Diagnostics of supersonic two-phase flows applied to the cold spray process

Kurzfassung

The cold gas-dynamic spraying method (or simply cold spray) is a high-rate material deposition method which was developed in the mid-1980s at the Institute of Theoretical and Applied Mechanics (ITAM, Russia, Novosibirsk). The deposition of coatings is performed by the impact of high-velocity particles that are in a solid state. Powder particles are accelerated by a supersonic free jet. The temperature of the gas is sufficiently lower than the melting temperature of the materials/particles. The method enables one to produce coatings made of most metals and alloys (Al, Zn, Cu, Fe, Ti, Ni etc.) on various substrates of metals and dielectrics. Because cold spray does not use high-temperature heating to melt the feed material, the method is very attractive for depositing oxygen-sensitive materials such as copper and titanium.The actual mechanisms by which solid-state particles deform and bond on impact has not been well characterized. The quality of coatings depends on particle velocity, size and density, on the reservoir pressure and temperature of the gas, on the nozzle shape and length, and other process parameters. Studying supersonic two-phase flows and shock wave structures produced by jets impinging on substrates is important for an interpretation of the process. The complexity of a mathematical description of multiphase jets requires experimental measurements covering a wide range of parameters. In particular, the importance of the character of jet-substrate interactions, the influence of particles on shock structures, particle velocities and trajectories, and particle distributions and concentrations in the jet need to be studied. We have developed diagnostics of supersonic two-phase flows that are applied to the cold spray facility. The set-up of the facility and preliminary results were described at the STAB-Workshop 2001. Main features of our facility are: axisymmetric jets; working gas: air, flow Mach number: M=2.6-2.8; reservoir temperatures: 20-500 , degree of underexpansion: 0.3-1.2; powder materials: Zn, Al, Cu; particle size: 20-40 ?m. in the cold spray process extremely slender nozzles are applied. Therefore, a major issue is the strong influence of boundary layers formed within the nozzle, on the flow. The methods of diagnostics include: -measurement of stagnation pressure in the free unloaded jet by Pitot gage; -methods of shadowgraph and schlieren visualization; -measurement of particles velocity in the loaded jet by the L2F method. Ergebnis: Unloaded and loaded jet parameters and conditions of jet impinging on the substrate. were studied in the cold spray facility of DLR. Optimal parameters and conditions for cold spray processes were identified. Further investigations of the influence of particles on the flow and the shock structures will be performed. Based on these investigations new nozzles appropriate for different applications will be designed.