Corrosion Properties of Powder Bed Fusion Additively Manufactured Stainless Steels

Abstract

Metal additive manufacturing (AM) has recently become a desirable process for complex parts across a broad range of applications. Laser powder bed fusion (LPBF) is an AM process for metals whereby a laser is used to selectively melt a pattern in successive layers of powder material as a means of building a three dimensional structure. The resultant material often contains pores and locally high cooling rates produced highly non-equilibrium solidification conditions which lead to solute segregation and formation of terminal solidification phases.[1,2] As a result of these varied microstructures, AM alloys may exhibit mechanical properties vastly different to their traditional wrought or cast counterparts.[3,4] While some adjustments have been made in manufacturing to enhance mechanical traits, such as incorporation of carbide particles into the powder mixture for strengthening, very little attention has been directed at understanding or enhancing the corrosion properties of these materials.[5,6] Initial studies of LPBF and powder bed injection molded steels have suggested that porosity and/or elemental segregation, similar to effects seen in more traditional powder metallurgy materials, result in varied microstructures.[7,2] These characteristics may lead to reduction in corrosion resistance of the AM stainless steels compared to their conventional, thermomechanically-processed counterparts. Studies to date have not produced results relating both the corrosion characterization of these materials in full immersion and atmospheric environments, or provided direct evidence of the possible microstructural influences on corrosion. This presentation explores the corrosion resistance of a range of LPBF stainless steels including 304L, 316L, and 17-4 PH in aqueous sodium chloride solutions. Comparison is made against their wrought plate counterparts. Samples were electrochemically characterized in both their as-printed state (image in Figure 1-a) and after grinding and polishing to a range of surface finishes. Initial potentiodynamic polarization measurements show considerable deviation between the behaviors of the AM steels vs. plate material (Figure 1-b), with about a 100 mV shortening of the passive region and up to a half order magnitude increase in the passive current density for the AM steel. This suggests influences of the porosity and microstructure of the LPBF alloy on the corrosion behavior, which may also affect these materials in the polished condition. In-situ and ex-situ characterization is being carried out to directly examine the influence of microstructural features on realized corrosion behavior. Finally, electrochemical behavior in full immersion is related to their performance in ASTM B-117 salt fog exposures as indication of their atmospheric corrosion performance. Results from this work will provide insight into the microstructural influences of AM materials on their corrosion susceptibility compared to traditional materials of the same alloys. Furthermore, the insight gained will inform processing pathways for optimization of corrosion performance. Acknowledgements Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000 References [1] L. Thijs, F. Verhaeghe, T. Craeghs, J.V. Humbeeck, and J.P. Kruth, Acta. Mater., 58 (2010) 3303. [2] M. Raza, F. Ahmad, M. Omar, and R. German, J. Materials Processing Technology, 212 (2012) 164. [3] M. Mani, B. Lane, A. Donmez, S. Feng, S. Moylan, R. Fesperman, National Institute of Standards and Technology, Gaithersburg, MD, NIST Interagency/Internal Report (NISTIR) 8036 (2015). [4] A. Wegner, G. Witt, Physics Procedia, 39 (2012) 480. [5] A. Hedge, A. Patil, and V. Tambrallimath, IJERT, 3 (2014) 14. [6] A. Poopola, Int. J. Electrochem. Sci., 9 (2014) 1273. [7] J. Trelewicz, G. Halada, O. Donaldson, and G. Manogharan, JOM, 68.3 (2016) 850. Figure 1