When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Ti-6Al-4V components that are used extensively in aero engines, particularly as turbine blades, are prone to damage. When a part is damaged, it can either be repaired or replaced. Compared to replacing damaged components, 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 overcome this drawback, researchers have attempted to increase sample density by enhancing impact velocity and/or applying a post-sintering heat treatment process. Although the former attempt was successful, post-spray sintering was still required to reach the desired porosity level. Since heat treatment processes are expensive, further particle deformation is required to increase cost-effectiveness of repairing damaged parts. To accomplish this, particles should be preheated and thermally softened during deposition process. As cold spray is unable to deposit particles at elevated temperatures, this thesis proposed to use high-velocity air-fuel (HVAF) as a solid-state additive manufacturing technology. HVAF process uses a flame that can be used to deposit solid particles at elevated temperatures and velocities. As a result of the flame, the substrate temperature would be also elevated during deposition. The first objective of this study was to investigate how increasing particle and substrate temperature can improve deformation of deposited particles and density of as-sprayed samples. In this context, a numerical analysis is preferred over experimental methods because particle deformation occurs at very high strain rates (up to 10-9 s-1).
The numerical analysis reveals that through thermal softening effect, particle deformation will increase by elevating the deposited particle temperature, which allows a denser structure to be fabricated. Nevertheless, the increase in particle deformation does not necessarily indicate better particle adhesion to the substrate. For this to happen, the oxide layer of particle and substrate must break and eject. To have a better understanding of the oxide layers failure, a numerical simulation was performed. The results showed that the particle's oxide layer fractures at the first nanosecond upon impact due to the particle severe deformation. In this sense, the area where the substrate oxide layer fails, which is positively affected by the particle temperature and velocity as well as the substrate temperature, defines the bonding area.
To examine the potential of the inner-diameter HVAF process (ID-HVAF) as a solid-state additive manufacturing method, it is first necessary to understand the effects of spraying parameters on in-flight particle characteristics and the density of the deposited Ti-6Al-4V coatings. 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. At the as-sprayed samples consisted of the brittle α-Ti phase, 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. The application of heat treatment decreased the 1.18% initial porosity level to 0.98%. By analyzing the as-fabricated sample's tribological properties, it was found that the deposited particles were easily detached resulting in a low wear resistance. It is therefore necessary to sinter the samples using the mentioned heat treatment to increase their wear resistance. Lastly, the results show the presence of vanadium oxide in both as-fabricated and heat-treated samples. Oxygen content measurements revealed 1.6 wt% oxygen in both samples which acts as an α-phase stabilizer and negatively affects the hardness of the heat-treated samples. In conclusion, for HVAF to be a viable solid-state additive manufacturing technique, the oxygen content of the as-fabricated samples must be further reduced by a better selection of spray parameters.