Powder Bed Fusion

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The powder bed fusion method uses a high-intensity energy source (laser/ electron beam) to selectively join powders of alloys/ceramics/metals/polymers on a powder bed[1]. The first technology to be discovered under this category is SLS in the year 1989[2]. The 3D cad file for the object to be created in sliced into several thin layers and sent to the SLS machine. The laser source is then focused onto regions on the powder bed selectively line by line, based on the spliced 3D CAD layer. Each line scanned on the powder bed is called a track. Prior to the drawing of the layers, the entire powder bed is heated to a temperature just below its melting temperature. This is done to control deformations due to temperature variations and allow for fusion between successive layers. A roller mechanism is then used to distribute the powder onto the bed evenly. The un-sintered powder within the bed provides support for the layers as they are fused, and can be cleaned away or reused after the part has been built[3]. The schematic in the figure below describes the basic outline of a powder bed system.

SLM schematic.jpg


Direct Material Laser Sintering (DMLS) is a trademarked name of another commercial AM technology operating on the same principles as SLS. However, it is compatible only with metallic powder materials. It was developed by Electro-Optical Systems (EOS) Gmbh and was commercially available as EOSINT 250 laser sintering machine from the year 1995 [4]. Selective Laser Melting (SLM) is a variation of the laser sintering process (SLS) in which the part is fabricated by complete melting of the powder. Hence, this process is ideal for metallic material systems (due to their higher melting points) when compared to polymer material systems. Due to the high intensity of the laser power source, parts produced through SLM often have less to no pores. The power requirement for the SLS technique ranges from 7 W for plastics [5] to 200 W for metallic systems[3]. The EOSINT machines have powers ranging between 200-400 W[6] and the lasers used in the SLM process operate around 400 W[3]. It is observed that SLM is invariably used in the literature to describe all the AM processes under powder bed fusion.

References

  1. Gao, W., Zhang, Y., Ramanujan, D., Ramani, K., Chen, Y., Williams, C. B., Wang, C. C., Shin, Y. C., Zhang, S., and Zavattieri, P. D., 2015, "The status, challenges, and future of additive manufacturing in engineering," Computer-Aided Design, 69, pp. 65-89.
  2. Hong, C., Gu, D., Dai, D., Gasser, A., Weisheit, A., Kelbassa, I., Zhong, M., and Poprawe, R., 2013, "Laser metal deposition of TiC/Inconel 718 composites with tailored interfacial microstructures," Optics & Laser Technology, 54, pp. 98-109.
  3. 3.0 3.1 3.2 Bikas, H., Stavropoulos, P., and Chryssolouris, G., 2016, "Additive manufacturing methods and modeling approaches: a critical review," The International Journal of Advanced Manufacturing Technology, 83(1-4), pp. 389-405.
  4. Lohner, A., 1997, "Laser sintering ushers in new route to PM parts," Metal Powder Rep, 52(2), pp. 24-27.
  5. Kruth, J.-P., 1991, "Material increase manufacturing by rapid prototyping techniques," CIRP Annals-Manufacturing Technology, 40(2), pp. 603-614.
  6. Khaing, M., Fuh, J., and Lu, L., 2001, "Direct metal laser sintering for rapid tooling: processing and characterization of EOS parts," Journal of Materials Processing Technology, 113(1), pp. 269-272
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