Figure 1a shows the 3D scheme of the BJM3DP system, which has four main components: an inkjet cartridge, powder gantry boxes, a roller, and x- and y-axes stages. Prior to the 3D printing process, the powder and inkjet cartridge were prepared as follows (see Supplementary Fig. 1 for photographic images of each component). First, the metal powder was mixed with the chelator. Here, the nozzle clogging issue was prevented by avoiding use of a generally preferred method of jetting a binding agent solution, which precipitates inside the inkjet cartridge and clogs the printing nozzle. Next, two gantry boxes were filled with the mixture of the metal powder and chelator. Each of these gantry boxes has a different purpose. One is the builder gantry box in which objects are 3D-printed, and the other is the feeder gantry box that stores and supplies the powder to the builder gantry box. Then, the inkjet cartridge was filled with deionized (DI) water, which activated the chelation reaction. After the preparation process, the printing cycle was initiated through deposition of a powder layer on top of the builder gantry box. During the printing process, the builder platform moved one step downward to provide space for one powder layer and the feeder platform simultaneously moved one step upward to push up the powder. Then, the roller module positioned at the feeder gantry box moved toward the builder gantry box to supply powder and flatten powder protruding on top of the latter box, as shown in Figure 1b. Once a powder layer was deposited, the module returned to its original position. Subsequently, DI water was jetted from the inkjet cartridge onto the powder layer deposited on top of the builder gantry box at programmed positions (Figure 1c). This cyclic process was repeated until the uppermost layer of the designed 3D object was deposited. Upon completion of the printing cycle, the 3D-printed object was removed from the pile of metal powder and the unchelated powder was subsequently removed through air blowing. Photographic images of the overall metal 3D printing process are shown in Supplementary Fig. 2.
As binding agents, nature-based chelators, which are crucial to ensure eco-friendliness of metal 3D printing, were used. Unlike polymer binding agents, which are hazardous, the chelators utilized here are food-grade organic materials because they are salts of nature-based fruit acids (e.g., of fruits such as lemons, cherries, and grapes)37. For example, citric acid derived from citrus fruits has three carboxyl groups, and it transforms into sodium citrate upon replacement of the hydrogens in these groups with sodium ions through a salt-formation reaction (Figure 1d)38. Here, the carboxyl group of sodium citrate plays an important role in the metal chelation reaction. Upon wetting of the uniform mixture of the metal powder and chelator (Figure 1e), metal chelation occurs on the surface of the metal particles, which induces the formation of metal-chelate bridges between the particles. Figure 1f and Supplementary Fig. 3 show the chemical structure of metal-chelate bridges formed on metal particle surface and the change in the microstructure of Al powder. Successful chelation imparts structural integrity to the 3D-printed object and its architecture is consequently maintained, as a result of which the object has a precise and sophisticated shape, as shown in Figure 1g.
Formation of metal-chelate bridges between metal particles.
Figure 2a shows the mechanism of chelate complex formation between Al particles and the underlying chemical reaction. When water droplets are jetted onto Al powder, water permeates between the particles and gradually dissolves the chelator. Then, the ionized chelator solution preferentially attacks intrinsic defects in the Al particles, consequently producing Al-chelate compounds, which results in bridging of the Al particles. Figure 2b shows FT-IR spectra of Al objects printed using the following proposed chelators: sodium salts of citrate (NaCit), tartrate (NaTar), succinate (NaSuc), and ascorbate (NaAsc). These four chelators have three, two, two, and one coordinate donor sites, respectively, and the difference in the number of coordination sites affects the formation of metal-chelate bridges. In the spectra of the carboxyl-based chelators (NaCit, NaTar, and NaSuc), two major bands, which originate from the asymmetric stretching vibration (νas(COO-)) and symmetric stretching vibration (νs(COO-)) of the carboxylate group (COO-), are present in the frequency ranges of 1540–1720 cm-1 and 1320–1470 cm-1, respectively39. In the spectra of NaCit, νas(COO-) at 1592 cm-1 and νs(COO-) at 1393 cm-1 are blue-shifted by 4 cm-1 and red-shifted by 11 cm-1, respectively, after chelation. The increase in the molecular mass due to the chelation of the carboxylate group and Al causes a change in the vibration frequency, which results in a band shift40. The peak shifts of νas(COO-) and νs(COO-) thereby indicate the formation of the chelate complex on the Al particle surface (the FT-IR spectrum of pristine Al powder in the same frequency range is shown in Supplementary Fig. 4). The separation of peaks (∆ν = νas - νs) was further analyzed to identify the difference in the types of coordination between Al and the carboxyl-based chelators41. Because NaCit, NaTar, and NaSuc chelated with Al have a lower value of ∆ν(COO-) than do the chelators themselves (Supplementary Table 1), these chelators coordinate with Al in the bidentate chelating form, which enables a single metal atom to have two bonds with a carboxylate group, as illustrated in the left panel of Figure 2b. Unlike with these carboxylate-based chelators, NaAsc forms only a single bond with the Al atom, as indicated by the non-split νC=O, the band broadening of νC-O, and the decreased intensity of the hydroxyl group after chelation42.
The XPS spectra of chelated Al provide information about the extents of chelate complex formation when the different chelators are used. The left panel of Figure 2c and Supplementary Fig. 5 show the Al 2p and O 1s spectra of the chelated and pristine Al powders, respectively. The deconvoluted Al 2p spectra have three distinct components: Al 2p3/2 (71.6 eV), Al 2p1/2 (72.0 eV), and Al‒O (74.2 eV). The Al‒O peak originates from the oxides and hydroxides formed on the Al surface43. As depicted in Figure 2d, the four types of chelated Al show a larger Al‒O/Al‒Al atomic ratio than the pristine Al particles. Furthermore, the atomic ratio increases as the number of coordination bonds increases. These results indicate that the chelators play a pivotal role in Al‒O formation, and greater the number of coordination bonds, higher is the thickness of the Al‒O layers formed on the Al particle surface, as observed in the SEM images (right panel of Figure 2c). This tendency of formation of a thicker Al‒O layer is also confirmed from the O 1s spectra. The O 1s peak has three components, which correspond to O‒Al, HO‒Al, and the chemisorbed water and are positioned at 530.7, 531.8, and 532.4 eV, respectively44. As can be seen from the plot in Figure 2d, the relative amount of O‒Al increases as the number of coordination bonds increases; this trend reveals that a chelator with more coordination sites is favorable for the formation of the Al-chelate complex. The promoted chelation results in stronger bonding between the metal particles, and therefore, increases the mechanical strength of the 3D-printed objects (Figure 2e). The object printed using NaCit has the highest compressive strength (0.88 MPa) and compressive modulus (40.66 MPa). The outstanding mechanical properties of this 3D-printed object can be explained by the fact that coordinate donor sites contribute to the strength of the 3D-printed object. Changes in the atomic ratio and mechanical properties with the type of chelator because of the difference in the number of coordination groups are depicted in Supplementary Fig. 6. NaCl was subsequently added to release more metal ions from the intrinsic defects in the Al particles and increase the binding strength of the printed object. Upon the addition of NaCl, the chloride ions attacked the Al defects to form Al ions, which, in turn, promoted chelation on the surface of the metal particles in an aqueous environment (Supplementary Fig. 7)45. The enhancement of chelation through NaCl addition was also confirmed by SEM, FT-IR spectroscopy, and XPS (Supplementary Fig. 8 and Supplementary Fig. 9), all of which revealed numerous metal-chelate bridges. The increase in the density of metal-chelate bridges, in turn, led to an increase in the compressive modulus to 68.36 MPa; this is a 1.7-fold increase compared to that of the 3D-printed object not treated with the NaCl additive, as shown in Supplementary Fig. 10.
Various objects 3D-printed using NaCit chelator.
To demonstrate the elaborateness of our developed BJM3DP system, we designed and 3D-printed various 3D structures, as shown in Figure 3a and Supplementary Fig. 11. The 3D printing resolution is an important parameter in the fabrication of objects with sophisticated geometries, such as a gyroid cell, an impeller, an ammonite shell, and a skull. A square-plate-shaped object suitable for measurement of layer thickness was 3D-printed to verify the smallest possible feature size of our 3D printing system. Theoretically, a layer thickness 2–3 times the powder size provides good powder flow and spreadability46,47. In our system, coarse Al powder (<355 μm) was used and the minimum thickness of the 3D-printed object was about 1 mm, as indicated by the thin structures in Figure 3b and Supplementary Fig. 12. In addition to the 3D printing resolution, the bleeding issue—a phenomenon in which the binding agent solution flows out of the printed object—should also be addressed to ensure high printing quality24. The use of water as a solvent for the binding agent prevents this issue: it has higher vapor pressure than conventional binding agent solutions, because of which it evaporates faster and therefore prevents the solution from flowing out of the prescribed structure. Consequently, use of water ensured high dimensional accuracy of the 3D-printed objects in this study”s” and “O-Al / O” as “O-ntered.ir abbreviations used iwthin the body of the manuscript.,(for example, rewrite "f acceptable., as shown in Figure 3c. Figure 3d shows a photographic image of an as-printed 3D object and its cross-section. The high packing density of the powder by itself not only enables the 3D object to retain its shape but also promotes the solidification of the structure during the subsequent post-treatment process, thereby providing structural integrity.
Post-treatment process and several 3D-printable metals.
The post-treatment process performed for strength improvement in the standard metal BJM3DP technique can also be applied to the chelation-based metal 3D printing technique proposed herein. The post-treatment consists of a two-step process: humidification followed by thermal sintering. First, the unreacted chelator between particles was fully reacted through humidification (see Supplementary Fig. 13 for the SEM image of the humidified 3D-printed object). Subsequent thermal sintering caused solid-state diffusion of metal particles, which led to a decrease in the interparticle porosity, as shown in Supplementary Fig. 14. In other words, the metal particles came into closer contact with their neighboring particles, as shown in Figure 4a. Consequently, the compressive strength and compressive modulus of the thermally sintered 3D-printed objects improved by factors of 4.0 and 4.7, respectively, compared to those in the as-printed state (Figure 4b). However, because of the issue of low intrinsic porosity in the BJM3DP technique, which arises from constrained sintering of coarse particles, the mechanical strength of the printed object is typically lower than that of objects printed by other 3D printing techniques22. Therefore, additional strategies to improve strength through lowering of interparticle porosity are required. It is possible to increase the packing density by using powder with an optimized particle size distribution48. In general, the use of bimodal powder mixtures having an appropriate coarse-to-fine particle size ratio enables the fine particles to fill the interstitial voids between the coarse particles. Moreover, molten metal can readily infiltrate the interparticle pores of the printed object through the capillary force after the sintering process49. These strategies are expected to contribute to improvement in the mechanical properties of printed objects through minimization of the final porosity. Figure 4c shows objects 3D-printed using copper (Cu), iron (Fe), and a titanium–aluminum–vanadium alloy (Ti–6Al–4V). The successful 3D printing using various metals demonstrates that our developed 3D printing system is universally applicable to a wide variety of elemental metals and their alloys.