The Effect Of Ag, Sc Alloying And Heat Treatment Processes On Microstructures And Properties Of 7055 Aluminum Alloy

7055 aluminum alloy was a new type superhigh strength 7000 series alloy developed by Alcoa. In combination with proprietary T77 temper, it had applied in Boeing 777 aircrafts. Under the financial support of National 973 Program "Improving the quality of aluminum and its sections", the effects of alloying, heat treatments on the mechanical properties and microstructures of 7055 aluminum alloy were investigated with various experimental and testing methods, such as optical microscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, microhardness testing and properties testing. The major conclusions were drawn as follows:The Ag addition in 7055 aluminum alloy promoted the formation of AlZnMgCuAg constituent phase along grain boundaries and AlFeCuZn interior the grains. AlFeCuZn phases were not removed completely during homogenization and became the trapping sites of Ag and Mg, leading to a new AlZnMgCuFeAg phases. Excessive Ag addition ( 0.5% ) increased the amount of non-equilibrium eutectic phases and depleted major alloying elements. The presence of the non-equilibrium phases, which could not dissolve completely during homogenization and solution treatment, were detrimental to strength. The Ag addition accelerated the early age-hardening response, increased hot stability of precipitates and retarded over-ageing of 7055 aluminum alloy.The addition of Sc formed primary Al₃ScZr composite particles combined with Zr atoms. The as-cast grains of 7055-0.2Sc alloy were refined greatly due to primary Al₃ScZr particles. Sc atoms in solid solution during chill cast precipitated in the form of coherent secondary Al₃ScZr particles with Zr atoms during homogenization and rolling. The secondary Al₃ScZr particles strongly pinned deformed structures and inhibited recrystallization and grain growth. 7055-0.2Sc alloy could attain higher strength than 7055 alloy because of higher degree of supersaturation, dispersion strengthening of fine and disperse Al₃ScZr particles and grain boundary strengthening of finer grain sizes.To take full advantage of RRA and achieve practical application, the influence of pre-ageing time and retrogression temperature on 7055 alloy with/without Ag addition. The results showed, after pre-ageing for 120℃/12h, 7055 alloy containing Ag had anearly hardness drop when retrogression at 160°C and 180°C. But for 120°C/18h and 24h, the early hardness drop disappeared. For three pre-ageing conditions, 7055 alloy without Ag addition had an early hardness drop. When retrogression at higher 200 °C, 7055 alloys with and without Ag addition had an early hardness drop. With higher retrogression temperature, the hardness drop rate that caused by precipitates re-dissolving at the early stage and precipitates coarsening at the prolonged stage accelerated. But 7055 alloy containing Ag had a slower drop rate than that of 7055 alloy due to the higher stability of precipitates as a result of Ag addition.Though 7055-0.2Ag alloy retrogressed at 200 °C and 220 °C for a short while contributed to higher strength than at 180°C, prolonged time would lower strength more rapidly. Under the condition of short-time retrogression at 180°C and re-ageing, 7055-0.2Ag alloy after 120°C/12h and 24h pre-ageing owned higher strength than that after pre-ageing for 120°C/18h. When retrogression for prolonged time, the strength of reduced.After 120°C/24h+ 180°C/45min+120°C/24h RRA treatment, ultimate strength, yield strength, elongation and electrical conductivity of 7055-0.2Ag alloy were 635.6MPa, 623.7Mpa, 7.4% and 38.1%IACS respectively. The strength and electrical conductivity were close to that in T6 temper and that in T73 temper respectively. Besides, the process was adaptable industrial application.The fracture mechanism of 7055-0.2Ag T6 alloy consisted of both shear-type transgranular fracture and intergranular fracture. After T73 temper, the fracture mechanism was mainly dimple-type transgranular fracture. But for RRA treatment, the fracture mechanism was intergranular crack with small amount dimples on the grain boundary interfaces. The difference in fracture mechanisms was related to different precipitation types and width of PFZ.After under-ageing, even peak-ageing at elevated temperature, then followed by secondary ageing at lower temperature, 7055 alloy attained strength and hardness close to, even higher than that in T6 temper respectively. Increasing secondary ageing temperature could shorten the time to peak ageing, but not favorable to the improvement of fracture toughness. Shortening pre-ageing time was beneficial to fracture toughness.The optimal secondary ageing process was 120°C/10min+60°C/240h. The ultimate strength, tear strength and unit initiation energy were 52.3MPa> 489.2MPa and 57.0 Nmrn'1, and higher than those of T6 temper by 4%,100% and 50% respectively.Microstructural observations showed higher density of precipitates were precipitated inside grains by low temperature secondary ageing, which contributed to higher strength than that of T6 temper. The volume fraction of grain boundary precipitates was smaller than that in conventional T6 temper, and their morphologies were transferred from needled to granular, which contributed to the improvement of fracture toughness of 7055 alloy. Higher secondary ageing temperature led to faster atomic diffusion and higher volume fraction of grain boundary precipitates, which was detrimental to toughness improvement.After cold rolling and T6 temper, the strength of 7055 alloy increased slightly under small rolling reduction. With more heavier reduction, the strength decreased. As for RRA temper, its strength decreased continuously. The electrical conductivity of cold rolled 7055 alloy decreased with reduction. After T6 and RRA treatment, the electrical conductivity increased, and at the same rolling reductions, the alloy in RRA temper had highest electrical conductivity.The lattice distortion produced by dislocations during cold rolling reduced the electrical conductivity, and therefore heavier rolling reduction corresponded to lower electrical conductivity. Due to the interaction between dislocation and solutes / vacancy, coarse precipitates formed on the dislocations, which increased the electrical conductivity of 7055 alloy and reduced its strength. During T6 and RRA temper, dislocation density lowered, which compromised its strengthening effects.A novel process was developed, that was overageing-resolutionization-reageing. The experimental results showed after 470°C/lh and 120°C/24h treatment, 7055-0.2Ag alloy in the 120°C/3h+180°C/8h over-ageing condition could maintain T6 strength and increase the electrical conductivity from 32.2%IACS to 36.7%IACS. Both lowering resolutionization temperature and prolonging overageing time decreased the strength and increased the electrical conductivity of 7055-0.2Ag alloy.The reseasons that overageing, resolutionization and reageing treatment could increasethe electrical conductivity were that coarse precipitates both inside grains and on the grain boundaries formed during overageing could not redissolve completely during resolutionization. Overageing and resolutionization treatment favored the further dispersion of Fe, Si and Zr atoms and reduced the density of residual dislocation. The results of intercrystalline and exfoliation corrosion experiments showed that for overageing, resolutionization and reageing treatment, the treatment temperature and time had significant influence on the corrosion behavior of 7055-0.2Ag alloy. Proper increase in resolutionization temperature was favorable to reduce intercrystalline corrosion and exfoliation corrosion. After 120°C/3h+180°C/4h> 8h> 12h+480°C/lh+120°C/24h, the corrosion resistance of the alloy was superior to that of that alloy in T6, T73 and RRA temper. In the 3.5%NaCl solution, the polarization curve of the alloys in various temper conditions were characterized as the control of activation polarization and satisfied Tafie relationship.