Understanding multi scale defect formation during solidification of alloys by integrated computational modeling
|Asle Zaeem, Mohsen
|Thomas, Brian G.
|Includes bibliographical references.
|Solidification defects have a detrimental influence on mechanical and physical properties of metals and alloys. These defects can form in both macro and microscale inside the alloy or at the surface. In this Ph.D. research, by means of computational modeling, we aim to understand the formation mechanisms of surface defects that form during continuous casting of steels especially peritectic steels. To this end, we developed macro-scale computational framework based on finite element method to understand the effect of carbon on surface defect features of steels. A finite element model including non-linear temperature-, phase-, and carbon-content-dependent elastic–viscoplastic constitutive equations is applied to study the effect of steel grade and interfacial heat flux on thermal distortion of a solidifying steel droplet. Due to thermal contraction, the bottom surface of the droplet bends away from the chill plate and a gap forms. It is shown that, regardless of the nature of the heat flux, the gap forms and grows the most very early during solidification (~0.1s) and remains almost unchanged afterward. The highest gap depths are predicted in ultra-low carbon (0.003%C) and peritectic steels (0.12%C), and agree both qualitatively and quantitatively with the experimental measurements. Thus, the current thermal-mechanical model, including its phase-dependent properties, captures the mechanism responsible for nonuniform solidification, depression sensitivity and surface defects affecting these steels. In this work we also investigated microsegregation and second phase formation mechanism in Al-Cu alloys with copper content ranging from 3 to 11 at% using a phase field model implemented in MICRESS program. Our results show that the θ-phase fraction decreases with increasing the cooling rate, and this reduction is more drastic in alloys with a higher Cu content. Also, the microstructure features are influenced by the growth dynamics, where seaweed structure formation results in a more homogenous distribution of θ-phase and a finer microstructure. The results show that, irrespective of Cu content and cooling rate, the seaweed structure formation is halted at CM interfacial anisotropies larger than 0.005. As the anisotropy decreases, different seaweed structures can form regarding the constitutional supercooling. At low anisotropies (Al-3 and Al-8.4 at% Cu) and low supercooling (Al-3 at% Cu) fractal or degenerate seaweed is dominant while at high supercooling (Al-8.4 at% Cu) compact seaweed forms. This difference in supercooling stems from different solute atom transport rates.
|Colorado School of Mines. Arthur Lakes Library
|2022 - Mines Theses & Dissertations
|Copyright of the original work is retained by the author.
|Understanding multi scale defect formation during solidification of alloys by integrated computational modeling
|Ciobanu, Cristian V.
|Brice, Craig Alan, 1975-
|Doctor of Philosophy (Ph.D.)
|Colorado School of Mines