Recrystallization kinetics, mechanisms, and topology in alloys processed by laser powder-bed fusion: AISI 316L stainless steel as example

Author(s)

Aota, Leonardo Shoji ^{[1, 2]} ; Bajaj, Priyanshu ^[3] ; Zilnyk, Kahl Dick ^[4] ; Jaegle, Eric Aime ^{[1, 5]} ; Ponge, Dirk ^[1] ; Sandim, Hugo Ricardo Zschommler ^[2] ; Raabe, Dierk ^[1]

Affiliation

^[1] Max Planck Inst Eisenforsch GmbH, D-40237 Dusseldorf - Germany

^[2] Univ Sao Paulo, Lorena Sch Engn, Dept Mat Engn, BR-12602810 Lorena - Brazil

^[3] M4p Mat Solut GmbH, A-9181 Feistritz - Austria

^[4] Inst Tecnol Aeronaut, BR-12228900 Sao Jose Dos Campos - Brazil

^[5] Univ Bundeswehr Munchen, Inst Mat Sci, D-85577 Neubiberg - Germany

• Document type: Journal article
• Source: MATERIALIA; v. 20, DEC 2021.
• Web of Science Citations: 1

Abstract

Alloys manufactured by laser powder-bed fusion have intrinsic and hierarchical microstructural features inherited from the fast solidification (up to 10(4) K/s) and subsequent thermal cycles. This creates epitaxed grains, dislocation cell structures, and second-phase oxide nanoparticles. Epitaxed grains follow a pattern where finer grains are found in the melt pool centerline along the laser track. Upon further annealing, this characteristic microstructure has pronounced consequences on the recrystallization mechanisms and thus on grain topology. By changing the scanning strategy, we control the emerging grain patterns in a representative alloy (AISI 316L austenitic stainless steel) by creating linear strings for unidirectional scans, while a chessboard grain pattern arises by applying a 90 degrees-rotation between layers. Upon post-processing annealing (at 1150 degrees C from 15 min to 8 h), we study the relationship between the as-built and recrystallized microstructures. Recrystallization starts with fine nuclei in regions with high dislocation density along the melt pool centerlines, resulting in early-stage linear impingement (linearly clustered nucleation), as revealed by microstructural path analysis. Recrystallization is sluggish, due to dynamic Zener-Smith pinning. This effect leads to jerky boundary motion due to periodic pinning and depinning from oxide particles, caused by their gradual coarsening. Lower nuclei number density slows kinetics for the case of unidirectional scanning, while twinning aids in the nucleation of grains with mobile grain boundaries. Our findings show that changes in the laser scanning strategy are a suitable design tool for tailoring recrystallization and thus microstructure. (AU)

• FAPESP's process: 19/19442-0 - Microstructural stability of AISI 316L stainless steel processed by selective laser melting using different scanning strategies
• Grantee: Leonardo Shoji Aota
• Support Opportunities: Scholarships abroad - Research Internship - Master's degree
• FAPESP's process: 18/23582-9 - Microstructural stability of 316L stainless steel processed via selective laser melting using different scanning strategies
• Grantee: Leonardo Shoji Aota
• Support Opportunities: Scholarships in Brazil - Master
• FAPESP's process: 19/06679-1 - Microstructural characterization of precipitation hardening stainless steels processed by additive manufacturing
• Grantee: Kahl Dick Zilnyk
• Support Opportunities: Regular Research Grants