Multivariable control of laser alloying of Ti6A14V Available to Purchase

The results of laser surface alloying of titanium with nitrogen gas may vary significantly during processing due to variations in absorptivity and geometrical variations 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 applied. The quantities, which have been measured, are the temperature and the melt pool surface area. For the process result, however, quantities as layer thickness and homogeneity are of more interest. Dynamic and steady state models have been developed to relate the processing results to the measured quantities. A thermographic CCD camera was applied to measure the melt pool surface area. One and two color pyrometers were used to measure the surface temperature. The effects of the laser power, the beam velocity and disturbances in absorptivity and work piece geometry were analyzed theoretically, as well as experimentally. Both the width and length of the pool area varies 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. As a result, it was found that the laser power is the appropriate actuator to control absorptivity disturbances. The beam velocity can be best applied to suppress the effects introduced by geometrical variations of the work piece. Based on these results, several controller algorithms were implemented and tested. It is shown that a multivariable mode-switch controller produces the most stable results.