Surface and subsurface modification of selective laser melting built 316L stainless steel by means of severe shot peening

Metal additive manufacturing is a cutting-edge manufacturing technology which enables production of complex shaped geometries in layer-by-layer method. In addition to intricate shapes, it facilitates minimum material wastage, consolidated assemblies as well as topology optimization [1], [2]. However, the as-printed parts especially produced by laser powder bed fusion (LPBF) have poor surface finish when compared to the conventional manufacturing methods such as hot or cold rolling [3]. Therefore, in the present work the as-printed LBBF 316L stainless steel components were subjected to severe shot peening (SSP) in an attempt to improve the surface and subsurface properties.
The as-printed LPBF 316L parts were shot peened with 2 and 42 number of passes. The effect of SSP on the surface roughness as well as grain refinement was studied with the help of scanning electron microscopy, optical profilometry and electron backscatter diffraction (EBSD). In addition to the microscopic investigations, the samples were analysed for residual stresses as well as microhardness in near surface areas. Subjecting the sample to SSP smoothened the surface by evening out un-melted powder particles (refer Fig.1). It resulted in significant improvement in the surface roughness value (Rz = 29 µm) when compared to the as printed condition (Rz = 71 µm). Furthermore, SSP resulted in grain refinement depth of ̴ 40 µm which was evident from the EBSD results. Moreover, beneficial large compressive residual stresses were also induced in near surface areas. The SSP caused work hardening and thereby significantly increased the hardness values in near surface areas. These advantageous improvements make SSP a reliable method for surface and subsurface modifications in LPBF built 316L stainless steel components.

References:
[1] J. Gausemeier, N. Echterhoff, and M. Wall, “Thinking ahead the Future of Additive Manufacturing – Innovation Roadmapping of Required Advancements,” Univ. Paderborn Direct Manufacuring Res. Cent., p. 110, 2013, [Online]. Available: http://www.hni.uni-paderborn.de/en/pe.
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[3] S. Santa-Aho et al., “Additive manufactured 316l stainless-steel samples: Microstructure, residual stress and corrosion characteristics after post-processing,” Metals (Basel)., vol. 11, no. 2, pp. 1–16, 2021, doi: 10.3390/met11020182.