Study On The Influence Of Impurity Element Si On The Exfoliation Corrosion And Stress Corrosion Cracking Behavior Of7050Aluminum Alloy

Due to its high strength, good plasticity and good corrosion resistance,7000series ultra high strength aluminum alloys are now widely used inaircrafts for civil and military use and serve as an important structuralmaterials in the field. In recent years, the research on the7000series Alalloys become popular, but lack of systematic research on the influence ofimpurities on the corrosion-resistant properties. Different content ofSilicon was added to the nominal7050alloy and the effect of Si on themicrostructure and properties of the alloy were studied systematically byexfoliation corrosion experiments, polarization curve test experiment,electrical conductivity testing and stress corrosion experiments, duringwhich optical metallography （OM）, stereo microscope, scanning electronmicroscope （SEM） and energy-dispersive spectrometry （EDS） were used.Microstructure observation shows that grains were arranged along themachining direction after aging treatment. The main second phases areAl₂CuMg and Mg₂Si. The amount, distribution and shape of the secondphases are closely related to the content of Si. When Si content is0.094wt.%, the most amount of S phase with irregular large size about40-70μm are observed, while the Mg₂Si phase is hard to find, and usuallyin small size. When Si content increases to0.134wt.%, the amount and sizeof S phase decrease, Mg₂Si phase is found an obvious increase and keep onincreasing with the Si content while the opposite with S phase.Electrical conductivity test results show that the more Si content, thegreater the conductivity. Si atoms increase the solid solubility as well as the amount of second phase which bring out other atoms in the matrixduring precipitation, reducing the solid solubility of the alloy, thus givingthe alloy a higher electrical conductivity.The polarization curve test shows that the alloy with the most Sicontent has the highest corrosion potential, which in thermodynamicsimproves the corrosion resistance of the alloy. The results mainly causedby the higher potential of Si atoms and the Si-rich phase Mg₂Si.Exfoliation corrosion properties of alloys with different Si contentwere studied. The results show that when the content of Si is0.094wt.%,the alloy has a good corrosion resistance. With the content of Si increasefrom0.134wt.%to0.261wt.%, the followed bigger potential differencebetween the second and the matrix would accelerate the corrosion. Inaddition, the self-corrosion potential increases which will delay theinitiation of corrosion. But the following corrosion will be faster, givingthe alloy a poor anti-corrosion property.The slow strain rate tensile results show that the tensile mechanicalproperties in the air decrease with the increase content of Si from0.094wt.%to0.261wt.%, which main due to the susceptible crackinitiation caused by the concentration of second phase at the grainboundaries. The effect of second phase strengthening proposed by thesmall increase of Si from0.094wt.%to0.134wt.%is similar with themicrocracks effect, and give the alloy a fairly properties both testing in theair and in corrosive environment. But when the Si content reaches0.261wt.%, the coarsening of the Si-containing second phase is likely tocause stress concentration and reduce the mechanical strength. When Sicontent is0.094wt.%, the S phase tend to be continuous distributed andmay be continuous dissolved in corrosive environment, giving the alloy arapid cracking and the highest SCC susceptibility. With the increasecontent of Si, the distribution of S phase become discontinuous, hinderingthe continuous dissolution process, resulting a reduced SCC susceptibility.