Search

Abstract: Side assisting gas plays a very important role in the laser-induced plasma suppression and the gas mixture ratio directly influences the formation and behavior of the laser-induced plasma during the laser welding process. In this paper, a photography system was set up with three synchronous CCD cameras to record the plasma plume during CO2 laser welding under different He–Ar ratios for helium–argon mixed side assisting gas. Three-dimensional reconstruction of the laser-induced plasma based on the computed tomography (CT) technology was achieved from the images shot by the cameras. Four characteristics, including the volume, uniformity, parameter P A associated with plasma absorption and parameter P R associated with laser refraction, were extracted from the 3D plasma and analyzed to investigate the effect on the plasma plume morphology as well as the laser energy attenuation. The results indicated that the He–Ar ratio of the side assisting gas has a considerable influence on some characteristics while some other characteristics are not sensitive to the mixture ratio. In addition, the effect of He–Ar ratio on the laser-induced plasma varies a lot with the flow rate of the side assisting gas. [Copyright &y& Elsevier]

By matching with the welding speed, the distance between the laser focus and butt seam was adjusted to obtain various fusion ratios of the two base metals. The main phases in weld metal were martensite and retained austenite whose proportions and morphology were measured by XRD and EBSD, respectively. The proportion of retained austenite, both at grain boundary and among martensite laths in core region of grain, increased with the fusion ratio of SUS304. The maximum content of retained austenite at grain boundary and in core region of grain could reach about 22% and 5.8%, respectively. The low-temperature impact toughness of the joint was improved with the increment of retained austenite content. The low-temperature impact energy at −196 °C had displayed a significant raise from 15 J to 44.3 J. The corresponding impact fracture section also transformed from cleavage fracture to entirely ductile fracture. [ABSTRACT FROM AUTHOR]