The retrogression-reaging (RRA) and hot forming behavior of AA7075 were studied. AA7075 is a high-strength alloy used in applications where weight is of particular importance, such as in automobiles. Like many of the high-strength aluminum alloys, AA7075 requires elevated temperature forming to achieve ductility comparable to steels at room temperature. Since AA7075 is a precipitation hardening alloy, heat treatments during forming and production need to be closely controlled to limit any loss of strength due to changes in the microstructure. Two new forming concepts are introduced to explore the feasibility of forming AA7075 in manners compatible with current automotive manufacturing processes. They are RRA forming and solution forming. These concepts seek to improve upon the room-temperature formability of AA7075-T6 and incorporate the paint-bake cycle (PBC) into the heat treatment process. The PBC is a mandatory heat treatment used to cure the paint applied to automobiles during production. Currently, the PBC is conducted at 180 °C for 30 minutes. RRA behavior was studied with molten salt bath treatments between 200 and 350 °C. The PBC was used in lieu of the standard 24 hour reaging treatment conducted at 121 °C. It was determined that retrogression treating below 250 °C was acceptable for RRA forming, with retrogressing at 200 °C producing the hardest material after reaging by the PBC. The formability of AA7075-T6 during RRA forming was evaluated by tensile testing at 200 and 225 °C. Ductility of AA7075-T6 at RRA forming temperatures was double compared to those produced at room temperature. RRA forming was demonstrated to achieve this improved ductility and a final material hardness after the PBC of only slightly less than the peak-aged condition. In addition, solution forming behavior was studied at 480 °C. Solution forming can increase ductility compared to RRA forming, but it requires aging at 121 °C prior to the PBC to produce peak-aged hardness. / text

[pt] O objetivo deste trabalho é avaliar a estabilidade microestrutural da liga 8090 (Al-Li-Cu-Mg-Zr) submetida a tratamentos térmicos de retrogressão e reenvelhecimento em diferentes condições de tempo e temperatura. Caracterização da morfologia e da estabilidade das fases endurecedoras foi realizada por microscopia eletrônica de varredura (MEV), utilizando a técnica EBSD (Electron Backscattering Diffraction). Microscopia eletrônica de transmissão (MET) foi também usada devido a ordem de grandeza nanométrica das fases precipitadas. Os resultados obtidos foram correlacionados com a propriedades mecânicas através de ensaios de microdureza e tração. Foi observado que a microestrutura da liga 8090 é estável, tanto na sua constituição policristalina quanto na sua microestrutura. O efeito de textura em virtude da deformação sofrida durante o processo de laminação permanece após o tratamento de retrogressão. Além disso, a evolução dos estágios de precipitação é pouco perceptível até o pico de endurecimento. As fases predominantes nesta liga são as fases delta (Al3Li), beta (Al3Zr) e T1 (Al2CuLi). Durante um reenvelhecimento mais prolongado é observado a precipitação da fase S (Al2CuMg) e do precipitado duplex delta/beta. Os ensaios de tração indicam a ocorrência do efeito Portevin-Le Chatelier para as amostras como recebida e envelhecidas a curtos intervalos de tempo. Este fenômeno dinâmico é resultado da interação de discordâncias com átomos de soluto e partículas de segunda fase. / [en] The purpose of this work is to evaluate the microstructural stability of alloy 8090 (Al-Li-Cu-Mg-Zr) when submitted to heat treatments of retrogression and reaging at different temperatures and for different time intervals. Characterization of the morphology and stability of the second phases was carried out by scanning electron microscopy (SEM), making use of the electron backscattering diffraction (EBSD) technique. Transmission electron microscopy (TEM) was also used for this purpose in virtue of the nanometric size of the second phases precipitated in the alloy. The results obtained were correlated with the mechanical properties determined by means of microhardness measurements as well as tensile tests. It was noted that the alloy exhibits a remarkable stability, not only in regard to its polycrystalline composition but also to its microstructure. The deformation texture introduced in the alloy due to its fabrication process (rolling) was found to persist after the retrogression treatment. In addition, the evolution of precipitation stages did not very considerably until the peak aging was reached. The main phases observed in the alloy were the phases delta (Al3Li), beta (Al3Zr) and T1 (Al2CuLi). During extended reaging, one can observe the precipitation of other phases such as S (Al2CuMg) and the duplex phase delta/beta. The tensile results indicated the occurrence of Portevin-Le Chatelier effect for the alloy in the as-received and short time reaged conditions. This dynamic effect, results from the interaction of dislocations with solute atoms as well as second phases particles.

The effect of retrogression and reaging heat treatment on corrosion fatigue crack growth behavior on AA7050 T73651 aluminum alloy is investigated. CT (Compact Tension) specimens are prepared in LS direction for fatigue crack growth tests . Samples are solution heat treated at 477 &deg / C and aged at 120 &deg / C for 24 h (T6 condition). After that, samples are retrogressed at 200 &deg / C for times of 1, 5, 30, 55 and 80 minutes in a circulating oil bath. Then, samples are re-aged at 120 &deg / C for 24 h (T6 condition). Hardness measurements are taken at different retrogression times and at the end of the heat treatment. Fatigue crack growth tests are performed at as received condition and at different retrogression times with sinusoidal loading of R=0.1 and f=1 in both laboratory air and corrosive environment of 3.5% NaCl solution. The highest fatigue crack growth resistance is observed for 30 min. and 5 min. retrogression for laboratory air and corrosive environment respectively. It is concluded that RRA can successfully be used to improve fatigue performance of this alloy.