Mechanical Behavior of AA6061 Aluminum in the Semisolid State Obtained by Partial Melting and Partial Solidification

International audienceThe tensile properties of a 6061 aluminum alloy have been studied in the semisolid state at large solid fractions. The tests have been carried out either after a partial melting treatment or after partial solidification. Results show the following: (1) the mechanical behavior depends on the liquid-phase distribution and, therefore, on the way the semisolid state has been achieved (melting or solidification); (2) there is a critical solid fraction range where the semisolid alloy is relatively brittle; and (3) the mushy alloy exhibits viscoplastic behavior with the occurrence of micro-superplasticity at low strain rate. Modeling of this behavior is carried out by considering either the area fraction of grain boundaries wetted by the liquid or a cohesion parameter of the solid phase, which depends on solid fraction and thermal treatment

Effects of heat treatments on the microstructure and mechanical properties of a 6061 aluminium alloy

International audienceThis paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy

Microstructural characterisation of partial melt alloys by in-situ X-Ray microtomography

International audienc

Rheological and metallurgical discussion of superplastic behaviour

The analysis of the deformation of superplastic materials shows that the structure tends to evolve towards an equiaxed state and to undergo grain growth. This behaviour influences the constitutive equations : apparent strain-hardening, sigmoidal variation of the Log stress- Log strain rate curve, low apparent activation energy values at intermediate strain rates. This deformation behaviour can be explained when the structural change is taken into account. The analysis described leads to a constitutive equation which presents a metallurgical basis since it takes into account the structural changes. By contrast, the generally proposed relationship is rheological and can be used only within a strictly mechanical approach. An hydrodynamic analogy of superplastic flow is then presented. It shows that superplasticity occurs, on the one hand when the hardest phase is not continuous, and on the other hand when the most ductile phase has a mean free path no greater than a few microns. The analogy also demonstrates that the textural evolution is different for the two phases and accounts for the observed interface sliding. Furthermore, this study suggests that superplastic behaviour with elevated values of the coefficient m can be observed, although large elongations cannot be obtained in cases where the material undergoes cavitation resulting from difficulties in interface sliding.Une analyse de l'évolution de la structure des matériaux superplastiques au cours de la déformation montre que celle-ci tend à devenir équiaxe et à présenter du grossissement. Cette évolution a une influence sur la loi de comportement : taux d'écrouissage apparent, allure sigmoidale de la courbe Log σ — Log ε, valeur faible de l'énergie d'activation apparente dans le domaine des vitesses intermédiaires. Il est montré qu'en tenant compte de cette évolution, il est possible d'interpréter le comportement observé. L'analyse présentée permet de mettre en évidence une loi de comportement à caractère métallurgique puisqu'elle tient compte de la structure en évolution. Par opposition, la loi généralement proposée est rhéologique et ne peut être utilisée que dans une approche strictement mécanique. Une analogie hydrodynamique à l'écoulement superplastique est ensuite présentée : elle montre que ce comportement apparaît d'une part lorsque la phase la plus dure n'est pas continue, et d'autre part lorsque la phase la plus ductile a un libre parcours moyen limité à quelques microns ; elle explique les évolutions de texture différentes pour les deux phases et rend compte du glissement observé au niveau des interfaces. De plus, cette étude permet de suggérer qu'un comportement superplastique à valeur élevée du coefficient m peut être observé, bien que des allongements importants ne puissent pas apparaître dans le cas où le matériau présente de l'endommagement résultant d'un glissement difficile au niveau des interfaces

Influence of the silicon content on the mechanical properties of AA6xxx laser welds

International audienc

Shear Behavior of AA6061 Aluminum in the Semisolid State Under Isothermal and Nonisothermal Conditions

International audienceThe shear behavior of a 6061 aluminum alloy was studied in the semisolid state at large solid fractions. The tests were carried out either at constant temperature after partial solidification (i.e., isothermal shear tests) or during solidification at low cooling rate (i.e., nonisothermal shear tests). In isothermal conditions, results show that (1) the mechanical behavior depends on the volume fraction of the solid phase present in the sample at the temperature of the test, (2) there is a critical solid fraction corresponding to the coalescence of the solid grains beyond which shear stress increases very sharply with increasing solid fraction, and (3) the mushy alloy exhibits viscoplastic behavior with a strain-rate-sensitivity parameter close to about 0.17. In nonisothermal conditions, results show that stress increases continuously with decreasing temperature whatever the strain rate. However, at high strain rate, it was observed that cracks developed when the solid fraction approaches 1, leading to a slower stress increase compared to that observed at low strain rate. Finally, modeling of this behavior is carried out by considering a cohesion parameter of the solid phase, which depends on solid fraction and strain rate

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