For the paraxial vision sensing in laser welding: the sensor features easy and convenient positioning and installation, with a straightforward optical path for image acquisition. Conventional paraxial illumination is also simple to implement. However, its major drawback is the inability to capture the planar profile of the keyhole. Additionally, paraxial vision sensors require a relatively large installation space.
In contrast, coaxial vision sensing for laser welding enables observation of the keyhole directly from its top. By processing the coaxial visual images of the molten pool and keyhole acquired during operation, the real-time welding status can be monitored and evaluated. Compared with paraxial vision sensing, coaxial vision sensing boasts multiple advantages: compact structure, integratable design with laser output lenses, and a small footprint. Nevertheless, separating and extracting coaxial imaging signals from the laser beam remains its greatest technical challenge.
Currently, advanced optical component manufacturing technologies can effectively address this issue. For solid-state lasers with short wavelengths such as Nd:YAG lasers, beam splitters are installed in the laser optical path to deflect optical signals from the molten pool or the laser beam, thus separating the coaxial imaging path from the laser beam path. For long-wavelength CO₂ lasers, micro-holes are fabricated on focusing mirrors to transmit imaging signals from the molten pool, which are then extracted to characterize variations in keyhole depth. This solution has prominent limitations, as its detection results are highly susceptible to welding parameters and plasma interference.
Vision sensing technology can be applied to investigate workpiece penetration status, keyhole variations with welding speed, as well as the correlation between penetration depth and the dimensions of keyholes and molten pools, so as to indirectly predict laser welding quality. For instance, analyzing the evolutionary rules of keyhole images when the weld status transitions from non-penetration or partial molten pool penetration to adequate penetration (full keyhole penetration) can provide a theoretical basis for closed-loop control of penetration depth in laser welding.
