Additively manufactured, long, serpentine submillimeter channels by combining binder jet printing and liquid-phase sintering

Abstract

Metallic microfluidic devices made from powder-bed additive manufacturing systems have received increasing attention, but their feasible channel geometry and complexity are often limited by lack of an effective approach to removing trapped powder particles within the channels or conduits of the sintered parts. Here, we present an innovative approach to fabricating long serpentine, high-aspect-ratio submillimeter channels made of stainless steel 316L (SS) by binder jet printing (BJP) and liquid-phase sintering. We leverage the unique nature of the BJP process, that is printing and consolidation steps are decoupled, enabling us to join two or more parts during the sintering step. Instead of constructing the channel device as a single part, we print multiple parts for easy powder removal and later join them to form enclosed channels. The key innovation lies in adding sintering additives like boron nitrides (BN) to the SS stock powder-at the SS/BN interfaces, liquid phase is locally generated at temperature much lower than the SS melting temperature, facilitating the bonding of the multiple parts as well as the consolidation of parts for near-full density. We systematically vary the sintering temperature to show how it affects the joining quality and the channel shape distortion. The joining quality such as the fracture strengths of the joined samples is measured by a pull test while the shape distortion is characterized by various imaging techniques. The feasibility of the proposed approach is demonstrated by fabricating a 400-mm-long, fully enclosed serpentine channel with a rectangular cross-section of 0.5 mm in width and 1.8 mm in height. The pressure drop across this 3D-printed SS serpentine channels is measured for air flow and compared to a standard gas flow model, showing that the device is free of leakage or clogs.

Keywords: Additive manufacturing; Binder jet printing; Depowdering; Joining; Microchannel; Sintering.

Figure 1

Schematic diagrams of the proposed serpentine submillimeter channel device, consisting of two flat cover plates and a core part with serpentine grooves on both sides. Two serpentine channels are stacked to double the effective channel length for a given planform area (pink arrows indicating a flow direction).

Figure 2

Step-by-step illustration of the proposed process steps for device fabrication using binder jet printing and diffusion bonding: (a) binder jet printing of three green parts—two cover plates and one core, (b) binder phase curing, (c, d) removal of loose powder, (e) binder phase burn-out, (e, f) full sintering and joining of the three parts to form the submillimeter channel device, (g) welding of fluidic interconnects.

Figure 3

Photograph images of the four SS joined block samples at different post-processed stages: first column—as-joined of two rectangular bars (each bar with the dimension of 15 mm by 8 mm by 8 mm); second column—EDM cut of the joined sample; third column—after pull testing. Each joined sample has different sintering conditions (a) 1130 °C for 6 h, (b) 1135 °C for 6 h, (c) 1135 °C for 12 h, and (d) 1140 °C for 6 h.

Figure 4

(a–d) Plots of the stress vs. extension of the SS joined parts (three duplicates for each condition) sintered at different temperatures (1130, 1135, and 1140 °C) and soaking durations (6 and 12 h); (e) Plot of the fracture strengths [maximum stresses at breaking points from (a–d)] as a function of the sintering temperature.

Figure 5

Photographs of the SS submillimeter channel device at each processing step: (a) after binder phase curing and loose powder removal, (b) after binder phase burnout, (c) after full sintering and joining, (d) after the fluidic interconnects welded. Scale bar in (a)–(d) = 10 mm.

Figure 6

Photographs and SEM images of the SS submillimeter channel devices sintered and joined (a) at 1130 °C for 6 h, (b) at 1135 °C for 6 h, (c) at 1135 °C for 12 h, (d) at 1140 °C for 6 h, (e) at 1150 °C for 6 h; micro-CT images showing the planar cross-section of the submillimeter channel devices sintered and joined (f) at 1135 °C for 12 h and (g) 1150 °C for 6 h. Scale bar: 5 mm in photographs and 0.5 mm in SEM images, and 4 mm in micro-CT images.

Figure 7

Schematic diagram and photograph of the varied channel widths and wall thicknesses (nominal dimensions indicated in mm) and SEM images with the measured dimensions in mm and shrinkage in %. This SS channel was sintered at 1135 °C for 12 h. Scale bar: photograph = 3 mm, SEM images = 0.5 mm.

Figure 8

Volumetric flow rates of air measured as a function of pressure difference across the SS submillimeter channel device (sintered at 1135 °C for 12 h). The solid line indicates the Poiseuille flow model for the rectangular channel under the low-Re-number flow regime.