Proof of full-strength bonding in material extrusion additive manufacturing
thesisposted on 08.11.2021, 10:22 by James Allum
Material extrusion additive manufacturing (MEAM) is a versatile and valuable manufacturing process, which has seen significant technological developments in recent years. Supporting affordable, rapid, and personalised production, MEAM is well suited to meet the demands of high-value industries such as medical devices and aerospace. MEAM offers exciting opportunities for innovation, but does have limitations, the most significant of which is mechanical anisotropy of 3D-printed parts, limiting the capacity for mechanical application. While it was established that the direction normal to the print-platform plane was weakest, there is no consensus for the cause of the issue or methods for resolving it. Numerous studies attributed this limitation to poor bonding between deposited layers, but this was not proven in tests on individual bonds with accurate geometric characterisation. Additionally, attempts to understand the relationship between parametric modification and mechanical performance of MEAM-produced parts resulted in significant contradiction, with understanding hampered by broad ranges of complex specimen/toolpath designs and characterisation methods employed. In this thesis, the bond strength in MEAM is proven to be full strength, i.e. equivalent to that of the extruded filament loaded in the direction of deposition which has physical properties which approximate that of the bulk material. To enable improved manufacturing control and reduce geometrical complexities in previous studies, novel notched, uniaxial tensile specimens were designed by direct GCode scripting and utilised for mechanical and geometrical characterisation. Test specimens, formed by stacked individual extruded filaments, enabled the precise characterisation of interlayer bonding. Untested and tested specimens were microscopically characterised to accurately identify the cross-sectional area, over which the applied load was distributed, and to understand the geometrical relationship between the extruded-filament geometry - layer height, extruded-filament width, and aspect ratio - and mechanical performance. Through this series of mechanical studies, filament-scale geometry was identified as the cause of mechanical anisotropy. The groove features of the interlayer bond region were responsible for the reduced contact area between extruded filaments (compared to the one measured commonly with calipers) and caused stress concentration. These features, occurring between deposited layers, reduced the load-bearing capacity of specimens loaded normal to the direction of extruded filaments, but did not affect specimens loaded in the direction of their deposition. Furthermore, the filament geometry was found to reduce strain-at-fracture and toughness for specimens loaded normal to the direction of extruded filaments because the strain concentration caused by filament-scale grooves significantly reduced ductility. Strain-at-fracture was found to be sensitive to the interlayer bond angle between the extruded filaments, with more acute angles causing increased strain concentration and reduced strain-at-fracture. The new understanding of bond strength and the identification that mechanical weaknesses of MEAM parts are caused by the filament-scale geometrical features of the interlayer bond region informed new unconventional strategies, which significantly improved their mechanical performance. These studies are summarised along with the thesis conclusions to guide the future work and provide readily implementable applications of the new understanding provided.
- Mechanical, Electrical and Manufacturing Engineering