Cold spray of high-performance titanium and titanium alloys
Creators: name: Hanna, Magdi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401607
subject: Metals and alloy materials

description: This thesis develops high-performance cold sprayed (CS) titanium (Ti) parts for a broad range of engineering services that meet current industry standards. Contributions of key variables to micro-and macrostructure and mechanical performance of titanium parts will be investigated systematically, including the grades of titanium stock powder, a range of fabrication conditions, and post-treatments. Both local properties (such as porosity, microstructure, hardness, and residual stress) and the overall mechanical properties of the bulk component will be examined (i.e., tensile strength, elastic and plastic deformation and elongation). Post-treatment will be performed in two distinct ways, pore-sealing with silica sealers and heat treatment at appropriate temperatures to optimise the desirable properties of the components, including removal of residual stress, homogenised microstructure and reduction in porosity. The expected outcomes include a new understanding of mechanistic roles of processing variables in microstructure and mechanical properties and a set of optimal processing conditions to yield high-performance titanium components with great potential for commercial applications, such as but not limited to biomedical devices. 

Three distinct research objectives have highlighted the framework of this research project, i.e. optimisation of CS parameters, appropriate post-heat treatment, and pore densification with ceramic sealant. The microstructural investigation of CS CP Ti deposits and CS Ti-6Al-4V coatings was carried out using optical microscopy and scanning electron microscopy (SEM). The percentages of oxygen and nitrogen in the feedstock powders and CS CP Ti deposits were determined by LECO analysis. Hardness, porosity, and bulk density were performed. Tensile strength tests were performed on machined CS CP Ti samples before and after heat treatment. 

The thesis is categorised into three key result sections that address the research work achieved to develop high-performance Ti via CS.
The first results section investigates the effects of gas temperature (GT) and post-processing heat treatment on the microstructure and mechanical characteristics of CS Ti deposits. Diffusion of oxygen and nitrogen in CS CP Ti deposits profoundly deteriorates their mechanical properties mainly due to the oxides and nitrides formations at the splats boundaries. Two approaches were adopted to reduce oxygen and nitrogen diffusion: increasing the gas pressure (GP) to the operational limits (6 MPa) of the CS system, which increases particle velocity, and optimising the gas temperatures to a minimum. Herein, we examined porosity, bulk density, oxygen and nitrogen diffusion and hardness characteristics of CS CP Ti deposits produced at varying processing GT (700, 800, and 900 C), which significantly influence the interactions of CP Ti with oxygen and nitrogen.

The second section investigates the effects of powder morphologies and post-processing heat treatment on CS CP Ti deposits' microstructure and mechanical performance. The results showed that CS CP Ti deposited from spherical shape powder exhibit lower porosity and hardness than that produced from irregular shape powder. According to LECO analysis revealed that CS CP Ti deposits manufactured from irregular–shaped powder had higher oxygen and nitrogen contents than those made from spherical powder. The irregular–shaped powder had a higher reactivity with oxygen and nitrogen than spherical powder due to its lower flowability and increased surface area. Post-processing heat treatment in a high-vacuum furnace at 920, 990, and 1100 °C dissipated particle boundaries, eliminated sub-micron pores, and greatly enhanced the tensile characteristics of CS CP Ti deposits. The results demonstrated that heat treatment greatly reduced the porosity of CS CP Ti deposits formed from spherical and irregular shape powders. The results revealed that spherical-shape powder significantly improved CS CP Ti deposits' microstructure and mechanical characteristics. Regarding the mechanical properties, heat treatment in a high-vacuum oven at 1100 °C promoted the elongation to break (strain %) of CS CP Ti deposits.

The final section explores a low-cost chemical sealing technique for densifying CS Ti-6Al-4V coatings on mild steel using a silica-based liquid ceramic sealer. The pore formation associated with the CS process requires the development of an economical sealer to enhance the corrosion resistance of CS Ti-6Al-4V coatings on a mild steel substrate. Herein, a sound method was developed to seal pores in the CS Ti-6Al-4V coatings with silica sealer. Potentiodynamic polarisation tests in 3.5 wt% sodium chloride electrolyte were employed to investigate the corrosion resistance of CS Ti-6Al-4V coatings fabricated at two stand-off distances (30 and 70 mm) before and after the sealing process. Polarisation resistance of CS Ti-6Al-4V coatings significantly increased by 80% after the sealing process. Electrochemical responses of CS Ti-6Al-4V were dependent on sealing the pores with agglomerated silica nanoparticles as observed by SEM and EDX analysis. The increment in polarisation resistance makes the sealer an effective treatment for CS Ti-6Al-4V coatings used in marine environments and other engineering applications. This sophisticated sealing process can reduce the deposition cost by reducing the thickness of CS Ti-6Al-4V coatings and increasing their lifetime on metal components. 

Moreover, the stand-off distance contributes to the bow shockwave strength. The intensity of the bow shockwave increases as the stand-off distance decreases, lowering the velocity of the gas and entrained particles. According to the research findings, shockwave effects increased porosity while decreasing hardness when spraying at 30 mm SoD. Optimising the stand-off distance is required to reduce the porosity in CS Ti deposits.

Summary of the Key Scientific Outcomes

Influence of GT

As the GT increased, the bulk density increased, and the volume fraction of porosity decreased. On the other hand, the oxygen and nitrogen concentrations of CS CP Ti deposits did not change after spraying at 700 and 800 °C GTs. At GT 900 °C, oxygen increased by 23%, and nitrogen increased by 1.5 times. The highest hardness value (214 HV (0.3)) was obtained at GTs of 800 and 900 C. Post-heat treatment at 800, 900, and 1000 °C dissipated the splat boundaries and eliminated most submicron pores in CS CP Ti deposits. Increasing the heat-treatment temperature from 800 to1000 °C resulted in a linear increase in grain size from 13 to 35 m. According to LECO analysis, low spraying temperatures (i.e., 700 °C) maintained oxygen and nitrogen levels in the CS CP Ti deposits at the same level as the stock powders. The bulk density of CS CP Ti produced at 900 C matched that of wrought CP Ti metal. 

Influence of Powder Morphology

Irregular-shape powder was more reactive with oxygen and nitrogen than spherical-shape powder, owing to differences in particle velocity and surface area. Post-processing heat treatment in a high-vacuum furnace at 920, 990, and 1100 °C dissipated particle boundaries, removed sub-micron pores, and greatly enhanced the tensile characteristics of CS CP Ti deposits. After post-heat treatment, CS CP Ti deposits made from spherical powder have a higher elongation to break and a slightly lower tensile strength than CS CP Ti made from irregular-shape powder. Furthermore, SEM microstructure images revealed that grain size increases when the temperature of the post-heat treatment rises. On the other hand, CS CP Ti manufactured with spherical-shape powders had a larger grain size than CS CP Ti made with irregular-shape powders.

Influence of SoD and Sealing Process

The porosity decreased, and hardness increased as the SoD increased (30–70 mm) due to the shockwave effect on impact particles' velocity. Despite its high corrosion resistance, CS Ti–6Al–4V coatings exhibit low corrosion protection for mild steel substrate, mainly due to open pores in the coating. The corrosion current density decreased following the silica sealing process, mainly due to the densification of pores with silica nanoparticles. Silica sealer significantly increased the polarisation resistance of CS Ti–6Al–4V coatings from 3579 to 24 381 Ω/cm2 (&gt;80%). Linear polarisation resistance (LPR) tests revealed a high polarisation resistance for CS Ti–6Al–4V coatings on mild steel following the sealing process. The results showed that CS Ti–6Al–4V coatings sealed with silica sealer exhibited low porosity and high hardness compared to the unsealed CS Ti–6Al–4V coatings. SEM/EDS observation indicated the silica sealer densified the CS Ti–6Al–4V coatings with silica nanoparticles.