The research objective of this thesis is to present an on-line monitoring and control method to obtain uniform hardening layer quality in laser surface hardening processes. In this process, a black coating on the workpiece should necessarily be used as a laser beam energy absorber to efficiently absorb expensive laser power. In this case, it is evident that the coating thickness as well as the composition of coating material can greatly affect the hardening process.
Currently, since very little researches are reported on identifying the effect of coating thicknesses in this regard, the effects of coating thickness variations not only on the hardening layer quality and the laser beam absorptivity but also on the process monitoring are experimentally investigated. To assess the integrity of the hardened layer quality, the geometrical parameters of the hardened layer size such as the depth and the width are utilized. The experimental results reveal that a range of optimal coating thickness can be identified and becomes thinner as the hardening velocity increases. The experimentally measured absorptivities are proven to be within about a range of 55%-85% and increase and decrease according to the increasing coating thickness.
Since the hardening layer is determined by the temperature distribution and its time history of the workpiece being hardened, it can be predicted through a heat flow analysis of the hardening workpiece. Therefore, a new analytical solution is introduced to the transient temperature distributions of a finite thickness plate in a laser surface hardening process. To prove the validity of the developed model, the calculated results are compared with the experimentally obtained data under various hardening conditions. The results show that the model predicts the temperature distribution with good accuracy.
Monitoring of the geometrical parameters such as the depth and the width is very important for on-line process control as well as for on-line quality evaluation. It is, however, a difficult problem because of the inherent characteristics of the hardening process. The monitoring variables used are the surface temperatures measured at various points on the top surface of the hardening workpiece, which are strongly related to the formation of the hardening layer. The surface temperatures are measured using an infrared temperature sensing system. The relationship between the measured temperatures and the hardened layer size is implemented on the multilayer perceptrons which are powerful for realization of complex mapping characteristics. The experimental results reveal that the measured temperatures are greatly influenced by the variations of coating thickness as well as the basic process variables such as the laser beam power and travel velocity. With thick surface coating, the infrared sensing system rather measures the higher temperatures greatly affected by the coating layer than the actual surface temperatures of the hardening workpiece. Therefore, the uniform coating within the optimal range of coating thickness should be necessarily achieved for surface temperature monitoring and for efficiency and reliability of the process.
The control purpose is to obtain uniform hardening layer quality in laser surface hardening process. In this research, the hardening layer size is directly regulated to a desired one. The proposed controller is composed of the neural hardening layer size estimator and the fuzzy and PD (Proportional and Derivative) controller. The simulation study shows that the proposed control method improves the overall system response in the presence of changes of laser travel velocity during laser hardening and guarantees the uniform hardening layer quality.
