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

Severo Ochoa and the Biomedical Sciences in Spain under Franco, 1959-1975

The influence of Severo Ochoa in the establishment of biochemistry and molecular biology in Spain is the central topic of this essay. From the time he was awarded the Nobel Prize in Physiology or Medicine in 1959, Ochoa's links with Spanish scientists and top authorities in education and science became instrumental to the development of these areas in the country of his birth. Ochoa's influence is analyzed through investigation of three "events": the reception of the award in Spain and some of its immediate consequences; his role in the VIth Meeting of the Federation of European Biochemical Societies, held in Madrid in 1969; and the international scientific symposium, held in Madrid and Barcelona, that celebrated his seventieth birthday in 1975. After an account of Ochoa's biography up to 1959, analysis of these events shows that Ochoa's influence cannot be understood without taking into account the political and scientific context of its reception.Peer reviewe

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

Microtomographie in situ appliquée à la déformation et la solidification d'alliages d'aluminium

La fissuration à chaud est un défaut majeur intervenant en fin de solidification d'alliages métalliques, lorsque le matériau est encore à l'état semi solide. L'objectif de cette thèse est d'apporter une contribution à la prévision de ce phénomène. Pour cela il est nécessaire de caractériser la déformabilité de la phase solide d'un matériau à l'état semi solide et d'apporter des informations au niveau local sur la microstructure lors de sa déformation (fractions de liquide et de pores, épaisseur de films de liquide, vitesses d'écoulement du liquide, indice de coalescence des pores...). Le comportement macroscopique de la phase solide sur la ligne de solidus a été déterminé et validé par des essais de compression à chaud. Cette loi de comportement a également été comparée avec plusieurs modèles analytiques. La différence de comportement mécanique entre une microstructure de composition homogène et une microstructure à gradient de composition a été mise en évidence. Des informations microscopiques en temps réel sur des échantillons à l'état semi solide soumis à un effort de traction ont été obtenues par microtomographie in situ standard en conditions isothermes et en solidification. Un modèle analytique prédictif de la fissuration à chaud a été modifié pour mieux rendre compte des valeurs expérimentales. Pour gagner en résolutions temporelle et spatiale, un dispositif de traction destiné à l'utilisation de la microtomographie in situ rapide ou ultra rapide a été conçu et réalisé. Les essais en conditions isothermes et en solidification sont désormais reproductibles sur une microstructure obtenue par refusion. Des phénomènes rapides au sein d'une microstructure très fine sont maintenant observables.Hot tearing is a defect which forms at high solid fraction in solidifying alloys, when the material is still semi solid. This work aims at bringing a contribution to the prevision of this phenomenon. To be able to do that, it is necessary to characterize deformability of the semi-solid alloy solid phase, and also bring local information on microstructure during its straining (liquid and pore fractions, liquid films thickness, liquid flow velocity ). Solid phase macroscopic behaviour on solidus line has been determined and validated by hot compression tests. The calculated behaviour law has also been compared with several analytical models. Mechanical behaviour differences between homogeneous and composition gradient microstructure has been put in evidence. Real time microscopic information on tensile tested semi solid samples have been obtained by in situ standard microtomography, in isothermal and solidification conditions. An analytic hot tearing predictive model has been modified to better fit the experimental data. An experimental device has been developed and realized to be able to carry out high and ultra high speed microtomography. These two methods bring better both time and space resolution. High velocity phenomenons in very fine microstructures can thus be visible. Isothermal and solidification conditions tests and now reproducible on remelted samples.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Nucleation and Growth Dynamics of the α-Al / β-Al5FeSi Eutectic in a Complex Al-Si-Cu-Fe Alloy

Secondary-sourced recycled aluminium alloys can exhibit high levels of different impurities. It is well known that the presence of iron, the most common impurity, can lead to the formation of hard and brittle intermetallic phases which are detrimental to the machining properties and the mechanical behaviour of the material in service. The purpose of this work is to study the nucleation and growth of the β-Al5FeSi intermetallic phase in the framework of the eutectic reaction: Liquid → α-Al + β-Al5FeSi. In situ X-ray microtomography has been used to investigate the formation of the irregular eutectic β-phase plates during the solidification at low cooling rate of an Al-8Si-4Cu-0.8Fe alloy. The results show that only a few plates form, nucleating early near the sample surface. Next, growth occurs very rapidly in the principal growth direction and slowly in the thickness direction. The plates are highly branched and appear to form as a divorced eutectic, i.e. not coupled with the α-Al. These features are inconsistent with the common irregular eutectic solidification theory based on the Jackson and Hunt model

Metal matrix syntactic foams produced by pressure infiltration—The effect of infiltration parameters

Metal matrix syntactic foams (MMSFs) were produced by pressure infiltration. Two parameters of the infiltration process (pressure and time) were varied and the infiltrated length was measured as the function of infiltration parameters in order to get data for the implementation of pressure infiltration as mass-production of MMSFs similar to injection mould casting, especially in the short infiltration time range (<10 s). The infiltrated length was found to be linear function of pressure and square-root function of time. The effect of the infiltration parameters on the microstructure and mechanical properties of MMSFs were investigated by optical microscopy and standardised compression tests. The microscopic images were used to qualify the pressure infiltration and showed that more than one combination of infiltration parameters can be found for successful production of a part with given required dimensions. Considering the compression tests, the main characterising properties were mapped as function of infiltration parameters. The registered values showed dependency on the infiltration parameters and indicated that a given infiltration length produced by higher pressure and shorter time has better mechanical properties. The infiltrated specimens were isotropic, anisotropy was not observed in the reference measurements

Modeling of Semisolid Flow

Numerical simulation of semisolid processing can be carried out using one- or two-phase modeling. Various models based from either fluid or solid mechanics formalisms were developed. The most sophisticated one-phase models account for shear thinning, thixotropy, and solidification/melting phenomena. They are of great interest to investigate the flow front and optimize die geometry, temperature of die, and billet. Liquid-solid segregation prediction requires two-phase modeling. Basic equations and constitutive models currently used for numerical simulation of semisolid processing are presented and discussed