Doctoral Dissertations

Date of Award

8-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Sudarsanam S. Babu

Committee Members

Steven Zinkle, Hahn Choo, Michael M.Kirka

Abstract

A complex interaction of process variables in an evolving geometry during Additive Manufacturing (AM), can bring about spatial and temporal transients of temperature and stress within each layer in a part. Although AM shares commonalities with conventional processing techniques such as casting, welding, and thermo-mechanical process, published literature has shown that the steady-state conditions are not strictly valid during AM process. Macro-scale fluctuations of thermal gradients (dT/dx: 103 to 107 K/m) combined with local changes in thermal expansion coefficients, crystallographic strains and localized stress-strain constitutive properties in conjunction with thermal cycles, can bring about a plastic strain gradient within the part. In the current scope of AM literature, microstructural gradients, defects, and textural variations in an AM build have not been correlated to the dynamic flow behavior as a result of fluctuating macro and micro-scale phenomenon’s, i.e., temperature reversals and associated thermal stresses brought about by large thermal gradients and transformation strains. The current research focuses on understanding the thermo-mechanical-metallurgical aspect of AM during complex TM gyrations.

In this research, we analyzed the behavior of an AM Ti6Al4V alloy under externally imposed thermo-mechanical (TM) reversals between 400ºC and 650/700ºC with alternating compressive and tensile strains (0.8\%, 1.0\%, 1.2\%). The macro stress-strain response resulted in an overall softening behavior with an increase in the beta phase fraction. Computational thermodynamic and diffusion models validated this increased beta phase stability due to the added stored energy in the alpha phase, triggering a phase transformation. A quantitative analysis of the interface concentration profile indicated that the alphabeta phase transformation occurred via a reconstructive mode. The ex-situ analyses are then corroborated with a time-resolved study using High Energy X-ray diffraction where the preferential plastic strain accumulation between the alpha and beta phases is rationalized. The new insights on the coupling between micro-scale phase stability of the alpha and beta phases and associated elemental distributions with macro-scale softening in a sample subject to TM gyrations, is indeed relevant to the physical metallurgy of alloys processed via AM. The phenomenon's outlined in this research is pertinent to the description of history-dependent material flow properties in two+ phase alloy system during complex thermal and mechanical signatures, such as experienced during AM.

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