The Research About The Quench Induced Precipitation And Quenching Sensitivty Of Al-Zn-Mg-Cu Alloys

Al-Zn-Mg-Cu alloys were widely used as structural material in the aerospace industry because of their high strength properties, low density, and excellent formability, combined with sufficiently high fracture toughness, fatigue and corrosion resistance. As the precipitation hardening alloys, the properties of Al-Zn-Mg-Cu alloy strongly depend on quenching process after solution. The precipitation of coarse particles by means of non-homogeneous nucleation leads to rapid decomposition of the supersaturated solid solution which results in the fast decline of materials mechanical performance, the corrosion property as well as the fracture toughness property, when reducing the quenching rate during quenching process after solution. In particular, regard to large size products during the quenching process, the cooling rate of the center layer is slower than the surface, which results in the micro structures and properties non-homogeneous along the thickness direction, seriously affecting the overall performance. Meanwhile, with the development of the aerospace industry, the size demanding of the astronautic structure materials gradually increased. Furthermore, for the hardenability (quenching sensitivity), properties homogeneous requirements are increasingly stringent, presented new challenge to the production's process technology.This paper aimed at1933aluminum alloy forging, AA7085, AA7050and AA7055hot-rolled plates. The influences of the compositions and the processing parameters on the microstructure evolution and the quenching sensitivity are studied by means of the optical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, differential scanning thermal analysis, mechanical property examination and hardness testing. As the results showed, when the external environment (especially temperature) changed, the compositions of the solute atoms can influence the diffusion rate of the atoms and the energy barrier of phase transformation, which will affect the precipitation rate and the transformer temperature of the second phases. The thermal treatments can influence the second phase heterogeneous nucleation sites and density by controlling the microstructures evolutions. The main conclusions are as the following:By the microstructure observation and property comparison of slow quenched AA7085hot-rolled sheets (with different elements composition), the influences of major alloying elements Zn, Mg and Cu content on the precipitation mechanism during low rate quenching were revealed, and the influences of the Zn, Mg, Cu elements content, and Zn/Mg content ratio on the quenching sensitivity were investigated. The influence of Zn/Mg content ratio on the activation energy of equilibrium phase η precipitation is determined by the DSC analysis. The relationship between the activation energy and quenching sensitivity has been confirmed. The micro structure analysis of slow rate quenched specimens reveals that the Mg element content play an important role in the precipitation behavior during slow rate quenching process after solution. The influence of Mg content on the mechanism of η, T phase precipitation has been confirmed. By the diffraction pattern of S'phase and the properties comparison, the influence of mechanism of Cu element content on the quenching sensitivity has also been studied.The microstructure evolutions during homogenization, slow quenching process after solution of AA7050sheet have been studied by a large number of TEM work. The Al3Zr precipitation behavior during homogenization has been studied, and the influence of Al3Zr particles distribution on the recrystallization behavior also has also been confirmed. The results of the microstructure analysis of slow rate quenched specimens show that the equilibrium rj phase s mainly precipitate at areas such as the grain boundaries, sub-grain boundaries, incoherent Al3Zr particles within the recrystallization areas and some regions where the Al3Zr particles interacted with the dislocations. The mechanisms of the recrystallization and polygonization behavior during process have been carefully investigated. By optimizing the homogenization treatment, rolling reduction, deformation speed, solution and other specific parameters, the recrystallized fraction and the sub-area microstructures have been controlled to decrease the quenching sensitivity. The recrystallization is inhibited efficiently by fine size and high density of Al3Zr particles when the ramp heating is concerned. Small equilibrium phases of η mainly precipitate at the sub-grain boundaries and some regions within sub areas when the processing parameters were controlled. In addition, a better quenching sensitivity is achieved.The TTP (time-temperature-property) curves of Al-Zn-Mg-Cu alloys were determined by an interrupted quench method. The TTP curves of several alloys (AA1933,7050,7055) were measured and their precipitations behaviors were compared. The results indicated that the alloy composition can influence the nose temperature, quenching-sensitive range. The mechanism of the second phase particles precipitation behavior during isothermal holding treatments has been confirmed, and the relationship of second phase particles and the transformation temperature, holding time has also been concluded. The influences of aging treatments and rolling reduction on the precipitation behavior during isothermal holding treatments have been investigated. The effect of rolling reduction on the activation energy of equilibrium phase η has been measured and studied by DSC measurement. The alloy composition and the treatments during process (rolling, aging) also act an important role to the nose temperature, quench sensitivity area of TTP curves.To model the industry cooling system, one end-spray-cooling method is adopted to characterize the industry quenching process of AA1933forgings and AA7050hot-rolled plate.At the studied quenching conditions, the TEM results show η and T phases show a different regulation due to different cooling rate corresponding to several distances away the cooling end, and the distinction of alloy composition, micro structure inevitably lead to quite difference of inhomogeneous nucleation and precipitation behavior. The micro structure differences along the thickness direction of AA7050plate also represent the different quenching sensitivity subjected to the surface,1/4, and middle layers. To characterize the quenching sensitivity of thick plate in the laboratory conditions, the cooling rates of AA7050little specimens are measured subjected to different quenching conditions. By using the same cooling methods, the cooling rates of thick plate can be molded, and it is helpful to understand the quenching sensitivity and meaningful to the application of Al-Zn-Mg-Cu alloys in aerospace industry.