Influence Of Quenching Rates And Natural Aging On Inhomogeneity Of Microstructures And Properties Of7055Aluminum Alloy Thick Plate

ABSTRACT:7055aluminum alloy has been widely used as structural materials in the aerospace, because of its high strength, high fracture toughness, good stress corrosion resistance and low density. In recent years, many attemps have been made to improve the comprehensive properties of the alloy through alloying, heat treatment and working process. However, high quenching sensitivity will give rise to the differences in properties and microstructures between the middle and the surface part of thick plates, thus in general only plate with thickness below38mm can be produced. Decreasing quenching sensitivity and inhomogeneity of thick plate have become a research focus in material science and engineering field over the recent years. In this work, the influence of quenching rates and natural aging on inhomogeneity of microstructures and properties of7055aluminum alloy thick plate were investigated by means of end-quench test, hardness test and exfoliation corrosion test, the microstructures were characterized by optical microscope(OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimeter (DSC). Main conclusions are listed in the following:(1) During slow quenching, coarse η equilibrium phase forms, and the solutes and vacancy concentration decrease, leading to the precipitation of larger and fewer η’hardening precipitates. Decreasing quenching rate gives rise to grain boundary precipitates with a larger size and lower copper content, wider precipitate free zone near grain boundaries. There is a linear relationship between width of precipitate free zone near grain boundaries and logarithm of quenching rate.(2) With the decrease of quenching rate, the resistance to exfoliation corrosion, the corrosion potential and the polarization resistance of the alloys decrease, while the corrosion current density increase. There is a linear relationship between polarization resistance and quenching rate. With the decrease of quenching rate from1250℃/min to138℃/min, exfoliation corrosion rating of the alloys evolves from EB to ED, and corrosion depth increases from167μm to558μm.(3) The inhomogeneity of thick plate increase firstly and then decrease with natural aging time. The decrease of hardness values were11%、7%and4%from spray end to the other one after natural aged for4320h、8640h and17280h, respectively; it was10%.8%and4%after subsequent artificial aging; while it was14%after artificial aging. The long time natural aging before artificial aging reduced inhomogeneity effectively. There was a linear relationship between hardness and logarithm of quenching rate.(4) The long time natural aging before artificial aging is favorable for formation of stable GP zones at slowly-cooled locations. Consequently, more amounts of dispersed and fine η’phase precipitates, which gives rise to higher hardness and simultaneously better homogeneity.