Effects of Grain Morphology on Flow Behavior of Semi-Solid Slurries

The flow behavior of semi-solid slurry determines the quality of the castings produced by the semi-solid forming process. Many studies have done to investigate the flow behavior of slurry under different conditions, and results show that the rheological behavior of slurry with dendritic structure is inappropriate for semi-solid forming. In this study, slurries with varying morphologies of grain for the same alloy with the same fraction solid have tested using a partial filling method. The SEED process was employed, and the pouring temperature adjusted to prepare semi-solid slurries with different grain morphologies. The flow pattern, entrapped air during the filling process, and also microstructure of the samples were examined to characterize the macro and micro flow behavior. The results show that a turbulent macro-flow, leading to entrapped air, and severe segregation appeared in the sample using slurry of Tpour ≥ 660 °C . For the slurry of Tpour < 660 °C, none of the three phenomena found in the sample. This investigation further showed that the detriment of dendrite on the semi-solid forming process, and implied that large size dendrite in semi-solid slurry must avoided.

Control of Amount of α-Al Phase Particles in near Eutectic Al-Si Alloy by Electromagnetic Stirring

Al-Si alloy is widely used as a casting alloy. The α-Al phase in the semi-solid state has low Si content in the Al-Si alloy. Then by separation of these α-Al phases from semi-solid Al-Si alloy, refining of aluminum can be possible. But, in near eutectic Al-Si alloy, few primary α-Al phases can be crystallized. If the fraction ratio of the α-Al phase can be increased, near eutectic Al-Si alloy can refine, and this method can be used for recycling. In this study, the effect of electromagnetic stirring (EMS) on the microstructure, especially the amount of the α-Al phase particles was investigated. A rotational magnetic field was applied to JIS ADC12 alloy which has near eutectic content during slow cooling from the liquid state to the solid-state, by using a three-phase AC coil. By applying EMS at solidification, the shape of the α-Al phase became particle shape from dendrite shape, and the amount of α-Al phase particles was increased. Moreover, by applying unidirectional intermittent EMS, the volume fraction of α-Al phase particles was decreased with increasing intermittent applying time. In ADC12 alloy, the primary α-Al phases can be crystallized only 10% generally, but it could be obtained over 40% by applying EMS. This means that the semi-solid slurry of near eutectic alloy with over 40% of fraction solid can be obtained by applying EMS.

Recent Industrial Application and Perspectives of Rheo-Diecast Process in China

The Rheo-diecast process has been rapidly developed and increasingly used in China in the recent 5 years. The high solid fraction (solid content close to 50%) rheo-diecast components were commercially used in the transportation markets mainly because of lightweight. The mechanical properties of the high solid fraction rheo-diecast components are obviously superior than that of the conventional liquid diecast parts. The defects such as oxide, gas entrapment, shrinkage porosities are well prevented in the high solid fraction rheo-diecast parts. The high solid fraction rheo-diecast parts can be fully T6 heat treated. A comparison between high solid fraction rheo-diecast part and conventional liquid diecast part will be described in detail. The low solid fraction (solid content 5-20%) rheo-diecast components were widely used in the 5G communication markets. The future perspectives of Rheo-diecast process will be described at last. 1. Cost reduction. 2. Production consistency. 3. New Rheo-diecast alloys development. 4. Numerical simulation of Rheological filling.

Effect of Stirring on Crystal Morphologies and on Macro-Segregation

To minimize the macro-segregation in continuously cast steel slabs, the effects of the stirring on the macro-segregation were studied. Industrial findings by the metallographic observations of the steel slabs showed macro-segregation was improved by the refinements of crystals. It was also found that the stirring at low fraction solid refined crystals. The unsolidified liquid core of the continuously cast slab with the optimum stirring was well packed with the refined crystals whereas it was not with coarse equiaxed crystals. The analogue study with experiments of Pb-Sn alloy showed, similarly as with steel slabs, the stirring at low solid fraction refined crystals. Also, the artificially created cavity in a mush was well packed with the refined, globular, crystals whereas it was not with the coarse dendritic crystals. Thus, it is considered the stirring at the low solid fraction is advantageous to refine crystals and to improve macro-segregation.

Effect of Superheat and Oxide Inclusions on the Fluidity of A356 Alloy

The effect of melt superheat and oxide inclusions on the fluidity of a commercial A356 alloy has been investigated. Fluidity measurements have been performed by means of Archimedean spiral in sand moulds. The specific testing method and the experimental apparatus show a good reproducibility. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes and the amount of defects occurring at the tip of the spirals. The results reveal that oxide films increase the variability in the fluidity results obtained at the same apparent experimental conditions. A long permanence in the holding furnace and the introduction of some turbulence during sampling increase the oxide formation and entrapment in the molten bath, thus decreasing the repeatability of the fluidity results. The fluidity increases linearly with superheat and it extrapolates to zero at the temperature corresponding to a fraction solid of about 23%. The initial Ti content in the alloy produces an independent crystallization during freezing of the fluidity spirals.

Impact of major alloying elements on the solodification parameters of cast hypoeutectic AlSi6Cu (1–4 wt.%) and AlSi8Cu(1−4 wt.%) alloys

The present work displays the potential of cooling curve analysis to characterize the solidification path of cast hypoeutectic series of Al-Si6-Cu(1−4 wt.%) and Al-Si8- Cu(1−4 wt.%) alloys. The aim of this work was to examine how variation in chemical composition of silicon and copper may affect characteristic solidification temperatures, fraction solid, and thermal freezing range of investigated alloys. Eight different Al−Si−Cu alloys (Al-Si6-Cu1, Al-Si6-Cu2, Al-Si6-Cu3, Al-Si6-Cu4, Al-Si8-Cu1, AlSi8-Cu2, Al-Si8-Cu3 and Al-Si8-Cu4) have been analyzed applying cooling curve analysis technique. Characteristic solidification temperatures have been determined using cooling curves or their corresponding first derivative curves along with ΔT curves. Fraction solid curves determined from recorded cooling curves have been used to calculate terminal freezing range and estimate crack susceptibility coefficient for each alloy. Theoretical mode for prediction of the cracking susceptibility coefficient developed by Clyne and Davies has been considered in this work. In addition, a novel mathematical model for prediction of crack susceptibility coefficient based on data collected from cooling curve analysis has been proposed.

Development of New Type Semi-Solid Injection Process for Magnesium Alloy

We have developed new type semi-solid injection process for magnesium alloy. This process does not require to use any cover gases and the special magnesium billet such as thixo-billet. In this study, plate specimens were produced by injecting the semi-solid billet with different fraction solid. The microstructure observation, detection of casting defects by an X-ray computed tomography scanner, and tensile test were carried out. With increasing fraction solid, the size and shape of α-Mg solid particles became smaller and more spherical. In the condition of low fraction solid or forming in liquid state, the casting defects were located in the center of the specimen at the thickness direction. Additionally, the volume fraction of the casting defect decreased with increasing fraction solid. Moreover, the casting defects can be reduced by preventing solidifying and clogging of the top of the nozzle. Then, the specimen which has few casting defects could be obtained by injecting the slurry of fraction solid 0.5. However, the tensile strength and yield strength were highest in fraction solid 0.4. It is contemplated that the composition of the solid solution component element in the matrix was increased in fraction solid of 50%, therefore the matrix became brittle.

Thermal Profiles and Fraction Solid of Aluminium 7075 at Different Cooling Rate Conditions

In order to determine suitable processing conditions for semi-solid aluminium 7075 thermal analysis (TA) was performed in order to obtain the relationship between fraction solid and temperature. During experimental work, the alloy was heated to 750°C by induction furnace and solidified at various cooling rates. Cooling curves for the metal were recorded with two thermocouples, one at the centre of the melt volume and one beside the containing crucible wall. A specially designed chamber with kaowool blanket was used to achieve the slowest cooling rate. The faster cooling rate was achieved with the crucible in open atmosphere with a set air flow rate over the crucible. A Data Acquisition (DAQ) system controlled by LabVIEW software was used to record the temperature-time profiles. From these cooling curves, the phase change at any corresponding time and temperature was estimated. The temperature difference between centre and wall of crucible was used to determine dendritic coherency point (DCP). Results show that, the slowest cooling rate with the kaowool blanket was at 0.03°C/s. An intermediate cooling rate of 0.21°C/s was achieved by leaving the melt to cool without kaowool blanket or forced air flow, and the fastest cooling rate was 0.43°C/s. The change in cooling rate altered the temperatures at which phase changes occurred, including those for eutectic and solidus. It was found that for lower the cooling rates that the DCP occurred at lower temperatures. The DCP for the cooling rate of 0.03 °C/s was found to be 574°C (corresponding to 0.85 fraction solid) whereas the DCP for 0.43 °C/s was found to be 623°C (corresponding to 0.55 fraction solid).

Improving Casting Integrity Through the Use of Simulation Software and Advanced Inspection Methods

Casting integrity is essential for providing components that meet design criteria for strength and fatigue performance. As the leading method of manufacturing metal components in the rail industry, maintaining quality and consistency is a continuing struggle for car owners and builders. Internal shrinkage and voids due to insufficient metal flow are issues commonly found in casting molds which are not designed or utilized properly. Using casting simulation software, potential issues can be discovered upfront and robust mold designs can be created that offer a tolerance for the variance or variations in casting conditions that are present in the real world. Strato, Inc. has extensively studied the effectiveness of these simulations in foundries through advanced inspection techniques. It is evident that casting simulations can not only locate, but also explain shrinkage cavities and voids through material density plots and inspection of directional solidification via critical fraction solid time plots. This approach is markedly more efficient than the traditional trial and error method, where mold makers rely on experience and destructive testing to develop acceptable mold designs. With recent advances in simulation software, the labor and time-intensive ways of the past have been supplanted by a more scientific approach to the problem. Understanding the fluid dynamics and thermodynamics of the casting process provides a means of creating a stable, repeatable final product. This higher quality final product can be delivered faster to the customer and at a far less expense by identifying problem areas prior to the tooling and sampling processes. Case-studies explored by the Strato engineering team suggest that using this software decreases the fallout rate.