This study investigated the accelerating behavior of spray particles during cold spraying (CS) by employing a computational fluid dynamics program, FLUENT. Optimization of the dimensions of CS nozzle was conducted to maximize particle velocity. The results show that the expansion ratio, divergent length, particle density and size, operating temperature significantly influence particle acceleration. It is found that the spray particles in nozzles with long divergent length can obtain a relatively higher impact velocity, but too long divergent length will reduce the particle velocity. Besides, the particle impact velocity shows a downward trend with increasing the particle size or density. Hence, the optimal divergent length should increase with the increase of particle mass. Moreover, higher gas temperature leads to a higher particle velocity, but it has no influence on the optimal divergent length.

Low-temperature HVOF spraying finds its potential applications in the preparation of high-performance metallic coatings with low oxygen content and porosity. In this study, the spraying method is used to deposit fine WC-10Co4Cr powders on stainless steel substrates at different spraying distances and the effects on microstructure, microhardness, fracture toughness, splat morphology, and surface roughness are investigated and discussed.

The kinetic spray process is a coating process that involves impingement of a substrate by metallic particles at high velocities. In this work, we investigated the kinetically sprayed Ti coatings, which were deposited onto different metallic substrates using gas-atomized powders. The powder particles were characterized in terms of size distribution, morphology, hardness, and explosibilty index. The propellant gas used for coating deposition was compressed nitrogen. The substrates were placed inside an enclosure filled with nitrogen gas during deposition. It was observed that Ti coatings can be deposited at relatively high deposition efficiencies using large particles (median size~ 100 mm). Ti coatings with a wide range of thickness and good macroscopic appearance were readily prepared. The particle temperature appears to have strong effects on the coating formation; deposition efficiencies of ~90% were achievable. Microscopic examination of the coatings revealed deposited Ti particles with significantly lower deformation when compared to ductile materials such as Al and Cu. As a result, the Ti coatings exhibited a high degree of porosity. Several methods were exploited in order to make the Ti coatings denser, including the incorporation of heavy, hard particles for in-situ peening during the coating deposition, and post deposition laser heating. Abstract only; no full-text paper available.

The kinetic spray process is a coating process that involves impingement of a substrate by metallic particles at high particle velocities. During the last several years, significant advancements in the fundamental understanding of the process have been made at Delphi Research Labs, which allowed us to greatly to enhance our capability in spraying difficult-to-deposit materials, especially with relatively large particles as feedstock and compressed nitrogen as the propellant gas (a challenging combination). In this work, we investigated Cu coatings that were kinetically sprayed using large Cu particles (53-150 microns). The emphasis is placed on understanding the effect of powder particles on the coating formation by the kinetic spray process. The examined particle characteristics in the present study include particle size, shape and morphology, and microstructure. The results indicated that the coating formation by the kinetic spray process is controlled primarily by two fundamental variables of sprayed particles: particle velocity and particle temperature. Very large sized particles, in spite of their relatively low particle velocities (as a result of acceleration by nitrogen rather than helium), can be rendered to exhibit enhanced spray ability, comparable to smaller size particles. This can be achieved by properly controlling the particle temperatures. It is found that kinetic spray using large size Cu particles (~100 microns) can lead to dense Cu coatings, with 50-80% deposition efficiency. Abstract only; no full-text paper available.