Solid State Phenomena Vols. 116-117

Abstract: This paper presents experimental results on steel thixoforming. The influence of thermal effects on the semi-solid response is analysed through the influence of thermal exchanges with tools and environment. Several rheological experiments such as compression, extrusion or radial filling test were developed to understand the semi-solid steel behaviour and determine the parameters that have a major influence on thixoforming. Actually, in our experiments, the temperature of the slug and consequently the solid fraction were found first order parameters while; the morphology of the solid phase plays a minor role.

Abstract: In this paper the authors propose a new technology for high volume fraction ceramic/metal composites—pseudo-semi-solid thixoforging, based on metal semi-solid process combing with traditional powder metallurgy technology, and explain the differences with other technologies and characters in detail. At the same time the authors prepare the cup shells with high volume fraction Al2O3/Al composite by this technology. It proves that it is feasible for the high volume fraction ceramic/metal composites forming through metallographic analyses and three-point bending tests. The results show that the tough strength and degree of hardness of the samples with certain plasticity are high. It will play a very big role in accelerating the ceramics composites and high melt point materials being used in more fields and tamp the foundation for further studies of this technology.

Abstract: This paper presents semi-solid processing of Cu-Ag alloy to produce squirrel cage for small and medium induction motors. Complete die filling could be achieved with the slug at 1065 oC, casting pressure of 9 MPa and ram speed of 0.08 m/s. There were no defects in squirrel cage and slug temperature and ram speed affected the slot filling of rotor importantly, however, casting pressure rarely has an effect of the filling in this experiment. As the same thixoforming condition, when the ram speed was 1.5 m/s, slurry could not fill the slot and solidified like a needle shape. The rapid flow like this would be the cause of non-filling defect of slot because the slurry injected and solidified in the slot firstly was became the obstacle not to flow the rest of the slurry.

Abstract: The aim of the paper is to demonstrate a control scheme by which it is possible to reproducibly reheat steel billets into the semi-solid state. Usually a heating program is used to reheat the billet into the semi-solid state. Our experiments showed that this control scheme leads to varying semi-solid fractions from one experiment to the next. To gain information about the billet’s state its temperature is often used since there is a known relationship between the temperature and the liquid fraction. Direct measurement of the temperature via thermocouples is not feasible in a production environment, therefore a radiation pyrometer has been used as a contact-less measurement device. The accuracy of the pyrometer depends heavily on the exact knowledge of the radiation coefficient, which can vary from billet to billet due to different surface properties and which is subject to change during the heating process. These uncertainties prohibit the implementation of a closed-loop control scheme since the exact temperature cannot be measured with the required accuracy. In order to be independent of the measurement errors the proposed control scheme only relies on the slope of the temperature. By detecting the distinct change of slope which occurs when the solidus temperature is crossed, the beginning of the melting process can be determined. The energy fed to the billet from this point onward determines the resulting liquid fraction. By detecting the entry into the solidusliquidus interval and then feeding the same amount of energy to each billet, it is guaranteed that the billet reaches the desired liquid fraction even by uncertain absolute value of the temperature and by small variations of the alloy composition. For the experiments the steel alloy X210 has been used and measurement data demonstrate the feasibility of the proposed control scheme.

Abstract: Static shear-rheology for self-made semi-solid AZ91D magnesium alloy slurry was studied by using the tester and a universal electronic machine. Shear-stress along with cylindrical surface in the sample was produced by using the tester and then shear-rheologic deformation happened. It showed that on the condition of the same loading, the longer the loading time and holding time were, the larger the deforming rate was. When the holding time attained a certain value, small grains acquired enough energy to grow up or amalgamate with prolonging the holding time and coarse grains started to melt from intergranular or grain boundaries, but the critical shear-stress kept a constant. The sample didn’t flow but appeared to instantaneous shear-strains with starting to load. When the loading exceeded the critical value, the sample started to flow and had the phenomena of elastic after-working, elastic before-working and remained deformation. The rheologic characteristics of semi-solid AZ91D magnesium alloy were expressed by the five element mechanical model: H1—(N1/H2)—(N2/S).

Abstract: The billet of magnesium AZ31B was made by twin-screw rheo-cast, and the specimen cut from the rheo-cast billet and normal cast billet were compressed in Gleeble-3000 hot simulator. The deformation temperature was 300, 350
, and the strain rates were 0.001~1s-1. The relationship among flow stress, deformation temperature and strain rates were analyzed. The variation of the microstructure was observed and compared after the specimen was hot compressed. The results show that the surface of the rheo-cast specimen is much more smooth than the normal cast specimen after the compression. The dynamic recrystallization had taken place during the compression and the recrystallized grain of rheo-cast sample is more tiny than normal cast specimen.

Abstract: Semi-solid slurry of A7075 aluminum alloy with fine solid granules was tried simply to make by passing the melt through upright tube followed by inclined water-cooled tube. Structures of solidified slurries were granular and mean grain sizes of about 0.06 to 0.11mm could be obtained. When the slurries introduced into an extrusion container kept at semi-solid temperature 873K were extruded to round bars at various extrusion ratios and press ram speed of 10mm/s, just after cooling to 833K, they could easily be extruded to bars with smooth surfaces at lower forces. Although every tensile strength of extruded bars were lower than that of hot-extruded one, there was a tendency that finer the solid granules in slurry, higher the tensile strength of extruded bar.

Abstract: This study sought to examine the suitability of the continuous strip casting process as a new fabrication method for the Ag-Cu brazing alloy. Several continuous strip casting processes such as planar flow casting, melt dragging, and twin roll strip casting were applied to the manufacture of the Ag-32%Cu strip. In particular, twin roll strip casting was found to realize the best strip shaping ability and microstructural homogeneity. The effect of aging heat treatment on the microstructure and mechanical properties of a Ag-Cu strip was also investigated through tensile test, hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations, and electron probe microanalysis (EPMA). Tensile results showed gradually decreasing strengths and simultaneously increasing elongation with longer aging time. An appropriate TS strength of 283 MPa and a significantly enhanced plasticity of 41% were realized with aging condition of 670oC/2hrs. Finally, twin-rolled strips 1 mm thick could be cold-rolled to a very thin strip (80~100 μm thickness). The microstructural evolution and crystalline morphologies of Ag-Cu metastable strips were also investigated and related to the mechanical properties.

Abstract: An experimental apparatus using cooling slope and thin metal container has been developed to study controlled rheocasting process. The microstructural characteristics of Al-8%Si industrial alloy processed by this route were determined statistically. The statistics and quantitative analysis of the dispersions of particle’s diameters were used for process optimization. Emphasis is placed on factors which determine the final grain size in a billet. It is suggested that much of the refinement of the microstructure in the process may be caused by temperature fluctuations in solid shell formed at cooling slope. The nuclei which could be formed by melting of eutectic phase in solid shell and transferring by overheated liquid metal to the container mostly survived and contributed to final globular microstructure of a billet and ring type feedstocks.