Mono-disperse Aluminum Droplet Generation and Deposition for Net-Form Manufacturing of Structural Components

This paper focuses on molten aluminum and aluminum alloy droplet generation for application to net-form manufacturing of structural components. The mechanism of droplet formation from capillary stream break-up provides the allure for use in net-form manufacturing due to the intrinsic uniformity of droplets generated under proper forcing conditions and the high rates at which they are generated. Additionally, droplet formation from capillary stream break-up allows the customization of droplet streams for a particular application. The current status of the technology under development is presented, and issues affecting the microstructure and the mechanical properties of the manufactured components are studied in an effort to establish a relationship between processing parameters and properties. ∗  High precision droplet-based net-form manufacturing of structural components is gaining considerable academic and industrial interest due to the promise of improved component quality resulting from rapid solidification processing and the economic benefits associated with fabricating a structural component in one integrated operation. A droplet based net-form manufacturing technique is under development at UCI which is termed Precision Droplet-Based Net-Form Manufacturing (PDM). The crux of the technique lies in the ability to generate highly uniform streams of molten metal droplets such as aluminum or aluminum alloys. Though virtually any Newtonian fluid that can be contained in a crucible is suitable for the technology, this work concentrates on the generation and deposition of molten aluminum alloy (2024) droplets that are generated and deposited in an inert environment. Figure 1 is a conceptual schematic of the current status of PDM. Droplets are generated from capillary stream break-up in an inert environment and are deposited onto a substrate whose motion is controlled by a programmable x-y table. In this way, tubes with circular, square, and triangular cross sections have been fabricated such as those illustrated in Figure 2. Tubes have been fabricated with heights as great as 11.0 cm. The surface morphology of the component is governed by the thermal conditions at the substrate. If we denote the solidified component and the substrate the "effective substrate", then the newly arriving droplets must have sufficient thermal energy to locally remelt a thin layer (with dimensions on the order of 10 microns or less) of the effective substrate. Remelting action of the previously deposited and solidified material will insure the removal of individual splat boundaries and result in a more homogeneous component. The thermal requirements for remelting have been studied analytically in reference . It was shown in that work that there exists a minimum substrate temperature for a given droplet impingement temperature that results in remelting. The "bump iness" apparent in the circular cylinder shown in Figure 2 is due to the fact that the initial substrate temperature was insufficient to initiate the onset of remelting. As the component grows in height by successive droplet deliveries, the effective substrate temperature increases due to the fact that droplets are delivered at rates too high to allow cooling before the arrival of the next layer of droplets. Therefore, within the constraints of the current embodiment of the technology, there exists a certain height of the component for which remelting will occur. This height is demarcated at the location where the "bumpiness" is eliminated and relative"smoothness" prevails, as can be seen in the circular cylinder. As the component grows beyond this height, the remelting depth will continue to increase due to increased heating to the effective substrate. Hence, the component walls will thicken due to slower solidification rates. The objective of Eighth International Conference on Liquid Atomization and Spray Systems, Pasadena, CA, USA, July 2000 ongoing work (not presented here) is to identify the heat flux required for the minimum remelting of the effective substrate, and to develop processing conditions for which this heat flux seen by the substrate remains constant for each geometry desired. In this manner, the fidelity of the microstructure, mechanical properties, and geometry will remain intact. As is evident from Figure 1, the research presented in this work did not employ electrostatic charging and deflection. However, in the final realization of the technology, charging and deflection will be utilized in order to control the droplet density as a function of the component geometry, or to print fine details at high speed and at high precision. The charging and deflection of droplets bears many similarities to the technology of ink-jet printing, except that in the current application of PDM, large lateral areas are printed, thereby requiring significantly higher droplet charges than in ink-jet printing. The high charges on the closely spaced droplets result in mutual inter-droplet interactions that are not apparent in the application of ink-jet printing. Recent experimental and numerical results on the subject of droplet interactions due the application of high electrostatic charges are presented elsewhere . Though not yet utilized for net-form manufacturing of structural components, droplet charging and deflection has been successfully applied to the "printing" of electronic components such as BGA's (Ball Grid Arrays). These results can be found in reference . Research on controlled droplet formation from capillary stream break-up over the past decade has enabled ultra-precise charged droplet formation, deflection, and deposition that makes feasible many emerging applications in net-form manufacturing and electronic component fabrication [4-9]. Unlike the Drop-on-Demand mode of droplet formation, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second, from capillary stream break-up and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ± 12.5 μm. Other net-form manufacturing technologies that rely on uniform droplet formation include 3D Printing (3DP) [10-12] and Shape Deposition Manufacturing (SDM) [13-15]. In 3DP, parts are manufactured by generating droplets of a binder material with the Drop-on-Demand mode of generation and depositing them onto selected areas of a layer of metal or ceramic powder. After the binder dries, the print bed is lowered and another layer of powder is spread in order to repeat the process. The process is repeated until the 3-D component is fabricated. Like PDM, the process of SDM relies on uniform generation of molten metal droplets. However the droplet generation technique is markedly different than droplet generation from capillary stream  Examples of preliminary comp onents fabricated with PDM. The tall square tube shown horizontally is 11.0 cm.  Conceptual schematic of cylinder fabrication on a flat-plate substrate with controlled droplet deposition. Molten droplet