Spectroscopic Studies and Mathematical Modeling of Laser Material Interaction for Development of Intelligent Quality Monitoring System.

Abstract

This research investigates the fundamental physics of laser processing of multi-coated materials, through spectroscopic studies and a mathematical modeling of laser material interaction. This work focuses particularly on developing an in-situ quality monitoring system by detecting defects generated in the processing, understanding the effect of coated materials on defects formation, and further characterizing differences between newly developed lasers in regard to energy transfer. First, several welding defects generated in CO2 laser processing of a multi-coated material are monitored using Optical Emission Spectroscopy (OES). Tracking the specific constituent behaviors that induce the defects is proposed as a novel way to monitor the process. Second, in order to obtain promising results in both defect detection and defect classification, a machine learning algorithm, a Support Vector Machine (SVM), is adopted for the spectral data analysis using the richness of the available data. The richness is a major benefit in the use of the optical emission spectroscopy because the spectrometer can resolve and distinguish each spectral line of the constituents of the target materials. Third, a numerical simulation study is presented to investigate the effect of the coating material for understanding the interfacial phenomena in the laser processing of the multi-coated material. These interfacial phenomena are important because they determine the processed qualities of the target samples in the laser material interaction. The interfacial phenomena such as recoil pressure, capillary and thermo capillary force are investigated by comparing a coating free material with a coated material. Finally, characteristics of the energy transfer of the disk laser and the fiber laser are identified to provide users with insight into which laser might be more suitable for a given application. To assess the laser systems, two factors are considered: energy absorption by the laser induced plasma, which is an inevitable phenomenon in laser material interactions, and the penetration features of the samples irradiated by the attenuated laser beam after absorption by the plasma. The work presented in this study can be utilized to achieve the quality assurance, to understand energy transfer in the laser material processing, and thus eventually to control the process.