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Nowadays it is common to see the production of complex and critical automotive and aeronautical components reduced in weight for energy efficiency using light alloys with improved microstructural and mechanical properties. The casting processes involved in this trend are strong; in this study, an optimized design of a vertical serpentine channel and a novel design of a horizontal serpentine channel to produce semi-solid slurry (S2S) with thixotropic behavior by self-stirring for rheocasting of A380 and A356 alloys are tested. Simultaneously, chilling during solidification, flow development, and shearing on the alloys to improve the performance of solid fractions and self-stirring at high shear rate are applied. The effects of these conditions on the modification of the morphology transition of the α(Al) phase from dendrite to equiaxed grain are discussed. The results suggest the ability of the mentioned processes to promote the morphological transition of the primary solid due to the produced equiaxed grains of α(Al) phase having sizes between 25–50 µm from A380 alloy processed by vertical self-stirring. On the other hand, the treatment of the A356 alloy using the new horizontal serpentine channel produces equiaxed grains with an average size of 39 µm. Unexpected Si crystals, trapped in the α(Al) phase using both methods with both alloys, are detected. The applied operation parameters were aided by gravity-pouring close to the liquidus temperature, and the obtained microstructural results show the ability for S2S to form alongside thixotropic behavior and non-dendritic solidification by mean of self-stirring in the serpentine channels, suggesting the potential for further experiments under die-casting conditions. [ABSTRACT FROM AUTHOR]

In the present paper, the microstructure evolution of semi-solid metal slurry was investigated. The material was tested by rheocasting route. In order to better estimate the materials behavior, the investigation was performed on three series of samples. The rheocasting was conducted different stirring speed of 500, 1000 and 1500rpm and 0.3 solid fraction, before casting. The microstructure was analysed using an optical and scanning electron microscope. The results show that the relationship between the equivalent diameter of Si and α - Al particles with stirring speed can be well fitted to quadratic or linear equation. The results from this study can also be used to optimize the process and study mechanism of the rheocasting process. [ABSTRACT FROM AUTHOR]

In this paper, the effects of pouring temperature of magnesium melt, preheating temperature of the barrel of the screw mixer, and shear rate on the solidified microstructures of semi-solid slurry were investigated by a mechanical stirring semi-solid process. The appropriate processing parameters of slurry preparation were obtained, and the mold filling ability of semi-solid slurry for thin-walled casting was examined. Results indicate that the solid volume fraction of non-dendritic mi-crostructure increases with a decrease in pouring temperature of magnesium melt and the barrel preheating temperature of the screw mixer. Also the grain size of primary a-phase is reduced. Furthermore, the solid volume fraction of semi-solid non-dendritic structure decreases with an increase of shear rate. The fine and round granular microstructure with 30一50 Nm insize of semi-solid AZ91D magnesium alloy was presented. Finally, a 1.0 mm thin-walled casting with a clear contour and good soundness was successfully made by semi-solid rheo-diecasting.

Abstract: Understanding the flow behavior of semi-solid slurries containing low solid fractions is key to the success in applying this process in the die casting industry. With these low initial solid fractions, the flow behavior of semi-solid slurries is quite complicated, making it difficult to model accurately. This present work developed and studied characterization methods for the flow behavior of semi-solid slurries at low solid fractions in high-pressure die casting. A new parameter, the ratio of gate speed to initial solid fraction (V g/f s), was proposed to be correlated to the normalized flow interface length, blister area and tensile properties. Results from the flow pattern analysis suggest that the flow behavior can be controlled to achieve laminar flow by varying the initial solid fraction. Blister test results show the trend that slurry die casting conditions with high V g/f s values exhibit high blister areas. Die casting conditions with excessively high gate speeds and insufficient solid fractions result in turbulent flow patterns and high levels of blister defect. The results of tensile test and fracture surface analysis are consistent with other analysis results. The samples formed by liquid die casting and slurry die casting with high V g/f s values have gas porosity due to turbulent flow pattern during die filling. On the other hand, the samples formed by slurry die casting with too low V g/f s values contain shrinkage porosity. This is because of insufficient time for shrinkage feeding due to a combination of a high solid fraction and a low gate speed. This study has demonstrated that die casting with slurries containing low initial solid fractions gives die casters another process parameter to adjust, which can help reduce and control the gas and shrinkage porosities. [Copyright &y& Elsevier]

Abstract: A key success of semi-solid metal forming by the rheocasting route is having an efficient and effective technique to prepare semi-solid slurries. To achieve that it is necessary to be able to characterize the microstructure during the early stages at different solid fractions as the slurry is being cooled. This present work applied the rapid quenching method to study microstructure evolution during the early stages in a rheocasting process. Nine stages of microstructure evolution were characterized by applying rheocasting times of 5, 10, 12, 15, 20, 30, 35, 40 and 45s during cooling. Analysis to evaluate the solid fraction, particle density, particle size, particle shape factor, and particle distribution was performed. The results show that the relationship between the solid fraction and the rheocasting time can be well fitted to a quadratic equation. For particle density, the relationship is better fitted to a linear equation. The change in particle size with time can be modeled using a ripening model with an upper bound value for the ripening coefficient. In addition, the particle distribution may be quantified by the dilation and counting technique to determine different levels of particle clustering. The results from this study suggest that this new characterization method can be used as a process and quality control tool in the rheocasting process. It can also be used to optimize the process and to study the mechanism of the rheocasting technique. [Copyright &y& Elsevier]

Abstract: This paper reports a new slurry-making process with indirect ultrasonic vibration (IUV), in which the horn is vibrated under the outside of the metallic cup containing alloy melt. Semisolid slurry of A356 Al alloy was prepared by this process, and the effects of IUV treatment time on the morphology of the primary α-Al phase and mechanical properties of rheo-diecasting samples were studied. The results indicate that excellent semisolid slurry of A356 Al alloy could be obtained within 50s by applying IUV near its liquidus temperature. The sample rheocasted with the slurry made by IUV for 50s had the maximum tensile strength and elongation, which were 254MPa and 7.5% respectively. [Copyright &y& Elsevier]

Abstract: The indirect ultrasonic vibration (IUV) method, in which the horn was vibrated under the outside of the metallic cup containing alloy melt, was employed for preparing semisolid slurry of 5083 Al alloy in this work. The effect of IUV on the microstructure of semisolid slurry and mechanical properties of rheo-diecasting samples were studied. The results indicate that good semisolid slurry could be obtained when the melt treated by IUV for 50s, and the average diameter and shape coefficient of primary α-Al particles were 60μm and 0.54 respectively. Apart from preparing semisolid slurry with fine microstructure, IUV had degassing effect on semisolid slurry of 5083 Al alloy. The tensile strength and elongation of the rheo-diecasting samples were 283MPa and 9% respectively, which were increased by 11.4% and 32.4% respectively compared to conventional liquid die-casting samples. [Copyright &y& Elsevier]

Immersion cooling and stirring are two elements of the internal cooling process recently proposed to produce aluminium rheocasting feedstock. They were employed in various combinations to identify the impact of each on the evolution of globular structures. AlSiMg melt sample water quenched above the liquidus point and after cooling into the semisolid temperature range produces predominantly dendritic dendritic features, typical of conventionally cast structures. Improvement in slurry features is noted when cooling below the liquidus temperature involves either stirring or immersion cooling. Internal cooling process which employs both immersion cooling and stirring produces smaller globules than those possible with stirring alone. Globular structures are obtained earlier and are refined further when internal cooling is extended to achieve higher solid fractions before quenching. [ABSTRACT FROM AUTHOR]

Abstract: Rheocasting is becoming the choice of the casting industry which relies on the semi-solid processing for high integrity structural parts. It is thus of great technological interest to identify simple methods to prepare slurries at reduced cost. The potential of internal cooling to produce slurries for rheocasting was investigated in the present work for several aluminium casting alloys which are technologically important for aluminium foundries. Alloys quenched directly from the liquid state were predominantly dendritic while the slurry samples quenched after an initial fraction of solid phase was first formed, were dominated by α-Al rosettes and globules. The solidification of the remaining liquid phase occurred through the growth of the α-Al rosettes and globules which have formed during internal cooling before quenching. [Copyright &y& Elsevier]

The RheoMetal process, based on the rapid slurry forming (RSF) technology, is a new rheocasting process enabling the production of semisolid slurry of high quality and with high efficiency. The RSF technology is based on an enthalpy exchange between two alloy systems rather than applying external cooling as is done in many other rheocasting processes. In this work, several process parameters important for the RSF technology have been investigated on aluminium A356 alloy slurry microstructures. During slurry preparation, the thermal history for the different parameters studied was also recorded. It was found that the process parameters tested (amount of solid addition, melt superheat, rotation speed) have an effect on the thermal history during processing and on the microstructures formed after slurry formation. [ABSTRACT FROM AUTHOR]

A new rheocasting process, low superheat pouring with a shear field (LSPSF), to achieve sound semisolid slurry is reported. Results show that a fully grain refined spherical structure can be obtained using proper processing conditions within 25 s. The efficient grain refinement should be attributed to the continuous nucleation events, enhanced crystal separation from the nucleation sites and crystal survival, as a consequence of low superheat pouring combined with localised rapid cooling and positive mixing–shearing during the initial stage of solidification. The discussion based on the morphological instability theory indicates that the combined effects of both low cooling and high grain density can enhance the stabilisation of solid/liquid interface and promote grains to grow spherically to a larger size scale. Suppressing dendrite growth and coarsening through the overlapping of diffusion fields and the Gibbs–Thomson effect are the main dynamic conditions that lead to the formation of spherical primary a α-Al particles. [ABSTRACT FROM AUTHOR]

Magnesium and silicon concentrations in the interior of primary α-Al of Al-7Si-Mg alloys were studied at temperatures in the liquid-solid range and just after solidification was completed. Analysis of the results showed that the magnesium concentration in the interior of primary α-Al is very low in the temperatures range from the liquidus to the start of the Al-Si eutectic reaction. Formation of silicon-rich phases during eutectic reactions, such as eutectic silicon and β-Al5FeSi, phases trigger a significant increase in the magnesium concentration in the interior of primary α-Al, when sufficient time is allowed for solid-state diffusion to occur. When fast cooling rates are applied during the Al-Si eutectic reaction, most of the magnesium is retained in π-Al8FeMg3Si6 and Mg2Si phases formed during solidification. Semi-solid Al-7Si-Mg castings were produced with varying magnesium contents, and the mechanical properties were evaluated in the as-cast, T5 and T6 conditions. It was found that the 0.2% offset yield strength of the semi-solid Al-7Si-Mg castings in the T5 and T6 conditions increases linearly with the square root of the magnesium concentration in the interior of the α-Al globules formed during the slurry preparation process. [ABSTRACT FROM AUTHOR]