Modelling and control of laser surface treatment

The results of laser surface treatment may vary significantly during laser surface processing. These variations arise from the sensitivity of the process to disturbances, such as varying absorptivity and the small dimensions of the work piece. To increase the reproducibility of the process, a real-time feedback control system was designed and tested. Process models were developed to gain insight in the process behavior. As a test case, laser alloying of titanium (Ti6Al4V) with nitrogen was considered. Unfortunately, not all the desired processing results, such as the thickness of the alloyed layer, can be measured during processing. The quantities, which can be measured, are temperature related, e.g. the melt pool temperature and the melt pool surface area. Dynamic and steady-state models were developed, which relate the processing results to the measured quantities. A thermographic CCD camera was developed to measure the melt pool surface area in real-time. Pyrometers were applied to measure its temperature. The effects of the laser power, the beam velocity and the disturbances (absorptivity, thin work piece) on the temperature distribution and melt pool surface area, were analyzed theoretically, as well as experimentally. The width and length of the temperature distribution and the melt pool vary due to the disturbances. In the case of a thin work piece, the length varies more than the width. In the case of an absorptivity disturbance, the variation of the length and width are of the same order. In addition, it was found that the laser power can be best applied to counteract an absorptivity disturbance. The beam velocity can be best applied to suppress the negative effects introduced by small dimensions of the work piece. Based on these results, several controller algorithms, including multivariable algorithms, were implemented and tested. A mode-switch controller was able to produce a constant melt pool depth despite disturbances. This controller applied the laser power to suppress an absorptivity disturbance, and the beam velocity to counteract a geometrical disturbance. Hence, although it is not possible to measure the thickness of the alloyed layer directly, it is possible to control it by measuring and controlling temperature related quantities (temperature, melt pool area) at the surface.