Investigation of remelting effects on controlling microstructure morphology in powder bed fusion

This research investigates the effects of remelting on 316L stainless steel fabricated using laser powder bed fusion (L-PBF), specifically selective laser melting (SLM). Surface roughness, hardness, porosity, UTS and max elongation were assessed in this study, in addition microstructure morphology w...

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Main Author: Wong, Wendell Shun Yin
Other Authors: Yeong Wai Yee
Format: Thesis-Master by Research
Language:English
Published: Nanyang Technological University 2022
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Online Access:https://hdl.handle.net/10356/163107
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spelling sg-ntu-dr.10356-1631072023-03-11T18:10:32Z Investigation of remelting effects on controlling microstructure morphology in powder bed fusion Wong, Wendell Shun Yin Yeong Wai Yee School of Mechanical and Aerospace Engineering Singapore Centre for 3D Printing WYYeong@ntu.edu.sg Engineering::Mechanical engineering This research investigates the effects of remelting on 316L stainless steel fabricated using laser powder bed fusion (L-PBF), specifically selective laser melting (SLM). Surface roughness, hardness, porosity, UTS and max elongation were assessed in this study, in addition microstructure morphology will be compared to simulation. Remelting leads to improved hardness in low energy density (ED) specimens from 216.41 ± 10.12 HV for single scan to 234.12 ± 7.43 HV for remelted specimens. Remelting has a positive effect on density due to decreased defect occurrence across the layers for regular ED specimens and better fusion. Density is improved from 7.876 ± 0.06 g/m3 to 7.910 ± 0.03 g/m3. The porosity of the specimens decreased from 0.15 % to 0.12 % and their surface roughness also decreased from 22.65 ± 2.04 µm to 14.5 ± 2.08 µm. In addition, variability was substantially reduced. Analysis of Variance (ANOVA) has highlighted that main factor such as laser power, scanning speed and hatch spacing and their interactions which affect ED improve the specimens’ physical properties. Ultimate tensile strength (UTS) has improvements between 16.58 % to 21.45 % from 528.5 ± 58.5 MPa to 641.9 ± 16.4 MPa while maximum elongation, has improvements between 8.78 % to 19.55 % from 38.70 ± 1.56 % to 46.27 ± 2.74 %. Optical micrography have shown remelting is able to produce consistent repeating melt pools forming zig zag pattern. Simulation is able to approximate melt pool width and depth within a difference of 3.620 ± 0.637 % and 13.825 ± 1.180 %. Rough approximation of expected microstructure morphology can be approximated by corroborating with simulated thermal gradient. ED below 30 J/mm3 shows grains that are constraint within the melt pool and shows a shallower thermal gradient via simulation. Above 30 J/mm 3 shows grains growing beyond the melt pool and a steeper thermal gradient. Master of Engineering 2022-11-24T07:49:59Z 2022-11-24T07:49:59Z 2022 Thesis-Master by Research Wong, W. S. Y. (2022). Investigation of remelting effects on controlling microstructure morphology in powder bed fusion. Master's thesis, Nanyang Technological University, Singapore. https://hdl.handle.net/10356/163107 https://hdl.handle.net/10356/163107 10.32657/10356/163107 en This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). application/pdf Nanyang Technological University
institution Nanyang Technological University
building NTU Library
continent Asia
country Singapore
Singapore
content_provider NTU Library
collection DR-NTU
language English
topic Engineering::Mechanical engineering
spellingShingle Engineering::Mechanical engineering
Wong, Wendell Shun Yin
Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
description This research investigates the effects of remelting on 316L stainless steel fabricated using laser powder bed fusion (L-PBF), specifically selective laser melting (SLM). Surface roughness, hardness, porosity, UTS and max elongation were assessed in this study, in addition microstructure morphology will be compared to simulation. Remelting leads to improved hardness in low energy density (ED) specimens from 216.41 ± 10.12 HV for single scan to 234.12 ± 7.43 HV for remelted specimens. Remelting has a positive effect on density due to decreased defect occurrence across the layers for regular ED specimens and better fusion. Density is improved from 7.876 ± 0.06 g/m3 to 7.910 ± 0.03 g/m3. The porosity of the specimens decreased from 0.15 % to 0.12 % and their surface roughness also decreased from 22.65 ± 2.04 µm to 14.5 ± 2.08 µm. In addition, variability was substantially reduced. Analysis of Variance (ANOVA) has highlighted that main factor such as laser power, scanning speed and hatch spacing and their interactions which affect ED improve the specimens’ physical properties. Ultimate tensile strength (UTS) has improvements between 16.58 % to 21.45 % from 528.5 ± 58.5 MPa to 641.9 ± 16.4 MPa while maximum elongation, has improvements between 8.78 % to 19.55 % from 38.70 ± 1.56 % to 46.27 ± 2.74 %. Optical micrography have shown remelting is able to produce consistent repeating melt pools forming zig zag pattern. Simulation is able to approximate melt pool width and depth within a difference of 3.620 ± 0.637 % and 13.825 ± 1.180 %. Rough approximation of expected microstructure morphology can be approximated by corroborating with simulated thermal gradient. ED below 30 J/mm3 shows grains that are constraint within the melt pool and shows a shallower thermal gradient via simulation. Above 30 J/mm 3 shows grains growing beyond the melt pool and a steeper thermal gradient.
author2 Yeong Wai Yee
author_facet Yeong Wai Yee
Wong, Wendell Shun Yin
format Thesis-Master by Research
author Wong, Wendell Shun Yin
author_sort Wong, Wendell Shun Yin
title Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
title_short Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
title_full Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
title_fullStr Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
title_full_unstemmed Investigation of remelting effects on controlling microstructure morphology in powder bed fusion
title_sort investigation of remelting effects on controlling microstructure morphology in powder bed fusion
publisher Nanyang Technological University
publishDate 2022
url https://hdl.handle.net/10356/163107
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