| ABSTRACT:The age-hardenable7050aluminum alloy plate has been widely used in aerospace industry due to its high mechanical properties. However, the alloy has a low stress corrosion cracking resistance when its strength is high. The effects of single-step, multi-step solution treatment on the microstructures, mechanical properties and corrosion resistance of the hot-rolled7050aluminum alloy plate were investigated. Hardness testing, ambient temperature tensile testing, polarization curves determination, alternating current impedance measurement, intergranular corrosion testing, DCB stress corrosion testing, optical microscopy scanning electron microscopy and transmission electron microscopy were used for investigation and analysis of the microstructures, the mechanical properties and corrosion resistance of the plate. The main conclusions are as follows:(1) The solution temperature for the optimal single-step solution is470℃. With increasing of solution temperature from440℃to490℃, the volume fraction of the residual phase decreases, the volume fraction and the size of the recrystallized grains increase, the hardness of the plate increases first, and then decreases. At the temperature of470℃, the yield strength(Rp0.2) and tensile strength (Rm) reach the peak value503MPa and536MPa respectively, the corrosion potential of the alloy goes up to the peak value, and the depth of intergranular corrosion is very shallow, which means that the plate has higher corrosion resistance at the solution temperature of470℃.(2) Double-step solution and step-solution can improve the strength of the plate, while high temperature pre-precipitation treatment results in the strength decrease about4%. According to the different solution treatments which are applied here, the tensile strength decreases in accordance with the following order:step-solution treatment(SST)>double-step solution treatment(DST4)>single-step solution treatment(470℃/2.5h)>high temperature pre-precipitation treatment(HTPT). Compared with the single-step solution, through the double-step solution and step-solution treatment, the7050aluminum alloy has less coarse second phases and lower volume fraction of recrystallization grains. The alloy can reach the highest strength after step-solution treatment, the yield strength(Rp0.2) and tensile strength(Rm) reach the peak value529MPa and551MPa, respectively.(3) The optimal corrosion resistance is obtained by the step-solution treatment. According to the different solution treatments, the corrosion resistance of the alloy decreases in accordance with the following order step-solution treatment(SST)> high temperature pre-precipitation treatment(HTPT)> double-step solution treatment(DST4)> single-step solution treatment(470℃/2.5h). After step-solution treatment, the intergranular corrosion depth is20μm, the critical stress intensity factor KISCCis16.82MPa·m1/2.(4) Considering the mechanical properties and corrosion resistance, the optimal solution treatment of the hot-rolled7050aluminum alloy plate is the step-solution treatment. And the process parameters of this treatment is:150℃/12h+200℃/12h+300℃/12h+400℃/12h+470℃/4h (heating rate is30℃/h). |
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