Modeling and control of an experimental HVOF thermal spray process

This work focuses on the modeling and feedback control of an experimental high velocity oxygen-fuel (HVOF) thermal spray process that is similar to the Metco Diamond Jet (DJ) hybrid gun. Initially, a fundamental process model, which is computationally tractable while capable of capturing the dominant physicochemical phenomena occurring in the HVOF thermal spray process, is used to determine the influence of controllable process variables such as combustion pressure and oxygen/fuel ratio, as well as the effect of powder size distribution, on the values of particle velocity and temperature at the point of impact on substrate. Based on model predictions and available experimental studies, the control problem is formulated as the one of regulating volume-based averages of the liquid fraction 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 liquid fraction 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 control problem formulation and the feedback control system are robust with respect to disturbances in spray distance and particle injection velocity, and variations in powder size distribution.