High-throughput, high-quality laser engraving using nanosecond pulsed fibre lasers
Dondieu, Stephen Donbai
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The work presented in this thesis investigates the use of 100 W and 200 W nanosecond pulsed fibre lasers for high-throughput and high-quality engraving of metals. The use of these lasers for most applications is a challenge, owing to laser-induced thermal accumulation within the workpiece. Over the years, researchers have tackled this issue, but focused on ultrashort pulsed lasers, leaving a research gap in terms of nanosecond pulsed lasers, which was the motivation for this thesis. This study features a single line machining technique, to aid understanding of the fundamental laser-material interactions in the high-power nanosecond regime, and to establish a phenomenological model through which an outline parameter space for area engraving can be deduced. Full factorial and Taguchi experimental methods are then used to optimise process parameters during area engraving. The influence of pulse duration, energy dose (i.e. total energy deposited per unit area), pulse repetition frequency, spot size, and laser beam scanning strategies during the engraving of aluminium (5251), brass (508L), and stainless steel (316L) were examined. Analysis of the material removal rate, surface quality, surface elemental composition, and microstructural changes, was conducted using an Alicona surface profilometer, optical microscope, SEM-EDX, and XRD. The study has found that the pulse duration, pulse repetition frequency, spot size, energy dose, and scanning strategies significantly influence process performance. In particular, the use of interlaced laser beam scanning reduces thermal accumulation and offers higher throughput and better surface quality when compared to conventional scanning methods. It was found that there is a trade-off between throughput and surface quality, and hence the optimised parameters will to some extent always depend on the application requirements. Ultimately, this study demonstrates high-throughput, high-quality laser engraving using higher average power pulse tuneable nanosecond fibre lasers, whilst discussing existing challenges and proposing potential remedies.