Powder Bed Fusion Processes
Powder bed fusion (PBF) processes were among the first commercialized AM processes. Developed at the University of Texas at Austin, USA, Selective Laser Sintering (SLS) was the first commercialized powder bed fusion process. Its basic method of operation
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Powder Bed Fusion Processes
5.1
Introduction
Powder bed fusion (PBF) processes were among the first commercialized AM processes. Developed at the University of Texas at Austin, USA, Selective Laser Sintering (SLS) was the first commercialized powder bed fusion process. Its basic method of operation is schematically shown in Fig. 5.1, and all other PBF processes modify this basic approach in one or more ways to enhance machine productivity, enable different materials to be processed, and/or to avoid specific patented features. All PBF processes share a basic set of characteristics. These include one or more thermal sources for inducing fusion between powder particles, a method for controlling powder fusion to a prescribed region of each layer, and mechanisms for adding and smoothing powder layers. The SLS process was originally developed for producing plastic prototypes using a point-wise laser scanning technique. As described in this chapter, this approach has been extended to metal and ceramic powders; additional thermal sources have been utilized; and variants for layer-wise fusion of powdered materials now exist. As a result, PBF processes are widely used world-wide, have a broad range of materials (including polymers, metals, ceramics and composites) which can be utilized, and are increasingly being used for direct digital manufacturing of end-use products, as the material properties are comparable to many engineeringgrade polymers, metals, and ceramics.
5.2
SLS Process Description
In order to provide a baseline description of powder fusion processes, Selective Laser Sintering will be described as the paradigm approach to which the other powder bed fusion processes will be compared. As shown in Fig. 5.1, SLS fuses thin I. Gibson, D.W. Rosen, and B. Stucker, Additive Manufacturing Technologies, DOI 10.1007/978-1-4419-1120-9_5, # Springer ScienceþBusiness Media, LLC 2010
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5 Powder Bed Fusion Processes
CO2 Laser
X-Y Scanning Mirrors Laser Beam
IR heater Counter-Rotating Powder Leveling Roller
Powder Bed
Feed Cartridges
Build Platform
Fig. 5.1 Schematic of the Selective Laser Sintering process
layers of powder (typically ~0.1 mm thick) which have been spread across the build area using a counter-rotating powder leveling roller. The part building process takes place inside an enclosed chamber filled with nitrogen gas to minimize oxidation and degradation of the powdered material. The powder in the build platform is maintained at an elevated temperature just below the melting point and/or glass transition temperature of the powdered material. Infrared heaters are placed above the build platform to maintain an elevated temperature around the part being formed; as well as above the feed cartridges to pre-heat the powder prior to spreading over the build area. In some cases, the build platform is also heated using resistive heaters around the build platform. This pre-heating of powder and maintenance of an elevated, uniform temperature within the build platform is necessary to minimize the laser
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