Modeling the Solidification Microstructure in Laser Deposition Processes

Mississippi State University, Mississippi State MS

Investigators

Abstract

0931801 Felicelli The mechanical properties of laser-deposited materials depend on the temperature history during the deposition process. All points in the built part go through many temperature cycles which cause complex solid phase transformations until the final cooling to room temperature. The numerical simulation of these transformations is difficult because they occur under high cooling rates, preventing the use of the readily available software based on diffusion-controlled transformations. One aspect of the difficulty is that the initial state, i.e., the microstructure that results immediately after solidification in the melt pool, is not known. This research involves modeling the solidification phenomena in the melt pool of a laser deposition process. The research will attempt a direct numerical simulation of dendritic solidification in the pool. This is deemed possible given the small size (approximately 1 mm) of the melt pool in most laser deposition processes. A new cellular automaton technique coupled with a lattice Boltzmann method will be developed to handle the huge computational demands expected from modeling solidification with fluid flow and solute transport in a complex solid network. The developed model will be used to calculate the solid structure after solidification, including microsegregation, microporosity and the eventual presence of eutectic and peritectic phases. A particular application and validation will be performed for steel alloys deposited with the Laser Engineered Net Shaping (LENSTM) process. Intellectual Merit. The outcome of the research will be a first-time tool to numerically simulate the growth of a dendritic structure in a macroscale domain under the effect of strong fluid flow. Because of the high temperature gradients developed in the melt pool, it is known that intense Marangoni convection occurs, with velocities in the range of meters per second. The influence of this convection and of the high cooling rates on dendritic growth in the pool has never been investigated. The correct determination of the solidification microstructure is of critical importance to understand the subsequent solid phase transformations during cooling. The knowledge developed will advance the state of understanding of solidification phenomena in the microscale and will contribute to improved numerical predictions of defects like segregation and porosity. The large computational requirements of the calculations will demand maximizing the capabilities of forefront methods of interface tracking, microscopic cellular automata, lattice Boltzmann models, and parallelization algorithms, contributing to the advance and wider acceptance of these techniques. Broader Impact. The findings of this research will directly impact the solidification research community, software developers and several technologies involving solidification processing. Currently, all major commercial codes simulating alloy solidification rely on continuum-type mushy zone models developed in the 1980s that use unrealistic microstructure approximations based on empirical correlations of permeability. The incorporation of new numerical developments is needed for more reliable predictions of defects during solidification. This research will make a significant contribution to this end, and in particular to the scarcely investigated area of solidification phenomena in laser welding and deposition processes. Two students, including one underrepresented minority, will be supported, and several topics of the research will be included in courses taught by the PI at graduate and undergraduate levels. Results will be disseminated into the scientific community through an active collaboration with colleague researchers, participation in workshops, seminars, and conferences, as well as through publications in specialized refereed journals. This project is jointly funded by the Thermal Transport Processes (TTP) Program, of the Chemical, Bioengineering, Environmental, and Transport Systems (CBET) Division and by the Materials Processing & Manufacturing (MPM) Program, of the Civil, Mechanical, and Manufacturing Innovation (CMMI) Division, within the Directorate for Engineering (ENG).