Abstract

Ti-6Al-4V components used extensively in aero engines are prone to damage. When a part is damaged, it can be repaired or replaced. Compared to replacing, repairing methods such as solid-state additive manufacturing processes are more cost-efficient. Among solid-state additive manufacturing methods, cold spray (CS) is more promising because it fabricates samples at high deposition rates without oxidation or phase transformation. Using CS, it is possible to manufacture samples with low porosity levels; however, the existing pores that result from insufficient deformation of the solid-state particles adversely affect the mechanical properties. To enhance sample density, particle deformation should be increased by thermally softening them during deposition. As cold spray cannot deposit particles at elevated temperatures, this thesis proposes using high-velocity air-fuel (HVAF) as a solid-state additive manufacturing technology.
The numerical analysis reveals that through the thermal softening effect, particle deformation will increase by elevating the deposited particle temperature. This allows a denser structure to be fabricated. Nevertheless, the increase in particle deformation does not necessarily indicate better particle adhesion to the substrate. In order for the particle and substrate to adhere better, both the particle and the substrate oxide layer need to be broken and ejected. Particle and substrate oxide layer failures depend on particle velocity and temperature and particle velocity and substrate temperature, respectively.
The inner-diameter HVAF process (ID-HVAF) has been used as a solid-state deposition technique. Results showed that increasing nozzle length and air/fuel pressure enhanced the velocity of in-flight particles and the density of as-sprayed Ti-6Al-4V coatings. Using the spraying conditions that produced the densest coating, the ID-HVAF process was used to fabricate four-mm thick Ti-6Al-4V samples. Heat treatment was required to enhance the mechanical properties of the samples by transforming the brittle α-Ti phase into the more deformable β-Ti phase. Applying heat treatment decreased porosity from 1.18% to 0.98%. Lastly, the results show vanadium oxide presence in both as-fabricated and heat-treated samples. Oxygen content measurements revealed 1.6 wt% oxygen in both samples. This acts as an α-phase stabilizer and negatively affects the hardness of heat-treated samples.