Study On Corrosion Property And Mechanism Of Spray Formed7055Aluminum Alloy

In this thesis, by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) etc. Rockwell hardness, conductivity, tensile specimens at room temperature, stress corrosion test and electrochemical corrosion test methods. Effects of hot extrusion and aging heat treatment on the microstructure of the alloy, chemical composition, mechanical properties were researched. Studied effects of aging on stress corrosion sensitivity and electrochemical corrosion behavior of spray formed7055aluminum alloy. The conclusions are as follows:The microstructure of spray formed as-deposited billet is composed of equiaxed grains with no macrosegregation and the grain size is between20μm and50μm. After hot extrusion, the primitive grain boundary of as-deposited billet vanished and reticular phases and grains were broken which comes with more densified microstructure.The variation of surface corrosion morphology of alloy, polarization curves, EIS, phase angle and impedance modulus in Bode diagram was contrasted. The result shows that the resistance to electrochemical corrosion of single-stage aging is worst, while the dual-stage aging shows the best electrochemical corrosion resistant. In the beginning, the corrosion process was hindered by oxide film, phase angle is in the range of80°, the radius of capacitance arc is large. With the extension of etching time, oxide film was broken, phase angle came down, capacitance arc got smaller, corrosion occurs more readily.It is shown that the spray formed7055aluminum alloy has the highest strength after the single-stage aging, the density of precipitated phase in the grain is very high, tiny precipitation was disputed on the boundary continuously, leading good strength but poor SCCR. After dual-stage aging, the structure in the grain was dispersed with tiny particles of GP zone and η’, while at the grain boundary, the precipitation were made and broken and the PFZ was narrow. So the strength was lower than the T6, while the SCCR was improved. Compared with T6, the precipitations after RRA treatment were coarser and more stable, while η precipitates distributed discontinuously at the grain boundary, with wider PFZ, which was similar to those of over ageing. This microstructure of the studied alloy combined the advantages of both T6and T76treatment, leading to ultra high strength and SCCR.