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Evaluation and optimization of surface finishing methods toward improving the corrosion and fatigue performance of additively manufactured laser-powder bed fusion metals
Prochaska, Stephanie O.
Prochaska, Stephanie O.
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2022
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The intrinsically rough surfaces of laser powder bed fusion (L-PBF) additively manufactured (AM) metal parts are detrimental to both mechanical and corrosion properties. Surface finishing is a critical post-processing step to ensuring part reliability and performance in real world conditions. Conventional surface finishing methods can be labor- and cost-intensive, often eliminating some of the efficiency benefits of utilizing AM. Recently, novel methods of bulk processing have been developed which rely on the application of certain chemistries to smooth part surfaces. However, the effect of these processes on the corrosion and fatigue performance has been previously unstudied.
This work evaluates the resulting material properties of L-PBF metals having undergone two different surface finishing processes. After a chemically accelerated vibratory finishing (CAVF) process, fatigue life is improved for 316L stainless steel and Inconel 625 parts. CAVF increased the near-surface compressive stresses in the 316L parts, impeding corrosion pit initiation and significantly increasing the breakdown potential. The effect on the corrosion properties of Inconel 625 was minimal due to the superalloy’s inherently good corrosion resistance. However, a standard hot isostatic pressing (HIP) treatment resulted in carbide segregation to the grain boundaries, caused formation of surface microcracks, and destabilized the passivation layer—ultimately reducing the corrosion resistance.
Evaluation of dislocation density and initial microstructure on a second method, a self-terminating etching process (STEP), revealed sensitization depth of L-PBF 316L to decrease with increasing dislocation density. Subsequent material etching was shown to terminate along grain boundaries at specific carbon concentrations, but varying amounts of remnant carbides remained on the surfaces after etching was complete. Therefore, corrosion resistance decreased from the pre-STEP condition, but could be improved through a post-STEP solution anneal to dissolve carbides back into the matrix. STEP improved the fatigue life of the evaluated specimens by up to 3×, with the smaller-grained specimens having the lowest surface roughness and best fatigue performance. While future work should focus on reducing remnant carbides post-STEP to improve corrosion performance, both STEP and CAVF represent promising methods for surface finishing of AM L-PBF metal components.
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