Diamond jet hybrid HVOF thermal spray: Gas-phase and particle behavior modeling and feedback control design

This paper focuses on the modeling and control of an industrial high-velocity oxygen-fuel (HVOF) thermal spray process (Diamond Jet hybrid gun, Sulzer Metco, Westbury, NY). We initially develop a fundamental model for the process that describes the evolution of the gas thermal and velocity fields and the motion and temperature of particles of different sizes and explicitly accounts for the effect of the powder size distribution. Using the proposed model, a comprehensive parametric analysis is performed to systematically characterize the influence of controllable process variables such as the combustion pressure and oxygen/fuel ratio, as well as the effect of the powder size distribution, on the values of the particle velocity, temperature, and degree of melting at the point of impact on the substrate. (These are the variables that directly influence coating microstructure and porosity, which, in turn, determine coating strength and hardness; see the second article of this series for details.) A feedback control system that aims to control the volume-based average particle velocity and melting ratio by directly manipulating the flow rates of fuel, oxygen, and air at the entrance of the HVOF gun is developed and applied to a detailed mathematical model of the process. Closed-loop simulations show that the feedback controller is effective in driving the controlled outputs to the desired set-point values and is also robust with respect to various kinds of disturbances in the operating environment.