Effect of Cryogenic Treatment on Microstructural Evolution and Corrosion Mechanism in Subsurface Layer of 7075-T6 Aluminum Alloy during High-speed Machining
In the realm of aviation and aerospace,7075 aluminum alloy has emerged as a pivotal choice for aircraft structural components,engine parts,and aerospace assemblies,owing to its remarkable attributes such as elevated strength and lightweight characteristics. However,confronted with intricate operational conditions,its components are often subject to elevated temperature,high pressure,and frequent load,rendering them susceptible to fractures and failures. Cryogenic treatment can enhance the dislocation density and crystalline structure of 7075 aluminum alloy,thereby bolstering its resistance to stress concentration and corrosion properties. In this paper,7075-T6 aluminum alloy was selected as the research object,and its microstructure evolution,surface morphology and corrosion resistance were investigated by transmission electron microscopy (TEM),HRTEM,X-ray diffractometry (XRD),scanning electron microscopy (SEM) and electrochemical analysis. The present investigation delved into the influence of cutting speed on the microstructural evolution mechanism and corrosion resistance properties of both cryogenically treated and untreated 7075-T6 aluminum alloy.The results showed that the surface defects of the workpiece at different cutting speeds were mainly characterized by chip adhesion and microcracks,and the surface quality of the workpiece was better when the cutting speed was increased to 1500 m/min compared with the cutting speed of 500 m/min. The surfaces of both T6 and T6-C aluminum alloys,after undergoing machining processes,were found to display the presence of α-phase (Al matrix) and η-phase (MgZn2) upon examination. In terms of the diffraction intensity of the α-phase peaks,the T6 aluminum alloy exhibited its highest diffraction peak on the (200) crystal plane when the cutting speed was set at 500 m/min. After cutting of 7075-T6 aluminum alloy without deep-cooling treatment,the precipitation phase was dominated by bar and block morphology,which produced a grain boundary free zone (PFZ) with a width of about 12-20 nm. After the cutting of 7075-T6 aluminum alloy after deep-cooling treatment,the precipitation phase was dominated by a massive morphology,and there existed a discontinuous non-precipitated grain boundary band with a width of 9-17 nm. Combined with the corresponding diffraction pattern (SAED) and XRD analysis,it could be seen that the precipitated phase was mainly η-MgZn2. Both the deep-cooling treatment and the increased cutting speed significantly improved the corrosion resistance of the 7075-T6 aluminum alloy,there was significant cracking in the corrosion morphology. Through electrochemical analysis,it was learned that at a cutting speed of 1500 m/min,the current density of the deep-cooled 7075-T6 aluminum alloy decreased by 3.24×10-6 A/cm2 and the polarization resistance increased by 1.68×105 Ω·cm2 compared with that without deep-cooling treatment. At the same time,with the increase of cutting speed,the radius of the capacitive showed an increasing trend of change,and compared with T6 aluminum alloy,T6-C aluminum alloy had a larger radius of arc resistance on the surface. Deep-cooling treatment can effectively refine the grain structure of 7075-T6 aluminum alloy,which in turn strengthens the dislocation density and thus significantly improves the corrosion resistance of the alloy.
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