Thesis
Development of microstructure, crystallographic texture and residual stress in the interface region between substrate and a layer made by additive deposition during hybrid manufacturing of IN718
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2024
- Thesis identifier
- T17157
- Person Identifier (Local)
- 201889116
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- This research investigates how process variables in Laser Metal Deposition (LMD) of IN718 influence microstructural characteristics, residual stress distribution, and mechanical properties across the interface region in both as-deposited and post-weld heat-treated conditions for repair applications and feature additions. Specifically, the study explores the viability of LMD for IN718 components, such as hot compression disks, with a focus on the interface region. Microstructural analysis was performed using optical and scanning electron microscopy, electron backscatter diffraction, and energy-dispersive spectroscopy to characterize the metallurgical properties. Both as-deposited and post-weld heat-treated conditions were evaluated, with additional analysis of microhardness to understand the influence of laser power. Residual stresses were quantified using the multiple-axis contour method and X-ray diffraction (XRD) measurements. A thermal model was developed to predict in-situ temperature histories, correlating with microstructural features and solidification patterns. A sequential de-coupled thermo-mechanical FEA model, calibrated experimentally, was employed to analyse residual stress behaviour and the influence laser power had on residual stress fields. The microstructure was shown to exhibit large columnar γ-grains oriented with the build direction and columnar dendrite. The laser power was shown to influence both the γ-grain size and crystallographic texture, which varied between a cube and a <001>//BD fibre. Sub-structure columnar dendrites were observed in the deposited material, extending across multiple layers in some instances. EDX analysis showed migration of alloying elements, with dendrite cores enriched in nickel, iron, and chromium, and inter-dendritic regions enriched in niobium and molybdenum. This migration led to niobium-enriched nucleation sites for Laves phase precipitation, forming networks throughout the build. Furthermore, the laser power was shown to influence the microstructural features like the volume of Laves phase produced and dissolution of precipitated phases in the interface region. Post-weld heat treatment successfully precipitated strengthening γ’’ phases, enhancing phase distribution. Additive manufacturing induced complex residual stress fields, primarily from thermal gradients and rapid heating/cooling rates, with the Thermal Gradient Mechanism explaining the phenomenon comprehensively. Maximum tensile stress below the melt pool penetration depth was attributed to extreme thermal gradients and cooling rates. Residual stresses were alleviated during heat treatment, enhancing component integrity. This comprehensive investigation provides insights into optimizing LMD processes for IN718 repair applications and component enhancements.
- Advisor / supervisor
- Violatos, Ioannis
- Konkova, Tatyana
- Resource Type
- DOI
- Embargo Note
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