In laser beam welding, excessive evaporation leads to bulging of the capillary tip which results in the generation of metal vapor-filled bubbles in the melt pool. These bubbles either collapse and dissolve, or solidify and remain as pores in the weld seam, thereby degrading the quality of the weld seam. We investigated the mechanism of bubble formation and collapse in detail for laser beam welding of aluminum by means of online X-ray videography. Capillary shapes were reconstructed from these high-speed videos and correlated to two kinds of processes either sensitive to pore formation or unsusceptible to pore formation. We show that the fluid dynamics in the melt pool is strongly influenced by subsequent bulging and collapsing of bubbles. Its influence on the melt flow was quantified by analyzing the trajectories of tracer particles in the melt pool. We found that the generation and collapse of bubbles is a major driver of the dynamics in the melt pool. The melt is accelerated to velocities of up to several hundreds of millimeters per second by collapsing bubbles. Similar effects were found in laser beam irradiation of transparent media, such as ice and water, which allows to resolve the generation and collapse of capillary bulging with higher temporal and spatial resolution.
Laser welding is the state of the art joining technology regarding productivity and thermal loads and stress on the workpiece. In deep penetration laser welding the quality of the resultant welds strongly depends on the stability of the capillary. The highly dynamic depth fluctuations are of major influence on the controllability of the laser welding process and on the prevention of weld defects. In the present paper the capillary dynamics is investigated by means of time- and spatially resolved in-process X-ray imaging and optical coherence tomography. The X-ray diagnostics allows measuring the geometry of the capillary with frame rates of 1 kHz, while the optical coherence tomography enables the determination of the capillary depth with an acquisition rate of up to 70 kHz. These measurements are correlated to time varying input laser power to provide profound insight in the dynamics of the laser welding process. The measurements are performed for copper, aluminum and mild steel. The capillary depth resulting from arbitrary laser power modulation was investigated. Thereby, the response of the capillary depth to laser power changes was determined. Based on these measurements the changes of the capillary depth in deep penetration laser welding were described by methods known from control theory. These analyses can be utilized to optimize control strategies, to calibrate transient simulations of deep penetration laser welding and to identify the influence of material properties.
Fundamental process monitoring is very helpful to detect defects formed during the complex interactions of capillary laser welding process. Beside the monitoring and diagnostics of laser welding process enlarges the process knowledge which is essential to prevent weld defects. Various studies on monitoring of laser welding processes of aluminum, copper and steel were performed. Coaxial analyses in real-time with inline coherent imaging and photodiode based measurements have been applied as well as off-axis thermography, spectroscopy, online X-Ray observation and highspeed imaging with 808 nm illumination wavelength. The presented diagnostics and monitoring methods were appropriate to study typical weld defects like pores, spatters and cracks. Using these diagnostics allows understanding the formation of such defects and developing strategies to prevent them.