The effect of friction stir processing on the hydrogen susceptibility of AA5083 specimens after hydrogen cathodic charging

Abstract

The present study investigated the effect of hydrogen on the mechanical degradation of friction stir processed (FSPed) 5083 aluminum alloy by intense hydrogen cathodic charging (HCC). The effect of different numbers of FSP passes was investigated: 3 and 8 passes, respectively. Hydrogen-charged and uncharged specimens were subjected to tensile testing and microhardness evaluation analysis, and were examined through optical microscopy, focus variation microscopy (FVM), and scanning electron microscopy (SEM) both on the microstructure and fracture zone. The results showed that the FSP process introduced a refined microstructure with finer grains. This led to an improved mechanical response during tension tests of the uncharged specimens; the energy absorption increased from 85 MJ/m3 of the base material to 94 MJ/m3 and 97 MJ/m3 for the 3 and 8 FSP passes, respectively. The introduction of hydrogen through the HCC process led to a more brittle mechanical response with a decrease in the energy absorption capability for all the charged specimens. The more prone specimen was the 8 FSP passes specimen where the energy absorption dropped by 20% and 71% for the two different charging current densities. The 3 FSP passes specimen presented a reduction of energy absorption of 4% and 18%, respectively, where the base material presented a reduction of 8% and 14%, respectively. This brittle response is also evident from the microhardness testing where the hydrogen charging led to increased surface hardness values. The 3 FSP passes specimen presented a better mechanical response with respect to the base material specimen (and the 8 FSP passes specimen) for all the charging conditions, and this led to the conclusion that a small number FSP surface modification could be a beneficial surface modification process as it improves the mechanical response of the material and is not significantly affected by hydrogen charging environments.