Femtosecond laser-enabled precison additive manufacturing: Process parameter optimization and surface roughness reduction

Résumé

AM is one of the most advanced technologies in recent years, transforming the paradigms of many industrial processes. Among the various techniques available, the use of lasers has stood out for its versatility and efficiency; however, there are still certain design limits in terms of the materials that can be used, as well as a constant need to improve the quality and precision of manufacturing methods. Femtosecond lasers, belonging to the family of USP lasers, have the ability to affect substrates of many kinds with the singularity of a very low or negligible thermal effect; this minimal effect, which can a priori be a disadvantage if a significant heat build-up is desired, has been exploited to perform additive manufacturing studies and achieve higher degrees of precision in the LPBF technique. The studies carried out in this thesis demonstrate that it is possible to fuse stainless steel powder of different sizes with an appropriate choice of processing parameters, using low values of pulse spacing and accumulating the pulses along the powder bed. Although it is necessary to use a small powder size, this thesis has succeeded in obtaining high surface quality cast stainless steel structures below 1 W laser power and with a pulse repetition of 500 kHz, the lowest ever achieved to date, and in a material never studied before. This thesis also analyses the ability of the optimised parameters to fabricate structures with reduced thickness profiles, matching the records of other similar techniques for structures with a thickness of less than 100 µm. Finally, the femtosecond laser has also been used, in burst mode, to reduce the surface roughness of different substrates with different morphologies. Using the maximum pulse repetition and the highest number of pulses per burst, these studies have succeeded in reducing the roughness of metallic substrates by more than 60 %, obtaining different degrees of final finish depending on the processing conditions. It has also been demonstrated that the polishing process does not affect the morphology of the processed profiles, reducing the surface roughness and improving the appearance, but without damaging or affecting the original design of the polished substrate. The results of this thesis demonstrate that the femtosecond laser is a very powerful tool for applications beyond surface ablation and/or texturing, achieving very promising results both in the field of additive manufacturing and in the reduction of surface roughness and improvement of the final finish of parts, also linked to and necessary in many additive manufacturing processes.