NUMERICAL INVESTIGATION OF HVAF-SPRAYED Fe-BASED AMORPHOUS COATINGS

A numerical analysis was performed to predict the effect of the convergent section geometry of a gun nozzle on the high-velocity air-fuel (HVAF) thermal spray Fe-based amorphous coating (AC) process. A computational fluid dynamics model was applied to investigate the gas-flow field and the behavior of in-flight particles at nozzle entrance convergent section length ranging from 28 mm to 56.8 mm and different shapes of the Laval nozzle convergent section (a straight line and Vitosinski convergence curve). On the one hand, the change in the gas-flame flow characteristics for the Vitosinski curve shows a uniform and stable flame compared with the straight-line curve in the convergent section. The straight-line curve shape of the Laval nozzle convergent section has a higher particle temperature compared with the Vitosinski-curve shape of the Laval nozzle convergent section. The particle dwell time for the straight-line curve shape of the Laval nozzle convergent section is longer than that for the Vitosinski curve shape of the Laval nozzle convergent section. On the other hand, the nozzle entrance convergent section length obviously affects the particle temperature, and the particle dwell time increases with the increasing nozzle entrance convergent section length. By analyzing both the melt status of the particles and particle velocity, the optimal gun configuration (0.7 V) producing low-porosity coatings was predicted. These calculations were experimentally verified by producing a low-porosity (1.37 %) Fe-based AC, fabricated with HVAF using the predicted optimal gun configuration. 
 