Inline Coherent Imaging of Laser Keyhole Welding
Galbraith, Christopher Mark
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Laser keyhole welding is an emerging materials processing technique offering many advantages over more conventional joining methods. High intensity laser light is applied to a sample, melting and evaporating the metal in its path. The resulting recoil pressure drives a narrow capillary (a 'keyhole') of vapour into the metal, allowing delivery of laser energy deep beneath the surface. The resulting weld seams are deep, narrow, precisely positioned, and often require no filler material. However, the process is inherently unstable and its underlying physics are not straightforward. The keyhole has traditionally proven difficult to measure, which hinders efforts to model, understand, and control this process. Inline coherent imaging (ICI), a laser-based ranging technique, has previously been demonstrated as an effective tool for in-situ measurement of the keyhole depth. In this work, I seek to improve our understanding of ICI's measurement of the keyhole, and enhance its utility. A new algorithm that tracks keyhole depth with zero resolution loss is demonstrated. Mechanisms responsible for signal losses in the weld image are explored, and a modified ICI system producing doubled line-rates near 600 kHz is realized. Active imaging beam scanning is implemented, increasing the resolving capability of the ICI system to three spatial dimensions. With this new capability, I measure keyhole morphology in-situ--an entirely novel way of observing this elusive and volatile physical system.