Investigation into the stress corrosion cracking properties of AA2099, an Al-Li-Cu alloy

Recently developed Al-Li-Cu alloys show great potential for implementation in the aerospace industry because of the attractive mix of good mechanical properties and low density. AA2099 is an Al-Li-Cu alloy with the following composition Al-2.69wt%Cu-1.8wt%Li-0.6wt%Zn-0.3wt%Mg-0.3wt%Mn-0.08wt%Zr. The environmental assisted cracking and localized corrosion behavior of the AA2099 was investigated in this thesis.

The consequences of uncontrolled grain boundary precipitation via friction stir welding on the stress corrosion cracking (SCC) behavior of AA2099 were investigated first. Using constant extension rate testing, intergranular corrosion immersion experiments, and potentiodynamic scans, the heat-affected zone on the trailing edge of the weld (HTS) was determined to be most susceptible of the weld zones. The observed SCC behavior for the HTS was linked to the dissolution of an active phase (Al₂CuLi, T₁) populating the grain boundary. It should be stated that the SCC properties of AA2099 in the as-received condition were determined to be good.

Focus was then given to the electrochemical behavior of precipitate phases that may occupy grain and sub-grain boundaries in AA2099. The grain boundary microchemistry and micro-electrochemistry have been alluded to within the literature as having significant influence on the SCC behavior of Al-Li-Cu alloys. Major precipitates found in this alloy system are T₁ (Al₂CuLi), T₂ (Al_7.5Cu₄Li), T_B (Al₆CuLi₃), and θ (Al₂Cu). These phases were produced in bulk form so that the electrochemical nature of each phase could be characterized. It was determined T₁ was most active electrochemically and θ was least. When present on grain boundaries in the alloy, electrochemical behavior of the individual precipitates aligned with the observed corrosion behavior of the alloy (e.g. T_B was accompanied by general pitting corrosion and T₁ was accompanied by intergranular corrosion attack). In addition to the electrochemical behavior of the above-mentioned intermetallics, the phenomenon of Zn substituting for Cu in T₁ was also studied. It was determined that for Zn substitutions up to 8.4wt%, improved corrosion behavior of T₁ would occur by dealloying of Zn and Li.

Lastly, isothermal aging treatments at 160°C and 190°C were conducted to vary the grain boundary structures in a systematic way. The SCC behaviors for the under-aged, peak-aged, and over-aged condition were studied using CERT and alternate immersion testing at each temperature. The severely under-aged condition (≤ 9 hours) at 160°C proved to have poorest SCC resistance. The over-aged condition at 160°C (≥ 24 hours) also had degraded SCC resistance. In contrast the 190°C isothermal aging condition produced better SCC resistance than 160°C isothermal aging temperature for all conditions. The differences in behavior were linked to the species populating the grain boundaries in each isothermal aging condition (T₂ at 190°C and T₁ at 160°C).