Effect of Microstructure and Dislocation Density on Material Removal and Surface Finish of Laser Powder Bed Fusion 316L Stainless Steel Subject to a Self-Terminating Etching Process

• Published in: 3D printing and additive manufacturing Vol. 10; no. 3; pp. 373 - 382
• Main Authors: Prochaska, Stephanie, Walker, Michael, Hildreth, Owen
• Format: Journal Article
• Language: English
• Published: United States Mary Ann Liebert, Inc., publishers 01.06.2023
• Subjects: surface finishing; electrochemical etching; self-terminating etching process; microstructure; sensitization; dislocations
• ISSN: 2329-7662; 2329-7670; 2329-7670
• DOI: 10.1089/3dp.2022.0190

Postprocessing of additively manufactured (AM) metal parts to remove support structures or improve the surface condition can be a manually intensive process. One novel solution is a two-step, self-terminating etching process (STEP), which achieves both support removal and surface smoothing. While the STEP has been demonstrated for laser powder bed fusion (L-PBF) 316L stainless steel, this work evaluates the impact of pre-STEP heat treatments and resulting changes in dislocation density and microstructure on the resulting surface roughness and amount of material removed. Two pre-STEP heat treatments were evaluated: stress relief at 470°C for 5 h and recrystallization–solution annealing at 1060°C for 1 h. Additionally, one set of specimens was processed without the pre-STEP heat treatment (as-printed condition). Dislocation density and phase composition were quantified using X-ray diffraction along with standard, metallurgical stain-etching techniques. This work, for the first time, highlights the mechanisms of sensitization of AM L-PBF 316L stainless steel and provides fundamental insights into selective etching of these materials. Results showed that the sensitization depth decreased with increasing dislocation density. For samples etched at a STEP bias of 540 mV SHE , material removal terminated at grain boundaries; therefore, the fine-grained stress-relieved specimen had the lowest post-STEP surface roughness. For surface roughness optimization, parts should be stress relived pre-STEP. However, to achieve more material removal, pre-STEP solution annealing should be performed.