Frequency-Band Exploratory Feature Analysis for Weld-Quality Correlation of Laser-Welded Steel–PMMA Joints Using Photodetector-Based In-Process Signal Acquisition

Real-time monitoring of laser welding of metal–polymer joints is essential for modern automated production, and effective online quality characterization is critically required for industrial deployment. In this exploratory laboratory study, in-process optoelectronic signals were acquired to capture the laser welding process of steel–polymethyl methacrylate (PMMA) joints. With fixed experimental parameters, four typical welding states (i.e., weak welding, sound welding, discoloration, and carbonization) and their corresponding optoelectronic signal datasets were obtained, and all signal analyses were performed via offline post-processing of the recorded data. On this basis, the time–domain characteristics of optoelectronic signals under different welding conditions were analyzed, followed by systematic frequency–domain analysis. The frequency–domain spectrum and spectral energy within the visible light band of 750–800 Hz were calculated to explore potential feature differences corresponding to various welding states. Observable discrepancies in frequency–domain features were observed among the four laboratory-defined weld categories. This study establishes empirical correlations under controlled laboratory conditions. The results primarily validate the exploratory potential of optoelectronic sensing for welding state characterization in laser-welded metal–polymer joints, while the industrial feasibility and real-time processing performance of the proposed approach remain to be further verified.