Reduction of pores by means of laser beam oscillation during remote welding of AlMgSi

The influence of a spatial beam oscillation on the dynamics of the capillary and the mechanism leading to an increased or reduced generation of process pores was investigated for deep penetration laser beam welding of the aluminum alloy AlMgSi. Welding with a feed rate of 4 m/min and a welding depth of 4 mm was examined with sinusoidal (longitudinal and lateral) and circular beam oscillation patterns at frequencies of 100 Hz and 200 Hz. The welding processes were analyzed by means of online X-ray imaging with a frame rate of 2 kHz, which provides the temporal and spatial resolution required to resolve the dynamics of the capillary. With conventional rectilinear welding, the weld seams are prone to the formation of process pores. By applying a circular beam oscillation, the weld seams were found to be virtually free from porosity. The mechanism leading to a reduced occurrence of process pores differed for the two investigated beam oscillation frequencies. At an oscillation frequency of 100 Hz bubbles are regularly formed in the melt pool but immediately removed at the subsequent pass of the laser beam by degassing into the vapor capillary. At 200 Hz the formation of process pores is completely avoided from the first. For a sinusoidal beam oscillation in longitudinal direction, the bubbles formed in the weld pool were found to be further inflated at each pass of the laser beam.