Multi-sensor Real-time Quality Diagnosis And Its Theoretical Basis For Laser Welding

Laser welding, an efficient and high-quality welding method, has been widely used in areas of automobile, metallurgy, and national defense industry, etc. However, because laser energy transition and physical processes of laser welding are notably complex, as well as, the welding quality is affected by various factors, merely adjusting welding parameters to guarantee the quality is not sufficient. Thus, it is laser welding quality monitoring that serves to be vital to obtain high-quality deep penetration welds, to exert laser welding's features of high speed and efficiency and do accelerate its industrial application process. Also, the essence of laser welding process can be better understood by the research. First of all, a multi-sensor laser welding process monitoring system has been built up according to the principle of Virtual Instrument. Ultraviolet Emission (400~440nm), Infrared Emission (1200~1700nm), and Audible Sound (20~20kHz) signals emitted during the laser welding process, have been picked up as detected parameters. The signals can be sampled and recorded with high speed, and by this system signals'characteristic information can be extracted in the time domain, frequency domain, and time-frequency domain. Moreover, the stability of the laser welding process can be monitored by this system. After a deep analysis of the three signals and the essence of welding process, it is found that they can reflect the laser welding nature in different aspects. In the experimental conditions: the UV signal changes with the laser power intensity linearly, and the laser power and assistant gas flow play a key role influencing the UV signal, while the welding speed a less important role. The IR signal properly reflects the state of the welding pool, such as weld hump, weld width, etc.. And it increases with the laser heat input. The AS signal appropriately reflects the metal vapor pressure in the "keyhole", and it corresponds well with the laser power intensity. The plasma optical signal can well reflect the penetration depth of partial penetration welding. With the different welding parameters, the statistic curve of penetration depth varying with the sensing signal has been set up for the first time. Moreover, the relationship between them has been analyzed profoundly, thereby providing theoretical direction and experimental reference for the real-time penetration depth monitoring or predicting during the partial penetration welding process. The frequency of plasma fluctuation can be divided into three sections. The signal's low frequency (<2000Hz) component reflects the intensity of plasma, and it can also be utilized as a criterion to judge the existence of the laser-induced plasma. The frequency band of 2000~6500Hz reflects existence of the "keyhole"well, therefore can be used as a criterion of classify whether it's the laser conduction welding or deep-penetration welding. The frequency band upper than 7000Hz can reflect stability of the weld pool and "keyhole", thus can be used as a criterion to judge the "keyhole"stability. Compared with a stable and sound deep penetration process, the three signals vary distinctly accordingly to different welding defects, such as gap, dissimilar sheet edges, misalignment, low assistant gas flow rate and laser conduction welding. According to signal features of different defects, a new method of identifying the welding defects has been put forward, so as to make laser weld defects'auto-detection possible. Behaviors of the laser-induced plasma and "keyhole"have been analyzed deeply corresponding to different sensing signals and different laser welding process. It has been found that the power intensity when the plasma comes into being is lower than that when the "keyhole"forms. Moreover, in deep penetration welding, the laser-induced plasma comes out first, then the "keyhole". Under the experimental conditions, the plasma forming time is about 3~4ms, while the keyhole forming time is about 14ms. A model of laser-induced plasma above the welding pool has been set up, in which the plasma is divided into a diffused part and a resident part. The diffused part, a factor severelyinfluencing laser energy transmission, can be blown away utterly by the assistant gas, while the resident part can hardly be blown away completely. The existence of a "keyhole"render the amplitude of plasma fluctuation increased and the fluctuation frequency fastened. This phenomenon proves that the "keyhole"itself fluctuates with a high frequency. With a profound analysis of factors relating to welding penetration, it has been found that the laser power intensity affects the penetration depth by altering the depth of "keyhole", and the heat-input affects the penetration depth by altering the amount of molten metal. Additionally, the former factor is the more decisive one.