Correlation between microstructure and corrosion behaviour of power bed laser beam melting 17-4PH stainless steel

摘要

In the past few years, additive manufacturing has become one of the most innovative processes tobuild metallic components because it allows complex geometry parts to be created in a short time.Especially, power bed Laser Beam Melting (LBM), also known as Selective Laser Melting, is themost popular metallic additive manufacturing process and significant progress was made in theunderstanding of the microstructure and the properties of LBM metallic parts like mechanicaltensile properties or fatigue behaviour. However, only few works focussed on the corrosionproperties of LBM metallic parts [1], [2].This work concerns 17-4PH martensitic stainless steel produced by power bed laser beam melting.The microstructure of the steel generated by the LBM process and that of a wrought steel used asreference were studied for the same metallurgical state. The classical thermal treatment calledH900, which is used for the industrial wrought 17-4PH steel to obtain homogenized microstructurewith the best mechanical properties (hardening due to the formation of copper precipitates), waschosen. The corrosion behaviour of the LBM steel was compared to that of the wrought 17-4PHsteel in order to determine the critical microstructural parameters concerning the reactivity of thesteel in an aqueous electrolyte.First results showed some differences in the corrosion behaviour between the LBM and thewrought 17-4PH steels. The corrosion potential values extracted from open circuit potentialmeasurements were more positive for the wrought sample than the LBM one. Anodic polarizationcurves showed higher pitting potential values and lower corrosion current values for the LBMsample compared to the wrought one. The results thus suggested a better corrosion resistance forthe LBM samples compared to the wrought steel. Results from the microstructural characterizationshowed a finer grain size and a higher percentage of austenite phase for the LBM steel which couldcontribute to explain the electrochemical results. To better understand the corrosion mechanisms,additional electrochemical tests were performed to characterize the corrosion damaging moreaccurately and generate more data about the microstructure / corrosion interactions. Particularattention was paid to metastable pitting which can be related to the stability of the passive film inthe solution. Results showed significant differences between the two steels.