Ziqiao Zheng

Effects of Li content on precipitation in Al-Cu-(Li)-Mg-Ag-Zr alloys

Although much attention has been paid to Al-Cu-Li-Mg-Ag-Zr alloys, there are sparse reports about the influence of Li on precipitation in these alloys. The aim of the present study is to determine the effects of Li on modifying precipitation in a baseline aluminum alloy 2195 and the accompanying variants with 0--1.6 wt.% Li.

Microstructure and mechanical properties of Mg, Ag and Zn multi-microalloyed Al–(3.2–3.8)Cu–(1.0–1.4)Li alloys

Abstract To develop super-high strength Al–Li alloy, the microstructures and mechanical properties of Mg, Ag and Zn micro-alloyed Al–(3.2–3.8)Cu–(1.0–1.4)Li alloys (mass fraction) with T8 temper were studied. The results showed that 1% of lower Li content restricted the strengthening effect of increasing Cu content, while simultaneous increase in Cu and Li contents contributed effectively to the enhancement of strength. The alloys were mainly strengthened by plenty of fine and well dispersed T1(Al2CuLi) precipitates. There were also some minor precipitates of θ ′(Al2Cu) and δ′(Al3Li), which became less in number density, even disappeared during the aging process. Meanwhile, higher Li content favored the formation θ ′ and δ ′ and a small amount of S′(Al2CuMg) phases. In addition, strengthening effect and microstructure variation were analyzed through total non-solution mole fraction of Cu and Li and their mole ratio. To obtain Al–Li alloy with super-high strength, the total mole fractions of Cu and Li should be increased, and their mole ratios should also be kept at a certain high level.

Effect of heat treatment process on tensile properties of 2A97 Al−Li alloy: Experiment and BP neural network simulation

Abstract Effects of heat treatment processes on tensile properties of 2A97 Al-Li alloy were investigated. The results show that one new heat treatment process which was developed from traditional T8 temper can effectively improve the tensile properties of Al-Li alloy. In the peak-aged condition, a large quantity of fine T1 dispersedly precipitated in the matrix. At the same time, few secondary phases precipitated at the grain boundaries, and precipitation-free zone was unobvious. The corresponding tensile strength, yield strength and elongation of alloy were 597 MPa, 549 MPa and 7.4%, respectively. In addition, BP neural network model was developed for prediction of the tensile properties of alloy subjected to different heat treatment processes. A very good correlation between experimental and predicted results was obtained, which indicates that the BP neural network can be used for the prediction of tensile properties of 2A97 Al-Li alloy.

Fatigue crack initiation and early propagation behavior of 2A97 Al–Li alloy

Abstract The fatigue crack initiation and early propagation behavior of 2A97 Al–Li alloy was studied. The smooth specimens were fatigued at room temperature under constant maximum stress control when stress ratio (R) is 0.1 and frequency (f) is 40 Hz. Microstructure observations were examined by optical microscopy, transmission electron microscopy, scanning electron microscopy and electron back scattered diffusion, in order to investigate the relationship between microstructure and fatigue crack initiation and early propagation behavior of 2A97 alloy. The results show that the fatigue cracks are predominantly initiated at inclusions and coarsen secondary phases on the surface of 2A97 alloy. The fatigue crack early propagation behavior of 2A97 alloy is predominantly influenced by the interactions between grain structure and dislocations or persistent slip bands (PSBs). When the misorientation of two neighbouring grains is close to the orientations of the favorable slip plane within these two grains, high-angle grain boundary severely hinders the PSBs passing through, and thus leads to crack bifurcation and deflection.

Microstructural evolution of Mg, Ag and Zn micro-alloyed Al–Cu–Li alloy during homogenization

Abstract The microstructural evolution of a Mg, Ag and Zn micro-alloyed Al–3.8Cu–1.28Li (mass fraction, %) alloy ingot during two-step homogenization was examined in detail by optical microscopy (OM), differential scanning calorimetry (DSC), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) methods. The results show that severe dendritic segregation exists in the as-cast ingot. There are many secondary phases, including T B (Al 7 Cu 4 Li), θ (Al 2 Cu), R (Al 5 CuLi 3 ) and S (Al 2 CuMg) phases, and a small amount of (Mg+Ag+Zn)-containing and AlCuFeMn phases. The fractions of intermetallic phases decrease sharply after 2 h of second-step homogenization. By prolonging the second-step homogenization time, the T B , θ, R, S and (Mg+Ag+Zn)-containing phases completely dissolve into the matrix. The dendritic segregation is eliminated, and the homogenization kinetics can be described by a constitutive equation in exponential function. However, it seems that the AlCuFeMn phase is separated into Al 7 Cu 2 Fe and AlCuMn phases, and the size of Al 7 Cu 2 Fe phase exhibits nearly no change when the second-step homogenization time is longer than 2 h.

Influence of AlCuSc Ternary Phase on the Microstructure and Properties of 1469 Alloy

Microstructure evolution and mechanical properties of a 1469 alloy and a Sc-free1469 type alloy were examined. SEM observation indicates that AlCuSc ternary phases (W) are formed after homogenization annealing, and cannot be dissolved during the following heat treatments. These coarse particles consume abundant Cu atoms from the Al matrix that are available for solutioning, which results in the decrease of precipitation of the T1 phase during aging treatment. The W phase has negative effects on the examined alloy’s mechanical properties. The tensile strength of Sc-added alloy is 40MPa lower than that of the Sc-free alloy. The formation of the W phase has a close relationship with Cu/Sc ratio, which shows the importance of controlling the concentration of Cu and addition of Sc. Formation of W phase suppress the effect of precipitation hardening of the T1 phase in high strength Al-Cu-Li alloys

A New Ultrahigh Strength Al-Cu-Li Alloy

In this work, a new ultra-high strength Al-Cu-Li alloy was investigated. The ultimate strength, yield strength and elongation of the newly designed alloy by artificial aging are 647.2MPa, 609.4MPa and 7.3% respectively. Among the main strengthening phases of T1, θ′ and S′ in the experimental alloys, T1 is the dominant one. The combined addition of Mg and Ag promoted the precipitation of T1 and increased the strength of the new alloy greatly. Zn had a similar effect as Ag during the aging strengthening progress, when added with Mg. Among the three micro-alloying elements, Mg, Ag and Zn, Mg had the strongest influence on age strengthening. Compared with the combined additions of (Mg +Ag) and (Mg + Zn), (Ag + Zn) had the weakest influence on aging strengthening. Pre-deformation before aging promoted the precipitation of T1 phase which weakened the influence of micro-alloying elements (Mg, Ag and Zn) on strengthening the alloys and minished the strength difference between alloy containing (Mg + Ag + Zn) and alloys containing two of them.

Microstructures and Mechanical Properties of an Al-Cu-Li-Mg-Zr Alloy Containing Zn and Mn

An Al-3.43Cu-1.28Li-0.49Mg-0.12Zr containing 0.62Zn and 0.29Mn was designed and the microstructures and mechanical properties of the alloy with various heat treatments were investigated. The precipitates of the alloy consist of T1 (Al2CuLi), θ′ (Al2Cu) σ (Al5Cu6Mg2) and δ′ (Al3Li). As solution temperature is changed from 485°C to 530°C, the solution degree of alloying elements in alloy increased, the amount of T1 in the alloy aged at 160°C for 18 h increased and that of θ′ is decreased, resulting in an increase of strength. After solution treatment at 530°C, the alloy aged for 18 h at 145°C is mainly strengthened by G P zones, and a little amount of T1 precipitates. As aging temperature is increased to 160°C and 175°C, the strength increased, due to the sufficient precipitation of σ and T1. The smaller amount of T1 in the alloy aged at 190°C is consistent with its lower strength. Meanwhile, it is found that the σ precipitate does not coarsen as aging temperature increases in the range from 160°C to 190°C.

The structural analysis of the combustion wave in Al–TiO2 self–propagation high temperature synthesis system

By the combustion wave quenching process, the dynamic structure of the combustion wave in Al-TiO2 self-propagation high temperature synthesis (SHS) system was maintained. A more detailed description of the machanism and dynamic of the structural formation was investigated with the help of scanning microscopy and electron microprobe. It was found that in the combustion wave, a special intermesh structure was formed. The phase compositions in the central parts and the edges of the meshs were Al, TiO2 and Ti, Al2O3, respectively. With the synthesis proceeding, the meshes was minified.

Role of Pre-Deformation on Aging Precipitation Behavior of Al-Cu-Li Alloys

Predeformation on tensile properties and microstructure of three Al-Cu-Li alloys, 1460, 2050 and 2A96 was investigated. The strength of the alloys is enhanced by the predeformation. Meanwhile, predeformation impacts the precipitation of T1 (Al2CuLi) and θ′(θ′′) (Al2Cu) phases. The precipitation (nucleation rate) of T1 is accelerated by the predeormation, and the effect is increased with the predeformation degree. While, effect on θ′(θ′′) precipitation is dependent on the alloy composition feature. In the 2A96 alloy with a medium Cu/Li ratio, it is decelerated by the predeformation. In the 2050 alloy with high Cu/Li ratio and 1460 alloy with low Cu/Li ratio but subjected to two-step aging at 130°C and 160°C, the θ′(θ′′) precipitation is accelerated at lower predeformation but decelerated at higher predeformation.