Effect of shielding gas plasma on monitoring signals in laser welding: Features of monitoring methods for laser welding and their application (1)

Summary

The shielding gas used in laser welding plays an important role in obtaining high-quality welds. Argon shielding gas, being cheap to use, has most notably entered widespread use, but it faces the growing problem of plasma generation in higher-power laser welding.

This series of papers describes investigations of the generation conditions of shielding gas plasma and its behaviour, the correlation between monitoring signals and welding properties, the correlation between highspeed photographic imaging and measured signals, the refractive index of argon plasma alone, the correlation between shielding gas flow velocity variations and laser welding process variations, as well as the development and application of a simple plasma simulation model and small spectroscopic sensor. The plasma and molten pool are directly observed by CCD camera, and the spectrum of radiation emission, acoustic emission and plasma potential are simultaneously detected. The plasma potential in this study is the potential difference between the insulated nozzle and testpiece, being obtained from the difference between the diffusion rates of the ions and electrons in the laser-induced plasma. During direct observations of the welding zone by CCD camera, both plasmas and molten pool are photographed in one image using a 25 W Ar laser back light and a band-pass filter with a large bandwidth. Through the latter's application during transmission of the two emission lines of the argon ion laser, the metal plasma, shielding gas plasma, and molten pool vicinity can be discriminated through the sufficient degree of contrast they produce.

For the optical signals, the spectrum emission of the Fe neutral atom (298.4 nm), as being typical of metal plasma behaviour, and that of the Ar neutral atom (738.4 nm) were detected.

Based on the results obtained, optimum conditions for the Ar shielding gas flow velocity and mixing ratio are proposed.