Direct observation of keyhole with an innovative layer-by-layer imaging method during deep penetration laser welding

In order to solve the difficult problem of observing the keyhole in deep penetration laser welding of metal materials, an innovative keyhole direct observation method based on layer-by-layer imaging technology is first proposed in this paper. In this novel method, a specially-designed composite workpiece consisting of an upper metal wedge and a lower glass trapezoidal prism is used. During deep penetration laser welding, a keyhole will form in and penetrate the metal wedge, whose cross section image that appears on the inclined bottom surface of the wedge is directly observed by a high speed video camera through the transparent glass prism. Then, the deep penetration laser welding is carried out with a fiber laser unit, by using the above proposed keyhole direct observation method, the keyhole cross section images at different depths are captured by the high speed camera. Finally, the images of the keyhole cross sections are processed and the outlines of the keyhole cross sections are extracted, from which the variation of the keyhole cross section during deep penetration laser welding of metal material is discussed. The result shows that from the top to the bottom, on one hand, the size of the keyhole cross section decreases gradually in general, but fluctuates to some extent, which indicates that the laser beam energy may not be uniformly absorbed into the keyhole along the depth and the evaporation site may change its position depending on the energy equilibrium there, on the other hand, the position of the keyhole cross section moves backward in the opposite direction of the welding velocity, whose motion tend agrees with the bending keyhole in the real case of laser welding.