Investigation Of Solution And RRA Treatments Of Thick Plate Of Aluminum Alloy7A55

Abstract:Aluminum alloy7A55mainly composed by Zn, Mg and Cu elements has been widely used in aerospace field for its excellent overall performance. Nowadays, the thick plate of aluminum alloy7A55is an essential structure material for large aircraft manufacturing. After heat treatment, the microstructures and properties along the thickness direction of the plates are inhomogeneous. And thus it will be detrimental to engineering applications. This thesis mainly studied the effect of solution treatment and RRA treatment on microstructures and properties of the plate of30mm in thickness for aluminum alloy7A55. Main conclusions are listed as the following:(1) The optimum double solution treatment is450℃/1h+480℃/30min for the sheet of aluminum alloy7A55. The corrosion resistance of aluminum alloy7A55is decreased by the residual coarse second phase and recrystallization. S(Al2CuMg) phase is easily corroded, particles and recrystallization structure accelerates exfoliation corrosion. Smaller residual second phase particles and smaller recrystallization fraction which present best mechanical properties and corrosion resistance can be obtained by using450℃/1h+480℃/30min solution treatment in aluminum alloy7A55.(2)The treatment300℃/10h+350℃/4h+450℃/2h+470℃/2h+480℃/0.5h increases the strength and homogeneities of properties for the thick plate of aluminum alloy7A55. After solution treatment, the properties of different layers of the thick plate of aluminum alloy7A55are different, which is related to the inhomogeneous of microstructures. With the treatment300℃/10h+350℃/4h+450℃/2h+470℃/2h+480℃/0.5h, the recrystallization area of the surface layer decreased. After T6aging, the tensile strength of the surface layer and the center layer is622MPa and645MPa, respectively. Both the layers’strengths are higher than other solution treatment. The difference of the tensile strength between the surface and center layer is23MPa which lower than that by other solution treatment. (3) In order to improve properties of the studied alloy, the pre-aging temperature should be105℃when the heating rate is3℃/min. With a slow heating rate, the hardness and conductivity curves after retrogression and reaging treatment are similar among different pre-aging temperature. For example, the retrogression temperature should be190℃while the pre-aging temperature is105℃under slow heating rate.(4) From the surface to the center layer, the conductivity and strength of the thick plate increase significantly. The conductivity of the surface layer,1/4layer and center layer is36.7%IACS,37%IACS and37.8%IACS, respectively. The strength of the surface layer,1/4layer and center layer is607MPa,614MPa and618MPa, respectively. The corrosion resistance of surface layer and center layer are both good.(5) During RRA treatment, the coarsening rate of the precipitates in the center layer is higher than in the surface layer. Thus the strength of the center layer decreases faster than surface layer. The difference of the strength between the surface and the center layer is11MPa after RRA treatment, which reduces12MPa than that of T6(120℃/24h).