On the Nature of Powder Spreading Defects and Their Detectability and Impact to Part Quality
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Massey, Caroline Ellen
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Abstract
Laser powder bed fusion (PBF-LB) metal additive manufacturing allows a new paradigm for design creativity and supply chain logistics. There are many process variables that affect part quality in powder bed fusion including powder quality, layer height, laser speed and power, and hatch spacing. Spreading defects can lead to many process defects such as voids, energy density changes, and part topography variations. The overall goal of the present dissertation is to identify the critical size of notches on the recoater blade via investigation of part quality and powder bed topography. The first study was a combined simulation and experimental study investigating recoater damage and spreading defects in PBF-LB. In the experimental study, notches were machined into the recoater to investigate the effect of spreading parameters on the powder bed via a laser line scanner. A simulation model via the discrete element method (DEM) modeled the spreading conditions, resultant topography, and the distribution of particles. This investigation will aid in understanding the spreading behavior at the particle level. The second study investigated the ability of optical based methods to predict recoater damage from in-process signals. A range of machine learning methods were employed to study this behavior. Key features from the optical images were used to identify height and width variations within the powder bed. The third study investigated the effect of recoater defects on part quality characteristics of PBF-LB printed parts. Witness specimens were analyzed via computed tomography and optical profilometry for their roughness and porosity content. This investigation will aid in determining the criticality of the size of the recoater damage and its influence on porosity distribution. Spreading deviations of size 0.0241 mm were not shown to cause swelling at the surface, whereas spreading defects of size 0.0707 mm, which was almost two times the layer thickness, showed noticeable signs of swelling. Altogether, these studies will inform a comprehensive understanding of powder spreading with a damaged recoater and its subsequent effects on powder bed and part level defects.
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2024-12-03
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Dissertation (PhD)