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Powder Bed Fusion
ISO/ASTM definition: “powder bed fusion, —an additive manufacturing process in which thermal energy selectively fuses regions of a powder bed.”
Powder Bed Fusion can also be known as (in alphabetical order):
➢ Direct Metal Laser Remelting or DMLR
➢ Direct Metal Laser Sintering or DMLS® (EOS GmbH)
➢ Direct Metal Printing or DMP (3D Systems Corporation)
➢ Electron Beam Additive Manufacturing or EBAM*‡
➢ Electron Beam Melting or EBM (Arcam AB)
➢ High Speed Sintering or HSS
➢ LaserCUSING® (Concept Laser GmbH)
➢ Laser Metal Fusion (TRUMPF Laser Technology)
➢ Micro Laser Sintering or MLS (EOS GmbH)
➢ Selective Electron Beam Melting or SEBM, *
➢ Selective Heat Sintering or SHS
➢ Selective Laser Melting or SLM
➢ Selective Laser Sintering or SLS® (3D Systems Corporation)
* Occasionally used in literature for describing EBM.
‡ EBAM is officially a trademarked DED process.
Powder bed fusion contains a range of technologies, all of which have core similarities, the main three are SLS, SLM and EBM. These originated from work at the University of Texas at Austin in the early 1980s which was awarded a patent in 1989, . In 1995, SLM and DMLS were developed in Germany as part of a project between the Fraunhofer Institute, EOS and others. In 1997, Arcam AB of Sweden was founded to commercialize the idea of EBM with their first machine being sold in 2001 and delivered in 2002. HSS was invented in the UK in 2003 and shows promise of being an up and coming AM technology.
Figure 1: Powder Bed Fusion example setup
Powder bed fusion methods all start with a powder bed, another parallel to binder jetting. A thin layer of loose powder between 0.001mm (EOS) and 0.2mm (Arcam) with an average of 0.02mm to 0.1mm is smoothly spread flat over a build platform. This layer of powder is then passed over by either a laser or electron beam which supplies significant heat to the powder. The powder is then either partially melted (sintered) or fully melted, to a point where the powder fuses to itself and to the layers below. Then the build platform changes height, a new layer of powder is deposited, and the process is repeated layer by layer until the part is complete. The main differences between all these methods are the heat source that causes melting, the environment in which the melting takes place, and the degree to which things are melted. SLS, DMLS, SLM and all other methods that contain laser in the name, use a laser to provide the thermal energy. SHS uses only a thermal print head. HSS uses an infrared lamp and a radiation absorbent material to do the sintering. EBM uses an electron beam. SLS and HSS generally work only with plastics and operate in a heated nitrogen atmosphere with the powder bed at an elevated temperature. DMLS and SLM primarily work with metals, and due to the reactive nature of some powdered metals, the process takes place in an inert oxygen-free environment with the powder bed being either at room temperature or at a low-temperature set-point. EBM works with metals and takes place in a vacuum and at elevated temperatures much closer to the metals melting temperature. SLS, HSS, and SHS all sinter the material resulting in a final part that has some porosity, but low to no residual stress in the part. Residual stress is energy contained within the material itself that causes it to deform and move. This lack of residual stress allows the parts do not need any type of support system as the powder bed provides the needed supports for overhangs. Although DMLS contains sintering as part of its name, its process does involve melting and not sintering of metals. SLM, DMLS, LaserCUSING and EBM fully melt the material and can result in fully dense parts. SLM, DMLS, and LaserCUSING all can result in significant residual stress depending on materials, geometry, and laser parameters, and thus need significant support structures to hold parts down. EBM utilizes its fast scan speed to preheat the entire layer with the electron beam to just below melting before actually melting the selected portions. This not only preheats the powder but also causes the powder to become loosely aggregated and reduces dust ‘smoking’ which is a repulsive reaction of the powder once it is hit with negatively charged electrons, . EBM results in parts that have little to no residual stress and thus uses very little if any supports.
Advantages of this process are mostly about material properties. First, there is a wide range of materials that can be processed from plastic parts like Nylon to a wide range of different metals like copper and Inconel to even ceramics. Next, parts that are fully or even partially melted can have significant strength advantages over non-melted processes as the material properties can be close to that of stock material. Then, depending on the process and material, support structures may not be needed, as the powder bed becomes the support. Build speeds can also be fast depending on materials and process, with processes that preheat the powder bed to just below melting being the quickest because it allows a very fast scan speed. Electron beam scan speeds are the fastest of any process due to the lack of mechanical parts to direct the beam. Processes that use lasers result in a high level of detail and fine features. EBM can process materials that are highly reactive in oxygen, and thus can be made quicker and cheaper than subtractive methods.
Disadvantages depend on the process; however, all of these processes can only utilize a single material in the final part. Some methods require inert gases as an additional consumable material. Layer heights are a function of the powder diameter and thus have a medium resolution in the build direction compared to other processes. EBM surface finish is generally rough and will need some post-processing in order to achieve tight tolerances. Some methods require support structures that need additional manual work to be removed from the final part. EBM requires more than an air blasting to remove unsolidified material as the remaining powder no longer acts as a light metal powder but clings together more like wet sand. Thus it needs either powder blasting, ultrasonic vibration, or mechanical methods to remove the powder, with certain geometries like deep narrow cavities being especially difficult.
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