Selective Laser Melting (SLM), a laser-based Additive Manufacturing technique has appealed to the bio-medical, automotive, and aerospace industries due to its ability to fabricate geometrically complex parts with tailored properties and high-precision end-use products. The SLM processing parameters highly influence the part quality, microstructure, and mechanical properties. The process-structure-property relationship of the SLM process is not well-understood. In the process-structure study, a quasi-2D model of Micro-Selective Laser Melting process using molecular dynamics is developed to investigate the localized melting and solidification of a randomly-distributed Aluminum nano-powder bed. The rapid solidification in the meltpool reveals the cooling rate dependent homogeneous nucleation of equiaxed grains at the center of the meltpool. Long columnar grains that spread across three layers, equiaxed grains, nano-pores, twin boundaries, and stacking faults are observed in the final solidified nanostructure obtained after ten passes of the laser beam on three layers of Aluminum nano-powder particles. In the structure-property study, the mechanical deformation behavior of the complex cellular structures observed in the SLM-fabricated 316L Stainless Steel is investigated by performing a series of molecular dynamics simulations of uniaxial tension tests. The effects of compositional segregation of alloying elements, distribution of austenite and ferrite phases in the microstructure, subgranular cell sizes, and pre-existing (grown in) nano-twins on the tensile characteristics of the cellular structures are investigated. The highest yield strength is observed when the Nickel concentration in the cell boundary drops very low to form a ferritic phase in the cell boundary. Additionally, the subgranular cell size has an inverse relationship with mechanical strength, and the nano-twinned cells exhibit higher strength in comparison with twin-free cells. / Master of Science / Additive Manufacturing's (AM) rise as a modern manufacturing paradigm has led to the proliferation in the number of materials that can be processed, reduction in the cost and time of manufacturing, and realization of complicated part geometries that were beyond the capabilities of conventional manufacturing. Selective Laser Melting (SLM) is a laser-based AM technique which can produce metallic parts from the fusion of a powder-bed. The SLM processing parameters greatly influence the part's quality, microstructure, and properties. The process-structure-property relationship of the SLM process is not well-understood. In-situ experimental investigation of the physical phenomena taking place during the SLM process is limited because of the very small length and time scales. Computational methods are cost-effective alternatives to the challenging experimental techniques. But, the continuum-based computational models are ineffective in modeling some of the important physical processes such as melting, nucleation and growth of grains during solidification, and the deformation mechanisms at the atomistic scale. Atomistic simulation is a powerful method that can offset the limitations of the continuum models in elucidating the underlying physics of the SLM process. In this work, the influence of the SLM process parameters on the microstructure of the Aluminum nano-powder particles undergoing μ-SLM processing and the mechanical deformation characteristics of the unique cellular structures observed in the SLM-fabricated 316L stainless steel are studied using molecular dynamics simulations. Ten passes of the laser beam on three layers of Aluminum nano-powder particles have unfolded the formation mechanisms of a complex microstructure associated with the SLM process. The study on the deformation mechanisms of 316L stainless steel has revealed the contribution of the cellular structures to its superior mechanical properties.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/104115 |
| Date | 13 January 2020 |
| Creators | Kurian, Sachin |
| Contributors | Mechanical Engineering, Mirzaeifar, Reza, Li, Ling, Elahinia, Mohammad H. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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