look at our special phase-field page for more information related to dendritic growth simulations.
An important tool in simulating microstructure development during solidification is the so-called "phase-field" method. In this method, the solid-liquid interface is smeared over a few computational cells by introducing a phase variable that smoothly varies from 0 to 1 (0=liquid, 1=solid). Then, the governing conservation equations can be solved on a fixed grid without having to track the solid-liquid interface explicitely. The evolution equation for the phase variable can be derived from Ginzburg-Landau theory, using an entropy formulation, or directly from the Gibbs-Thomson equation. The advantage of the phase-field method is that kinetic and curvature effects are automatically taken into account. This, of course, is critical for simulating dendritic solidification microstructures. Recently, we have extended the phase-field method to include convection in the liquid phase.
An example of two computations involving the coarsening of an alloy (Al-4%Cu) mushy zone is shown below. The upper panels are with convection, the lower ones without. The thick solid lines are the solid-liquid interfaces, and the colors indicate species concentrations. In both cases, the system relaxes into a state with less interfacial area in accordance with LSW theory. However, convection significantly alters the species redistribution in the liquid phase and causes a different coarsening pattern. The evolution of the permeability of this system was also investigated. More details can be obtained from publications (send e-mail).
One project in this area deals with the design and control of a so-called 'soft reduction' section in the thin-slab continuous steel caster presently being installed by IPSCO in a new mini-mill near Muscatine, Iowa. The soft reduction section is located at the end of the caster and serves to reduce the slab thickness to make up for solidification shrinkage and prevent centerline segregation. Modeling involves (i) a Neural Network and (ii) a heat transfer and segregation simulation code.
Another area of emphasis is thermomechanical modeling, especially of deformations and segregation in the partially solidified region (mushy zone) in continuous casting of steel. This is important not only for soft reduction, but also for a new process called "continuous forging."
Finally, we are developing a new heat transfer model which can be used as a control tool to dynamically (during operation) adjust the spray cooling in continuous casting. More detail later.
The goal of this effort is to implement capabilities for solidification simulations within the Telluridecode. The Telluride code is being developed under the ASCI program at Los Alamos National Laboratory. This code is a high-accuracy, modern CFD code that can track material interfaces. More detail can be found on the Telluride home page.
Please also look at our special freckling page.
This project deals with the simulation of flow, heat transfer, micro-/macro- segregation and grain defects in single-crystal directional solidification of Ni-based superalloy turbine blades, where the model includes a thermodynamic data-base and first-principle phase equilibrium calculations. The thermodynamic model was obtained from W.J.Boettinger at NIST.
The main objective is to predict the nature and occurance of freckle defects in single-crystal superalloys. Images of freckles are contained in the following thumbnails (photos courtesy of A.F. Giamei, UTRC):
The superalloy project is sponsored by the U.S. Advanced Research Projects Agency (ARPA) through the Investment Casting Cooperative Arrangement (ICCA). The ICCA is a consortium of five companies: General Electric Aircraft Engines, Pratt and Whitney, Howmet, PCC, and UES. In addition to these five member companies, a number of research institutes are participating members of the program. The present ICCA project is entitled "Materials Science-Based Microstructure Modeling of Multicomponent Superalloys."
Our present task is led by Tony Giamei of the United Technologies Research Center.
An informative article on modeling of superalloy investment casting can be found in a recent issue of the JOM.
This research is primarily concerned with the effects of convection on coupled columnar and equiaxed solidification of metal alloys. The goal of this study is to understand the basic physical phenomena (such as grain fragmentation, growth, and sedimentation) and to create a simulation model that can be used to predict the compositional and structural features in a solidified material. The emphasis is on the effects of melt convection and solid (grain) transport during solidification.
The figure below shows a simulation of solidification of an Al-4wt%Cu alloy using a two-phase model. The growing solid phase is assumed to have a globulitic (spheroidal) morphology (sometimes also referred to as 'equiaxed'). The cavity is cooled from the left sidewall. The nucleation, growth, and movement of the globulitic crystals is calculated together with the thermosolutal convection in the melt. Final results include the macrosegregation pattern and the grain size distribution, both being caused by the solid and liquid motion during solidification (from J. Materials Processing and Manufacturing Science, Vol. 2, pp. 217-231, 1993).
This project investigates the micro-/macro-segregation in the casting of multicomponent steel alloys. One present application is the prediction of macrosegregation in the large tool steel ingots manufactured by Lukens Steel Company. Capabilities for melt flow and segregation calculations during solidification of steel alloys are also being incorporated in the Magmasoft commercial casting simulation software. An example of flow/heat transfer/species transport/micro-/macro-segregation calculations during the solidification of a steel casting can be viewed by clicking on the images below.
(sponsored through the American Metalcasting Consortium, AMC, by the Defense Logistics Agency, DLA)
This project is completed as of Summer 1996. A summary paper of the results is available in Modern Casting, September 1996, pp. 29-31. To obtain a copy of the full report, contact the AFS Research Dept. at (847) 824-0181.
Goals and Objectives:
One of the casting software packages
examined is MAGMAsoft. Click here
to view the result of a simulation example of the casting of a roadarm
(from a tank).
The goal of this project is to develop a simulation model that can be used to predict the flow of a molten metal alloy/particulate reinforcement mixture, its solidification, and the final particle distribution in the solidified part. A typical example is a Al-Si alloy with 20vol% SiC particles having a size of about 20 micrometer. Applications include high-wear automotive parts and aerospace components.
The IJEMS is a Cooperative Experiment on the Wake Shield Facility which flew on the Space Shuttle STS-69 mission in September, 1995.
The objective of the IJEMS is to provide experimental data on the interaction between the solid/liquid interface and suspended Ni particulates during the directional solidification of eutectic and near-eutectic Sn-Cd alloys under microgravity conditions. Microgravity eliminates convective melt motion and buoyancy forces on the particulates, and thus allows for a more conclusive test of available (but inconclusive) theories on particle/solidification front interactions, particularly for a eutectic microstructure (instead of just a plane front for pure substances).
The project is unique in its (i) small budget (~$100,000), (ii) short leadtime (about 1/2 year), (iv) heavy student participation, and (v) cooperation between the two major state universities (IU and ISU).
As of January 1996, the four samples have been succesfully retrieved from the Shuttle (after its flight in Sep 95), and are presently being analyzed and compared to corresponding ground-based experiments.
