Abstract
The corrosion fatigue properties and fracture characteristics of friction stir welding joints of 7075 aluminum alloys were studied via corrosion fatigue tests, electrochemical measurements, and corrosion fatigue morphology and microstructure observations. The results show that the corrosion fatigue crack of the friction stir welding (FSW) joint of 7075 aluminum alloys originated in the junction zone between the thermomechanically affected zone and the weld nugget zone. The corrosion fatigue life of the joint decreased with increasing stress amplitude, with an S–N curve equation of lgN = 5.845 − 0.014S. Multiple crack sources were observed in the corrosion fatigue fracture. The main crack source originated from the corrosion pits at the interface between the thermomechanically affected zone and the weld nugget zone due to the influence of the coarse microstructure and the large potential difference between both zones. Corrosion morphologies of a rock candy block and an ant nest appeared in the crack propagation zone and the grain boundary of the weld nugget zone. In addition, fatigue speckles and intergranular fractures were observed, as well as brittle fracture characterized by cleavage steps and secondary cracks in the final fracture zone.
Highlights
The 7075 aluminum alloys are widely used in marine engineering and other fields due to their good plasticity, high specific strength, and high specific stiffness
All samples were fractured in the friction stir welding (FSW) joint regions, and the exact locations of the fracture were determined by subsequent microscopic structure observation
The results indicate that the corrosion fatigue life decreased greatly with increasing stress amplitude
Summary
The 7075 aluminum alloys are widely used in marine engineering and other fields due to their good plasticity, high specific strength, and high specific stiffness. Aluminum alloy structures are prone to corrosion fatigue failure due to the corrosive environment and cyclic loading. The corrosion fatigue of aluminum alloy FSW joints can cause mechanical fatigue damage under cyclic stress. The electrochemical reaction between the corrosion medium and the aluminum alloy results in corrosion damage When these two kinds of damages coexist, the corrosion effect of the corrosive medium on the crack-tip metal significantly increases the corrosion fatigue crack growth rate of the welded joint surface, leading to the premature failure of the FSW joint. It is necessary to study the corrosion fatigue performance of FSW joints of aluminum alloy under marine environments. The corrosion fatigue, S–N curve, and corrosion fatigue performance of 7075-T6 aluminum alloy FSW butt-welding joints were studied. According to the observation of the fracture morphology of the crack source region, the crack propagation region, and the transient fracture region, the fracture characteristics were revealed
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