In today's highly competitive marketplace, it is of great significance for the manufacturing industry to reduce lead-time and costs for the product development. Since first emerged in 1986, as a new time-reduction method, Solid Freeform Fabrication (SFF) technology, also called Rapid Prototyping (RP), has helped successfully to address the challenges. To date, various direct metal SFF processes have been developed that are able to produce metal parts or molds directly from the CAD data by using metals as part material. Since 1998, some of them have been commercialized and many others are still in research pursuing commercialization. In comparison with the conventional manufacturing processes, they have unique capabilities stemming from the layer-by-layer addition of metal. They are capable of building injection molds with the conformal cooling channels that considerably reduce cycle time and improve part quality; and functionally gradient parts that comprise multiple metals; and smart structures that contain electronic devices at intended locations.
Despite such distinctive advantages, direct metal SFF processes have not yet met manufacturers' requirements completely due to dimensional inaccuracy, poor surface finish, long build time and stress-induced deformation. This is primarily because in most of the direct metal SFF processes, each layer undergoes overall phase change. The objective of this study is to develop a new direct metal SFF process, called Selective Infiltration Manufacturing (SIM), in which superheated microscopic metal droplets are 'selectively' infiltrated into a layer of microscopic metal powder that is preheated by a laser. During the entire process, the powders that serve as a base (or matrix) material remain solid and thus contribute to reduce thermal deformation and internal stress due to the accordingly reduced amount of molten metal at each layer.
First of all, to produce superheated μ -droplets of Sn-37Pb wt.%, infiltrant, a drop-on-demand generator was fabricated. It consists of a solenoid vibrator, a μ-drilled 130-mm diameter nozzle, an impulse-transmitting rod, and a tubular heating element. An experimental study of the effects of the dominant parameters on the droplet diameter was conducted. They include the superheating temperature of metal, the amplitude of the solenoid, and the dimensions of the rod's head end. The smallest droplets was 301 ± 10 mm in diameter at a temperature of 260℃, and 299 ± 12 mm at 290℃, and 301 ± 12 mm in diameter at 320℃.
Using the fabricated droplet generator, experiments on the selective infiltration of a single droplet were carried out with varying values of the dominant parameters, such as the superheating temperature $T_{sup}$, the substrate temperature $T_{sup}$, the substrate velocity vsub, and the mean laser power $P_M$ of a pulsed Nd:YAG laser. Results have shown that the selective infiltration occurred successfully under the following conditions: $T_{sup} = 260, 290℃, $P_M$ = 33 W, and $T_{sub} = 120℃.
To provide the design data, a numerical analysis of the selective infiltration of a single μ -droplet was carried out using CFD-ACE, a commercial code. With the optimal values of the dominant parameters obtained from the experiments, the analysis also yielded a successful selective infiltration. For computation, the temperature of the laser-preheated powders was measured by a K-type thermocouple on which a powder was bumped. Computational results have predicted the remelting of the substrate surface, which enables the analysis to help to design the SIM process before experiments.
Based on the conditions for the successful selective infiltration, several example SIM parts were infiltrated to demonstrate the feasibility and applicability of the SIM process as a new direct metal SFF process. They include the process initials (SIM), which possess various infiltration paths, rectangular and circular shell structures, and shell structures with inclined walls at the given certain angle. Results have shown that the remelting thickness is 15㎛ at $T_{sup}$ = 260℃ and 21 mm at 290℃. The average surface finish is 20㎛ $R_a$ and the density 7.82 g/㎤, 92.8% of cast Sn-37Pb whose density is 8.42 g/㎤.
