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Influence of laser parameters on the depositions of 316L stainless steel using laser powder bed fusion

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Title: Influence of laser parameters on the depositions of 316L stainless steel using laser powder bed fusion
Authors: Vecchiato, Filippo
Item Type: Thesis or dissertation
Abstract: Laser powder bed fusion (LPBF) is an additive manufacturing technique by which complex structural parts can be built-up in a layer-by-layer process, selectively melting the powder on previously deposited layers. The high energy density applied by the laser beam to a confined volume of powder not only results in high and directional thermal gradients, but also requires extensive calibration to produce uniform melt pools, essential to produce fully dense components. The additive nature of the process and the significant thermal gradients results in a unique microstructure that can be altered by changing the process parameters used. In this work, the melt pool morphology, microstructure and texture of LPBF austenitic stainless steel of type AISI 316L has been characterised by morphological (LOM and SEM),microstructural (SEM and EBSD) and thermal history (thermal cameras, XRD, neutron diffraction) investigations using a variable energy input comparison. The influence of the temperature gradients were also investigated using a combination of XRD and neutron diffraction to evaluate the residual stresses in LPBF-produced parts. The absorbed specific energy used controls the size of the melt pools deposited on the substrate. However, the deposition mode (keyhole mode >150W or conduction mode <150W) was shown to be controlled by the laser power, while the penetration depth is controlled by the laser exposure time. The deposition mode also affects the solidification process. Temperature gradients to the top of the deposition dome were found during conduction mode depositions, producing columnar grains growing bottom-to-top in the melt pool, while keyhole mode presented variable temperature gradients that resulted in columnar grains below the substrate surface but more equiaxed grains in the deposition dome. An increase in laser idle (off) time resulted in a much finer microstructure due to lower temperature gradients, however it also produced lower parts density (94.95%) compared to standard idle time of 5 μs (99.95%). The thermal residual stresses were affected by the build direction strategy of the parts, with 28% larger residual stresses in vertically printed specimens compared to horizontally printed specimens.
Content Version: Open Access
Issue Date: Sep-2019
Date Awarded: Jan-2020
URI: http://hdl.handle.net/10044/1/95894
DOI: https://doi.org/10.25560/95894
Copyright Statement: Creative Commons Attribution NonCommercial Licence
Supervisor: Wenman, Mark
Davies, Catrin
Sponsor/Funder: Engineering and Physical Sciences Research Council
Atomic Weapons Establishment (Great Britain)
Funder's Grant Number: 1857790
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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