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Title: The effect of changing particle size distribution and layer thickness on the density of parts manufactured using the laser Powder Bed Fusion process
Author: Alfaify, Abdullah
ISNI:       0000 0004 7960 3612
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Powder Bed Fusion (PBF) is a metal additive manufacturing process where parts, described using a CAD data file, are fabricated layer by layer by melting metal powder. Selection of the ideal process parameters for the pulsed laser powder bed fusion (L-PBF) processes is of paramount importance, as there are a vast amount of parameters that have a direct impact on an eventual quality of fabricated parts. The aim of the present study is to present a systematic method of optimising the selection of the process parameters. The research comprehensively investigates the effect on final part density of changing (i) the particle size distribution of the primary powder, (ii) the layer thickness and (iii) the location of the fabricated part on the build platform. All these should help the prediction of the density/porosity of the parts and control their density for their desired applications. In previous studies, volumetric energy density (VED) or scan speed have been used as the control variables for selecting the appropriate applied energy. The VED is not always completely accurate and able to identify the optimum energy that leads to fully dense parts. Consequently it should be used more as a guideline for finding the region in which to operate. A similar value of VED can be obtained using various combinations of laser power, scan speed and layer thickness. However, some values of scan speed, for instance, are not able to produce sufficient melt. To produce components with acceptable mechanical performance requires a comprehensive understanding of process parameters and their interactions. In this work, the process parameters (layer thickness (LT), laser power (LP), point distance (PD), exposure time (ET) and hatching distance (HD)) were considered and studied comprehensively, and individually, to provide a better understanding of the effects of each process parameter on the final built part. Titanium alloy Ti-6Al-4V ELI and 316L Stainless Steel are used throughout this research. The Taguchi experimental design method and the Response Surface Method (RSM) were used to determine and optimise the effect of the selected input parameters, and also to investigate the impact of changing critical parameters on the density of parts manufactured. The influence on the part density was selected as the output to be measured to identify the most statistically significant parameters. This was then followed up by in-depth studies focusing on individual parameters. The results show the ideal combinations of process parameters which can provide fully or near fully dense parts. These process parameter combinations are used to fabricate samples at different layer thicknesses and from a different range of particle size distributions. The effect of changing the layer thickness and particle size distribution was then investigated. It was found possible to fabricate parts with relative density above 99% for layer thickness ranges from 30µm up to 100µm, with appropriate tuning of the process parameters. A clear correlation between the number and shape of pores and the process parameters was identified. Generally, large and irregular pores are usually a result of the lack-of-fusion process due to inadequate melt energy being applied, which can be a result of large PD, HD and LT or short ET. The small and circular pores are mainly due to a result of exaggerated overlapping in HD, PD or elongated ET. These relations are not only due to the effect of the individual process parameter (e.g. PD, HD etc.), but also the interactions between process parameters themselves. These were also found to critically affect the porosity. Then, the study develops regression models and verifies them experimentally. The proposed models were validated and used to accurately predict part density. Finally, a first-of-its-kind study about build location effect was conducted. This shows that part location has a significant impact on sample quality. Potential reasons for the effect of build location are discussed. Spatter was found to be the major factor that causes a variation of the density of parts built in different locations on the platform. The best build location for both materials was found to be close to the inlet of the gas flow. This position minimises the influence of spatter. The study provides a deeper understanding of the variation of the density of a part built in different locations on the build platform and the effect of that on reproducibility. The study also raises awareness about building in angled orientations. The results from this research offer a valuable understanding of ways to optimise the selection of processing parameters, for fabricating parts using PBF with high density/low porosity. It leads to a better understanding of concerns which need to be taken into account for optimal operation. The focus is not restricted to the process parameters of the PBF machine and the PSD of the primary powder used, but also highlights the significance of ensuring correct part orientation/location as all these parameters influence the part density of the fabricated part.
Supervisor: Ridgway, Keith Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available