Using the potentiodynamic polarization analysis, the fatigue crack propagation behavior of A7N01P‐T4 aluminum alloy metal inert gas welded joints cut from a high‐speed train underframe after 1.8 million km operation was studied in air and in a 3.5 wt% NaCl solution. The fracture surface and crack growth path were analyzed using optical microscopy, scanning electron microscopy, and electron backscattered diffraction. The results reveal that the corrosion fatigue crack growth rate of an A7N01P‐T4 welded joint in a 3.5 wt% NaCl solution is higher than that in air. Furthermore, the corrosion fatigue crack growth rate is noted to be the fastest in the heat‐affected zone, followed by the base metal, whereas it is the slowest in the weld metal, which is consistent with the corrosion resistance of the A7N01P‐T4 joints. The second phase is observed to exhibit a significant influence on the corrosion fatigue crack propagation path. The cracks are noted to grow toward the soft orientation and have obvious plastic deformation during the propagation process, which indicates that the anodic dissolution is the main cause of the corrosion fatigue crack growth.

中文翻译：

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180万公里运行后高速火车底架A7N01P-T4铝合金焊接接头的腐蚀疲劳裂纹扩展行为

使用电位动力学极化分析，研究了在空气中和在3.5 wt％的NaCl溶液中从高速列车底架上切割下来的A7N01P-T4铝合金金属惰性气体保护焊缝在180万公里运行后的疲劳裂纹扩展行为。使用光学显微镜，扫描电子显微镜和电子背散射衍射分析了断裂表面和裂纹扩展路径。结果表明，在3.5 wt％的NaCl溶液中，A7N01P-T4焊接接头的腐蚀疲劳裂纹扩展速率高于空气中的腐蚀疲劳裂纹扩展速率。此外，腐蚀疲劳裂纹的生长速率在热影响区中是最快的，其次是母材，而在焊接金属中是最慢的，这与A7N01P-T4接头的耐腐蚀性一致。观察到第二阶段对腐蚀疲劳裂纹扩展路径表现出显着影响。注意到裂纹在扩展过程中朝着软取向生长并且具有明显的塑性变形，这表明阳极溶解是腐蚀疲劳裂纹扩展的主要原因。