Modeling and control of laser cladding by powder injection.

摘要

This thesis is concerned with automated laser cladding by powder injection. The thesis addresses different aspects of the technology, including system development, modeling, control, and experimental analysis.; A state-of-the-art automated laser cladding apparatus was established, which combined a laser cladding technique and an automated direct feedback control system to monitor and control the clad characteristics in real-time. An optical CCD-based detector along with a pattern recognition algorithm was developed to provide the clad height and angle of the solid/liquid interface in real-time.; In addition to the apparatus development, a mathematical model was developed to address the dependency of the clad geometry on process parameters. For the numerical solution of the model, a finite element technique was used to study the dynamic behavior of laser cladding by powder injection. The model was then used to investigate the correlation between the clad geometry and the process parameters such as laser pulse shaping, process speed, and powder feedrate.; In addition to the numerical analysis, different experimental-based techniques, including stochastic and artificial neural network analyses were used to identify laser a cladding dynamic model. A Hammerstein-Wiener nonlinear model structure was proposed in which a physical knowledge of the process was incorporated in the model structure to relate the process speed to the clad height. A second-order linear model was also identified using the auto regressive exogenous method to reflect the dependency of the clad height to the laser pulse energy. An other experimental-based model was developed in this thesis based upon an Elman recurrent neural network to obtain a comprehensive model to relate the main process parameters to the clad height and the rate of solidification.; Using the identified models, different controllers including PID and fuzzy logic were developed. The performance of the PID controller was examined on the apparatus in the presence of different process disturbances.; An experimental analysis was developed to identify the clad bead quality using two combined parameters: effective energy density and effective powder deposition density. This analysis provided the critical states that should be met in the intelligent controller to obtain good quality clads. The strategy was exploited through application of iron-aluminide coatings on mild steel substrates. Furthermore, the effects of the individual process parameters on the clad characteristics of iron-aluminide on the mild steel were studied.; The developed PID control system over the laser pulse energy was successfully applied to the prototyping of two simple shapes. Results showed that the closed-loop controller significantly improved the geometry of the produced parts compared to an open-loop control system.