ARL Center for Cold Spray - Cold Spray Process
Cold Spray Process
The cold spray process as shown in Figure 1 imparts supersonic velocities to metal particles by placing them in a heated nitrogen or helium gas stream that is expanded through a converging–diverging nozzle. The powder feed is inserted at high pressure at the nozzle entrance. High pressures and temperatures yield supersonic gas velocities and high particle acceleration within the gas stream. The particles, entrained within the gas, are directed towards the surface, where they embed on impact, forming a strong bond with the surface. The term "cold spray" has been used to describe this process due to the relatively low temperatures (100-500°C) of the expanded gas stream that exits the nozzle.
Figure 1. Typical Cold Spray Assembly
Subsequent spray passes increase the structure thickness. The adhesion of the metal powder to the substrate, as well as the cohesion of the deposited material, is accomplished in the solid state.
Sprayed particles must reach a "critical velocity" before impact will result in consolidation with the surface. This required minimum velocity varies among metal types and is typically between 500 and 800 m/s. The gas used for particle acceleration is generally nitrogen, helium, or a mixture of the two. While considerably more expensive, helium gas produces much higher particle velocities. An example of the acceleration of a 20 micron diameter copper particle in a nitrogen gas stream within a nozzle is shown in Figure 2, which has been created by 1-D isokinetic and 2-D, CFD modeling.1 In this case the initial temperature and pressure of the gas are 500 C and 30 bar, respectively. The gas expands and accelerates through the 174 mm nozzle as its temperature decreases. Very rapid changes take place at the nozzle throat, where gas sonic velocity is reached. Particle velocity and temperature approach gas values as drag and heat transfer occurs.
Figure 2. Particle and gas velocities and temperatures within a cold spray nozzle
Upon impact with an aluminum surface, the sequence of deformation of a 20 micron copper particle, traveling at 650 m/s, is shown in Figure 3.2 The simulation of the impact process is carried out using the CTH computer code. The figure shows that as the particle/substrate contact time increases, the particle flattens while the substrate crater depth and width increase. At the same time, a jet composed of both the particle material and the substrate material is formed at the particle/substrate contact surface. Simultaneously, there is a temperature rise, concentrated at the particle/surface interface. This temperature rise is an indication of shear instability, which causes extensive flow of material at the corresponding surfaces, and the estimated impact velocity to induce shear instability compares fairly well with the experimentally determined critical velocity of copper. This means that, as in the case of explosive welding of materials, bonding in cold spray is a result of the shear instability at the interacting surfaces.
Figure 3. Deformation of a 20 micron copper sphere striking an aluminum surface.
The attributes of cold spray include low temperature deposition, dense structures, and minimal or compressive residual stress. In addition to these characteristics, the deposited material possesses strength close to or above that of wrought material. An example of the consolidated deposit produced by the cold spray deposition of 6061 aluminum ally is shown in Figure 4. Helium gas was used to accelerate the 6061 aluminum particles. Individual particle splats can be identified and the dense, non-porous nature of the consolidated particles is clear, with measured porosity less than 1%.
Figure 4. 6061 aluminum powder (left) and its consolidation by cold spray (right)
In the as-deposited state, cold spray deposits can exhibit higher strengths than wrought alloys. Figure 5 compares the strength characteristics of cold sprayed 6061 aluminum alloy with those of wrought 6061 alloys of various heat treatments.3 When annealed, cold spray deposit strength decreases, but elongation and ductility increase.
Figure 5. Comparison of cold sprayed 6061 aluminum characteristics with those of wrought materials.
These examples show that cold sprayed materials can have physical characteristics similar to wrought materials. Such characteristics allow cold spray repairs to closely mimic or surpass in strength the material that is repaired. In addition to good strength characteristics, the repairs can be easily accomplished and cosmetically acceptable.
- 1. V. Champagne, D. Helfritch, S. Dinavahi, and P. Leyman: "Theoretical and experimental particle velocity in cold spray," Journal of Thermal Spray Technology, 2011, 20, (3), 425-431./li>
- 2. M. Grujicic, J. Saylor, D. Beasley, W. DeRosset, D. Helfritch: "Computational analysis of the interfacial bonding between feed-powder particles and the substrate in the cold-gas dynamic-spray process," Applied Surface Science, 2003, 219, 211–227.
- 3. B. Gabriel, V. Champagne, P. Leyman, D. Helfritch, R. Kestler, C. Sauer, and K. Legg: Cold spray for repair of magnesium components," Final Report, ESTCP Project WP-0620, July, 2011.