Machinery's Handbook, 31st Edition
METAL ADDITIVE MANUFACTURING PROCESSES 1557 After the heat source, the most important components of SLM systems are beam deflec - tion optics, which provide scanning capability for selectively melting areas of the powder bed. With scan speeds of up to 23 ft/s (7 m/s), the scanning mirror must be fast, accurate, and reliable. The final significant optical element is the correction lens, which ensures that the beam is round as it traverses the build platform at different angles. Most systems use an f-theta lens design with anti-reflection coatings to prevent damage from the beam reflecting back into the laser. A schematic illustration of the laser-beam powder bed fusion process is shown in Fig. 25. The system consists of a powder bed build area on a build plate that moves in the Z-direction. For each printed slice (see Table 8), the plate moves down by one layer thick- ness. The powder deposition system then spreads a new layer of powder, and the laser traces the profile of that part slice on the layer of powder. This sequence is repeated until the full 3D part is completed. Because the laser melts the metal powder, to prevent oxidation, the SLM mechanism is enclosed in a gas-tight chamber containing argon or other nonreactive gases. Typically, powder particle size used in SLM processes is between 0.0008 and 0.0024 in. (0.02 and 0.06 mm). Titanium, stainless steel, and nickel-base superalloys are commonly processed by SLM; aluminum, copper, and gold are more challenging, but some newer machines feature lasers with colors in a green range (~500 nm) that are more readily absorbed by these metals. Laser spot size is typically 0.0027 in. (0.07 mm) in diameter but may be up to 0.006 in. (0.15 mm) in diameter. At any given instant, metal particles within the laser spot are melted, forming a small pool of molten metal. This pool quickly solidifies, as heat trans - fers to the solid metal build plate on the first layer or to the previously solidified material in the layers below the current one. Thus, the molten metal pool within the laser spot zone continuously forms and solidifies as the laser spot moves rapidly across the powder layer, much like a mini-welding process. The high rate of solidification of the molten pool results in very fine microstructure and excellent mechanical properties. Strength of SLM-processed material is often greater than for the same alloy in conventional casting due to this fine microstructure. On the other hand, because the process involves melting and solidification, gas porosity and so - lidification porosity may form, leading to fine pores throughout the material. Such defects significantly reduce fatigue and impact strengths, though without reducing the final part’s yield and tensile strength. One advantage of AM, and SLM in particular, is that the processes can produce over- hangs and internal channels or cavities. In SLM, when material is printed in an overhang, the molten pool sits on loose powder from previously created layers. Even though molten metal in such an overhang cools and solidifies rapidly, some of the metal seeps down into the spaces between the loose particles, leaving a rough surface on
Fig. 25. Schematic Illustration of Laser Beam Powder Bed Fusion, also known as Selective Laser Melting (SLM)
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