Design of multistep aging treatments of 2099 (C458) Al-Li alloy

Journal of Materials Engineering and Performance (Impact Factor: 0.98). 10/2005; 14(5):641-646. DOI: 10.1361/105994905X64594

ABSTRACT Multistep artificial aging treatments coupled with various natural aging times for aluminum lithium 2099 alloy (previously
called C458) are discussed to obtain mechanical tensile properties in the T6 condition that match those in the T861 condition,
having a yield strength in the range of 414–490 MPa (60–71 ksi), an ultimate strength in the range of 496–538 MPa (72–78 ksi),
and 10–13% elongation. Yield and ultimate tensile strengths from 90–100% of the strength of the as-received material (in the
T861 condition) were obtained. The highest tensile strengths were consistently obtained with two-step, low-to-high temperature
artificial aging treatments consisting of a first step at 120 °C (248 °F) for 12–24 h followed by a second step between 165
and 180 °C (329–356 °F) for 48–100 h. These T6-type heat treatments produced average yield and ultimate strengths in the longitudinal
direction in the range of 428–472 MPa (62.1–68.5 ksi) and 487–523 MPa (70.6–75.9 ksi), respectively, as well as lower yield
strength anisotropy when compared with the as-received material in the T861 condition.

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    ABSTRACT: The flow behavior of 2099 alloy was investigated by means of hot compression tests in the temperature range of 300–500 °C and strain rate range of 0.001–10 s−1. Processing maps were developed on the basis of the dynamic materials model (DMM). The results show that the flow stress increases with increasing strain rate and decreasing temperature. At a given deformation condition, the flow stress composed three stages: work hardening stage, softening stage and steady stage. Microstructure observation indicates that in the deformation temperature range of 300–420 °C and strain rate of 0.001–10 s−1 shows a dynamic recovery (DRV) character; while that deformed in the deformation temperature range of 420–500 °C and strain rate of 0.001–10 s−1 shows a dynamic recrystallization (DRX) character. According to the processing map and microstructure observation, DRV and DRX are the main deformation mechanisms in the safe regions, while flow instability is manifested as adiabatic shear bands and flow localization at the strain of 0.7. The optimum processing parameters for 2099 alloy at different strains conditions are given with the efficiency dissipation more than 0.3.
    Materials Science and Engineering A 09/2014; 613:141–147. DOI:10.1016/j.msea.2014.06.085 · 2.41 Impact Factor
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    ABSTRACT: Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s−1 and quasi-static compressive loading at a strain rate of 0.0032 s−1. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.
    Engineering Failure Analysis 12/2013; 35:302-314. DOI:10.1016/j.engfailanal.2013.02.015 · 1.13 Impact Factor
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    ABSTRACT: With its combination of high specific strength, good machinability and excellent weldability, AA2219 direct chill (DC) cast alloy has become a new category of materials for manufacturing large molds for the plastics and automotive industries. The effect of two-step aging on the microstructural evolution and mechanical properties of AA2219 DCcast alloy was investigated. The precipitate microstructure was characterized under different heat treatment conditions using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM).The poor mechanical properties of the air-quenched alloy were attributed to the presence of quench-induced coarse θ' and θ precipitates, which had very limited contribution to the precipitation hardening during the aging treatment. The two-step aging treatment of the air-quenched AA2219 alloy involved the precipitation of GP zones in the first step followed by their transformation into fine θ'' strengthening precipitates in the second step, which considerably improved the mechanical properties. After undergoing120 ˚C/36 h+190 ˚C/8 h two-step aging, the hardness, YS and UTS of the air-quenched alloy were increased by 27%, 46% and15%, respectively, compared with 190 ˚C/8 h one-step aging.
    Materials Science and Engineering A 12/2014; 625:213-220. DOI:10.1016/j.msea.2014.12.002 · 2.41 Impact Factor

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