Feedback control of HVOF thermal spray process accounting for powder size distribution

This work presents a novel formulation of the control problem and a feedback control system for the high velocity oxygen-fuel (HVOF) thermal spray process, which explicitly accounts for the effect of powder size distribution. Initially, based on model predictions and available experimental data, the control problem is formulated as one of regulating appropriate averages (with respect to the particle volume distribution) of the temperature and velocity of the particles at the point of impact on substrate (these are the variables that directly influence coating microstructure and porosity, which, in turn, determine coating mechanical and thermal properties) by manipulating the oxygen/fuel ratio and the combustion chamber pressure, respectively. Then, a feedback control system is developed and applied to a detailed mathematical model of the process. Closed-loop simulations show that the average particle velocity and temperature at the point of impact on substrate reach the desired values in a short time, which validates the feasibility of real-time implementation of feedback control on HVOF thermal spray systems. It is also shown that the proposed formulation of the control problem (which accounts for the effect of powder size distribution) leads to a solution of the control problem that is superior (with respect to the achievement of the desired control objectives) to a solution that assumes a monodisperse powder size distribution. Finally, the proposed control problem formulation and the feedback control system are shown to be robust with respect to disturbances in spray distance and particle injection velocity, and variations in powder size distribution.