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The mechanical properties of thixoextrusion components can be improved by controllable processing parameters such as the solid fraction of alloy, holding time, punch velocity, heat treatment and die temperature. In this study, the effects of thixoforming parameters on the microstructures and mechanical properties of thixoextrusion ADC12 alloy Aptomat Contact are studied. ADC12 has excellent castability with high fluidity and low shrinkage rate, so it is widely used in industry, especially in automotive and motorcycle engine part casting. It is a near eutectic alloy with high strength and low ductility (1%). The optimization parameters mechanical properties were investigated by changing the punch velocity, specimen temperature and holding time. The results also indicated optimal value at punch velocity (15 mm/s), specimen temperature (560 °C) and holding time (5 min) which was changed microstructure from eutectic dendrite to globular grain, increasing the ductility (3.3%) of this alloy during the semi-solid forming process while the remaining mechanical properties lead to an increase in the quality of finished parts.

International audience; The behavior of model granular materials (glass beads) wetted by a small quantity of liquid forming capillary bridges is studied by one-dimensional compression test combined with X-ray computed tomography (XRCT) observation. Special attention is paid to obtain very loose initial states (initial void ratio of about 2.30) stabilized by capillary cohesion. XRCT-based analyses involve spherical particle detection adapted to relatively low-resolution images, which enable heterogeneities to be visualized and microstructural information to be collected. This study on an ideal material provides an insight into the macroscopic compression behavior of wet granular materials based on the microstructural change, such as pore distance distribution, coordination number of contacts, coordination number of neighbors and number of contacts per grain.

The thixoforming process is becoming more and more important in the field of metal-forming. In this process, the most difficult part is heating the sample while keeping the temperature uniform. The pressing speed is one of the main parameters that impacts strongly on product quality. In this paper, the die for studying the effect of pressing speed was designed and manufactured. The Deform 3D software was applied for observing the relationship between the pressing speed and the force history, stress, and temperature distribution. The results show that a pressing speed of v = 8 mm/s results in the smallest stress. The stress depends on many elements. For example, the pressing speed of v = 6 mm/s makes the heating process happen fast. The billet is cooled faster so the stress increases and even changes the structure unexpectedly. When the pressing speed is too high, the metal on the surface of the billet flows with high speed so it collides with the folding segments causing great resistance thus creating high stress.

An approach combining the Discrete Element Method (DEM) and experimental techniques (oedometric compression test and X-ray Computed Tomography (XRCT)) is proposed to observe the mechanical behavior of wet granular material, modeled as frictional spherical glass beads, in very loose state under growing of applied external force. An experimental observation combining one-dimensional compression test and XRCT is first presented. The plastic responses of real material (spherical glass beads) are depicted with different values of very low initial densities. The grain-scale behavior of this material is then characterized by using the XRCT and in-situ compaction test. We propose a method to detect the spherical structures from the 3D tomography images. Microstructure properties of the loose system are then analyzed and computed from the detected spherical structures. This simplified material gives further a better understanding of the macroscale behavior of wet granular soils based on their microstructure characterization. The DEM simulation then allowed us to predict accurately in 3D the plastic response of very loose assemblies of modeled wet beads to an isotropic/ oedometric compression in the presence of a small amount of an interstitial liquid, which gives rise to capillary menisci and attractive forces. The plastic responses along the compression curves are then shown in particular without and with the appearance of rolling/ pivoting resistances in contacts and also the effect of size polydispersity. The evaluation of microstructure and force transmission along the compression curve is also characterized. Finally, we compare the predictions of the experimental results with the DEM simulations results. Our comparisons show that the predictions of experiments are underestimated for the DEM simulations. Nevertheless, the combined approach in this work also provides an comprehensive characterization of the plastic and grain-scale compression behavior of wet granular soil at very loose state; Une approache combinant la méthode des élements discrets (DEM) et expériences techniques (essai oedométrique et microtomographie aux rayons X (XRCT)) est proposée pour observer le comportement mécanique des matériaux granulaires mouillés (des billes de verre mouillées) en état très lâche sous le chargement extérieur. Une observation expérimentale combinant l'essai oedométrique et XRCT est premièrement présentée. La réponse plastique du materiau réel (des billes de verre) est illustrée avec des certaines valeurs de densité initiale très faible. Le comportement à l'échelle du grain de ce matériau est puis caractérisé en utilisant à l'aide de microtomographie aux rayons X et d'essais mécaniques in-situ. Une méthode pour détecter des structures sphériques qui ont obtenu depuis des images de la tomographie est proposée. Ensuite, les propriétés micro-structurales sont analysées et calculées avec les données des sphères détectés. Ce matériau simplifié donne une meilleure compréhension du comportement à macro-échelle des sols granulaires mouillés basée sur leur caractérisation micro-structurale. La simulation numérique a permis de prévoir précisément en 3D la réponse plastique d'assemblages très lâches de billes de verre modélisé soumis à une compression isotrope/ oedométrique. Ce maériau a été préparé avec la présence d'une petite quantité d'un liquide interstitiel qui contrôle à capillaire ménisque and des forces attractives. Les réponses plastiques le long des courbes de compression sont présentées dans les cas avec et sans des résistance au roulement/ pivotement en contacts. L'effet du grain polydispersité en taille a aussi mentionné. L'évaluation de la microstructure et transmission de force le long de la courve de compression est également caractérisé. Finalement, nous comparons les prévisions des résultats expérimentals avec ceux de la simulation numérique. Nos comparaisons montrent que les prévisions de l'expérience sont sous-estimées pour celles de la DEM. Néanmoins, cette approache dans ce travail également fournit une caractérisation complète de la réponse plastique et du comportement à l'échelle du grain de sol granulaire mouillé à l'état très lâche

The presence of liquid menisci between soil particles in wet granular soils plays a key role in the macroscopic behavior. This liquid bridge between particles may also allow the existence of metastable microstructures that lead to mechanical instabilities when the soil in subjected to external loadings. In this work, the authors present an investigation into macroscopic and microscopic behaviors of wet granular soils in very loose state under one-dimensional (1D) compression by using oedometer test combined with the X-ray computed tomography (X-ray CT) observations. The specimen is prepared in the presence of a small amount of a water meniscus by using the controlled pluviation method. The influences of various mechanical parameters are first studied on the compression curve. The microstructure along those compression curves are then analyzed by using X-ray CT to observe the arrangement of microstructure under growing of applied force.

We present an investigation into macroscopic and microscopic behaviors of wet granular soils using the discrete element method (DEM) and the X-ray Computed Tomography (XRCT) observations. The specimens are first prepared in very loose states, with frictional spherical grains in the presence of a small amount of an interstitial liquid. Experimental oedometric tests are carried out with small glass beads, while DEM simulations implement a model of spherical grains joined by menisci. Both in experiments and in simulations, loose configurations with solid fraction as low as 0.30 are prepared under low stress, and undergo a gradual collapse in compression, until the solid fraction of cohesionless bead packs (0.58 to 0.6) is obtained. In the XRCT tests, four 3D tomography images corresponding to different typical stages of the compression curve are used to characterize the microstructure.