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Title:Characterizing the tensile behavior of additively manufactured Ti-6Al-4V using multiscale digital image correlation
Author(s):Foehring, David
Advisor(s):Lambros, John; Chew, Huck Beng
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Additive manufacturing
Direct metal laser sintering
Abstract:In this work, the tensile behavior of an additively manufactured (AM) alloy, titanium 6Al-4V, or Ti-6Al-4V (by wt. 90% titanium, 6% aluminum, 4% vanadium), is examined on multiple length scales using two-dimensional (2-D) digital image correlation (DIC) techniques. Ti-6Al-4V is one of the AM metals that is currently commercially available for use in a variety of engineering applications. This work focuses on characterizing the relationship between the microstructure and bulk mechanical properties of the AM Ti-6Al-4V specimens and understanding how unique microstructural features pertaining to AM metals, such as columnar grain morphology and nonequilibrium phase structures, influence the micro-scale deformation behavior. A low magnification in-situ DIC study (referred to as the macro-scale study) was performed during tension experiments to assess the bulk properties of most specimens while the specimens were loaded, and a high magnification ex-situ DIC study (referred to as the micro-scale study) was performed on a single sample using a microscope after the sample was unloaded and removed from the load frame to observe the heterogeneous strain accumulation at this smaller scale. The experimentally-measured mechanical properties of AM Ti-6Al-4V were compared against conventionally processed Ti-6Al-4V. AM Ti-6Al-4V specimens exhibited greater strength than conventionally processed Ti-6Al-4V due to the acicular, or needle-like, grain structure. In the micro-scale study, alternating regions of high and low strains were observed to accumulate along these needles, or laths. The effects of build orientations (related to microstructure anisotropy), powder bed layering thicknesses (30 and 60 microns), and heat treatment, on the macro-scale properties were examined. A significant decrease in strength was observed in samples subjected to annealing heat treatment as compared to as-built specimens with no heat treatment. This behavior was due to the coarsening of the grain structure observed in scanning electron microscope images. AM Ti-6Al-4V tension specimens exhibited higher strength when the direction of applied tension was perpendicular to the AM build orientation. Upon further investigation in the microscale study, this observed anisotropy is thought to be related to interfaces of consecutively deposited layers as regions of high strain seen in the micro-scale study were observed to accumulate at changes in structure associated with the deposition of layers.
Issue Date:2017-07-17
Rights Information:Copyright 2017 David Foehring
Date Available in IDEALS:2018-03-02
Date Deposited:2017-08

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