Modeling the Microstructural Evolution of Parts During Metal Additive Manufacturing

Open Access
Author:
Mcnamara, Kevin R
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 16, 2018
Committee Members:
  • Richard Martukanitz, Thesis Advisor
  • Long-Qing Chen, Thesis Advisor
  • Allison Michelle Beese, Committee Member
Keywords:
  • Microstructural Modeling
  • Microstructure
  • Metal Additive Manufacturing
  • Johnson-Mehl-Avrami
  • Nonisothermal Phase Transformations
  • Kinetics
  • Inconel 718
  • Ti-6Al-4V
  • 3D Printing
Abstract:
Metal additive manufacturing is a promising technology for building complex functional parts for many industrial applications. A major challenge for the successful implementation of this technology is the currently limited ability of manufacturers to reliably predict the properties of the part before printing it. This challenge stems from the dynamic nature of the layer-by-layer build process, which causes a nonuniform thermal history and a complex microstructural history throughout the part. The task of predicting the final microstructure and related properties of the part is nontrivial due to the lack of robust kinetic models for nonisothermal phase transformations, particularly for multiphase alloy systems that undergo multiple simultaneous phase transformations. A computational microstructural model was developed to track the microstructural evolution of the part during the metal additive manufacturing build process. This model is a modified version of the Johnson-Mehl-Avrami model for nonisothermal phase transformations and can be applied to any material system that undergoes solid-state phase transformations if its transformation kinetics are known. The transformation kinetics for two common additively manufactured superalloys, Inconel 718 and Ti-6Al-4V, were derived using thermodynamic data and kinetic theory. Using the derived kinetics along with the part’s thermal history as inputs, the microstructural model outputs the phase fraction history of the part during the build. This model was applied to Inconel 718 and Ti-6Al-4V, each built by two different metal additive manufacturing processes: powder bed fusion and directed energy deposition. Mechanical property tests and characterization were performed on an Inconel 718 part built by powder bed fusion for validation of the microstructural model. A computational tool was developed using MATLAB that implements the microstructural model to calculate the phase fraction history of the part during the build and provides a user interface to inspect the thermal and microstructural history throughout the part. A link can be made between microstructure and properties in order to predict the properties of the part. Using the visualization tool, the user can analyze the microstructure throughout the part to develop methods to optimize the process in order to achieve improved part properties.