6005A-T6铝合金腐蚀疲劳裂纹扩展行为研究

Study on Corrosion Fatigue Crack Growth Behavior and Mechanism of 6005A-T6 Aluminum Alloy

摘要: 本研究聚焦于铝合金的腐蚀疲劳裂纹扩展行为，选用6005A-T6材料制备试件。试件在XOY平面内(X轴为轧制方向)分别沿X轴和Y轴方向切制改进型单边缺口拉伸(SENT)样品，并在室温空气及pH值为5的3.5 wt.% NaCl溶液中进行腐蚀疲劳测试。结果显示，材料经轧制后形成明显的轧制织构，X向(平行轧制)试样的晶粒呈长条状扁平晶，沿轧制方向定向排列。6005A-T6铝合金晶内分布着大量细小的β"-Mg₂Si强化相，晶界存在不连续的粗大析出相(主要为β-Mg₂Si)，晶界两侧存在明显的无沉淀析出带(PFZ)。与惰性气氛相比，NaCl介质显著提升了裂纹的扩展速率并降低了扩展阈值，表明腐蚀环境对疲劳裂纹有明显的加速作用。显微组织分析进一步表明，腐蚀介质使裂纹路径更为曲折，伴随显著的偏转、闭合及分叉现象；断口观察到泥纹状腐蚀产物、沿晶二次裂纹及腐蚀坑，呈现沿晶与穿晶混合的断裂模式。基于电化学–力学耦合机制，本文阐释了Cl⁻引发的点蚀在循环载荷作用下如何促进裂纹萌生与扩展，从而揭示了“腐蚀–疲劳”协同损伤的本质机制。

Abstract: In order to study the corrosion fatigue crack propagation performance of aluminum alloy, 6005A-T6 aluminum alloy was selected as the research object. The improved single-edge notch tensile (SENT) specimens parallel to the X-axis and parallel to the Y-axis were cut in the XOY plane (the X-axis direction is the rolling direction), and the corrosion fatigue crack propagation experiment was carried out in the laboratory air and 3.5 wt.% NaCl (pH = 5) solution environment. The results show that the material exhibits a distinct rolling texture with elongated and flattened grains in the X-direction (parallel to rolling) aligned along the rolling direction after rolling. The 6005A-T6 aluminum alloy contains numerous fine β"-Mg₂Si strengthening phases within the grains, while discontinuous coarse precipitates (primarily β-Mg₂Si) are present at the grain boundaries. The pronounced precipitate-free zone (PFZ) are observed on both sides of the grain boundaries. Compared with the inert air condition, the crack growth rate of the sample in the NaCl solution environment increases significantly, the crack growth threshold decreases, and the corrosive medium significantly accelerates the fatigue crack growth, showing obvious environmental acceleration effect. Further analysis of the fracture morphology shows that the crack propagation path in the corrosive environment is more tortuous, accompanied by complex behaviors such as crack deflection, closure and bifurcation. Mud-like corrosion products, intergranular secondary cracks and corrosion pits can be observed on the fracture surface, and the fracture mode shows the mixed characteristics of intergranular and transgranular. Based on the perspective of electrochemical-mechanical coupling, the process of pitting corrosion caused by Cl corrosion promoting the initiation and propagation of fatigue cracks under cyclic loading was expounded, and the internal physical mechanism of “corrosion-fatigue” synergistic damage was revealed.