Multiphase Transport and Grain Structure Development in Alloy Solidification 

Investigators:  C.Y. Wang,  and Juwen Gao
Sponsor:  NSF Career Program

Understanding the structural and compositional development is of interest to materials engineers working on the solidification of metal alloys. It is these structural and chemical features of the alloys that ultimately determine their physical and mechanical properties.This research project aims to address the intriguing couplings between the microscopic solidification phenomena, such as grain nucleation and growth, and macroscopic transport phenomena such as heat flow, melt convection, solid phase transport, and species redistribution. A mathematical model is being developed for the multiphase transport phenomena, while accounting for the micro- and macro-interactions. Experiments are being conducted to provide data on the final segregation and structure of solidified alloys. Dendrite/flow interactions and the bulk transport behavior during solidification are examined using a transparent model alloy. The data will be used to systematically calibrate, validate, and improve the micro-macroscopic model for alloy solidification. The capabilities of this model to predict macro-segregation and grain structure have been demonstrated for equiaxed dendritic solidification. Currently, the model is being extended to coupled columnar and equiaxed solidification in order to predict the columnar to equiaxed transition.The experimental and theoretical/numerical results will provide: (1) needed fundamental understanding of dendrite/flow interactions and their critical roles in establishing the structural and compositional characteristics of solidified metal alloys and (2) an experimentally validated model to predict segregation and structural zones in alloy castings.

Functionally graded materials (FGMs) Experimental and numerical studies of gravity casting of FGMs

Objective To understand fundamental transport phenomena underlying FGM solidification and graded structure formation. Technical Approach Use transparent analog systems to visualize particle migration, melt convect-ion, and microstructural development, to understand the interactions between the particles and the growth front, and to characterize the particle distribution and grain structure in the graded material. Perform ceramic-metal experiments and post-processing characterization. Develop a multiphase transport model to predict graded structures and to computer- design FGMs tailored towards specific functional requirements. Major Accomplishments Developed an optical fiber probe system for particle volume fraction measurement and a microscope video system for observation of particle/freezing front interactions. Experimentally and numerically studied 1-D and 2-D gravity casting of FGMs and validated against each other. Computationally prototyped various Al/SiC graded composites via gravity and centrifugal casting.

Optical fiber probe for particle volume fraction measurement Computer simulation of 2-D Al/SiC composite solidification

Click here for a slideshow of columnar to equiaxed transition and in dendritic alloy solidification.