Microstructure and the characterisation of mechanical properties of Wire + Arc Additively Manufactured nickel-base superalloys

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Additive manufacturing of large structures has the potential to shorten production lead times and reduce material wastage. One technique is Wire + Arc Additive Manufacturing (WAAM), a high-deposition rate process that uses an electric arc and wire feedstock to deposit material layer-by-layer into a desired geometry. Nickel-base superalloys 718 and 625 are two alloys which have been successfully deposited using WAAM, but their as-deposited microstructures contain detrimental Laves phase and long columnar grains with a strong fibre texture. The effect of these microstructural inhomogeneities on the material’s response to heat treatment and their subsequent mechanical properties are not yet well understood.
This work demonstrates, that WAAM Alloys 625 and 718 have micro-, meso- and macro-scale characteristics that are inherently different from their conventional equivalents, and consideration of microstructure and anisotropy is important in optimising and characterising the properties of these materials.
Although the as-deposited microstructure provides an unfavourable starting point, specialised heat treatments are crucial to optimising the tensile properties of WAAM Alloy 718. A modified heat treatment is effective in addressing microstructural inhomogeneities and reducing anisotropy in grain structure, resulting in near-isotropic elevated temperature tensile properties. In addition, unfavourable deposition conditions can lead to the formation of extensive crack-like defects in WAAM Alloy 718. These defects have hot cracking characteristics and result in “semi-stable” crack extension during fracture testing. Toughness values are direction dependent, attributed to interaction of the main crack with defects. Finally, the unique grain structure of WAAM Alloys 718 and 625 presented challenges in the use of neutron diffraction techniques. A new crystallite tracking method was developed to account for crystallite movement during in-situ loading, allowing for experimental determination of crack-tip strain fields. The results demonstrate that it may not be appropriate to treat WAAM materials as homogeneous or isotropic continua when predicting their mechanical behaviour.
Date of Award28 Sep 2021
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorHarry Coules (Supervisor)

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