Effects of friction stir processing on hydrogen storage of ZK60 alloy

In this paper we report the use of a novel processing route to produce samples for use as a hydrogen carrier. ZK60 alloy was produced by induction melting and sheets taken from the melt ingot were submitted to Friction Stir Processing (FSP) and subsequent manual filing with a rasp under ambient conditions. Samples from the Base Metal As-Cast (AC) and Stir Zone (SZ) were microstructurally investigated before and after filing including the alloy in as-cast state. The results showed that before filing SZ and AC samples presented equiaxial grains with the SZ sample having a much finer microstructure compared to AC. The values of grain sizes are around 150 μm and 1-2 μm for the AC and SZ samples, respectively. After filing, both samples presented similar grain sizes of only 60 nm. Although they attained similar grain sizes, the filings from SZ were more homogeneous and presented thinner innerlayers compared to its AC counterpart. The filings taken out from the SZ presented much faster kinetics for hydrogen absorption mainly due to its thinner innerlayers and its finer second-phase particle distribution, reaching up to 4.5 wt% of hydrogen uptake against only 1.0 wt% for AC sample after 10 h of absorption in the first cycle. Both samples presented similar behavior for full discharge time. These results show the possibility of using FSP with subsequent filing as a mean to obtain materials with suitable properties for use as energy carriers with enhanced kinetics and better oxidation resistance in shorter processing times.