Binder Infill Pattern Design Strategies for Increased Mechanical Properties in Binder Jet Additive Manufacturing Parts

Abstract

The use of a liquid binding agent is critical for forming part shape and providing green part strength in binder jet (BJT) additive manufacturing (AM). Traditionally, binder is homogeneously deposited throughout the entire part cross-section during printing in order to give sufficient green part strength for post-processing and to preserve part geometric accuracy. However, recent research suggests that, compared to sintering powder with no binding agent, the presence of binder limits densification during sintering. Specifically, the authors have shown that unbound powder – achieved by placing binder only around the part boundary (i.e., printing a "shelled" part) results in significantly higher sintered density compared to conventional bound powder. While this shell printing approach improves final part density and mechanical properties, the corresponding printed green parts are extremely fragile due to the low binder content. Thus, the overall aims of this research are to (i) identify techniques for simultaneous balancing of a part's green and sintered properties through binder pattern manipulation (i.e., toolpathing) and (ii) gain an understanding of their corresponding process-structure-property relationships. Specifically, the author explores three distinct strategies for binder jetted 316L: (1) architected lattice infill, (2) lattice unit cell size effect, and (3) topology optimized infill. • In architected lattice infill, a binder patterning strategy, comprised of an exterior contour shell with various internal strut based or triply periodic minimal surface (TPMS) lattice infill patterns, is applied to balance the tradeoff between green and sintered part properties that result from reduced binder usage. An octet infill part was found to have an 85% reduction in green part strength from that of a conventional solid infill, but a 2.6% higher relative density and 24% higher flexural strength once sintered. • In addition to lattice architecture choice, the effects of unit cell size choice for a lattice architecture on green and sintered part properties are studied. It was found that a larger unit cell had superior performance over smaller unit cells of equivalent bound volume fraction. Furthermore, binder deposition and part properties approached that of a conventional homogenous solid infill of lower saturation ratio at small unit cell sizes. • Finally, the author explores the use of topology optimization to generate infill for minimized compliance in green and sintered parts. Placement of bound and unbound regions are iteratively designed to achieve an optimal configuration, subject to a range of allowable bound volume fraction constraints. Overall, the concept of infill patterning, although common in other types of additive manufacturing, is unconventional in BJT. To the knowledge of the author, this work is the first to explore the structure-property effects of BJT infill design on green and sintered parts through several strategies of binder patterning.