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Title: Physical verification of the melt pool in laser-bed fusion
Author: Giordimaina, Andre
ISNI:       0000 0004 7961 1102
Awarding Body: Swansea University
Current Institution: Swansea University
Date of Award: 2017
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Laser Powder-Bed Fusion (LPBF) is an additive manufacturing process which fuses metal powder on a layer by layer basis to form complex three-dimensional components. As with other additive processes, LPBF is seeing a rapid evolution of machine design, scanning techniques, and materials development which has moved the process well beyond its origins in rapid prototyping to a process which can manufacture fit-for-purpose components. At the heart of the LPBF process lies the melt pool, and the way in which the laser properties, such as speed, power and beam diameter interact to form tracks fused to the substrate is integral to the way in which multiple tracks will fill the contours across each layer in the build sequence. Controlling the as-solidified bead shape is important to ensure optimal mechanical properties. A widespread technique for measuring the effect of laser properties on the mechanical properties and track formation is process mapping. Single-layer or single-track process maps, which measure the behaviour of the melt according to laser properties on a single layer of powder, have been limited to a base plate of same composition, but with a different microstructure, typically resulting from a rolling process. The work in this thesis describes the efforts to standardise a high-throughput method of creating process maps which measure the effects of these process parameters on, in a way which compliments and improves upon the usual technique of deposition of single line tracks directly onto a base plate. One result of this work is a new method where substrates are built using the LPBF process, on which single tracks are deposited with a controlled powder depth. This is done in such a way that the as-built tracks are representative of the process at regions away from the base plate, by building the substrate in-situ, before the forming of the tracks. It was found that the crucible single track method could be used quite effectively to control the powder layer depth at which tracks were deposited on. The additional benefit granted by the crucible substrate was the ease at which high quality topographical and cross-sectional metallography could take place in order to quantify and investigate the effects of changing the parameters. For example, by using the crucible method, it was found that titanium alloy Ti-6Al-4V, at a maximum laser power of 200W, could form relatively stable track formations at 100µm layer thickness at a scan speed of 500mms-1. At lower power values, faster scan speeds or larger layer depths, tracks would not form successfully. Another important outcome was that the crucible method predicted a much less severe transition between conductive and keyhole modes of melting than direct deposition of single tracks onto a baseplate, with shallower re-melting of lower layers. The crucible method also predicted a more forgiving transition between continuous lines and lines which had broken down due to poor wetting or insufficient temperatures.
Supervisor: Brown, Stephen G. R. ; Lavery, Nicholas P. ; Sutcliffe, Chris J. Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral