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.

The corrosion fatigue crack growth (CFCG) behavior of A7N01P-T4 aluminum alloy under different plastic damage was studied. The crystallographic orientation and microstructure along the crack propagation path were investigated by electron back-scattered diffraction (EBSD). In addition, molecular dynamics were employed to statistically analyze crystal deformation, and dislocation types and densities under different degrees of plastic damage. The results have revealed that compared with L0 (no plastic damage:Ɛ=0), the CFCG threshold of L1 (plastic damage:Ɛ=0.045) is lower, and the CFCG threshold of L2 (plastic damage: Ɛ=0.15) is increased. This can be attributed to a greater degree of work hardening after the occurrence of plastic deformation. Some plastic deformation can also cause local softening of the material. This results in differences in CFCG response under different degrees of plastic damage.

This paper studies the relationship between plastic damage and the ultrasonic nonlinear coefficient by changing the interior plastic strain of A7N01P-T4 aluminum alloys. An effective evaluation of the plastic damage of tensile test specimens was made by comparing with two different nonlinear ultrasonic detection methods, namely, nonlinear longitudinal wave method and surface wave method. The nonlinear coefficient of the stress wave factor was introduced to characterize the plastic damage for metallic materials. The results showed that with the increasing plastic damage degree, the nonlinear coefficient increased slowly at the early stage of yield hardening (3%) and then increased rapidly at the middle stage (3–5%). It is concluded that the nonlinear longitudinal wave is more suitable for point measurement and its sensitivity is lower than that of the longitudinal wave.

The corrosion fatigue cracks propagation behavior of A7N01P-T4 and A7N01S-T5 aluminum alloys in 3.5 wt.% NaCl solution and air were studied by single side notch corrosion fatigue tests (SNET). The crack growth rate (da/dN) was measured and the crack propagation mechanism was analyzed by scanning electron microscopic analysis. The results showed that the corrosion fatigue cracks growth rate of A7N01S-T5 alloy in air and 3.5 wt.% NaCl solution is faster than that of A7N01P-T4 alloy. The crack propagation follows a mixed intergranular and transgranular mode. The anodic dissolution and hydrogen embrittlement accelerate the propagation of corrosion fatigue crack of 7-series Al alloy in 3.5 wt.% NaCl solution. The corrosion resistance and fatigue crack propagation are related to the strength of the alloys and the density of grain boundaries.

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.

The cold metal transfer (CMT) technique was introduced to repair the heat-affected zone (HAZ) of the 7075-T651 aluminium (Al) alloy MIG welding joint using wire ER5356 as filler. The microstructural characteristics and fracture toughness of the welded joint before and after CMT repair welding was investigated. The results indicated that the sound welded joint was successfully produced through CMT; the repaired heat-affected zone (RHAZ) exhibited fine grain and small grain boundary precipitates (η phase) with a larger inter-particle spacing distance compared to that of the HAZ. Additionally, the J0 integral value of the RHAZ (43.38 KJ m−2) and the repaired weld metal (42.68 KJ m−2) were higher than that of the HAZ of the MIG welded joint (32.88 KJ m−2) and the CMT welded joint (32.81 KJ m−2). The fracture toughness properties of the joint repaired with CMT were improved without sacrificing its overall fracture toughness properties. This study proposes a new method for repairing MIG welding joints made of 7075-T651 Al alloy, further reducing the cost of manufacturing high-speed train.