Laser Powder Bed Fusion Additive Manufacturing and Corrosion Testing of Alloy 718 and Superduplex Stainless Steel
TWI Industrial Member Report 1214-2026
By Emily Davison, Briony Holmes and David Martelo
Industrial Need
Corrosion resistance in additively manufactured (AM) materials is a critical topic, which is increasingly gaining the interest of various industry sectors, especially in the oil, gas and energy industries. However, corrosion resistance in AM metal components is currently not well understood as it is sensitive to multiple factors including microstructure, porosity/voidage and surface roughness. The surface roughness of AM materials has been identified by TWI as a key parameter determining the corrosion resistance of AM parts which has not been well explored elsewhere. AM parts, particularly those produced by laser powder-bed fusion (L-PBF), are associated with rough surfaces which could initiate localised corrosion in demanding environments, such as pitting and crevice corrosion or stress corrosion cracking. Many of the advantages of AM rely on the production of complex designs, some with complex internal structures, which cannot be accessed for machining and so determining the relationship between the as-built surface quality and corrosion susceptibility is critical for these parts in demanding applications. In addition to measuring the significance of surface finish on corrosion resistance, this project also aims to develop advanced process control techniques which could be used to locally control surface finish in areas of AM components which are exposed to corrosive environments.
Similar to surface finish, voids have the potential to initiate localised corrosion and have been reported to inhibit re-passivation of AM materials when corroding (Sander, et al., 2017). Nearly all AM parts contain some level of voids, however, TWI has achieved AM Facility Qualification from Lloyd’s Register (LR) in the production of L-PBF stainless steel 316L (SS316L) components. During this qualification process, the volume of voids has been shown to be effectively reduced to ~0.02%. Much of the literature on corrosion performance results from testing on materials with higher voidage and so is not necessarily representative high quality parts. TWI’s excellent process control will allow the production of parts with a range of relevant voidage levels to determine the impact of voids on the corrosion performance of AM parts.
This project will develop optimisation procedures and quantitative experimental data on corrosion resistance of AM parts in 25Cr superduplex stainless steel and Alloy 718 as a function of surface roughness and voidage to provide informed consultancy and recommendations to TWI members in the adoption of AM technologies for applications in corrosive environments.
Key Findings
- Laser powder-bed fusion (L-PBF) material tested in the as-deposited (unground) state showed high %weight loss during corrosion testing.
- Using a L-PBF ‘re-melting’ strategy to improve surface roughness was generally found to be beneficial for corrosion resistance, particularly in as-built specimens.
- No one-to-one correlation was found between roughness or surface breaking voidage and corrosion in AM Alloy 718. However, high porosity (>3%) is thought to have a severe detrimental effect on corrosion properties of L-PBF Alloy 718, outstripping all other influences.
- The voidage content in superduplex stainless steel (SDSS) appeared to be less important than the phases present with respect to corrosion resistance. Heat treatment of the as-built SDSS was found to be largely beneficial for corrosion resistance.
- Existing corrosion testing Standards are inadequate for AM parts. For example, the standard NDT methods would be unlikely to detect the micro size-volumetric flaws present in AM parts, though such features would have an effect on localised corrosion susceptibility.
Impact
The main objective of this project was to explore the significance of localised surface finish and porosity on the corrosion resistance of AM materials (specifically Alloy 718 and superduplex stainless steel manufactured using laser powder-bed fusion).
The key findings from this work confirms that L-PBF components, and especially critical surfaces (i.e. those likely to experience corrosion) require additional post-processing prior to going into service. This is not necessarily a limiting factor and is common for both additive and more conventional subtractive manufacturing processes but does need to be accounted for when determining the suitability of components for AM.
Microstructure was not found in this study to be hugely influential on the corrosion properties. However, this may be due to the fact that porosity and surface roughness were specifically used as a key process variable in this study. For industrial applications, where the process is optimised for fully-dense material, microstructure is likely to become more important. In addition to machining, heat treatment might also be required, for example to get the desired duplex microstructure for SDSS or to reach the high strength requirements of a precipitation hardened nickel alloy. This may also be partially controlled through careful process parameter selection, feedstock chemistry and the development of AM-specific HT.
In terms of processing, voidage is a key variable that needs tight control and a robust assessment methodology in place as corrosion properties (Alloy 718 in particular) are largely dominated by the level of voidage in the material. The results from this work support the need for voidage characterisation as laid out in BS ISO 23669.