Binder Jet Additive Manufacturing Research Articles

A rapid development workflow for binder inks for additive manufacturing with application to polymer and reactive binder ink formulation

Binder jet additive manufacturing (BJAM) enables processing of metals, ceramics, polymers and composites through the deposition of a binding agent onto a powder bed in a layer-by-layer sequence. Most commonly, a polymer binder is utilized to provide temporary strength to the printed component enabling part removal and handling from the printer; however, it is challenging to manage distortion and shrinkage during sintering. Reactive binders, which create solid interparticle bridges upon decomposition, provide additional benefit beyond temporary green strength and can lead to enhanced densification kinetics, control of warping and shrinkage, and tailoring of local composition. This paper presents a process for the development of custom binder inks which includes exploration of binder ink rheology, wetting and infiltration interactions between the binder ink and powder, ink jetting and the effect of drop spacing on line formation, thermal decomposition of the binder, and evaluation of green strength using indentation. The binder ink development process is applied to the synthesis of custom polymer and reactive (metal salt) binder inks, showcasing the capability to control jetting behavior through rheological modifications and the favorable comparability of wetting, infiltration, and green strength of the reactive binders to the studied polymer binders. The viscosity and surface tension of the binder inks vary by 10 cP and 35 mN/m, respectively, with successful jetting and powder binding achieved by polymer and metal salt binder inks, with green strengths measured between 2 and 18 MPa. Through our presented process, rapid sampling of binders can be performed and through the implementation of reactive binders into BJAM in the future, a powerful tool for control of densification and warping is identified.

Multi-objective optimization of binder jet additive manufacturing of Co-Cr-Mo using machine learning

Multi-objective optimization of binder jet additive manufacturing of Co-Cr-Mo using machine learning

Effects of fumed silica flow aids on flowability and packing of metal powders used in Binder-Jetting additive manufacturing process

Binder-Jetting is a fairly new sinter-based additive manufacturing technology for metals. Due to sintering, small particle size distributions are favorable, yet they are limiting the printing process due to bad flowability and compaction. This paper aims to resolve this contradiction by adding flow aids to powders used in Binder-Jetting. Therefore 5 different powders with successively decreasing particle size distributions ranging from 32 µm (D90) to 5 µm (D90) are combined with 2 different fumed silica (aerosil) flow aids. Improvements are measured by the use of apparent and tap density as well as hausner ratio. Results show that the mixing process for flow aids and powders has significant influence on the improvements. Moreover, the amount of aerosil is a key criterion for optimized flow behavior. Improvements up to 42 % for apparent and 11 % for tap density are achieved. Hausner ratio can be improved up to 34 %. Preliminary printing results indicate that increased flowability and packing behavior also increases powder bed density and green density. The results are in line with existing theories regarding the use of flow aids in powders used in other industries and can be now be adopted for metal powders as well.

Numerical simulation and experimental measurement of pressureless sintering of stainless steel part printed by Binder Jetting Additive Manufacturing

Binder Jetting-Metal Additive Manufacturing (BJ-MAM) is a powder bed-based additive manufacturing technology which deposits liquid binder droplets to join powder particles to form complex shaped structures (i.e., green parts). As the printing process is done around room temperature, BJ-MAM is largely immune to the distortion and cracking issues that can be prevalent in melting based powder bed processes. However, a main issue for BJ-MAM is the part shrinkage and distortion during high-temperature sintering. The densification and deformation behaviors during pressureless sintering of green parts printed by BJ-MAM were studied in this paper. Experimentally, cantilever- and bridge-shaped coupons with varying beam lengths were printed using 316L stainless steel powder; these coupons produced different extents of deformation after sintering. Based on the existing modeling approaches in the powder metallurgy literature, a finite element model was developed incorporating an elastic-viscoplastic constitutive equation for computing both uniaxial equivalent creep strain and volumetric swelling strain. Two methods, a viscosity based and a power-law creep based, were further evaluated for calculating the uniaxial equivalent creep strain. Material property data used in the constitutive equation such as viscosity and creep parameters were collected from the literature, critically reviewed, and then inputted into the model. Other salient features of the model included thermal-mechanical property data that were dependent on both relative density and temperature as well as frictional contact between the part surface and the furnace wall under gravitational load. The calculated quantities such as shrinkage, final relative density, and deformed shapes were compared with the respective experimental data across different part geometries.

Binder-jetting additive manufacturing of Mg alloy densified by two-step sintering process

Difficult to effectively densify in a short time is an important issue that limits the application of magnesium (Mg) alloy fabricated by binder jetting additive manufacturing (AM) process. In this study, two-step sintering (TSS) process was utilized to improve the density of printed Mg alloy samples. Results show that the relative density, mechanical properties and corrosion resistance of the samples after different TSS process were dramatically improved in a short time. A suitable sintering temperature of first sintering step can rapidly increase the density of the samples in a short time. During the second step sintering process, little exceeding the liquidus temperature can further increase the density of the sample, but little lower than the liquidus temperature will lead to a sharp increase in grain size. The sample after sintering at 680 °C for 30 min then sintering at 610 °C for 6 h shows the highest relative density (0.995), the highest strength (394 MPa), the lowest corrosion rate (101 mm/year) and the smallest corrosion current density (60.4 μA/cm2). Compared with one-step sintering process, TTS process has higher sintering efficiency.

Heat Treatment of 17–4 PH Stainless Steel Produced by Binder Jet Additive Manufacturing (BJAM) from N2-Atomized Powder

Heat Treatment of 17–4 PH Stainless Steel Produced by Binder Jet Additive Manufacturing (BJAM) from N2-Atomized Powder

Binder Saturation as a Controlling Factor for Porosity Variation in 3D-Printed Sandstone

Binder jet additive manufacturing (BJ-AM) is a rapidly evolving 3D printing technique where the access to an array of powder materials is expanding. The use of silica sand has grown in popularity within the BJ-AM sector and has been shown to have a high potential of replicating physiochemical properties of natural materials for geoengineering applications. Consistent porosity is critical for 3D-printed samples used in rock testing since homogeneity between samples would provide unlimited capabilities in a laboratory setting. Binder saturation is one of the key user-set parameters that controls the ratio between the dimensional tolerance and porosity. Nonetheless, the binder saturation is an internal calculation by the printer’s software that relies on several assumptions, where the most important physical aspect is droplet spacing. This study establishes relationships between the droplet spacing, dimensional tolerance, binder saturation, and porosity. By holding the droplet volume constant and changing its spacing, better control of saturation was observed. Higher saturation reduced porosity and increased circularity of cylindrical samples, but overall dimensional tolerance of fine features was reduced. This study provides improvements of 3D-printed rock for the representation of porosity and geomechanical properties observed in natural sandstones.

Impact of inkjet printing parameters and environmental conditions on formation of 2D and 3D binder jetting geometries

While Binder Jet Additive Manufacturing (BJAM) has great potential, its implementation is limited by defects in the finished parts. An improved understanding of how printing parameters impact the quality of the printed parts is needed. Prior work on the droplet/powder interactions in BJAM printing focused on individual lines, this work shows that successful layer formation favors larger droplet spacing than is viable in individual lines. When printing layers, the first printed layer was significantly rougher than the spread bed indicating significant powder ejection. However, this difference was eliminated after printing 1–4 additional layers. These results show that prior printed layers have a strong impact on droplet impact and imbibition and that simple droplet or line geometries are not effective for testing printing parameters. The roughness of the first layer may contribute to large pores observed between layers. This paper further examines the impact of key printing parameters including layer thickness, droplet/line spacing, and droplet inter-arrival time on the effective saturation and surface roughness of 2D and 3D parts. The droplet inter-arrival time (print frequency) had negligible impact on surface roughness and saturation under the conditions tested. Printed layers behaved similarly at droplet spacing values comparable to the droplet diameter (44 μm). In contrast, droplet spacing of 60 μm, produced substantially lower saturation. Effective saturation generally increases with increased number of layers. Ambient humidity exposure during printing is shown to have a negligible impact on printing outcomes though higher moisture levels from steam exposure dramatically alter saturation. Drying powder before spreading reduced variation in the printed parts. All tests were based on gas atomized −22 μm 316 SS powder.

Additive manufacturing of strong silica sand structures enabled by polyethyleneimine binder

Binder Jet Additive Manufacturing (BJAM) is a versatile AM technique that can form parts from a variety of powdered materials including metals, ceramics, and polymers. BJAM utilizes inkjet printing to selectively bind these powder particles together to form complex geometries. Adoption of BJAM has been limited due to its inability to form strong green parts using conventional binders. We report the discovery of a versatile polyethyleneimine (PEI) binder for silica sand that doubled the flexural strength of parts to 6.28 MPa compared with that of the conventional binder, making it stronger than unreinforced concrete (~4.5 MPa) in flexural loading. Furthermore, we demonstrate that PEI in the printed parts can be reacted with ethyl cyanoacrylate through a secondary infiltration, resulting in an increase in flexural strength to 52.7 MPa. The strong printed parts coupled with the ability for sacrificial washout presents potential to revolutionize AM in various applications including construction and tooling.

Binder Jetting of Silicon Steel, Part I: Process Map of Green Density

Abstract This study focuses on developing and demonstrating a straightforward workflow for identifying pathways to increase green part density in binder jetting additive manufacturing (BJAM) using statistically driven process maps. The workflow was applied to investigate the effects of process parameters toward improving green part density, with a direct application in manufacturing of Fe-Si components. Specifically, a half-factorial experimental design was used to study the effects of four key parameters—layer thickness, powder spreading speed, roller rotational speed, and binder saturation—on Fe-Si spherical powder with D50 of 32.40 µm. Relative bulk density was estimated via three methods: geometrical and mass measurements, the Archimedes test, and CT imaging. The study discusses relative bulk density as well as localized density variation in the printed parts, which is attributed to both parameter selection and inherent process variability. A regression analysis was used to reveal the significance of main effects and second-order interactions. The regression model (R2 = 0.915) was used to derive an expression for green density as a function of the parameters and had a prediction error of 0.96%. Based on the regression model, an optimized set of parameters was obtained that would maximize green density up to 57.96% for the machine and material system.

A laboratory-scale binder jet additive manufacturing testbed for process exploration and material development.

Binder jet additive manufacturing (BJAM) is capable of fabricating complex three-dimensional components from a variety of material classes. Understanding the fundamentals of BJAM, including spreading of thin layers of powder, powder-binder interactions, and post-processing is critical to develop robust process parameters for BJAM. Toward meeting these needs, this work presents the design, fabrication, and qualification of a testbed for modular, mechanized, BJAM. The testbed seeks to replicate the operating conditions of commercial AM equipment and features fully programmable motion control including powder spreading using a precision roller mechanism, powder supply via a vibrating hopper, and gantry positioning of an inkjet printhead. The inkjet deposition system allows for the use of variable nozzle diameters, the exploration of novel binder compositions, and full control of jetting parameters. Validation of the accuracy and repeatability of the machine and its subsystems, as well as the fabrication of exemplary stainless steel components, are described. The precision engineered testbed can therefore enable the study of the BJAM process, exploration of novel binder compositions, and processing of custom powders to further scientific research and industrial applicability of BJAM.

Irradiation stability and thermomechanical properties of 3D-printed SiC

Neutron irradiation tests were carried out on 3D-printed SiC derived from binderjet additive manufacturing and chemical vapor infiltration. Irradiation was carried to 2.3 dpa over a temperature range of 400–850 °C. Anisotropy that had been observed in the thermal conductivity of 3D-printed SiC prior to irradiation vanished after irradiation as the irradiation defect thermal resistivity accumulated in the material. No degradation in strength was observed in the material before or after irradiation, at various temperatures, or in different orientations. Electron microscopy of the microstructure after neutron irradiation showed distinct defect morphologies in the heterogenous material, but no evidence for irradiation-induced cracking or degradation in the microstructure was observed.

Binder jetting of “Hard-to-Weld” high gamma prime nickel-based superalloy RENÉ 108

Abstract The ultra-high strength of high γ’ nickel (Ni)-based superalloys with low ductility comes with the price of high susceptibility to cracking and a high residual stress state when processed with conventional powder bed fusion technologies. Binder jet additive manufacturing (BJAM) is an interesting alternative to other additive manufacturing (AM) modalities since it does not require melting to fuse powder material but selectively adheres powder with a binder. While there are several studies on the use of melting and solidification-based AM technologies, limited literature is available on the use of BJAM for high γ’ Ni-based superalloys. This study presents the advantages and disadvantages associated with the BJAM of RENE 108 superalloy. The BJAM microstructure is characterized in the green, brown and sintered states. The formation of parallel primitive lines perpendicular to the build direction is observed after printing. This leads to persistent lines of porosity and a relative density above 96% after a sintering cycle. A fully dense microstructure is obtained after hot isostatic pressing (HIP) of the material. The formation of HfO2 and Ta-rich carbides along prior particle boundaries (PPBs) is also observed during the sintering stage. Precipitation along PPBs is intrinsic to the powder metallurgy of Ni-based superalloys and influenced by the debinding conditions of the parts. They have a strong impact on the mechanical properties of the material. At low temperatures ( 760 °C), the deformation becomes intergranular, and fracture occurs at the PPBs. The ductility of BJAM RENE 108 becomes slightly lower than the DS material while the strength of the materials remains comparable.

High Strength and Rapid Solution Mg Alloy by Adding Fe Element Fabricated by Binder Jetting Additive Manufacturing

High Strength and Rapid Solution Mg Alloy by Adding Fe Element Fabricated by Binder Jetting Additive Manufacturing

Influence of Particle Diameter on Sand Flowability in Binder Jetting Additive Manufacturing Process Using Atomized Artificial Sand

Influence of Particle Diameter on Sand Flowability in Binder Jetting Additive Manufacturing Process Using Atomized Artificial Sand

Binder jetting additive manufacturing of 316L stainless-steel green parts with high strength and low binder content: Binder preparation and process optimization

• A self-developed binder with A-stage phenolic resin as a key agent was prepared. • High strength 316 L green part with low binder saturation was fabricated by BJAM. • The uneven distribution of binder hinders the strength of green parts. Recently, binder jetting additive manufacturing (BJAM) was used to manufacture complex metal parts in an economical and large-scale way. However, the usage of resin to bond metal powder renders as-printed green parts with low strength, which is potential to cause the structural damages of final parts during post treatments. To overcome the intractable problem, a self-developed binder mainly consisting of A-stage phenolic resin was explored for BJAM to achieve relatively high-strength as-printed green parts. The binder can be cured as a C-stage phenolic resin with 190 ℃ and then provide a high strength for the green parts. The effect of layer thickness and binder saturation on the strength, dimensional accuracy, and surface roughness of the BJAM-printed 316 L stainless-steel green samples using the developed binder was investigated systematically. With the optimized parameters (a layer thickness of 100 μm and a binder saturation of 30.2 %), green samples can possess the ultimate bending strength (UBS) of 10.08 MPa and the ultimate compressive strength (UCS) of 25.56 MPa, which are beneficial for the structural stability of the printed samples during subsequent treatments. A reduction in strength and an increase in roughness are observed with increasing the layer thickness for the as-printed green samples. An increment in binder saturation results in the improvement in strength and surface roughness of the green samples. The relationships between the binder and the mechanical performance of the green samples were discussed from the viewpoint of the uneven distribution of binder in depth. This work explored a promising binder with a low saturation for BJAM to fabricate 316 L green parts with desirable mechanical strength and acceptable surface roughness.

Influence of droplet velocity, spacing, and inter-arrival time on line formation and saturation in binder jet additive manufacturing

Binder Jetting (BJ) is a low-cost Additive Manufacturing (AM) process that uses inkjet technology to selectively bind particles in a powder bed. BJ relies on the ability to control, not only the placement of binder on the surface but also its imbibition into the powder bed. This is a complex process in which picoliter-sized droplets impact powder beds at velocities of 1–10 m/s. However, the effects of printing parameters such as droplet velocity, size, spacing, and inter-arrival time on saturation level (fraction of pore space filled with binder) and line formation (merging of droplets to form a line) are unknown. Prior attempts to predict saturation levels with simple measurements of droplet primitives and capillary pressure assume that droplet/powder interactions are dominated by static equilibrium and neglect the impact of printing parameters. This study analyzes the influence of these parameters on the effective saturation level and conditions for line formation when printing single lines into powder beds of varied materials (316 stainless steel, 420 stainless steel, and alumina) and varied particle size (d50=10–47 µm). Results show that increasing droplet velocity or droplet spacing decreases effective saturation while droplet spacing, velocity, and inter-arrival time affect line formation. At constant printing velocity, the conditions for successful line printing are shown to be a function of droplet spacing and square root of the droplet inter-arrival time analogous to the Washburn model for infiltration into a porous media. The results have implications to maximizing build rates and improving quality of small features in BJ.

3D Printed Minerals as Astrobiology Analogs of Hydrothermal Vent Chimneys.

Hydrothermal vents, which are highly plausible habitable environments for life and of interest for some origin-of-life scenarios, may exist on icy moons such as Europa or Enceladus in addition to Earth. Some hydrothermal vent chimney structures are extremely porous and friable, making their reconstruction in the lab challenging (e.g., brucite or saponite in alkaline hydrothermal settings). Here, we present the results from our efforts to reconstruct a simplified chimney structure directly out of mineral powder using binder jet additive manufacturing. Olivine sand was chosen for this initial method development effort since it represents a naturally occurring seafloor material and is inexpensively available in large quantities in powder form. The crystal structure of olivine used for the print was not modified during the process, as confirmed by powder X-ray diffraction (XRD). To characterize the microstructure of our 3D printed precipitates, we used computed tomography (CT) X-ray scan techniques. We also evaluated a chimney precipitate from a sample collected from the Prony Hydrothermal Field (PHF), southern New Caledonia, an alkaline system driven by serpentinization with mineralogy composed of brucite and carbonates. While not directly comparable from a mineralogical point of view, the microstructure and porosity of both precipitates was similar, suggesting that our 3D printing technique may be a valuable tool for future astrobiology research on hydrothermal vent precipitates.

Effect of fine powder particles on quality of binder jetting parts

Abstract Despite their desired effects on quality metrics of powder metallurgy parts, fine powder materials are rarely used in powder-based additive manufacturing as powder feedstock materials due to their poor flowability and inefficient layer recoating. As such, process-structure-property relationships of powder-based AM processes have been explored primarily for coarse powder particle sizes (i.e., 25 μm–150 μm in diameter). With the new developments in powder recoating systems in modern binder jetting additive manufacturing (BJ-AM) printers, it seems now feasible to process fine powder materials that have average particle size of ∼10 μm (or smaller). In the current research, the use of fine copper powders (average particle size of 5 μm) in BJ process and its effects on green and final part properties are experimentally investigated. Specially, the authors studied the effects of different powder recoating settings on the density of printed green parts. The density of the sintered parts was also explored for various sintering parameters (i.e., heating rate and peak sintering temperature). Linear/volumetric shrinkage, microstructure and mechanical characteristics of sintered specimens were explored, and the results were compared to those of copper specimens made via coarse powder materials. The results indicated that fine copper powder resulted in parts with properties (UTS of 179.4 MPa and elongation of 42.2 %) greater than bimodal powder parts, which in turn eliminates extra time and work needed for powder mixing ratio optimization and blending process.

Microstructure evolution during binder jet additive manufacturing of H13 tool steel

H13 is one of the most used tool steels for both hot and cold work tooling applications. Binder jet additive manufacturing offers the potential to deposit complex tools at scale due to larger powder bed sizes and faster deposition rates. However, to date there is no published literature on the sintering of H13 to full densification. In this paper, we discuss the pressureless sintering of binder jet AM H13 steel to full densification via supersolidus liquid phase sintering (SLPS) while presenting appropriate process windows (1360 °C – 1380 °C) for densification without distortion. The process windows have been rationalized based on thermodynamic calculations of liquid volume fractions with temperature. We show that higher binder saturation results in higher carbon retention and subsequently early liquid formation that can initiate the sintering process at lower temperatures. We report abnormal grain growth during sintering and found that the solidification phase transformations play a critical role on microstructural evolution and must be considered to accurately model the kinetics of SLPS.