Powder Bed Fusion Metal Additive Manufacturing

The article below was extracted from my PhD thesis. All the content and figures are copyrighted. For more information, you can check the original document here.

Over the past two decades, the term “Additive manufacturing” (AM)—also known as “3D printing” or “Rapid prototyping”—has become increasingly popular both in academia and in industry due to the unmatched design freedom it provides compared to conventional manufacturing (CM) processes, such as lathing, milling, welding, etc. While CM generally involves the progressive removal of material from a workpiece to create the final shape, AM enables near-net-shape manufacturing of parts by fusing or melting the feedstock material together—be it polymer, ceramic, or metal-based—layer by layer [1]. This process offers the capability of fabricating internal chambers, internal channels, or complicated geometrical shapes that could never be achieved using CM [2]. Moreover, AM reduces material waste and provides a shorter time to market for applications that require low volume or parts-on-demand productions [3]. These benefits have helped to extensively expand the applications of AM to many different industrial sectors, including the marine, aerospace, military, and healthcare sectors, in which metal AM now plays a crucial part.

An overview of all up-to-date metal AM technologies and key manufacturers for each technology is illustrated in Figure 1. Metal AM can be classified into two major categories: one is based on melting or fusing of the material into the final part directly in a single stage; the other one requires a two-stage process where the material is first bound together using additional agent forming a “green” part and then post-processed (e.g., sintered) to produce the final metal part. In each category, several sub-categories are defined based on the underlying technologies, feedstock materials, and energy sources. While each category has its pros and cons based on different criteria, metal AM technologies based on full melting of materials stand out to be the prospective candidate due to their better material characteristics (i.e., mechanical behavior), which is a result of high relative density of the as-built part (see Figure 2).

Metal AM based on melting processes includes two main sub-classifications: powder bed fusion (e.g., Laser Powder Bed Fusion (L-PBF) and Electron Beam PBF (EB-PBF)) and directed energy deposition (DED). The main differences among these technologies lie in the energy source used to melt the metal powder (i.e., laser beam, electron beam, plasma, and arc) and the way materials are deposited [4], [5]. In powder bed fusion (PBF), metal powders are distributed as a thin, uniform layer on a substrate. In contrast, DED sprays powder or feeds metal wire directly into the molten metal pool. As reported in [6], among these technologies, PBF is currently one of the most frequently employed AM processes in the industry. Part of the success of PBF technology stems from its better relative density, higher accuracy, and greater geometrical freedom [5].

Laser powder bed fusion additive manufacturing

Similar to other additive modalities, PBF processes require an initial 3-dimensional (3D) Computer-aided design (CAD) of the desired object. This 3D model is then sliced into many layers in the vertical direction (the Z direction), where layer thickness can be configured from 30-100 um. Each layer of the sliced part consists of a cross-section view (in the XY plane) of the geometry to be printed at a specific Z-coordinate. Before the slicing process, support structures can be added to the designed part to assist overhanging geometries and prevent them from falling during fabrication [8]. The next step is to generate the laser scan pattern for each cross-section layer created by the slicing process. Different scanning patterns, or “scanning strategies”, can be selected or designed by the user to obtain the desired microstructures and mechanical properties, including hardness, ductility, toughness, fatigue, etc. [9]. Fig. 1‑3 depicts the L-PBF process with all the involved components. The building process starts by dispensing and coating a thin layer of loose metal powder on the build substrate in a thickness that equals that of the slicing process. Subsequently, a high-power beam is rastered on the surface of the powder bed following the selected scanning strategy. The rastering is performed by a laser galvanometer scanner in L-PBF technology. The regions on the powder bed that are exposed to the high-power source will selectively melt and consolidate to form one “slice” of the object to be printed. The process continues by lowering the build platform to a certain distance equal to the layer thickness, re-coating another layer of loose powders, and scanning the beam until the last layer is finished. The entire process occurs in the inert-gas-filled environment to prevent the molten material from oxidation [10], [11]. Particularly, during the L-PBF process, a circulation of inert gas flow (i.e., Argon or Nitrogen) is introduced on top of the layer to carry away the melting by-products (e.g., soot, spatter, smoke, etc.) to improve the laser absorption efficiency and melt pool stability [12], [13]. The build platform temperature can be changed within two hundred degrees Celsius (in L-PBF) [14]. The high-temperature process is sometimes effective at reducing residual stresses in the material [15].

References:

[1] W. E. Frazier, “Metal additive manufacturing: A review,” J. Mater. Eng. Perform., vol. 23, no. 6, pp. 1917–1928, 2014, doi: 10.1007/s11665-014-0958-z.

[2] N. Gardan and A. Schneider, “Topological optimization of internal patterns and support in additive manufacturing,” J. Manuf. Syst., vol. 37, pp. 417–425, 2015, doi: 10.1016/j.jmsy.2014.07.003.

[3] T. J. McCue, “Wohlers Report 2018 : 3D Printer Industry Tops $ 7 Billion,” Forbes, 2018.

[4] H. K. Rafi, N. V. Karthik, H. Gong, T. L. Starr, and B. E. Stucker, “Microstructures and mechanical properties of Ti6Al4V parts fabricated by selective laser melting and electron beam melting,” J. Mater. Eng. Perform., vol. 22, no. 12, pp. 3872–3883, 2013, doi: 10.1007/s11665-013-0658-0.

[5] A. Gasser, G. Backes, I. Kelbassa, A. Weisheit, and K. Wissenbach, “Laser Additive Manufacturing,” Laser Tech. J., vol. 7, no. 2, pp. 58–63, 2010, doi: 10.1002/latj.201090029.

[6] Wohlers Report, “Wohlers Report 2014. 3D Printing and Additive Manufacturing State of the Industry,” in Wohlers Report, 2014, p. 277.

[7] “Overview of Metal AM processes.” https://www.metal-am.com/articles/binder-jetting-fdm-comparison-with-powder-bed-fusion-3d-printing-injection-moulding/ (accessed Nov. 17, 2020).

[8] F. Calignano, “Design optimization of supports for overhanging structures in aluminum and titanium alloys by selective laser melting,” Mater. Des., vol. 64, pp. 203–213, 2014, doi: 10.1016/j.matdes.2014.07.043.

[9] L. N. Carter, C. Martin, P. J. Withers, and M. M. Attallah, “The influence of the laser scan strategy on grain structure and cracking behaviour in SLM powder-bed fabricated nickel superalloy,” J. Alloys Compd., vol. 615, pp. 338–347, 2014, doi: 10.1016/j.jallcom.2014.06.172.

[10] P. Bidare, I. Bitharas, R. M. Ward, M. M. Attallah, and A. J. Moore, “Laser powder bed fusion at sub-atmospheric pressures,” Int. J. Mach. Tools Manuf., vol. 130–131, no. January, pp. 65–72, 2018, doi: 10.1016/j.ijmachtools.2018.03.007.

[11] B. Zhang, H. Liao, and C. Coddet, “Selective laser melting commercially pure Ti under vacuum,” Vaccum, vol. 95, pp. 25–29, 2013, doi: 10.1016/j.vacuum.2013.02.003.

[12] A. Bin Anwar and Q. C. Pham, “Selective laser melting of AlSi10Mg: Effects of scan direction, part placement and inert gas flow velocity on tensile strength,” J. Mater. Process. Technol., vol. 240, pp. 388–396, 2017, doi: 10.1016/j.jmatprotec.2016.10.015.

[13] A. Bin Anwar and Q. C. Pham, “Study of the spatter distribution on the powder bed during selective laser melting,” Addit. Manuf., vol. 22, no. April, pp. 86–97, 2018, doi: 10.1016/j.addma.2018.04.036.

[14] L. E. Murr et al., “Metal Fabrication by Additive Manufacturing Using Laser and Electron Beam Melting Technologies,” J. Mater. Sci. Technol., vol. 28, no. 1, pp. 1–14, 2012, doi: 10.1016/S1005-0302(12)60016-4.

[15] K. Kempen, B. Vrancken, L. Thijs, S. Buls, J. Van Humbeeck, and J.-P. Kruth, “Lowering thermal gradients in Selective Laser melting by pre-heating the baseplate,” Solid Free. Fabr. Proc., vol. 24, 2013, doi: 10.1017/CBO9781107415324.004.