Repair of Nickel-base Superalloy Components using Direct Energy Deposition

  • Riddhi Sarkar

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

In the present study, repair of Inconel 718 (IN718) components was conducted using direct energy deposition (DED) technology. The deformation behaviour of Inconel 718 (IN718) components repaired using DED was examined using 4 – point bend tests and high cycle fatigue (HCF) tests. Investigations were performed on the entire repaired part- including the substrate and the deposit zone. A gradient microstructure was observed in the repaired IN718- the substrate exhibiting uniformly distributed equiaxed grains with grain size ~90 µm and high fraction of annealing twins; however, the deposit zone microstructure differed significantly from the wrought substrateconsisting of a mix of columnar and equiaxed dendrite with average grain size ~28 µm. Due to the sharp difference in grain size and morphology in the substrate and the deposit zone, the DED-repaired IN718 components are referred to as “repaired composites” in the thesis. Micro-hardness results highlighted the need for heat treatment as it can remove the heat-affected zone and hardness dip, creating a uniform hardness profile across the joint. The mechanical tests and microstructural investigation were performed in the as-deposited and under two different heat-treated conditions: direct aged (DA), where the repaired composites were aged at 720ºC for 8 hours and 620ºC for 8 hours, and solution-treated and aged (STA) where the repaired composites were solution treated at 980ºC followed by ageing. These heat treatmentswere chosen to enable solid-state phase transformation in the repaired composites while preventing coarsening of the substrate grains. The 4-point bend tests were performed on the repaired composites with two groove angles, and the strain evolution in the repaired components was compared using digital image correlation. In all the repaired composites, strain concentration was observed in the deposit zone – substrate interface due to NbC liquation resulting in Nb-rich film formation on the substrate grain boundaries and an effect of groove angle was observed. The repaired composited with a low/shallow groove angle exhibited higher deformability compared to the ones with a higher/steep groove angle.Corresponding room temperature high cycle fatigue (HCF) properties of DED repaired IN718 joints are studied using hour-glass HCF samples were machined which were half substrate and half deposit with the substrate to deposit zone interface at the When the fatigue stress was low (below 50% of the yield stress), the STA condition had a better HCF performance than the DA, however, the opposite trend appeared for the high stress (over 70% of the yield stress), resulting in a cross-over point on the stress-life S-N plot. Based on the S-N curve results, a series of interrupted fatigue tests were performed at selected maximum stresses to study the progress of fatigue cracks in terms of the initiation and growth at the corresponding stress levels. Using singlespecimen tests, the progression of fatigue cracks on the surfaces of the specimens was studied using scanning electron microscopy (SEM). Interrupted fatigue tests, combined with microscopy and fractography, revealed that the fatigue failure occurred in the substrate for the DED joint in the DA condition, whilst in the deposit zone for the STA condition due to the distribution and fracture of the Laves and δ phases. Grain boundary cracking in the substrate near the substrate-to-deposit interface can occur in both heat-treated conditions due to the Nb-rich liquid films.Additionally, this study investigated the effect of TiB2 particles on inducing columnar to equiaxed transformation (CET) in the as-deposit IN718, deposited using two sets of energy densities: high energy and low energy density. The addition of TiB2to IN718 resulted in significant grain refinement in the as-built microstructure. In the high-energy deposit, grains of size >300µm occupied ~22% of the total grain area and ~10% in the low-energy deposits, which were reduced to ~5% upon TiB2 addition. Moreover, a preferential {100} texture observed along the building direction in the asdeposit IN718 became weak and random in the IN718/TiB2 deposits. The resulting IN718/TiB2 microstructure was found to be sensitive to the wt% of TiB2 particle content. TiB2 content >1.5 wt% was optimal for inducing grain refinement. The Laves phase often observed in the as-deposit IN718, is greatly reduced upon TiB2 addition and replaced by needle-shaped phases rich in Cr, Mo, and B. The TiB2 particles and the needle-shaped secondary phases formed a high fraction of heterogenous nucleation sites, causing grain refinement. Moreover, the strength properties exhibited asignificant increase due to TiB2 addition. IN718/TiB2 exhibit significantly high hardness and strength properties from uniaxial tensile tests. The tensile strength increased from 570 MPa in IN718 to 988 MPa in IN718/TiB2 tested parallel to the building direction. When tested perpendicular to the build direction, increased yield strength of 840 MPa was observed in IN718/TiB2 compared to 475 MPa in IN718 deposits. The conventional strengthening mechanisms were presented to explain the effect of TiB2 on the refined microstructure. The major contributing strengthening mechanisms included grain refinement, Orowan strengthening and increased GND density due to TiB2 addition. narrowest section of the hour-glass specimens. Although the monolithic DED deposit had a similar tensile strength to the wrought substrate, the DED repaired joint exhibited an overall decreased HCF performance, regardless of the heat treatment conditions.
Date of AwardFeb 2023
Original languageEnglish
Awarding Institution
  • Coventry University
  • Deakin University
SupervisorDaniel Fabijanic (Supervisor), Tim Hildith (Supervisor), Michael Fitzpatrick (Supervisor), David Parfitt (Supervisor) & Bo Chen (Supervisor)

Cite this

'