Evaluating Sources and Root Causes of Pore Formation in Electron Beam-Based Directed Energy Deposition of Titanium Components

Open Access
Author:
Snyder, Kyle
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 19, 2018
Committee Members:
  • Jayme Scot Keist, Thesis Advisor
  • Allison Michelle Beese, Committee Member
  • Todd Palmer, Committee Member
Keywords:
  • Additive Manufacturing
  • Directed Energy Deposition
  • Electron Beam
  • Porosity
  • Pore Formation
  • Titanium
  • Ti-6Al-4V
Abstract:
Porosity is a common issue in additive manufacturing of titanium and its alloys, causing detrimental effects on mechanical properties and limiting potential applications for high performance components. The goal of this project was to establish sources and root causes of pore formation in electron beam processing and determine their influence on porosity. Potential sources were identified as, porosity from hydrogen evolution, aluminum vaporization, and melt pool lifetime. A factorial experiment was designed to evaluate the effects of hydrogen evolution through filler wire quality and the effects of aluminum vaporization by varying the aluminum content in the filler wire by using Ti-6Al-4V (Ti-64) and CP-Ti alloys in an electron beam-based directed energy deposition process. Several characterization techniques were implemented to determine the total degree of porosity for each run condition. Analysis of the raw data as well as statistical approaches were applied to improve the interpretation of the response variable (porosity) in terms of the independent variables. An analysis of variance (ANOVA) was conducted using different pore characteristics as responses for different variables, aside from the selected experimental variables. Next, a theoretical model for pore formation was developed to create a physical understanding of the experimental results. Three main mechanisms for pore formation were identified and used as the basis for this this model. The formation mechanisms depend on several factors including the amount of time available for each mechanism to act, temperature of process, and properties of the liquid metal. Time and temperature of the process are considered to be consistent for all samples, but the addition of alloying elements alters the properties of liquid titanium meaning Ti-6Al-4V will have different liquid properties than CP-Ti. Alloying titanium with aluminum alters the liquid metal properties of titanium to favor an increase in the degree of porosity based on the classical pore formation mechanisms.