| Author: | WONG, Bo Ching |
| Title: | Study of hydrogen transport, electrolytic charging and thermo-hydrogen processing of high pressure cold sprayed Ti6Al4V |
| Advisors: | Fu, Mingwang (ME) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Mechanical Engineering |
| Pages: | xxxi, 240 pages : color illustrations |
| Language: | English |
| Abstract: | Cold spray is a kinetic energy-based deposition method that has aroused interest in additive repair of high-value metallic aerospace assets, such as titanium components. Cold spray can fabricate ultra-thick deposits without melting and phase change, where the dense nanograins distribute across the entire deposit due to severely deformed particles. Hydrogen embrittlement in metallic parts is a critical safety risk in systems and processes involving hydrogen. Despite heat treatment being a common method to retrieve the ductility of cold-sprayed deposits, the effect of microstructural evolution in cold-sprayed deposits against hydrogen damage is unclear. This study used cathodic charging with high current density to investigate the hydrogen damage on samples of helium cold-sprayed Ti6Al4V coatings on CP-Ti substrates along with post-spray heat treatments. Hydrogen damage mechanisms varied with heat treatment temperature. The 750 °C heat treatment triggered diffusion bonding to increase the interparticle strength, but hydrogen damage occurred with blistering. Severe hydrogen pitting and splats debonding were observed in the 540 °C stress relieved samples due to the absence of metallurgical bonding. The as-sprayed coating was found to be the most resistant against hydrogen damage, since its highly dense nanocrystalline structure restricted the hydrogen mobility. There is a lack of hydrogen diffusion prediction methods for cold-sprayed deposits since their irregular microstructures are formed by the supersonic impact of particles. This study presents the image-based realistic modelling and hydrogen diffusion simulation with a grain-boundary network approach on cold-sprayed Ti6Al4V microstructure with heat treatments at 540°C and 750°C. Grain boundaries (GB) with a misorientation angle of less than 15° in EBSD mapping were categorized as special boundaries, or else the boundaries were random. The simulation was conducted by utilizing the accessible MATLAB Im2mesh tool and ABAQUS to obtain repeatable results. The high fraction of special boundaries in cold-sprayed deposits due to particle deformation shows its potential as a GB engineering method to improve hydrogen resistance. Dense special boundaries trap hydrogen and overcome the "short-circuit diffusion effect" by random boundaries. Heat treatment induces recrystallization and reduces the fraction of special boundaries, consequently deteriorating the performance of hydrogen mitigation. The simulation results reveal that the accuracy highly depends on the quality of EBSD characterization and the experimentally measured hydrogen properties. The image-based framework demonstrates its capability to simulate 2D diffusion across the complicated GB network of cold-sprayed deposits, as well as using GB maps from literature, despite the raw EBSD data not being provided. The fluid transport properties of cold-sprayed deposits have hitherto barely been addressed compared to the mechanical properties. This study used a small cold-sprayed Ti6Al4V "tsunami" deposit with a unique porous structure, which was consolidated by helium carrier gas on a mild steel substrate holder due to instantaneous fluctuations of nozzle traverse speed. The porosity was measured to be 15% by densitometer, 35.5-54% by optical microscope and 1-34% by XCT experiments with selected ROIs. The XCT tensor simulated porosity, mean absolute permeability and mean effective diffusivity were able to be correlated statistically with power functions. The transport properties posted a positive-linear relationship at porosities ranging from ~2.5-30%, where the ranges of permeability and diffusivity were from ~1.0E+2 to 5.0E+4 mD and from ~1.0E-9 to 1.0E-7 m²/s, respectively. It was also noted that the changing formation factor and the cementation factor in the computing algorithms led to unaligned diffusivity prediction at critical porosity, which was below 13% in this study. The range of fluid transport properties can thereby be potentially predicted on-site with a statistical correlation approach based on porosity. Developing cold spray additive repair for aerospace applications remains a significant challenge due to the lack of ductility and adhesion of hard metal deposits, such as titanium alloys. This study compares the effect of multi-step heat treatment (MSHT) in vacuum and hydrogen atmospheres on cold-sprayed Ti6Al4V coatings on CP-Ti using nitrogen and helium carrier gas. All the as-sprayed coatings were delaminated at strains of 1-3% during three-point bending tests, with detached fragments struck on surroundings. The nitrogen-sprayed Ti6Al4V sample after MSHT in vacuum was the only specimen exhibiting ductile deformation without fracture or delamination, which had an ultimate flexural strength (UFS) of 887 MPa at a strain of 11.1%. The superior ductility was contributed by the fully equiaxed microstructure in the coating with α grains <10 µm and submicron-thin β laths, where the adhesion was improved by the 40 µm thick interfacial diffusion bonding. However, the effectiveness of MSHT was limited by the as-sprayed quality. The porous helium-sprayed coating due to nozzle clogging underwent the same heat treatment but broke into powder and debris upon bending load. Hydrogen charging could enhance interfacial diffusion bonding, leading to the formation of pentagonal α2 grains. This study was also the first to capture the morphology of twinned-pentagonal α2 grains in Ti6Al4V particles, as observed through the SEM fractography of helium-sprayed Ti6Al4V coatings that underwent a MSHT in hydrogen. The thermo-hydrogen refinement of microstructure (THRM) was applicable to Ti6Al4V but not to CP-Ti, as it triggered excess grain growth and severe embrittlement in CP-Ti substrates. Overall, this thesis utilizes simulations and experiments to address the hydrogen degradation and integrity concerns about using cold spray as a novel additive repair method for aviation, energy and manufacturing industries. Simulation with realistic model provides insights into understanding hydrogen diffusion behavior in grain-boundary networks and developing techniques to predict the hydrogen-affected zone over the service life of components. Statistical correlation between porosity, permeability and diffusivity provides insights into developing comprehensible process and inspection guidelines for cold-spraying components for fluid transport and fluid-tight applications. Cathodic hydrogen charging reveals that the as-sprayed Ti6Al4V is resistant to hydrogen embrittlement during plating compared to the heat-treated deposits. Vacuum MSHT proves its capability to enhance ductility and adhesion of cold-sprayed Ti6Al4V coatings, where inevitable hydrogen uptake at high temperature can be utilized to further improve diffusion bonding followed by proper desorption. |
| Rights: | All rights reserved |
| Access: | open access |
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