Methods of rapid prototyping that can be used directly to fabricate metal objects include selective laser sintering, shape deposition, laser deposition, and droplet deposition. Droplet deposition is under development at the University of Southern California and also at a company called Incre. These techniques use electronically controlled jets to selectively deposit molten metal. Since no apparent development of this process is being conducted in either Europe or Japan, no further discussion will occur in this report.
By using higher temperatures and higher-power lasers (>50 W), selective laser sintering can be used to create high-density metal layers. Fig. 6.6 shows a relatively high-density layer (about 125 µm) of Inconel 625 superalloy. This layer was formed in the SLS process with a Nd:YAG laser at 60 W of laser power. Research is underway at the University of Texas using this process to fabricate high-density metal parts with a variety of powdered metals and alloys.
Shape deposition (also described in Chapter 2) is under development at Carnegie Mellon and Stanford universities. Shape deposition manufacturing (SDM) first deposits layers as near-net shapes and then machines them to accurate dimensions before additional material is added. Fig. 6.7 shows a part produced by this process. The part does not have detailed features yet, but it is composed of two different materials: a stainless steel core and an outer shell with copper inlays (not visible in the figure). Multiple materials offer the promise of building integrated components in one operation.
Fig. 6.8 is a schematic of a laser deposition process being developed at Sandia National Labs; a similar process is under development at Los Alamos National Labs. This process consists of a powder delivery system and a laser beam; the powder delivery is designed to intersect with the laser delivery simultaneously, to build up objects in a layer-like fashion. In essence, the process consists of 3D laser welding. Fig. 6.9 shows objects created by Sandia's laser deposition process. They are made of almost 100%-dense 316 stainless steel.
Fig. 6.9. Stainless steel objects created by laser deposition process at Sandia.
As described in Chapter 2, lamination of laser-cut sheets can also be used as a rapid prototyping process. If the sheets are metal, then metal parts and molds can be fabricated by stacking and bonding these sheets. Professor Nakagawa at the University of Tokyo Institute of Industrial Science has been the primary researcher of this technique.