Integrated modeling of friction stir welding of 6xxx series Al alloys: Process, microstructure and properties

International audienceCompared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has already replaced conventional welding methods in a variety of industrial applications especially for Al alloys. This provides the impetus for developing a methodology towards optimization, from process to performances, using the most advanced approach available in materials science and thermomechanics. The aim is to obtain a guidance both for process fine tuning and for alloy design. Integrated modeling constitutes a way to accelerate the insertion of the process, especially regarding difficult applications where for instance ductility, fracture toughness, fatigue and/or stress corrosion cracking are key issues. Hence, an integrated modeling framework devoted to the FSW of 6xxx series Al alloys has been established and applied to the 6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based strength and strain hardening model, and a micro-mechanics based damage model. The presentation of each model is supplemented by the coverage of relevant recent literature. The "model chain" is assessed towards a wide range of experimental data. The final objective is to present routes for the optimization of the FSW process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating the potential of chain models to support a "material by design approach" from process to performances

Various transformation modes observed in two-phase γ+α2 TiAl-based alloys

This paper deals with various transformation modes taking place in two-phase γ+α2 TiAl-based alloys. These transformation modes are : (1) decomposition of the α phase leading to the precipitation of the γ lamellae (α→α+γ), (2) ordering reaction of the α phase (α→α2), (3) massive transformation (α→γ), (4) formation of monolithic γ grains and (5) discontinuous coarsening. By analyzing their kinetics, three types of competitions were identified between these transformation modes : (1) vs. (2), (2) vs. (3) and (4) vs. (5). The occurrence of (2) or (3) and (4) or (5) in binary alloys is strongly dependent on the alloy composition. Therefore, by examining these two competitions in transformation modes, it is possible to evaluate the influence of small additions of ternary and quaternary elements on the response to heat treatments without a precise knowledge of the phase diagrammes of complex alloy systems. Another important finding of the present study is related to the formation of the γ phase from the α phase ; this can occur through either lamellar precipitation (α→α2→α2+γ, or α→α+γ→α2+γ) or massive reaction (α→γ). The mechanisms involved during these two transformations are investigated and some of the key issues are discussed in the present paper

Mapping the microstructure of a friction-stir welded (FSW) Al-Li-Cu alloy

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Atom-probe study Ti Al based alloy

Two phase α2+γ Ti Al based alloys have a better ductility than single phase alloys. Oxygen seems to have a key role with respect to the ductility properties of these alloys. This paper present an atom probe determination of oxygen concentration in α2 and γ phases of two phase alloys and in single phase alloys. The nearly constant observed oxygen concentration in γ phase is considered as the oxygen maximum solubility in this phase (~ 200 at. ppm). The oxygen excess which exists in normal alloys either leads to an oxide precipitation in single phase alloys or is absorbed in the [MATH] phase of two phase alloys. The absence of oxides in two phase alloys could explain the better ductility of these alloys

Microstructure and Mechanical Behaviour of NbTiAl based alloys doped with low additions of silicon.

Nb-base refractory intermetallic materials have potential interest for high temperature applications thanks to their low density and high temperature strength. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high temperatures, two-phase or multicomponent intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. In the present paper, Nb-base refractory alloys based on Nb-35Ti-15Al (at.%) were doped with small amount of Si (1 and 2 at% of silicon) addition to improve its high temperature strength by keeping an acceptable ductility at room temperature. The samples were prepared by arc-melting starting from pure elements (99.99%). The silicon addition effects on the microstructural features were investigated by using X Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) techniques. Its effects on the mechanical properties were assessed by compression tests at ambient and high temperatures. Compression tests show the beneficial effect of the Si addition on strength

Shape recovery in high temperature shape memory alloys based on the Ru-Nb and Ru-Ta systems

This work investigates the high-temperature behaviour of three RuTa alloys and three RuNb alloys. The first step was to determine how Ta or Nb content affects the MT temperatures. The monoclinic Ru50Ta50 and Ru50Nb50 alloys undergo two successive displacive transformations from the high temperature β phase field: β (B2) → β’ (tetragonal) → β” (monoclinic) whereas Ru45 Nb55 , Ru45Ta55 , Ru43Nb57 , and Ru43Ta57 exhibit a single transition from cubic to tetragonal on cooling. All alloys exhibit a highly twinned microstructure with a (011) compound twinning mode. The main feature of the β’ → β” transformation is the formation of domains boundaries separating translation variants instead of formation of new twin separating orientation variants. The shape memory effect was studied through compression tests performed in the β’ or β” phase. The total shape recovery is mainly due to the β’ → β transformation and appears to decrease from about 3% for the monoclinic alloys to about 0.1% for alloys with 43% Ru in accordance with the evolution of the lattice parameters of martensites. Note to the reader: On pages 05021-p6 and 05021-p7 several mistakes have been corrected on October 19, 2009

Precipitation microstructures in a 6056 alloy after FSW

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Mapping the microstructure of a friction-stir welded (FSW) Al-Li-Cu alloy

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