| A unique, dual camera video system was developed to provide real-time information about the keyhole penetration mode for use as a possible sensor for penetration control. To analyze the possibilities of this video-based sensor, an experiment was designed to vary the penetration modes during welding by continuously changing the focal point position at different power levels and travel speeds. Video recordings of the events allowed the transition into and out of the keyhole penetration mode to be carefully studied from two simultaneous vantage points resulting in the extraction of qualitative and quantitative details from the individual video frames. Features from the magnified, high resolution video images produced during the experiment were observed and identified relative to the transitions. Interpretation of these real-time images was based on the work of others. For example, the video images showed that the power density variations during the tests generated three types of pools (flat, depressed and keyhole), supporting the process model obtained from post-weld metallographic work of other investigators. A significant observation was the laser plume metamorphosis prior to and immediately after the keyhole penetration transition. Appearance of a wispy, fantail-shaped plume occurred simultaneously with the steady-state, depressed-pool genesis in the images. As the power density increased, the diverging fantail plume changed to a concentrated columnar shape. The keyhole cavity initiated when the base of the columnar plume, the coupling spot (beam/material interaction site) and the estimated beam spot size were identical. Upon keyhole penetration termination, the images showed the columnar plume formation and reversion to a fantail as the power density decreased. Also, keyhole penetration depth correlated with image feature dimensions, specifically coupling spot (beam/material interaction) width and weld pool length. Analysis of the video images confirmed that the unique perspective produced by the dual-camera video system could provide the basis for an in-process penetration control strategy and an added understanding of the physical processes that affect laser welding penetration mode and pool/plume dynamics during CO... |
