Stress corrosion cracking of additively manufactured Alloy 625

Laser Bed Powder Fusion is an Additive Manufacturing technology for the fabrication of semi-finished components directly from computer-aided design modelling, through melting and consolidation, layer upon layer, of a metallic powder with a laser source. This manufacturing technique is particularly indicated for poor machinable alloys, like Alloy 625. However, the unique microstructure generated could modify the resistance of the alloy to the environmental assisted cracking. The aim of this work is to analyze the Stress Corrosion Cracking and hydrogen embrittlement resistance behavior of Alloy 625 obtained by LPBF, both in as built condition and after a standard heat treatment (grade 1). U-bend test performed in boiling magnesium chloride at 155 and 170° C confirmed the immunity of the alloy to SCC. However, slow strain rate tests in simulated ocean water on cathodically polarized specimens highlighted the possibility of the occurrence of hydrogen embrittlement in a specific range of strain rate and cathodic polarization. The very fine grain size and dislocation density of the thermally untreated specimens appear to increase the hydrogen diffusion and the embrittlement effect on the pre-charged specimens that were deformed at high strain rate. Conversely the heat treatment appeared to mitigate hydrogen embrittlement at high strain rates however whilst at slow strain rate all the specimens showed a similar behavior.