Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. A classic case of difficult phase transformations is the nucleation of strengthening precipitates in high-strength lightweight aluminium alloys. Here, using a combination of atomic-scale imaging, simulations and classical nucleation theory calculations, we investigate the nucleation of the strengthening phase θ′ onto a template structure in the aluminium-copper alloy system. We show that this transformation can be promoted in samples exhibiting at least one nanoscale dimension, with extremely high nucleation rates for the strengthening phase as well as for an unexpected phase. This template-directed solid-state nucleation pathway is enabled by the large influx of surface vacancies that results from heating a nanoscale solid. Template-directed nucleation is replicated in a bulk alloy as well as under electron irradiation, implying that this difficult transformation can be facilitated under the general condition of sustained excess vacancy concentrations. Exactly how seemingly simple solid-state precipitation occurs in alloys remains elusive. Here, the authors show that excess vacancies introduced into a nanoscale, irradiated or deformed aluminium-copper alloy enable template-directed nucleation of the known strengthening phase θʹ.

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Transforming solid-state precipitates via excess vacancies

Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

Making sustainable aluminum by recycling scrap: The science of “dirty” alloys

In this work, a structural response of 6061-T6 aluminum alloy to friction-stir welding (FSW) was studied in wide range of processing conditions in order to establish a better foundation for the microstructure-strength relationship in produced welds. In contrast to the widely-accepted conception of the FSW-induced precipitation behavior, the welding temperature was found to be often below the dissolution threshold and thus no particle dissolution took place. Moreover, the peak temperature never exceeded 500 °C, and therefore the particles never dissolved completely. Considering a relatively low cooling rate measured in the stir zone, the dissolved precipitates were suggested to partially re-precipitate as solute clusters during weld cooling cycle thus imparting a substantial hardening effect. Accordingly, the precipitation coarsening was deduced to be the major softening mechanism. Due to the extreme sensitivity of this process to a duration of the weld thermal cycle, the welding speed was surmised to be the key factor controlling weld strength.

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Microstructure-strength relationship in friction-stir welded 6061-T6 aluminum alloy

Stress-level-dependency and bimodal precipitation behaviors during creep ageing of Al-Cu alloy: Experiments and modeling

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are applied to investigate the influence of 2% pre-deformation on precipitation in an Al-Cu alloy. This pre-deformation results in a significant hardness increase and an enhanced age hardening response, which is demonstrated to be associated with the heterogeneous formation of pre-/θ' precipitates on dislocations. Two early-stage pre-θ' phases (pre-θ'−1 and pre-θ'−2) are frequently observed by atomic scale imaging. Pre-θ'− 1 consists of three Cu layers with each separated by two {200}Al planes, while pre-θ'− 2 has a regular structure in between two Cu layers with a distance of 2aAl (~ 1.5cθ'). Pre-existing dislocations have a profound impact on the phase selection during aging, resulting in the unusual occurrence of pre-θ'− 1, pre-θ'− 2 and 1.5 cθ' thick θ'. The 1.5aAl (~ 1cθ') thick hitherto unknown pre-θ'− 3 and the θ'-approximant phase reported before in aged Al-Cu alloy with large pre-deformation also exist but with rare occurrence. The unveiled precipitation scenario leads us to sort out different precipitation reactions at dislocations: (i) direct formation of thick θ'; (ii) formation of pre-θ'− 1 and 2 cθ' thick θ'; (iii) formation of pre-θ'− 2 and 1.5 cθ' thick θ'. There are striking structural similarities between pre-θ' phase and θ' with the same thickness, while the only difference is the atom rearrangement inside the two stable interfacial Cu layers on {002}Al planes. This work provides not only new insights into the effect of pre-deformation on the precipitation behaviors of Al-Cu alloys, but also a guide to tailor the microstructure to achieve desirable mechanical behaviors.

Peipei Ma

Papers

Stress-level-dependency and bimodal precipitation behaviors during creep ageing of Al-Cu alloy: Experiments and modeling

Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

Multiple precipitation reactions and formation of θ'-phase in a pre-deformed Al–Cu alloy

Large creep formability and strength–ductility synergy enabled by engineering dislocations in aluminum alloys

Solute Sn-induced formation of composite β′/β″ precipitates in Al-Mg-Si alloy