Papers by Author: Michel Suéry

Abstract: Rheological properties of semi-solid alloys are closely knit to the solid-phase microstructure. Parameters such as particle size distribution are commonly determined by 2D cross section analysis. The determination of mechanisms such as particle deagglomeration with increasing shear rate however, requires information on the 3D spatial distribution. By means of synchrotron radiation tomography and SEM on AlCu samples, particle size distributions and the not yet microscopically observed interrelation of shear rate and particle agglomeration in thixo-material is investigated.

Abstract: This paper is concerned with the rheological behavior of aluminum alloys during solidification and the modeling aspects of this behavior. The whole range of solid fractions is considered and it is shown that the rheology can be divided into several domains exhibiting quite well-defined characteristics in terms of shear and tensile properties. Modeling based either on suspension approaches for small solid fractions or on porous medium approaches when the crystals mechanically interact is also considered.

Abstract: This paper is concerned with an investigation of the deformation behaviour of an Al-Cu alloy during tensile testing in the semi-solid state. It was carried out by fast in-situ X-Ray microtomography at ESRF, Grenoble. Deformation was performed at constant velocity, which was chosen to be small enough so as not to affect the acquisition of the images. It is observed that deformation is accompanied, initially, by some liquid flow from the adjacent regions towards the deformed zone. Then pores form in the liquid films and grow until they occupy a significant part of the cross section of the specimen. Quantification of this phenomenon was carried out thus leading to a better understanding of pore formation in semi-solid mixtures.

Abstract: The aim of this work is to study by X-Ray microtomography carried out at ESRF Grenoble the microstructure of an Al-4wt%Cu alloy which was previously cold rolled to obtain globules of the solid phase upon heating in the semi-solid range. Since this process produced entrapped liquid in the globules, 3D quantification of this liquid was performed. Moreover, the influence of the addition to the alloy of a small amount of Ba, which has been shown to decrease the contiguity between the solid globules as a consequence of the decrease of the solid-liquid interfacial energy σsl, was investigated. It is in particular shown that the amount of entrapped liquid is much larger in the Ba-containing alloy in agreement with the reduction of σsl, whereas the size of the liquid pockets is similar. In addition characterization of the interglobular liquid shows that the interface area between this liquid and the solid per unit volume is larger for the alloys containing Ba in agreement with previous observations carried out on 2D sections. The influence of strain during cold rolling is also reported but it is shown to have a quite limited influence on the previous parameters.

Abstract: The microstructural evolution and mechanical response in compression in the semi-solid state of previously extruded AZ91 alloys containing two levels of Ca additions (1mass%Ca (AZC911) and 2mass%Ca (AZC912)) have been studied. Ca additions have a significant effect on microstructural evolution and compression behavior of the AZ91 alloy. At 515°C, the liquid fraction in AZC911 is larger than that in AZC912, so that the compression stress of the AZC912 alloy was found to be much larger than that of the AZC911 alloy. This behavior is explained through DSC analysis which suggests that some solid Al2Ca phase remains in AZC912 alloy at this temperature. Increasing the remelting temperature for this alloy leads to more liquid and coarsening of the solid particles occurs with increasing holding time.

Abstract: The evolution of the microstructure of semi-solid Al-Cu alloys has been characterized during isothermal treatments in the semi-solid state by using in-situ X-ray microtomography experiments carried out at ESRF, Grenoble. This work is therefore a continuation of previous experiments reported at the 8th S2P Conference in Limassol. Quantitative data are presented dealing with changes with holding time of the solid volume fraction, of the solid-liquid interface area and of the mean and Gaussian curvatures of the solid phase. Discussion of the results is carried out on the basis of the mechanisms involved in semi-solid systems.

Abstract: In order to improve the understanding of hot tearing during laser welding of aluminium alloys, the rheology of the alloys in the mushy state must be characterized. The present work investigates the mechanical behaviour of the aerospace alloy AA6056 using a specially designed isothermal tensile test in the mushy state. Using a Gleeble thermo-mechanical machine, two different tests have been performed: i) tests during partial remelting and ii) tests after partial solidification at a high cooling rate. These tests have been carried out not only on the 6056 alloy but also on a mix between 6056 and 4047 Al-Si alloy which corresponds to the composition of the nugget of a laser using a filler wire. The increase of the solid fraction results in an increase of the maximum stress and a change on the fracture surface from a smooth dendritic to a more ductile one. Moreover, the alloys exhibit a typical visco plastic behaviour with an increase of the maximal stress with the strain rate. When the test is performed at a particular solid fraction of 0.97, the fracture is more erratic and the ductility is low. The results show the existence of a ductile/brittle/ductile transition with the fraction of solid. The fracture stress is shown to be higher when testing after partial remelting as compared to partial solidification for the same solid fraction. This is due to the difference in microstructure of the mushy zone and more particularly in the connectivity of the solid skeleton. An adapted creep law is used to describe the mechanical behaviour of alloys during the partial remelting test using the fraction of grain boundary wetted by the liquid given by Wray. This law is shown to be irrelevant to the partial solidification tests, as a result of the modified geometry of the liquid phase. From these tests, we have determined a new law relating the solid fraction to the fraction of grain boundaries wetted by the liquid. This law is a useful tool to predict the mechanical behaviour when mechanical loading occurs during solidification.

Abstract: Microstructure and mechanical properties of AM50+xTi (x=0,0.01,0.1wt%) magnesium alloys extruded from as-cast and solution treated conditions have been studied. Results show that Ti element obviously refines the microstructure of AM50 magnesium alloy and Mg17Al12 phase. Only 0.01 wt% Ti addition can make the Mg17Al12 phase turn into particles and small rod-like shape. Ti addition improves tensile strength at room temperature, and obviously improves elongation at elevated temperatures up to 200°C. The AM50+xTi alloys extruded from as-cast have better tensile strength at room temperature and better elongation at 100°C, 150°C and 200°C than that of AM50+xTi alloys extruded from solution treatment; The plasticity of AM50 magnesium alloys increases with Ti content increasing and temperature increasing for the tensile fractograph.

Abstract: Mechanical properties and microstructure of extruded AZ91(-Ca) alloys have been studied in this paper. The results showed that Ca has no significant effect on reducing grain size of the extruded AZ91 alloy. The ambient temperature tensile tests showed that the ultimate and yield strength of extruded AZ91 alloy decreased by addition of Ca. At elevated temperature, Ca addition improves the yield strength of both AZ91 alloy. The variations in microstructure and mechanical properties of the AZ91 alloy are also discussed in terms of the effects of Ca on grain refinement and formation of constituent phases.

Abstract: Effects of solution and aging treatment on microstructure and mechanical properties of rolled AM50+xCa alloys(x=0, 1, 2 wt. %) were studied. The results indicated that, with increasing solution time i, the secondary phase Mg17Al12 was dissolved into the Mg matrix and Al2Ca became thinner and shorter, then gradually broken and spheroidized.With an increase of aging time, Mg17Al12 precipitated from the Mg matrix in the form of particles and Al2Ca changed a little. After solution treatment, hardness and tensile properties of the alloy’s decreased. After the aging treatment, the alloy’s hardness increased first and decreased later while the tensile properties increased little. The solution and aging treatment can increase the ductility of AM50 and AM50+1Ca alloys. For AM50+2Ca alloy, the ductility increased after solid solution treatment and decreased after aging treatment.