Adaptive Grid Methods for Phase Field Models of Dendrite Growth
J. A. Dantzig,* N. Goldenfeld (Physics),* J-H. Jeong
National Aeronautics and Space Adminstration, NAG 8-1249

Phase field models offer the potential to examine new theories of dendrite growth. However, previous work has been hampered by computational limits imposed by the conflicting needs for spatial resolution and domain size. We apply adaptive grid methods to permit these calculations to examine new regions of parameter space, such as low undercooling. These calculations permit examination of theory and experiments not previously possible.

Bulging between Rolls in Continuous Casting
B. G. Thomas,* L. Yu
Continuous Casting Consortium**

Beneath the mold in the spray zone of a continuous caster, internal pressure causes plastic creep bulging of the solidifying steel between the support rolls. This leads to internal cracks, segregation, and permanent shape changes. To understand and quantify this phenomenon, finite element models thermal stress models are being developed using ABAQUS. The results will be implemented into a model to predict and control the final width of slabs, which is known to vary with casting conditions, and is likely affected by bulging.

Coordinated Application of Constitutive Models, Simulation, and Experiment for Study of Metal-Forming Processes
A. J. Beaudoin,* M. E. Bange
National Science Foundation, NSF 98-75154; Alcan International, Ltd.

There exists considerable difference between the laboratory setting used for characterization of metals and the industrial plant where the resulting material models are applied. The present work combines experimental test procedures and finite element simulations to pose a deformation history that lies between the simple uniaxial tests used in laboratory characterization and the complexity of a production process. Inhomogeneity is considered at the scale of the microstructure, as well as from the friction and geometric conditions induced during forming. The resulting experimental design serves to further the utility of existing constitutive models and provide a rigorous validation for the predictive capability of finite element codes.

Effect of Processing Conditions on Development of Oxide Film Microstructure in Electro-Oxidation of Aluminum Alloys
A. J. Pearlstein,* H. Gao, A. Scheeline (Chemistry)
U.S. Department of Energy, DE-FG02-96ER45607

Surface preparation of aluminum alloys for adhesive bonding is an important process in using aluminum in automotive structural applications. Proper choice of acid, oxidant, temperature, contact time, electrical potential, and other processing conditions is critical in forming porous oxide films on the alloy that simultaneously provide corrosion protection and high bond strength. Our goal is to understand how processing affects oxide microstructure and to use that information to develop a better process. We have developed a laminar-flow rotating cylinder electrode reactor for rapid evaluation of contact time and potential effects, and have used it to grow microstructures of remarkable uniformity.

Effect of the Reversed Bending on Subsequent Formability in Aluminum Stampings
A. J. Beaudoin,* K. J. Lian
Ford Motor Co.

Deformation of sheet metal prior to contact with the forming punch may contribute to premature failure. In particular, the formability of aluminum may be decreased due to deformation through a draw bead or in prior leveling operations. This work examines the impact of nonuniform deformation-through the sheet thickness-on formability. Particular focus is on the through-thickness plastic deformation developed when sheet passes through a draw bead or about a small radius. The subsequent loss of formability is related to the distribution of through-thickness plastic strain.

Fiber Orientation in Injection Molded Composites
C. L. Tucker III,* H. Huynh
Delphi Automotive Systems

Some injection-molded plastics are reinforced with short glass fibers. The flow patterns during mold filling cause the fibers to orient in specific directions, making the part stronger and stiffer in those directions, weaker and more compliant in others. Proper design of these parts requires that we know these orientation patterns. We have combined 2-D and 3-D computational fluid mechanics software with a theory of fiber orientation, to predict orientation patterns in molded features with complex geometry. Current work focuses on improving the accuracy of the model for small parts with short flow lengths.

Flow and Clogging in Continuous Casting Tundish Nozzles
B. G. Thomas,* H. Bai
Continuous Casting Consortium**

Clogging of tundish nozzles caused by deposition of alumina inclusions limits the productivity and adversely affects the quality of continuous casting. This project seeks to quantify, understand, and predict the formation of these clogs, particularly when they occur in the tundish well, and require changing the tundish. Mathematical models are being developed to predict flow in the nozzle and clogging, focusing on the initial formation of the argon bubbles, the effects of argon bubble motion, and turbulence. The results are being evaluated in conjunction with water-model and plant measurements of flow and clogging in nozzles.

Interface Heat Transfer
B. G. Thomas,* Y. Meng
Continuous Casting Consortium**

Heat transfer in continuous casting molds is controlled primarily by heat conduction across the interface between the solidifying steel shell and the water-cooled copper mold. A comprehensive model, CON1D, has been developed to predict this heat transfer, including mass and momentum balances on the interfacial powder layers, superheat delivery from the turbulent liquid pool, and gap formation between the shell and the mold walls. Lab experiments are being conducted to measure heat transfer across resolidified flux interfaces under conditions approximating those in continuous casting. These are needed to obtain fundamental powder and flux properties, so that the model can become a fully predictive tool to solve quality problems and interpret mold thermocouple signals.

Mixing and Microstructure Control in Polymer Processing
C. L. Tucker III,* S. Comas-Cardona
National Science Foundation, DMI 98-13020

A polymer blend consists, at the microscopic level, of droplets of one polymer dispersed in a continuous matrix of another. The microstructure of a blend, i.e. the size, shape and orientation of the droplets, has a major influence on the properties of the bulk material. We are developing theoretical models for how this microstructure arises during processing, from the deformation during mixing. We are also testing the model with carefully controlled experiments on droplet deformation, and writing finite element codes to predict the microstructure in complex flows, such as extruders and polymer mixers.

Modeling Filling of Wheel Casting
J. A. Dantzig,* A. J. Beaudoin,* M. Bloch
Reynolds Metals

In this project, we simulate the filling of automotive wheel molds during low-pressure dye casting. The objective is to optimize the process to eliminate defects and reduce scrap.

Modeling and Control of Residual Stresses in Cast Iron Solidification
J. A. Dantzig,* A. Chang
Caterpillar, Inc.

In this project, models for cast iron solidification are used to predict the behavior of casting processes. Casting residual stresses are computed, and a design optimization approach is used to improve product design.

Optimization of Injection Molding
C. L. Tucker III,* Y.-E. Yoo
Moldflow Pty. Ltd.

When a new plastic part is being developed, substantial time and effort is required to properly design the mold to make the part. Computer simulations of polymer flow during mold filling are routinely used, but can only tell an engineer that his design is bad. They cannot recommend a better design. We are combining mold filling simulations with design sensitivity and optimization methods, to produce software that automatically finds the best mold design for a given part. For instance, the software can automatically determine the best locations for the gates, which are the places where polymer enters the mold.

Processing Optimization for Dimensional Control in Polymer Composites
C. L. Tucker III,* S. R. White,* P. H. Geubelle,* M. Li, D. J. O'Brien, Q. Zhu
National Science Foundation, DMI 96-10382

The dimensional accuracy and residual stresses in parts made from composite materials depend on the temperature history of the material as it is cured. In this project we are developing methods to help optimize the curing of these materials. Aspects of the project include: characterizing the curing kinetics and viscoelastic properties of a typical epoxy resin; calculating the temperature and cure profiles during the curing cycle; and performing a viscoelastic stress analysis to determine residual stresses and dimensional changes. We are also using design sensitivity methods to permit rapid optimization of the processing conditions.

Residual Stress Control during Heat Treatment of Aluminum Alloys
J. A. Dantzig,* H. Sehitoglu,* M. Newman, F. Zhang
Ford Motor Co.

In this project, models and experiments are co-developed to describe residual stress in the heat treatment of aluminum alloys. Constitutive models for materials behavior are developed and applied to industrial processes.

Thermal Stress Analysis of Solidifying Steel Shells
B. G. Thomas,* C. Li
Continuous Casting Consortium**

A coupled, two-dimensional, transient finite-element model has been developed to predict temperature, shrinkage, and stress development in both horizontal and vertical sections through the solidifying shell as it moves down through the caster. The model includes the effects of the volume change during phase transformation, ferrostatic pressure, the generalized plane strain stress state, the constraining influence of the mold, creep plasticity, and the dynamic effect of solidification shrinkage on heat transfer across the interfacial gap between the mold and the shell. The model is being applied to simulate the early stages of solidification, ideal taper for different steel grades, and crack formation.

Transient Flow Structures and Heat Transfer in the Continuous Casting Mold
B. G. Thomas,* S. P. Vanka,* S. Sivaramakrishnan, T. Shi, S. Subramanian
National Science Foundation; Continuous Casting Consortium**

Transient events are important to the generation of quality problems in continuous cast steel. Large eddy simulation models are being developed to directly simulate three-dimensional transient flow in the mold region. Results are being compared with conventional K-e model results in addition to experiments on water models and in commercial casting machines. The models will be used to investigate multiphase flow effects, dissipation of superheat, and the movement of argon gas bubbles and solid inclusion particles. The results will help us understand and prevent flow-related quality problems, including surface defects, due to free-surface motion, meniscus freezing, and entrained inclusions and gas bubbles.