Corrosion fatigue crack growth in A7N01S−T5 aluminum alloy MIG welded joints

This paper studies the corrosion fatigue crack growth (CFCG) in 3.5 wt% NaCl solution of the local zones of A7N01S−T5 aluminum alloy metal inert gas (MIG) welded joints cut from high−speed train underframes after service lives of 1.8 million km. The results show that the base metal (BM) region exhibits the highest rate of CFCG compared to both the heat−affected zone (HAZ) and the weld metal (WM) zone. Crack features suggest that the corrosion fatigue failure mode of the BM is hydrogen−assisted propagation and that the failure mode for both the HAZ and WM is anodic dissolution. We calculate that the multi−site adsorption and substitution of Cl− on the surface of the passivation film can make the oxygen and aluminum atoms detach from the surface of the passivation film, causing its destruction and allowing the cracks to expand further. Hydrogen is more likely to accumulate and diffuse along the grain boundaries, reducing their strength and causing them to become the most likely orientation for crack propagation. This study provides the new insight into the specific CFCG mechanisms of each zone in aluminum alloy welded joints.