Machinery's Handbook, 31st Edition
1558 METAL ADDITIVE MANUFACTURING PROCESSES the bottom of the overhang. This roughness is particularly high if the angle of the over- hang, relative to horizontal, is less than 45 degrees. As a result, the as-printed finish of up-facing surfaces will be better than down-facing overhang surfaces. (Surface finish in SLM material may be improved by subsequent shot peening.) Typical surface finishes of up-facing and side-facing surfaces are equivalent to those of investment castings, so only bearing or mounting surfaces may need to be machined. Since the rate of heat transfer into loose powder is much less than the rate into solid metal, heat transfer from overhangs is reduced and the cooling rate is lower than in lay- ers in the body of a part. Structural supports printed beneath overhangs (Fig. 26) and connected to the base plate provide a path for increased heat transfer from overhangs. Supports also anchor the printed part to the base and reduce distortion due to differences in temperature throughout the part as it is printed. Without such supports, overhanging layers of the printed parts would deflect upward due to thermal stress. A downside of such supports is that they can interfere with printing more than one layer of parts in one build.
Where 3D printed layers overhang loose powder instead of solid layers, more heat is retained, changing the cooling rate.
When these overhanging layers are longer, they can bend upward during cooling, as a result of thermal stress.
This distortion can be prevented with added structures (to be removed later) to reduce thermal stress, support the overhangs, and anchor the part.
Fig. 26. Use of Supports to Anchor Parts in SLM Process After the part is completed, the unused powder is collected for future use, and the base plate with the parts attached is removed from the SLM machine. Parts usually are cut from the plate using a band saw or wire electrical discharge machining (EDM), supports are removed, and any surfacing is completed. Electron Beam Melting (EBM): Powerful electron beams have been used for welding since the late 1950s. At the beginning of the twenty-first century, this energy source was adapted to powder bed fusion (PBF) as the electron beam melting (EBM) process. EBM is commonly used to process titanium and superalloy powders into net-shaped parts— providing significant material savings over traditional manufacturing processes. In EBM, a tungsten filament emits electrons at a high voltage of 60,000 V (60 kV). A focusing coil or electromagnetic lens produces a converging Gaussian beam with a spot size of 0.008–0.04 in. (0.203–1.02 mm) and a power of up to 6,000 W; a deflection coil directs the scanning beam over the surface of the powder layer at a speed of 26,000 ft/s (8,000 m/s). These focus and deflection coils are the electronic counterparts of the f-theta and scanning optics of the laser beam systems in SLM. Schematically, the EBM system is similar to that of SLM; however, while the laser beam is moved optically by deflection
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