Laser welding process monitoring can be divided into side-axis and coaxial types according to the collection angle of the imaging light signal. The side-axis type extracts signals reflecting the welding process from obliquely above or on one side of the welding pool at a certain angle with the laser beam; the coaxial type extracts signals reflecting the welding process from directly above the welding pool and small hole, coaxial with the laser beam. Extract imaging signals in the direction. Depending on whether there is an illumination source or not, visual sensing in the laser welding process can be divided into active and passive. The active type uses an auxiliary lighting source to illuminate the molten pool and small holes off-axis or coaxially, while the passive type uses the radiation light of the plasma as the illumination light or the radiation light of the liquid metal in the molten pool as the imaging light signal.
In the process of side-axis visual sensing, the positioning and installation of the sensor are relatively convenient and simple. Its image acquisition light path is also very simple; conventional side-axis illumination is also relatively simple. But the inability to see the plane shape of the hole is its biggest flaw. In addition, the installation and positioning of the side-axis vision sensor require a relatively large space.
Coaxial visual sensing during the laser welding process can observe the small hole from directly above the small hole to monitor and judge the status of the welding process by processing the collected coaxial visual images of the molten pool and small hole. Compared with range-axis visual sensing, it has many advantages such as a compact structure, can be integrated with the laser output lens, and takes up little space. However, its biggest technical problem is to separate and extract the coaxial imaging signal from the laser beam.
Current advanced optical device preparation technology can effectively solve this problem. For solid lasers with shorter wavelengths such as Nd: YAG, a spectroscope is generally placed in the laser optical path to reflect and deviate the optical signal or laser beam from the molten pool to achieve separation of the coaxial imaging signal and the laser beam optical path; while for longer wavelength solid lasers The CO2 laser is generally extracted by focusing the micro holes on the mirror to transmit the imaging light signal from the molten pool to characterize the changes in the depth of the small holes. This treatment method has great limitations, and its treatment results are greatly affected by welding conditions and plasma.
Through visual sensing, we studied the welding penetration status of the workpiece, the change of the small hole with the welding speed, and the corresponding relationship between the penetration depth and the width of the small hole and the molten pool, which can indirectly predict the laser welding quality. For example, comparing the changing pattern of the keyhole image when the penetration state of the weld seam changes from "not penetrated" or "only molten pool penetration" to "moderate penetration (small hole penetration)" during the welding process can be calculated as the penetration rate during laser welding. Closed-loop control provides the theoretical basis.