Effects Of Impurities Content On The Microstructure And Stress Corrosion Cracking Susceptibility Of 7055 Aluminum Alloy

7xxx series aluminum alloys(Al-Zn-Mg-Cu) are extensively used in the aircraft structure due to their low density and high strength. Highly susceptible to stress corrosion cracking(SCC) in the peak-aged temper(T6), numerous researches have been focused on heat treatments in order to solve this shortcoming. They are nowadays heat-treated to overaged(T7), or retrogression and re-aged(RRA) also known as T77 type tempers, conditions that give them a much higher SCC resistance. However research on the impact of impurity elements on the corrosion resistance(SCC resistance) of such alloy is nearly absent. Recently, 7055 aluminum alloy, introduced in the T77 temper showed a particularly unique combination of mechanical properties, corrosion resistance and pricing. It was expressly elaborated for use in compression-dominated structures, such as upper wings surface, keel beams, horizontal stabilizer and seat tracks. Therefore, it made sense to focus our research on this alloy. The main objective of this present dissertation was to correlate the impurities content(silicon and iron) and the resultant microstructure with the SCC behavior of the T77-treated 7055 aluminum alloy.The study was conducted by casting 7055 aluminum alloys with different impurities content but constant Fe/Si ratio, submitting them to solution aging, and studying their microstructure and SCC susceptibility. That was done by means of SEM-EDS, slow stress rate testing, and various electrochemical testing such as open-circuit potential measurement, polarization curve experiment and electrochemical impedance spectroscopy testing. The main conclusions were as follows:1. Mg(Al,Cu,Zn)2 was identified as the main strengthening precipitate and its amount decreases while the impurities content increases. Moreover, the addition of iron led to the growth of Fe-rich second intermetallic particles(Al7Cu2Fe and Al3Fe), and the increase of silicon from 0.071% Si to the growth of Si-rich(Mg2Si) second intermetallic phases. The reduction of the content of Mg(Al,Cu,Zn)2 was attributed to the growth of these three particles.2. 7055 aluminum alloy is very sensitive to microstructural change stemming from the impurities content. At Fe/Si=2, when the impurities content was increasing, theultimate strength, elongation and time to failure were considerably decreasing while the SCC was increasing.3. The relative SCC resistance was studied through electrochemical testing and microstructural examination. From the microstructural examination, it was observed that the distribution became more discrete and the size of the second phases coarsened when increasing the impurities content, which was extensively reported as enhancing the SCC resistance. This increase of the inter-particle distance was due to the large reduction of the Mg(Al,Cu,Zn)2 area fraction, while the size of this particle remained constant with the increase of impurities. It is true that Mg2 Si and Fe-rich second phases area fractions were increasing, but the number of particles was low as they were coarse particles, and they coarsened even more with the increase of impurities content, and so didn’t affect significantly the spacing between intermetallic phases.4. Moreover, the study of Tafel plots and Nyquist plots indicated that the corrosion rate was decreasing when the impurities content was increasing, limiting both the crack velocity(anodic dissolution mechanism) and the hydrogen production. Indeed, since the corrosion potential of intermetallic phases differed from the one of the matrix, they formed local electrochemical cell. As the total area fraction of second phases increase, the decrease of the corrosion rate might come from the nature of the particles. It has been concluded that, at the potential of the 7055 aluminum alloy, the corrosion rate of Mg(Al,Cu,Zn)2 is superior of the ones of Mg2 Si, Al7Cu2 Fe and Al3 Fe.5. Some literatures have already shown that the ratio Fe/Si is affecting the casting properties, In the following dissertation, the change of ratio from Fe/Si=0.6(alloy 1) to Fe/Si=2(alloy 2) by adding some iron was not affecting the elongation and time to failure in the same direction than the addition of impurities at constant ratio. Similar observations were made for the corrosion rate, which increase from alloy 1 to alloy 2. That came up with the idea that the ratio Fe/Si is affecting the properties of 7055 aluminum alloy and by extrapolation the properties of 7xxx aluminum alloys.