Influence of welding-induced microstructure on fatigue crack growth in 5xxx aluminum alloys in air and hydrogen environments

• Fatigue crack growth studied in weld and heat affected zone (HAZ) of Al-5 Mg in H. • Fine grain (FG)-HAZ had superior fatigue performance in H and air environments. • Limited amount of precipitates in FG-HAZ limits H accumulation at GBs. • High amount of precipitates in coarse grain-HAZ & weld promotes H-induced intergranular fracture. This study investigates the influence of welding-induced microstructural variations on the fatigue crack growth behavior of Al-Mg aluminum alloys in mixed hydrogen/humidity environments, with specific relevance to welded structures used in ships. The weldment comprises three distinct regions: fine-grained heat-affected zone (FG-HAZ), coarse-grained HAZ (CG-HAZ), and weld metal, each exhibiting different resistance to hydrogen-assisted fatigue. Experimental analysis using compact tension testing, scanning Kelvin probe force microscopy, and advanced microscopy techniques reveals that the FG-HAZ demonstrates superior resistance to hydrogen embrittlement, retaining ductile fracture behavior under hydrogen exposure. This enhanced performance is attributed to its refined grain structure and the rare occurrence of Al-Mg-rich phase particles along grain boundaries. In contrast, the CG-HAZ and weld metal show intergranular fracture features in hydrogen environments, associated with the presence of these particles at grain boundaries. These results underscore the importance of microstructural control, particularly phase morphology and grain boundary characteristics, for improving the reliability of aluminum alloy welds in marine hydrogen environments.