| Scince the skin of aircraft bears cyclic loading and works in different corrosive environment constantly, the aluminum alloys used for aircraft-skin require not only high strength but also better corrosion and damage resistance to meet the design requirement of long-life and high-reliability for today’s aircraft. As a subject of National Basic Research Program, this research is aimed at exploring an new aluminum alloy with high resistance to damage and middle level of strength. The key indicator of damage resistance is expected to be improved by25%, and the anticipant performances of the new alloy are σb≥430MPa, σ0.2>360MPa,δ≥15%, da/dn≤1.5×10-3mm/cycle (△Kp=30MPa·m1/2). With these performances, the new alloy will stand out and be widely used in aerospace, transportation, military equipment and other fields.In this research, authors designed four series of aluminum alloy (namely Al-Cu-Li-Mg-Ag alloy, Al-Cu-Mg-Ag-Mn-Zr alloy, Al-Cu-Li-Mg-Mn-Zr alloy, Al-Cu-Li-Mg-Mn-Zn-Zr alloy) by controlling the content of Cu and adding some new elements, such as lithium, erbium, etc. Authors choose the approriate alloy which shows the best comprehensive property as the foundational alloy with high fatigue crack resistance. The effects of annealing, aging treatment and rare earth elements (Er) on changing microstructure and the properties of the designed alloys were investigated in this work. Besides, a great variety of aging treatments were adopted to improve the tensile and fatigue properties of the alloys. Finally, the fatigue crack initiation mechanism, early crack growth behavior and long crack propagation mechanism of aluminum alloys were studied and the effect of microstructure on fatigue crack propagation was discussed in this article. Here come the main conclusions;I Al-Cu-Li-Mg-Ag alloyi Static recrystallization will occur in the Al-Cu-Li-Mg-Ag alloy, when it is annealed at450℃for5hours. However, when annealed at385℃for4hours, the Al-Cu-Li-Mg-Ag alloy shows fibrous structure and high recrystallization temperature. When solutioned at a temperature below520℃annealing before solution will keep the Al-Cu-Li-Mg-Ag alloy away from recrystallized. Improving solution temperature accelerates aging response speed of the Al-Cu-Li-Mg-Ag alloy, and leads to a higher ductility of Al-Cu-Li-Mg-Ag alloy.ii Under the condition of equal strength, the ductility of the Al-Cu-Li-Mg-Ag alloy was increased with increasing predeformation. When20%of the predeformation adopted, the Al-Cu-Li-Mg-Ag alloy showed the best tensile properties. The Al-Cu-Li-Mg-Ag alloy showed better tensile property in double artificial aging than the single artificial aging (T8). Besides, Al-Cu-Li-Mg-Ag alloy showed better tensile property in lower double artificial aging temperature of RT+145℃than in double artificial aging temperature of70℃+145℃, while the fatigue crack growth rates were equal. Under heat treatment of540℃(1h)+12%+RT (5d)+145℃(20h), the Al-Cu-Li-Mg-Ag alloy shows the best comprehensive properties with σb=443MPa, σ0.2=397MPa,δ=16.5%, da/dn≈1.34×10-3mm/cycle (△K=30MPa√m)。iii Zr can suppress the growth of recrystallized grain, and lead to a fine grain structure of the Al-Cu-Li-Mg-Ag alloy. Nevertheless, the fatigue crack growth rate was not reduced by the addition of Zr. On one hand, high-angle boundaries impede the motion of slip bands, and result in microcraks. On the other hand, high-angle boundaries hinder fatigue crack propagation, induce deflection of crack and reduce the speed of crack propagating.II Al-Cu-Mg-Ag-Mn-Zr alloyi Intermediate annealing reduces the stored energy result from hot rolling, lowers the strength and improves the ductility of the Al-Cu-Mg-Ag-Mn-Zr alloy. The strength and ductility of the Al-Cu-Mg-Ag-Mn-Zr alloy improve with the increase of annealing temperature. It begins to recrystallize when the hot rolled plate is annealed over400℃, and perfect recrystallization with highest strength and ductility is showed when the hot rolled plates are annealed at450℃for5hours. When solutioned at a temperature over520℃, the Al-Cu-Mg-Ag-Mn-Zr alloy begin to recrystallize, even though the alloy was annealed at350℃for10hours before solution treatment. The volume fraction of recrystallization rises with solution temperature increase.ii Al-Cu-Mg-Ag-Mn-Zr alloy shows better tensile property by solution at520℃than530℃. In single aging treatment(T8), the fatigue crack growth resistance of the Al-Cu-Mg-Ag-Mn-Zr alloy in different aging conditions is under-aged> peak-aged> over-aged in sequence.On the same level of tensile property, the Al-Cu-Mg-Ag-Mn-Zr alloy shows better fatigue crack resistance in double artificial aging treatment than in single artificial aging treatment and natural aging. After the heat treatment of530℃(1h)+6%+120℃(8h)+145℃(24h),the Al-Cu-Mg-Ag-Mn-Zr alloy with higher copper content shows excellent comprehensive performance with σb=474MPa, σo.2=363MPa, δ=22.5%, da/dn≈1.0×10-3mm/cycle (△K=30MPa√m)iii Element Er mainly exist in the grain boundary of Al-Cu-Mg-Ag-Mn-Zr alloy in the form of eutectic phase (Al8Cu4Er), and reduce the strength and fatigue crack growth resistance of Al-Cu-Mg-Ag-Mn-Zr alloy.Ⅲ Al-Cu-Li-Mg-Mn-Zr alloyi It begins to recrystallize when the hot rolled plate of Al-Cu-Li-Mg-Mn-Zr alloy is annealed at400℃for5hours. Both predeformation and aging temperature improvement improves the strength of the Al-Cu-Li-Mg-Mn-Zr alloy, and predeformation seems to have a greater influence on strengthing. The comprehensive properties of the Al-Cu-Li-Mg-Mn-Zr alloy in peak-aged of T8temper is ab=494MPa, σ0.2=400MPa,δ=12.6%, da/dn=8.9×10-3mm/cycle(△K=27MPa√m), which is much lower than the Al-Cu-Mg-Ag-Mn-Zr alloy(alloy B2).Ⅳ Al-Cu-Li-Mg-Mn-Zn-Zr alloyi Al-Cu-Li-Mg-Mn-Zn-Zr alloy shows excellent fatigue crack growth resistance with a low fatigue crack growth rate of1.8×10-3mm/cycle (△K=30MPa√m)and da/dn=9.6×10-4mm/cycle(△K=30MPa√m) in nutural aging and T8UA condition. CoMPared with the single artficial aging, the Al-Cu-Li-Mg-Mn-Zn-Zr alloy shows better fatigue crack growth resistance in double artificial aging of120℃+145℃. A modest increase in predeformation improves the tensile properties rather than the fatigue crack growth resistance of the Al-Cu-Li-Mg-Mn-Zn-Zr alloy. When the predeformation are4%and 6%, the fatigue crack growth rates of the Al-Cu-Li-Mg-Mn-Zn-Zr alloy are3×10-3mm/cycle and1.88×10-3mm/cycle respectively.ii Lowering the first aging temperature of double artificial aging will result in a better ductility and fatigue crack growth resistance. When the Al-Cu-Li-Mg-Mn-Zn-Zr alloy (D2) was under the heat treatment of540℃(lh)+4%+RT(10d)+145℃(45h), it shows the best comprehensive properties of σb=441MPa,σ0.2=368MPa,δ=16%,da/dn=1.1×10-3mm/cycle△K=30MPa√m).Take an overall consideration of the comprehensive properties of the Al-Cu-Li-Mg-Ag, Al-Cu-Mg-Ag-Mn-Zr, Al-Cu-Li-Mg-Mn-Zr, Al-Cu-Li-Mg-Mn-Zn-Zr alloys in all heat treatment conditions, Al-Cu-Mg-Ag-Mn-Zr alloy(B2), with the heat treatment of530℃(1h)+6%+120℃(8h)+145℃(24h), shows the best comprehensive properties of σb=474MPa,σ0.2=363MPa,5=22.5%,da/dn≈1.0×10-3mm/cycle(△K=30MPa√m)... |
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