In-situ fabrication of laser additive manufactured titanium-aluminide based alloys doped with niobium-chromium for high performance engineering application.
Kanyane, Lehlogonolo
Kanyane, Lehlogonolo
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Abstract
The current commercial metal materials such as nickel-based super-alloys, titanium alloys and stainless steels that are used in gas turbine engines tend to experience in-service failures due to high-temperature oxidation, thermomechanical failure, high-temperature wear and hot corrosion. These failures necessitate that new alloy(s) be designed and their performance be optimized. Ti-Al based alloys hold useful properties in terms of high-temperature stability and enhanced strength. Laser Engineered Net Shaping (LENS) Technique is a manufacturing process that allows for the production of Ti-Al alloys through layer by-layer deposition, at the same time; it offers the freedom to print complex designed components for potential gas turbine applications. This work was aimed at developing novel in-situ gamma Ti-Al-x(Nb-Cr) alloy(s) using laser additive manufacturing (LAM) technique called LENS. The in-situ fabricated quaternary gamma Ti-Al-Cr-Nb alloy was compared to the commercially available General Electric (GE) cast alloy, and an in-house direct energy deposited (DED) pre-alloyed GE samples.
A predictive model was first developed, prior to alloy development and sample manufacturing for characterization, to analyze thermal gradients distribution during LENS additive manufacturing using COMSOL Multiphysics software. The model preempts the crack probabilities during LAM of Ti-Al based alloys and indicated the environmental temperature that is to be preserved during manufacturing if cracks and microstructural transformation were to be avoided. The samples were then printed at constant base-plate heating temperature, which was below homogenization and phase transformation, and hot enough to prevent cracking to occur in-built during manufacturing. Post manufacturing, the samples underwent heat-treatment (HT) for homogeneity, elemental dissolution, and microstructural transformation. The micro in-situ alloying of both Cr and Nb to the gamma Ti-Al binary alloy led to an equiaxed
microstructure with precipitates of β-phase along the lamellar boundaries with hardness of 621.01HV. The β-precipitants helped induced grain boundary pinning and dislocation resistance. The in-house DED printed GE sample and the commercial cast GE had microhardness values of 500.79HV and 401.02HV, respectively. The in-situ synthesized gamma quaternary alloy had a stiffness value of 1.06mN/nm with an indentation hardness value of 73790MPa, which was high when compared with the as cast GE. The modulus of elasticity (Er) of all the GE
samples shows that the samples are of the same family. In addition, the results showed that both Cr and Nb improved electrochemical behaviour of the gamma alloy in biodiesel environment. Impressively, the laser in situ manufactured gamma Ti-Al-Cr-Nb alloy was stable in an oxidation environment at temperatures of about 750oC. There were no cracks or initiation of stress present on the worn surface of the alloy. The hard-intermetallic nature of laser developed in-situ Ti-Al-Nb-Cr alloy was highly resistant to plastic deformation. High Coefficient of Friction (CoF) of 0.411μ was evident on cast GE sample while the DED and in-situ GE alloys presented CoF of 0.231μ and 0.31μ respectively. High CoF in cast GE alloy signals the generation of wear debris, this generated debris during wear of the sample are easily removed from the wear tracks and leads to exposure of fresh surfaces which increases direct contact
between the wear bodies as confirmed by the SEM micrograph. The yield strength (YS) of in-situ synthesized sample is 12.9GPa with elongation value of 5.8% while the cast GE alloy presented a YS of 11.34GPa with an elongation value of 5.7%. The in house DED GE presented YS of 13.03GPa at 5.7% elongation. The low YS in GE cast can be attributed to fully lamellar structure, which is known for low strength while the near gamma equiaxed structure evident in DED and In-situ GE are recognized for high strength properties. A numerical computational model to analyze thermo-mechanical behaviour of γ-TiAl based turbine blade using Abaqus CAE/2021 presented stress strain in turbine blade between blade and the root. Maximum stress of 147MPa was evident in the model which is below the yield stress of 172GPa and therefore the Ti-Al blade will withstand thermal stresses in gas turbine conditions for energy production application.
Description
Submitted in partial fulfilment of the requirements for the degree Doctor of Engineering (Metallurgical Engineering) at the Department of Chemical, Metallurgical and Materials Engineering
Faculty of Engineering and the Built Environment at the Tshwane University of Technology.
Date
2024-04-01
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Tshwane University of Technology
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Keywords
In-Situ Laser Additive Manufacturing, Computational Model, Thermal Gradients, Titanium Aluminides, Microstructure, Nano-Indentation, Corrosion, Oxidation and Tribology