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Title: Investigations into process monitoring for selective laser melting
Author: Ashton, I.
ISNI:       0000 0004 6059 8235
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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This thesis investigates process monitoring for Selective Laser Melting (SLM). Although SLM involves a dynamic melt pool, little in the way of process monitoring has been investigated. The fusion zone produced by the laser melting of metallic powders, emit a wide range of electromagnetic radiation from the near ultra violet to infrared. This study has focused on methods of collecting and interpreting the emission spectra to further understand the fusion process in SLM. Techniques were developed to separate two distinct emission radiation types, blackbody continuum radiation and discrete spectral line emissions, from the spectral data. This separation allowed for the temperature of the vapour cloud emitted from the melt pool to be calculated. An off-axis configuration was used to study the effect that changes to the SLM process parameters have on the melt pool and the vapour. Increases to hatch input energy resulted in increases to blackbody continuum radiation magnitude and spectral line intensity. However, the vapour temperature didn't rise with input energy as the fusion interaction acted as a steady state heat source. Varying laser beam diameter lead to contrasting trends in blackbody continuum radiation magnitude and spectral line emissions. The vapour temperature, using spectral line intensities, was found to be at a maximum for the smallest laser beam diameter, 52 µm. However this corresponded to a minima in blackbody continuum radiation output. When comparing the monitoring results to the physical specimens, no trend was found with respect to the beam diameter, with all samples of identical input laser power having equal track dimensions. It was concluded that melt pool dynamics changed with beam diameter which lead to variations in melt ejection, effecting surface finish. However, powder insulation maintained equal localised energy levels which lead to consistent track widths. To improve the effectiveness of emission radiation collection, an existing SLM system was modified to allow for a co-axial process monitoring. The internal optic system was redesigned such that a spectrometer could be coupled into the optical track. The effectiveness of the system was hampered by chromatic aberrations and signal attenuation from mirror coatings. A consistent spectrally resolved signal could not be achieved due to unavoidable chromatic aberration in the f-theta objective lens. This optic traditionally provides flat field focus required to maintain an equal laser beam diameter across the powder bed. The feasibility and the design an f-thetaless optical track for an SLM system is presented. By removing the f-theta objective lens, not only would co-axial monitoring performance increase, but higher power laser sources could be used, as thermal lensing would be reduced. A system which used a Galilean focusing unit, which dynamically altered system focal length, was design to maintain flat field focus. Using the prototype f-thetaless optical unit, consistent emission coupling was achieved across the powder bed.
Supervisor: Sutcliffe, C. J. ; Edwardson, S. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral