A Systematic Investigation On The Acceleration Behavior And Deposition Mechanism Of Cold Sprayed Particles

Cold spraying (CS), also called cold gas dynamic spraying (CGDS), is a new coating technique which has been developing during the past two decades. Powder particles (typically<50μm) are accelerated to a high velocity ranging from300to1200m/s by a supersonic gas flow issuing from a Laval type nozzle.In comparison with the conventional thermal spray technique, cold spraying is not yet mature. Some crucial issues including the supersonic gas flow field and the particle deposition mechanism are still not well understood. In this study, a comprehensive investigation on the particle acceleration behavior and deposition mechanism was conducted by both numerical and experimental methods. Besides, the heat transfer behavior between the high temperature gas and substrate was also studied in the present work. The major creative contributions are summarized as follows:1. Numerical simulations were employed to study the effect of substrate diameter, spray angle and nozzle cross-section shape on the supersonic gas flow field and particle acceleration behavior. It is found that substrate with smaller diameter results in weaker bow shock and thus higher particle impact velocity. As for the angular spray, the normal velocity component which is closely related to the deposition efficiency decreases sharply with decreasing the spray angle. Besides, it is also found that the rectangular nozzle can provide better velocity distribution of the driven gas than the elliptical nozzle.2. Several FEA models were built based on Lagrange, Euler and SPH methods, in order to simulate the high velocity impact process in cold spraying. The calculation precision is significantly dependent on the meshing size for Lagrange models. Corse mesh results in the low-fidelity resultant outputs, while if the mesh is extremely fine, the element distortion may lead to the abort of the program. For Euler method, meshing can be partitioned to a rather fine size and thus more deformation details can be captured. But the contact interface between different parts is hard to be distinguished, which brings some troubles to the post-precession. SPH method, as a new calculation method, is a sort of meshless method. The calculation precision is dependent on the arrangement of SPH particles. The preliminary result shows that SPH method also gives good resultant outputs.3. Based on the meaningful findings obtained from chapter2, some FEA models were chosen to simulate different impact processes in cold spraying. Then, a systematic investigation on the effect of particle interaction, spray angle, substrate hardness and oxide film on the particle deposition and coating formation was carried out by analyzing these simulated results. It is found that small inter-particle distance and spray angle significantly deteriorate the coating quality. Besides, the substrate with large hardness enables the particles to deform more intensively for both the first layer particles and subsequently depositing particles. With the coating thickness increasing gradually, such effect becomes increasingly weak and almost disappears when coating is thick enough. In addition, the simulated results also indicate that the formation of the viscous-like metal jet helps to remove the cracked oxides from the interface. Part of the cracked oxides, however, remains at the interface and mainly accumulates at the central region after particle deposition.4. Lagrange model combined with the experimental observation was employed to study the particle deposition mechanism under the condition of preheating treatment. It is found that particle and substrate preheating can promote the occurrence of thermal softening effect, and then enhance the plastic deformation and final bonding strength between particles and substrate. With increasing the preheating temperature, the bonding strength and the amount of the particles depositing on the substrate increases gradually.5. Lagrange model combined with the experimental observation was employed to study the effect of substrate hardness and spray angle on the formation mechanism and performance of Ti coatings. It is found that metallurgical bonding plays the dominant role in the bonding between Ti particle and Cu substrate. For the Ti particle depositing on the Al substrate, mechanical interlock can be clearly observed at the interface. As for the SS substrate, both bonding forms are absent; hence the bonding strength between particle and substrate is rather low. Furthermore, it is also found that with decreasing the spray angle, the coating thickness decreases, but the coating porosity increases gradually.6. Numerical simulation combined with experimental measurement was conducted to study the heat transfer behavior between the high temperature gas and substrate. It is found that substrates with smaller thermal conductivity can achieve higher surface temperature, but denser isothermal, which may lead to the larger coating residual stress after substrate cooling. Moreover, decreasing the standoff distance and substrate thickness, increasing the inlet temperature and substrate diameter, can increase the preheating temperature.