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Droplet-based manufacturing (DBM) process has great potential in directly fabricating metal parts. In order to enhance the practicability of DBM, decreasing droplet size becomes the research focus in this field. In the present work, a novel pneumatic-driven droplet generator structure was proposed and the mechanism of generating aluminum droplets smaller than the nozzle diameter was studied. By conducting simulation experiments, the effects of structure parameters on droplet formation process and droplet size were investigated. The results indicated that the aspect ratio of the nozzle hole (i) and the distance between inlet hole and nozzle hole (H 1) had significant effects on droplet size while the diameter of the inlet hole had little effect. Droplets smaller than the nozzle diameter could be produced only when i and H 1 were 1 and 1 mm, respectively. Based on the optimized structure parameters, a generator was developed and droplet-ejecting experiments were carried out. By using a nozzle with the diameter of 500 μm, aluminum droplets with the mean diameter of 359.9 μm were produced stably and then a rectangular tube was fabricated by depositing droplets sequentially. The results demonstrated the capability of the novel generator to produce small droplets and to fabricate parts directly.

7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interfacial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an extruded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.

Non-isothermal deposition of uniform molten droplets as basic building blocks has a great influence on the geometric profile and microstructure of metallic components fabricated by the drop-based three-dimensional (3D) printing technology. In this paper, the thermal and dynamic behaviors of molten aluminum droplets during non-isothermal deposition were studied numerically and experimentally. The result shows that local solidification and interfacial re-melting occur during the initial period of non-isothermal deposition. The re-melting in microseconds depends greatly on the impacting droplet temperature, the deposition surface temperature, and the thermal contact resistance. Further, the coupling action of subsequent solidification and oscillation behaviors of aluminum droplet fixed on the target surface was also investigated. It is interesting to find that the formation and distribution of the solidified surface morphology, such as the typical micron-sized ripples, are significantly affected by layer-by-layer solidification and underdamped oscillation in the remaining molten metal. Based on the above research, a semiquantitative relationship between external morphology and internal microstructure was proposed, which was further certified by investigating the piled vertical columns. The works should be helpful for the process optimization and non-destructive detection of drop-based 3D printing techniques.

Thermal accumulation in micro droplet deposition manufacturing (MDDM) has a significant influence on geometric profile and microstructure of the fabricated metal micro-parts. In this paper, thermal behavior of a new aluminum droplet on the deposit surface was investigated using one-dimensional heat transfer model. Then several thin-walled aluminum cubic pipes were fabricated by MDDM to verify the numerical analysis result. The result shows that the thermal accumulation would increase gradually with the increase of the deposit height. It associated with thermal input and output on the top surface of the deposit, which could be controlled or eliminated by optimizing processing parameters such as deposition frequency.

The deposition of uniform metal droplets has attracted extensive interest for potential application in rapid prototyping and manufacturing. In this paper, a three-dimensional model based on a volume of fluid (VOF) method was developed to investigate the successive deposition of molten Al droplets on a horizontally moving substrate. In this model, protecting gas surrounding the droplets was treated as a void region, and then the droplets were considered as a single-phase fluid. The spreading, cooling and solidification processes of droplets were investigated under different substrate velocities. The simulated final morphology of droplets agreed well with experiments carried out under the same conditions. It was found that the molten Al droplets solidified in a layer by layer mode due to the high heat conductivity. A series of L-shaped ridges appeared on the surface of solidified droplets, which resulted from the integrated effects from solidification and oscillation consisting of alternately spreading and recoiling of droplets.