Corrosion performance of an austenitic stainless steel fabricated by wire arc additive manufacturing.

Abstract

The emergence of Wire and Arc Additive Manufacturing opens new prospects for the marine industry as an Additive Manufacturing (AM) technology capable of fabricating large metallic components in a timely manner, at reduced cost and with greater flexibility. The microstructure resulting from the WAAM deposition is non-equilibrium and deviates from conventional wrought and cast, likely changing the corrosion behaviour as well. This thesis investigates the corrosion behavior of WAAM deposited 316L stainless steel (SS) based on the consideration of the microstructure, with a focus on the effects of the solidification sequence, feedstock composition, heat treatment, interpass temperature, and elemental segregation. The work was conducted in four main studies, each addressing a different aspect of the WAAM 316LSS corrosion performance.
Firstly, the corrosion behavior of a multi-layered WAAM ER316LSi wall was
examined, both as-deposited (AD) and after stress relief heat treatment (HT). The cyclic reheating during deposition was found to facilitate the transformation of delta ferrite into sigma leading to its complete transformation of during the heat treatment, decreasing corrosion performance. The study identified that microstructural differences between as-deposited WAAM and wrought alloys are linked to differences in pitting potential and pit nucleation with the WAAM having high metastable-like pit
activity.
Secondly, an innovative potentiostatic pulse technique (PPT) was developed to evaluate the pit nucleation behaviour of AD and HTWAAM 316LSi SS and compare to a wrought 316LSS. The AD WAAM had 20 times more nucleation sites than the wrought alloy explaining the high metastable like pit activity. The HT amplified the number of nucleation sites by a factor of 4, located near sigma phases.
Thirdly, the effect of interpass temperature on the microstructure and pit nucleation behavior of WAAM ER316L SS was investigated. Lowering the interpass temperature increased the ferrite content and decreased the sigma phase precipitation. PPT showed that the WAAM ER316L pits developed predominantly near sigma phases and had higher pit density compared to wrought 316L. The pitting susceptibility of WAAM ER316L increases with increasing interpass temperature due to the precipitation of larger sigma phase enhancing the elemental segregation. Finally, the last study explores the impact of solidification mode and elemental segregation on the corrosion performance of WAAM 316LSS using a customized feedstock and two commercially available welding wires with varying Creq/Nieq. The results show that the solidification sequence and feedstock composition have a significant impact on the corrosion behavior, confirming the predominant role of elemental segregation. Primary ferritic solidification and acicular delta ferrite to austenite transformation exhibited higher pitting resistance due to the reduced segregation of ferritizers in the austenite phase. Conversely, primary austenitic solidification showed increased pitting activity and was more sensitive to reheating cycles. This work provides new insight into the microstructure-corrosion property relationship of WAAM 316LSS, which will guide the development of future WAAM-specific feedstocks and better optimization of deposition parameters for improved corrosion resistance.

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Identifiers

ORCID for Corentin Penot: orcid.org/0000-0002-1609-1555

ORCID for Julian Wharton: orcid.org/0000-0002-3439-017X

ORCID for Yikun Wang: orcid.org/0000-0001-5619-7795

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