Mechanical finishing of additively manufactured metal parts

​The finishing step of additively manufactured metal components may be categorised into three mechanisms: 1) mechanical, 2) thermal and 3) (electro-)chemical processes. In this article, we review the effectiveness and impact of mechanical subtractive techniques on finished AM components.

In conventional near-net shaping processes (such as moulding, casting and die-casting), subtractive techniques such as machining, shot-peening and grinding have been widely used to enhance dimensional accuracy and surface quality. This has been naturally extended to additive manufactured components in order to meet stringent quality expectations such as typical surface roughness 0.8μm< Ra< 1.6μm for aerospace components.

CNC turning

CNC turning of parts built in AISI 316 and 15-5HP steels using selective laser melting results in a surface roughness Ra ~ 0.4μm. Experiments [1, 2] showed that finishing of AM parts had limited effects on the fatigue stress at 10^6 cycles, but significant effect at 10^7 cycles. 

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High speed milling

High speed milling (HSM) used to finish Al 22919 aluminium parts made using EBF3 [ref] was found to produce highly favourable surface roughness (8-56μin RMS) and waviness (400μin RMS) [3]. However, compared with other subtractive processes [3], HSS introduced larger residual stresses in the finished surface. It provides the best combination of low localized RMS surface finish and minimal long range waviness [3].

Glass bead blasting
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Glass bead blasting is usually a manual operation with minimal set-up time. Depending of the component geometry, it is not always possible to reach surfaces. In the tests, glass bead blasting was not aggressive enough to eliminate the ridged surface finish resulting from the EBF3 process.

Grinding

Using grinding with AISI 316L steel parts made with SLM and EBM [4] reduced the as-built surface roughness from Ra ~15μm to 0.34μm. On horizontal surfaces, the surface roughness (Ra) was reduced from 12 μm to 4 μm [5], and on vertical surfaces from 15 μm to 13 μm [5] in Nickel-Iron-Copper parts. This clearly illustrates the importance of orientation and build-direction. 

shape adaptive grinding set up [6]

​Shape-adaptive grinding [6] can be used to finish complex and intricate geometries that usually pose a challenge for conventional grinding. Ti6Al4V MAM parts made using EBM, a surface roughness (Ra) of ~10nm was achieved by using a 3-step process and varying diamond abrasive pellets and paste.

3 step shape adaptive grinding [6]

References
[1]Joseph M. Flynn1, Alborz Shokrani1, Stephen T. Newman1 and Vimal DhokiaHybrid Additive and Subtractive Machine Tools-Research and Industrial Developments INTERNATIONAL JOURNAL OF MACHINE TOOLS AND MANUFACTURE · NOVEMBER 2015 DOI: 10.1016/j.ijmachtools.2015.11.007
[2] A. B. Spierings, T. L. Starr, and K. Wegener, “Fatigue performance of additive manufactured metallic parts,” Rapid Prototyp. J., vol. 19, no. 2, pp. 88–94, 2013.
[3] K. M. B. Taminger, R. a Hafley, D. T. Fahringer, and R. E. Martin, “Effect of Surface Treatments on Electron Beam Freeform Fabricated Aluminum Structures Karen M. B. Taminger, Robert A. Hafley, David T. Fahringer, and Richard E. Martin NASA Langley Research Center, Hampton, VA.”
[4] L. Ler, C. Flache, R. Petters, U. K・n, and J. Eckert, “Comparison of different post processing technologies for SLM generated 316l steel parts,” Rapid Prototyp. J., vol. 19, no. 3, pp. 173–179, 2013.
[5] S. Rossi, F. Deflorian, and F. Venturini, “Improvement of surface finishing and corrosion resistance of prototypes produced by direct metal laser sintering,” J. Mater. Process. Technol., vol. 148, no. 3, pp. 301–309, 2004.
[6] A. T. Beaucamp, Y. Namba, P. Charlton, S. Jain, and A. a Graziano, “Finishing of additively manufactured titanium alloy by shape adaptive grinding (SAG),” Surf.  Topogr. Metrol. Prop., vol. 3, no. 2, p. 024001, 2015.