Influence of the focal position on the melt flow during laser welding of steel

For keyhole laser welding of tempered steel, we show that the characteristic formation of eddies in the melt flow significantly depends on the focal position. The local melt flow velocities and accelerations were analyzed in-situ by means of x-ray imaging. It was observed that the keyhole geometry as well as the direction of rotation of the eddy close to the weld pool surface changes when the focal-position is shifted by one Rayleigh length.