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Laser Powder Forming (LPF) technologies such as Laser Engineered Net ShapingTM [LENS ®]* and others are gaining in importance. The strength of these technologies lies in their ability to fabricate fully-dense metal parts with good metallurgical properties at reasonable speeds. The work was initiated at several government and university laboratories, both in the US and in Europe, and several variations on the technique have been explored. The methods are also sometimes referred to by the general term, laser fusing. Fig. 8 shows the approach adopted by Sandia National Labs and commercialized by Optomec. POM Group, Accufusion Inc., and other companies provide similar processes.
A high power laser is used to melt metal powder supplied coaxially to the focus of the laser beam through a deposition head (C). The laser beam typically travels through the center of the head and is focused to a small spot by one or more lenses (B). The X-Y table (D) is moved in raster fashion to fabricate each layer of the object. Typically the head is moved up vertically as each layer is completed. The laser beam may be delivered to the work by any convenient means. A simple right angle mirror (E) is shown, but fiber optics could also be used. Metal powders (A) are delivered and distributed around the circumference of the head either by gravity, or by using an inert, pressurized carrier gas (G). Even in cases where it’s not required for feeding, an inert shroud gas (F) is typically used to shield the melt pool from atmospheric oxygen for better control of properties, and to promote layer to layer adhesion by providing better surface wetting. A variety of materials can be used such as stainless steel, Inconel, copper, aluminum etc. Of particular interest are reactive materials such as titanium. Most systems use powder feedstocks, but there has also been work done with material provided as fine wires. In this case the material is fed off-axis to the beam. Materials composition can be changed dynamically and continuously, leading to objects with properties that might be mutually exclusive using classical fabrication methods. The building area is usually contained within a chamber both to isolate the process from the ambient surroundings and to shield the operators from possible exposure to fine powders and the laser beam. The laser power used varies greatly, from a few hundred watts to 20KW or more, depending on the particular material, feed-rate and other parameters. Objects fabricated are near net shape, but generally will require finish machining. They are fully-dense with good grain structure, and have properties similar to, or even better than the intrinsic materials. Laser powder forming has fewer material limitations than SLS, doesn't require secondary firing operations as some of those processes do, and can also be used to repair parts as well as fabricate them. Some typical applications are the fabrication and repair of injection molding tools and the fabrication of large titanium and other exotic metal parts for aerospace applications. The emphasis on these applications is partly due to the fact that support structures for overhanging sections generated by the technology are fully dense and hard to remove. These applications don't need to address that problem. Work is ongoing to find a more convenient means of generating supports.
* LENS ® and Laser Enginered Net Shaping (TM) are registered trademarks of Sandia National Labs. and Sandia Corp.
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