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

Abstract 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. Keywords Additive manufacturing; Powder bed fusion; selective laser sintering; polymer coalescence; process parameter prediction; physical gelation List of Abbreviations a Particle size a0 original particle size AM Additive manufacturing CCR Critical Coalescence Ratio De Deborah number DSC Differential scanning calorimetry ED Energy density EMR Energy melt ratio h Hatch spacing IR infrared l layer height P laser power PBF Powder bed fusion LPPBF Laser polymer powder bed fusion rchamber Dimensionless neck radius after isothermal time as a supercooled polymer melt rcritical Dimensionless neck radius at 0.94 relative density rlaser Dimensionless neck radius after laser scanning SC Scan count tAF, 1 Time available for fusion while the laser is scanning tAF, 2 Time available for fusion after the laser has finished scanning until a new layer begins tAF Time available for fusion Tb, inf Temperature of the powder bed bulk tCF Time for critical fusion (i.e., 0.835 dimensionless neck radius) Tfeed Temperature of the feed powder tX10 Time to 10% crystallinity Tm0 Equilibrium melting temperature Tb Surface temperature of the powder bed Tmax maximum temperature measured when laser scanning UCM Upper-convected Maxwell Vb Beam speed x neck radius η Viscosity (unspecified) η0 Zero-shear viscosity ηext Extentional viscosity ηss Steady shear viscosity Γ Surface energy λ Relaxation time Θ Angle between coalescing particles