Toward the study of additively manufactured functionally graded materials: Experimental analysis of microstructure, phase composition, and mechanical properties

Open Access
Author:
Bobbio, Lourdes Del
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 03, 2017
Committee Members:
  • Allison Michelle Beese, Thesis Advisor
  • Zi-Kui Liu, Committee Member
  • Hojong Kim, Committee Member
Keywords:
  • functionally graded materials
  • additive manufacturing
  • thermodynamic calculations
  • CALPHD
  • titanium
  • stainless steel
  • invar
Abstract:
Functionally graded materials (FGMs) are a class of material systems with intentional changes in elemental composition or structure within a single component, which result in location-specific properties that spatially change within the gradient zone. Metallic FGMs with changes in chemistry are composed of two or more terminal elemental metals or alloys. One method for fabricating metallic FGMs is through directed energy deposition (DED) additive manufacturing (AM). In DED AM, a melt pool is created by a laser on a substrate, or previous layer, and powder is deposited into this melt pool. When fabricating an FGM using DED, the relative fractions of two or more powders are varied as a function of position as they are deposited into the melt pool. It is important to understand how different metals and alloys combine with this fabrication method. This is assessed using a combination of experimental analysis and computational predictions used to evaluate the equilibrium phase diagram of a given FGM system. By using an experimental and computational approach, it is possible to fully characterize the system as well as identify gaps in agreement between the two approaches. Three FGM systems are investigated in this thesis: Ti-6Al-4V to Invar 36, Ti-6Al-4V to V to 304L Stainless Steel, and V to Invar 36. Each of these FGMs failed either during fabrication or during post-processing machining, which makes further analysis of great importance in order to understand what led to these catastrophic failures. The methodology for experimental characterization of these FGMs included elemental composition analysis, phase composition identification and analysis, and mechanical property analysis. In order to confirm that the deposited composition matched with the planned composition, which is important for computational analysis, the elemental composition was analyzed through energy-dispersive x-ray spectroscopy. To determine the reasons for failure of the FGMs, X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy were performed in order to identify the phases. Hardness was measured in order to determine how the secondary phases that formed in the gradient affected the mechanical properties of the system. Through these experimental techniques, it was determined that the two causes for failure in these FGM systems were the formation of high volume fractions of intermetallic phases present in all three FGM systems, while additionally, a low melting point region was present in the Ti-6Al-4V to Invar 36 FGM. Alternate gradient paths are proposed for future fabrication of these FGM systems that avoid the deleterious compositions resulting in failure.