The precipitation behavior of strengthening phases has a significant effect on the properties of Al–Cu–Li–X (X = Mg, Ag, Zr) alloys during aging treatment. In this work, the precipitation behavior and competing relationships among precipitates were studied, and the relationship between the microstructure and mechanical properties was analyzed in detail at four aging temperatures for an Al–Cu–Li–X alloy (AA2195). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC) were used to systematically explore the precipitation characteristics of the alloy. Then, the precipitation sequence and the interrelationship among the precipitates were determined in the under-aged and peak-aged states at various temperatures. The results show that the alloy exhibits different precipitation characteristics in various aging conditions, particularly for the under-aged state. Furthermore, the T1 phase (subject to aging temperature) preferentially multiplies to dominate the microstructure, governs the formation of other precipitates, and eventually gives rise to the multi-precipitate coexistence of the peak-aged state. Therefore, the influence of temperature on the precipitation behavior is realized by its effect on nucleation of the T1 phase. Moreover, the increase in the strength of the alloy arises from the comprehensive strengthening effect of all precipitates, in which the T1 phase plays a decisive role.

Precipitation behavior of an Al–Cu–Li–X alloy and competing relationships among precipitates at different aging temperatures

Aluminum–lithium alloys have been extensively used in the aviation and aerospace field. Now it is one of the critical materials for the production of large aircraft in China. Thermoplastic deformation is one of the key methods to solve the difficulty in the precise deformation of Al–Li alloy parts. The thermoplastic deforming behavior of the Al–Li alloys is characterized by a phenomenological model. The effects of deforming temperature, strain rate, and combining with strain are applied to fit and predict the flow stress in the model. However, the model does not explicitly explain the deformation mechanism. The physical constitutive model takes into account the physical parameters and the microstructure evolution of the material during deformation,such as dislocation density, grain size, and dynamic recrystallization, which is the development trend of the thermoplastic deforming constitutive relationship of Al–Li alloy. In this paper, the researches on the phenomenological constitutive models and physical constitutive models summarized commonly are adopted to characterize the thermoplastic deformation of Al–Li alloys. Besides, the characterization tendency is forecasted.

Progress in characterization methods for thermoplastic deforming constitutive models of Al–Li alloys: a review

In this paper, corrosion behavior of 2198 Al–Cu–Li alloy in different aging stages is investigated using immersion and electrochemical measurements in 3.5 wt% NaCl aqueous solution. The corrosion resistance is found to decrease from the solution-anneal to the peak-aged condition but increase after the peak-aged, which is due to microstructure evolution of three main kinds of precipitating phases with the aging process: T1 (Al2CuLi), θ′ (Al2Cu), and a few δ′ (Al3Li) phases. The anode T1 phase grows and increases with the aging treatment and becomes nearly unchanged after the peak-aged. Moreover, the cathode θ′ phase slightly decreases in the over-aged. The potentiodynamic polarization curves also indicate the most positive corrosion potential and the lowest corrosion current density in the peak-aged. The results of electrochemical impedance spectroscopy are in agreement with the corrosion morphologies. Furthermore, the related equivalent circuit is established to investigate the corrosion mechanism of this alloy.

Corrosion behavior of 2198 Al–Cu–Li alloy in different aging stages in 3.5 wt% NaCl aqueous solution

Aging treatment plays an important role in strengthening of 2198 Al–Li alloy. Through a serious of heat treatment processes, a large amount of precipitates emerge, mainly observed to be θ′(Al2Cu), Al3Zr, and T1(Al2CuLi), among which, T1 turns to be the most important precipitate that contributes to the strengthening of 2198 Al–Li alloy. While the temperature of the aging process is 175 °C, the density and size of T1 phase keep increasing through the process and reach peak in about 18 h. T1 phase tends to have a certain orientation relationship of ${\left({0001} \right)_{{{\rm{T}}_1}}}//{\left\{ {111} \right\}_{{\rm{AI}}}}$
 , ${\left\langle {1010} \right\rangle _{{{\rm{T}}_{\rm{1}}}}}//{\left\langle {110} \right\rangle _{{\rm{AI}}}}$
 and may have different kinds of multilayered structures. In most of the multilayered structures, the distance between two adjacent copper-rich laths is less than that in classical single-layered phase. Thus, it can be inferred that the microstructure of T1 phase might change in the process of developing from single-layered structure to multilayered structure. In addition, the interactions between different phases become relatively frequent when the density of T1 phase reaches a threshold.

Precipitation of T1 phase in 2198 Al–Li alloy studied by atomic-resolution HAADF-STEM

The influence of pre-strain (2%, 5%, 8% and 12.5%) and aging state (under, peak and over aging) on mechanical properties and stress corrosion behavior of 2297 Al–Cu–Li alloy was studied. The peak microhardness of the alloy reaches 156.3 HV at 15 h with 12.5% pre-strain, and that of the alloy is only 115.6 HV at 31 h without pre-strain. The slow strain rate testing results indicate the low stress corrosion cracking factor for the samples without pre-strain and high stress corrosion cracking factor for the samples with pre-strain, and the stress corrosion cracking factor of the peak-aged sample with 12.5% pre-strain reaches to 33.3%. From the transmission electron microscopy characterization, it can be inferred that the quantity and distribution of T1 (Al2CuLi) precipitates significantly affect the stress corrosion behavior of 2297 Al–Cu–Li alloy. The over-aged sample exhibits poor stress corrosion resistance, resulting from the large numbers of coarse T1 precipitates and precipitation free zone at the grain boundary.

Study of pre-strain before artificial aging on mechanical property and stress corrosion behavior of 2297 Al–Cu–Li alloy

Significant advantages in weight reduction and increased strength have place advanced aluminum-lithium alloys at forefront of aerospace materials research. These alloys are being developed to fulfill the ever increasing need for high strength, high properties, light weight and cost effective for aerospace industry. Conventional aluminum alloys has long been in service for aerospace application. The addition of lithium to aluminum improves modulus and decrease density compared to conventional aluminum alloys. Atomic weight of lithium is 7 mass units compared to aluminum 23 mass units, hence there is density reduction of about 3% for each weight percent addition of lithium and about 6% increase in Youngs modulus. In principle weight saving for aerospace structural parts could reach up to 15 %. This paper examines effect of lithium addition on properties, physical metallurgy; various phases developed during processing of these alloys. The addition of Lithium to aluminum form coherent, low density Al3Li (δ׳) precipitates. However the binary alloys have poor mechanical properties which are attributed to strain localization and shearing of soft Al3Li (δ׳) precipitates. This problem has been solved by development of ternary and quaternary alloys containing copper and magnesium. In all aluminum-lithium alloys, small addition of zirconium or scandium is done to improve recrystallization. The new developed aluminum lithium alloys series Al-Li-Cu-X are potential candidate to replace existing conventional alloys in terms of enhanced properties with reduced density.

Bilal Ahmed

Papers

Aluminum Lithium Alloys (Al-Li-Cu-X)-New Generation Material for Aerospace Applications