Study of inner porosity detection for Al-Mg alloy in arc welding through on-line optical spectroscopy: Correlation and feature reduction

Inner porosity of aluminum alloy in arc welding is usually invisible, transient and difficult to detect in real-time with weak physical information. This study investigated the key technologies for on-line inner porosity detection for aluminum-magnesium (Al-Mg) alloy in pulsed gas tungsten arc welding (GTAW) using arc optical spectroscopy and post-micro-characterization. A monitoring system for robotic GTAW was developed using a spectrometer, charge-coupled device camera, and microphone. Experiments for different degrees of porosity were designed before post-testing and microscopic characterization using scanning electron microscopy and energy dispersive spectroscopy. Two types of generation mechanism of inner porosity were discovered for Al alloy in GTAW. The line spectrums of metal and hydrogen components were selected before feature extraction by means of principal component analysis of optical spectroscopy. The correlation between the principal component coefficient of line spectrums (H I and Mg I) and inner porosity was quantitatively investigated using statistical values. Then, an improved feature parameter, e.g., the absolute coefficient of the H I spectrum from the first principal component, for inner porosity detection was proposed and experimentally validated at different levels of porosity. Finally, feature reduction and visualization were achieved by means of the proposed principal component analysis and t-Distributed Stochastic Neighbor Embedding. The correlation between microscopic analysis and spectrum features is also discussed.