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Development and analysis of a laser polishing strategy to support the industrialisation of additively manufactured aluminium parts

Mason, Benjamin ORCID: 2023. Development and analysis of a laser polishing strategy to support the industrialisation of additively manufactured aluminium parts. PhD Thesis, Cardiff University.
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While additive manufacture (AM) is an attractive technology for many areas of engineering, there are substantial barriers to its further industrialisation, one of which is the substandard surface roughness. Literature reviews showed there to be much interest in reducing the as built roughness through parameter optimisation, and various post processing options. One such post-processing technique is laser polishing (LP), sharing many of the benefits of AM (geometric freedom, waste-free operation). The literature reviews also highlighted a series of areas that needed specific attention to facilitate the continued development of LP for AM metal parts, guiding the progress of this thesis. Specific gaps identified were; how to effectively measure the surface roughness of AM parts, how to best laser polish AM aluminium, and what effect the LP has on the material in terms of structure and mechanical properties. This project started with developing a methodology to reliably and repeatably measure the surface roughness of AM metal parts. The developed methodology was benchmarked using AM aluminium test parts, across a range of measurement devices. It was shown to be robust to changes in measurement device across a wide range of standard roughness parameters. This work also highlights the need for greater understanding regarding the appropriate selection of filter nesting index. Furthermore, the influence of various measurement and post-processing options on the measurement uncertainty have been quantified in terms of pointwise height discrepancies, and the influence on calculated roughness parameters. Following that, this project developed a novel LP strategy for AM aluminium, utilising multiple steps designed to sequentially ablate and smooth the surface. This was shown to significantly reduce the surface roughness, with a dependency on the initial surface roughness. Minimum roughness values measured were 1.95 µm Sa, 18.83 µm Sp, 14.11 µm Sv, and 25.07 µm S10z. Maximum reductions found were 87.4%, 87.4%, 81.7%, and 81.4% for the roughness parameters in turn. The LP strategy resulted in an increase in microhardness in the remelted region, with an associated increase in porosity just below this remelted layer. The maximum hardness after the final polishing step was measured to be 142 Hv0.005 (120 Hv0.005 bulk), while the near surface porosity was estimated to have risen from 0.9% (bulk) to 4.5%. Evaluating the tensile properties showed mixed results, with LP giving increased, decreased, or no change in values, and different effects depending on whether the samples had been stress relieved, or T6 tempered (peak hardened) prior to polishing. As built samples were found to have increased E and decreased Efail after LP, with the effect negated through the application of heat treatment. No change in UTS was found due to LP in any condition This showed no significant change in fatigue strength due to the application of either LP or T6 tempering. Showing any benefit of the reduced surface roughness was offset by the increased surface hardness and porosity arising from the LP strategy used. It is hoped that these results will facilitate the further industrialisation of AM aluminium through the improved understanding of how to evaluate the surface roughness, with the smoothing potential of the LP strategy opening avenues for high value, aesthetic components. Furthermore, the additional data presented regarding the tensile and fatigue properties show there are no significant detrimental effects of the LP strategy.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: 1) Laser Polishing 2) Additive Manufacture 3) Mechanical Properties 4) Surface Roughness 5) Surface Roughness Measurement 6) Aluminium
Date of First Compliant Deposit: 12 April 2024
Last Modified: 12 Apr 2024 15:19

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