IVESTIGATING THE WELD DEPTH BEHAVIOUR USING DIFFERENT OBSERVATION TECHNIQUES: X-RAY, INLINE COHERENT IMAGEING AND HIGHSPEED OBSERVATION DURING WELDING ICE

摘要

For a comprehensive understanding of the deeppenetrationlaser-welding process, it is of fundamentalinterest to understand the dynamic behavior of thecapillary. The X-ray system of the Institut fuerStrahlwerkzeuge (IFSW) allows to record a twodimensionalprojection of the capillary with framerates up to 10 kHz.Hence the system is capable to gain information aboutthe welding capillary, such as size and shape.However, the laser-welding process is a highlydynamic process with significant changes in timeperiods shorter than 0.1 ms.Queen’s University and Laser Depth Dynamics (LDD)have developed a sensor based on inline coherentimaging [1] which provides direct geometricalmeasurements of the keyhole depth and associateddynamics at rates >300 kHz with micron-scaleprecision. The technique is based on spectral domainlow-coherence interferometry and is delivered througha camera port and combined co-axially with theprocess beam.The two measurement methods were set up in acombined experiment.The X-ray system recorded the shape and depth of thecapillary with a spatial resolution of about 100 μm anda frame rate of 1 kHz, whereas the depth sensor fromLDD provided 200 kHz at a single spot with axialresolution on the order of 10 μm.The present contribution compares the results of thetwo methods for steel and aluminum welds allowingnew insights to the short-timescale behavior of thecapillary.In order to understand the principal findings, theresults were compared with high speed videos, takenduring welding of the transparent material ice.Compared to the X-ray technique, a higher spatialresolution can be obtained at high repetition rates.At a first glance, it might be astonishing, that weldingof ice can be compared with welding of metals. In alarge number of experiments, however, we found thatduring welding of ice a lot of phenomena known fromwelding of metals (especially steel and aluminum) arealso present during welding of ice, but can be observedmuch more clearly because of the low temperature andthe transparency of the material.