Microstructure And Properties Of 7B85 Aluminum Alloy By Ultrasonic Surface Rolling

Aluminum alloys are widely used in aerospace,transportation,construction materials and other fields because of their low density,high specific strength and good specific stiffness.However,the changeable and bad service environment makes the surface of aluminum alloy easy to be damaged.such as crack,wear and corrosion,etc.resulting in deterioration of the performance of the aluminum alloy,which will limit its use range.Therefore,it is necessary to improve the corrosion resistance and mechanical properties of 7B85 aluminum alloy.In this paper,The effect of ultrasonic surface rolling treatment on the microstructure and mechanical properties of 7B85 aluminum alloy and the change of corrosion resistance in 3.5wt.%Na Cl solution were studied.The main research contents and results are as follows:(1)Surface nanocrystallization of 7B85 aluminum alloy was carried out by ultrasonic surface rolling treatment.The influence of main technological parameters of ultrasonic surface rolling on surface properties of 7B85 aluminum alloy was studied.The results show that with the increase of pressure or preloading depth in ultrasonic surface rolling,the scratches on the alloy surface gradually weaken,the surface of the alloy becomes smoother and smoother,the surface roughness decreases,and the surface microhardness increases;some small microcracks appear on the surface of the alloy when the pressure or preloading depth was too large.As for the feed speed,the surface finish of the alloy does not change much and the hardness value remain basically the same when the feed speed is from 50mm/min to 1200 mm/min.Considering the processing efficiency,the optimal ultrasonic rolling process parameters of 7B85 aluminum alloy are as follows:pressure is 0.2 mm,preloading depth is 0.3 MPa and feed rate is 1200 mm/min.(2)The microstructure and mechanical properties of 7B85 aluminum alloy treated by ultrasonic surface rolling were studied.The results show that there are mainly Al phase,?precipitate phase,Fe-rich Al₂Cu(?)or Al Cu phase in 7B85 aluminum alloy before ultrasonic surface rolling treatment.After rolling treatment,part of the?precipitated phase is dissolved into the Al matrix.At the same time,a large number of dislocation lines and dislocation entanglements appear in the surface layer,and the microstructure stress on the alloy surface is generated.After ultrasonic surface rolling treatment,the surface of 7B85 aluminum alloy was nanocrystalized,the thickness of the nanocrystalline layer was about 1.1?m,the grain size reached 22.52 nm,the surface microhardness was increased by 60.96%,the ultimate tensile strength was increased by 13.2%,and the tensile yield strength was increased by 11.7%.It shows that the mechanical properties of 7B85 aluminum alloy are enhanced by ultrasonic surface rolling treatment.which is mainly due to the dislocation strengthening caused by dislocation lines and dislocation entanglement,the fine grain strengthening caused by the formation of nanometer layer on the surface and the solid solution strengthening caused by the solid dissolution of?precipitated phase in the matrix metal.(3)The immersion test and electrochemical corrosion test of 7B85 aluminum alloy by ultrasonic surface rolling were carried out in 3.5wt.%Na Cl solution at room temperature.After ultrasonic surface rolling treatment,the open circuit potential of 7B85 aluminum alloy becomes higher,the arc radius of electrochemical impedance spectrum becomes larger,the thickness of corrosion product film becomes thicker,the film resistance becomes larger,the release rate of H₂becomes slower,the self-corrosion potential is higher,and the corrosion current density is lower.At the same time,immersion experiments at room temperature showed that pitting pits appeared on the surface of 7B85 aluminum alloy before and after ultrasonic surface rolling treatment,which was mainly due to the formation of galvanic cells between aluminum phase and Fe-rich Al Cu phase,which accelerated the dissolution of aluminum around Fe-rich Al Cu particles.After rolling treatment,the surface corrosion products are denser and more complete,and the surface corrosion products are Al(OH)₃and Al₂O₃,which indicates that the intergranular corrosion caused by nanocrystalline and stress corrosion caused by internal residual stress can quickly promote the formation of passivation film on the surface of aluminum alloy and slow down the formation of pitting pits.Immersion test at room temperature and electrochemical corrosion test show that 7B85 aluminum alloy has better corrosion resistance after ultrasonic surface rolling.