Study on corrosion fatigue properties and fracture characteristics of 7075 aluminum alloy FSW joint
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摘要: 以7075-T6铝合金搅拌摩擦焊接头为研究对象，对其显微组织结构、3.5% NaCl(质量分数)溶液腐蚀疲劳寿命和腐蚀疲劳断裂特征进行了研究，分析了7075铝合金搅拌摩擦焊接头的腐蚀疲劳性能及断裂过程.结果表明，7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳S-N曲线方程为lgN=5.845-0.014S，随着应力幅增大，腐蚀疲劳寿命大幅度降低；腐蚀疲劳裂纹起源于接头的热影响区，逐渐扩展最终断裂于接头的焊核区.腐蚀疲劳断口存在多个裂纹源，且受到应力集中作用的影响，裂纹源萌生于腐蚀坑处.高应力作用加剧了试样边角部分的腐蚀损伤，导致边角比平面位置腐蚀程度更严重.裂纹扩展区出现了明显的晶间断裂和疲劳辉纹；在腐蚀介质和交变载荷的共同作用下，裂纹扩展区腐蚀程度最重，晶界处产生了阳极溶解现象并产生了“冰糖块状”和“蚁巢状”的形貌特征；瞬断区产生了大量解理台阶和二次裂纹，为脆性断裂，在第二相粒子分布区域存在孔洞形貌特征.Abstract: Taking 7075-T6 aluminum alloy friction stir welding joint as the research object, the microstructure, 3.5 wt % NaCl solution corrosion fatigue life and corrosion fatigue fracture characteristics were studied, and the corrosion fatigue performance and fracture process of 7075 aluminum alloy friction stir welding joint were analyzed. The results showed that: the S-N curve equation of corrosion fatigue of 7075-T6 aluminum alloy friction stir welded joint is lgN=5.845-0.014S, With the increasing of stress amplitude, the corrosion fatigue life decreased greatly. The corrosion fatigue crack originated in the thermal affected zone of the joint, gradually expanded and finally broke in the welded core zone of the joint. There were multiple crack sources in the corrosion fatigue fracture, and with the influence of stress concentration, the crack source originated in the corrosion pit. The high stress aggravated the corrosion damage of the corner part of the sample, led to more serious corrosion than that of the plane position. Obvious intergranular fracture and fatigue striation appeared in the crack growth zone. Under the combined action of corrosive medium and alternating load, the crack growth zone suffered the most serious corrosion, and the anodic dissolution occurred at the grain boundary, resulting in the morphology characteristics of “rock candy” and “ant nest”. Instantaneous fault zone was brittle fault,and there were a lot of cleavage steps and secondary cracks in this zone, the pore morphology appeared in the second phase particle distribution region.
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图 4 FSW接头显微组织
Figure 4. Microstructure of FSW joint. (a) microstructure of NZ; (b) microstructure of TMAZ; (c) microstructure of HAZ
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图 9 腐蚀疲劳源断口形貌
Figure 9. Fracture morphology of corrosion fatigue source. (a) morphology map around crack source; (b) morphology of crack source region
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图 10 裂纹扩展区形貌图
Figure 10. Morphology of crack propagation zone. (a) fatigue striations; (b) intergranular corrosion; (c) ant-nest corrosion
表 1 焊接工艺参数
Table 1 Welding parameters
主轴转速n/(r·min−1) 焊接速度 $\nu $ /(mm·min−1)搅拌头倾角 $\theta $ /(°)搅拌针转动方向 700 150 2.5 逆时针 
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表 2 FSW腐蚀疲劳试验数据
Table 2 Corrosion fatigue test results of FSW
试样编号 应力幅
$\sigma $ /MPa加载频率
f/Hz应力比
R腐蚀疲劳寿命
N(周次)A1 250 0.3 0.06 190 A2 250 0.3 0.06 230 A3 250 0.3 0.06 326 B1 230 0.3 0.06 645 B2 230 0.3 0.06 732 B3 230 0.3 0.06 790 C1 180 0.3 0.06 2822 C2 180 0.3 0.06 2980 C3 180 0.3 0.06 3243 D1 150 0.3 0.06 6435 D2 150 0.3 0.06 7023 D3 150 0.3 0.06 8230 E1 130 0.3 0.06 9230 E2 130 0.3 0.06 10560 E3 130 0.3 0.06 12542
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表 3 S-N曲线方程及参数
Table 3 S-N curve equations and parameters
铝合金牌号 S-N方程 m C 7075—T6 lgN = 5.852 − 0.014S 0.032 0.71 × 106
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