INVESTIGATION OF FATIGUE AND MECHANICAL BEHAVIOUR OF ADDITIVELY MANUFACTURED INCONEL 718 AND ITS COMPATIBILITY WITH THERMAL BARRIER COATINGS

Abstract

This thesis investigates the impact of post-processing strategies conventional and new heat treatments, surface machining, and thermal barrier coatings on the microstructure, mechanical properties, and fatigue behavior of Laser Powder Bed Fusion (L-PBF) fabricated Inconel 718. The goal is to address challenges in the as-built state such as residual stresses, rough surfaces, and microstructural defects that limit performance in fatigue-critical and high-temperature applications. The as-built alloy exhibited a fine dendritic structure with Nb segregation and laves phase formation, resulting in high tensile strength (~970 MPa) but reduced fatigue life due to surface flaws acting as crack initiation sites. Machining significantly reduced surface roughness (from ~5.8 μm to ~0.82 μm), enhancing fatigue life without altering the bulk microstructure. Standard heat treatment (solutionizing at 980°C followed by double aging) led to laves phase dissolution, recrystallization, and precipitation of γ′/γ″ strengthening phases, improving hardness (~27%), tensile strength (~23.2%), and fatigue strength (from 200 to 400 MPa). However, the lower Basquin slope indicated greater fatigue damage accumulation. To further enhance performance, a new heat treatment (1130°C solutionizing + 730°C aging) was introduced, yielding equiaxed grains, increased CSL boundaries, and higher γ′ (16.78%) and γ″ (46.76%) fractions. This improved yield (1074 MPa), tensile strength (1238 MPa), and fatigue endurance (450 MPa), with enhanced resistance to crack initiation under cyclic loading. The study also examined plasma-sprayed lanthanum zirconate (LZ) thermal barrier coatings (TBCs) on AM and wrought IN718. Coated AM samples showed effective thermal insulation (~50–60% lower substrate temperatures) and durability over 113 thermal cycles. Mechanical integrity was preserved post-exposure, with coated AM samples achieving UTS of ~1253 MPa and 28.2% ductility. Additionally, LZ TBCs reinforced with 2 wt% CNTs were evaluated for hot corrosion resistance at 1130 °C in Na₂SO₄–V₂O₅ environments. CNTs reduced salt infiltration, enhanced coating density (to ~95%), and minimized porosity and residual stress, significantly improving corrosion resistance and structural stability.