Predicting mechanical property plateau in laser polymer powder bed fusion additive manufacturing via the critical coalescence ratio

• Main Authors: Camden A. Chatham, Michael J. Bortner, Blake N. Johnson, Timothy E. Long, Christopher B. Williams
• Format: Article
• Language: English
• Published: Elsevier 2021-03-01
• Series: Materials & Design
• Subjects: Additive manufacturing; Powder bed fusion; Selective laser sintering; Polymer coalescence; Process parameter prediction; Physical gelation
• Online Access: http://www.sciencedirect.com/science/article/pii/S0264127521000277

The state of the art in property-process relationships in the laser polymer powder bed fusion (LPPBF) subcategory of powder bed fusion (PBF) has derived relationships between the energy supplied and polymer thermal properties governing melting and degradation, so-called the “energy melt ratio (EMR).” The EMR provides a framework for process parameter value selection based solely on melting behavior. However, coalescence, and not merely melting, is the basis for mechanical properties in LPPBF printed parts. The authors present a method for (1) predicting polymer coalescence based on transient temperature profiles resulting from a combination of LPPBF process parameter values and (2) connecting the predicted coalescence response to the observed onset of a plateau in mechanical properties. This work tests the hypothesis that the observed onset of a mechanical property plateau corresponds with a transition in consolidation physics. Complete coalescence must be achieved prior to the onset of physical gelation. For this work, in situ transient temperature profiles were obtained using infrared thermography. Coalescence prediction, via the Upper-convected Maxwell model, and physical gelation prediction, via Lauritzen-Hoffman and Avrami equations, were found to successfully identify LPPBF parameter combinations resulting in parts with density and tensile strength inside the plateau region. The hypothesis that the plateau occurs at the onset of closed pore morphology is supported.