Abstract

The purpose of this work is to understand the reasons for varied mechanical properties among three polymer Powder Bed Fusion (PBF) Additive Manufacturing (AM) processes. Parts for this work were manufactured from Polyamide 12 (PA12) using the Laser Powder Bed Fusion (L-PBF), Multi-Jet Fusion (MJF), and the recently developed Large Area Projection Sintering (LAPS) processes. Previous works have shown that LAPS parts demonstrate significantly higher density, ductility, and toughness than parts from the L-PBF and MJF processes. A hot isostatic pressing (HIP) treatment was developed to reduce porosity in L-PBF and MJF parts and determine if increasing part density would improve ductility for these processes. It is found that density is not strongly correlated with mechanical properties for high density parts (Ï > 0.98 g/cm3) for the L-PBF and MJF processes, and a following study confirms that this is the case for the LAPS process as well. Differential Scanning Calorimetry (DSC) and microtome sectioning are performed on parts from all three processes, and it is found that neither percent crystallinity nor crystalline morphology are strongly correlated with mechanical properties. A heat treatment at temperatures well over the melting point for the material is developed and is shown to improve ductility and toughness for all three processes. It is concluded that the improved ductility observed in the LAPS process stems from long exposure to high temperature, rather than increased density or a specific crystalline structure, and is associated with post-condensation reactions increasing polymer chain length. Work is also presented on the development of a production-scale LAPS system at Ascend Manufacturing. This research focuses on the results of "tiling" and "scanning" methods for producing larger parts than previously possible with prototype LAPS systems. Tensile testing showed that the methods both produced parts with similar properties, though with lower ductility than previous LAPS parts. Analysis of thermal data identifies areas for improvement of the system to attain high strength and high ductility parts. Preliminary work is done to demonstrate avenues for process improvement. Analysis of data from across the entire powder bed shows that process defects can be observed from the thermal data available in the LAPS process, and that weak spots in parts may be related to this data. Finally, the potential for process improvement through a multiple-input, multiple-output (MIMO) control scheme is discussed.

Degree

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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