Search

Thermal conductivity of a rheocast component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. The thermal conductivity measurement in different locations showed variation of this property throughout the rheocast component. The results of microstructural investigation revealed that the ratio of solute-lean α1-Al particles formed during slurry preparation to fine solute-rich α2-Al particles formed during secondary solidification had significant influence on thermal conductivity. The reduced amount of solutes in the α1-Al particles was determined as the root cause of higher thermal conductivity. A linear relation between the fraction of precipitates and the increase in thermal conductivity was obtained and silicon in solid solution is shown to have a dominant influence. As silicon was precipitated during the heat treatment, thermal conductivity increased. For an optimal combination of thermal and mechanical properties, it is therefore important to use an ageing temperature above the temperature of Si precipitation. [ABSTRACT FROM PUBLISHER]

Rheocast and liquid cast AZ91 magnesium alloy were compared to explore the feasibility of Rheocasting commercial Mg alloys using the RheoMetal process. A close-to industrial scale squeeze casting machine was used, with a mold having rectangular shaped cavities of section thicknesses 7, 13 and 20 mm. The pore content and its distribution as well as the microstructure of the castings were analyzed. The average size of the globular primary α-phase particles in the rheocast samples was around 50 μm. The shape factor of the primary particles in the rheocast samples had a bell-shaped distribution with a peak at 0,4-0,6 while the conventionally cast material had more stochastic distributions. The porosity in the rheocast samples was coarser than in the liquid cast samples, even though both types had similar levels of overall porosity. There was a layer of segregated porosity at around 30% of the section thickness from the sample surface; severe in the liquid cast samples and slight or non-existent in the rheocast samples. [ABSTRACT FROM AUTHOR]

Commercial grade AM50 magnesium alloy was dlecast to varying wall thickness using both the RheoMetal process and conventional liquid casting. The resulting microstructure and pore structure were analysed. The rheocast material had larger pores than the conventionally cast material, scattered over the central 50% of the cross-section, while the pores in the conventionally cast material was concentrated to a segregation band at a depth corresponding to approximately 30% of the wall thickness. In the Rheocast samples, there was a correlation between thicker sections and larger primary a-Mg globules with a lower shape factor. [ABSTRACT FROM AUTHOR]

Rheocasting is on its way to establish as a cost efficient method for producing high integrity aluminium components. There is still, however, a need to learn more about the possibilities and limitations of this relatively new process in industrial use. This especially relates to ingate design and machine process parameters, e.g. shot profile, die temperature and intensification pressure, for obtaining sound quality. A component was produced by rheocasting at varying phase II velocities in a normal high pressure die casting machine supplied with a RheoMetal station. The component quality was investigated in the most massive section of the casting, using X-ray and metallographic techniques. The pore content and its distribution were found to strongly depend on the injection velocity where a low velocity is preferred due to the low risk for air entrapment and the higher fraction solid obtained at completed die filling, thereby reducing the solidification shrinkage. [ABSTRACT FROM AUTHOR]

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]

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]

Stir casting plus cooling plate technique has been used for the fabrication of Al matrix composites based on alloy 356. Improvement of the wettability of SiC particles was carried out, employing the oxidisation of SiC particles, the use of wetting agents by adding magnesium into the matrix and the coating of SiC particles using a sol–gel technique. The introduction of SiC particles into partially solidified alloy with high viscosity prevents the particles from floating and agglomerating. Unoxidised SiC particles are mostly detached from the Al matrix during the grinding suggesting poor adhesion and poor wettability between the matrix and the particles. Oxidised SiC particles and sol–gel silica coated SiC particles indicate good binding between composite and matrix. The use of magnesium promoted wettability of SiC with A356 alloy. The eutectic silicon phase formed on the surface of SiC particles during solidification may be due to nucleation effects provided by the particles. [ABSTRACT FROM AUTHOR]

Rheocasting processes produce a globular (non-dendritic) morphology in the primary phase of a metallic alloy. The authors have developed the direct thermal method (DTM) of obtaining this morphology, using only standard foundry equipment. In the DTM the alloy is poured, at low superheat, into a cylindrical thin walled metallic mould, and by careful selection of mould materials and geometry, a near isothermal situation occurs for sufficient time to render the primary phase globular. No active thermal management is required or used. The sensitivity of DTM rheocasting of aluminium alloy to variables such as mould inner and outer diameter, mould material, heat transfer at the system interfaces, and starting temperatures, is assessed in this paper. This is done through both experiment and computer modelling. It is concluded that steel is a superior mould material than copper for the DTM, both for operational reasons and to enable the casting of larger diameter billets. [ABSTRACT FROM AUTHOR]

Microstructural characterisation carried out on four non-dendritic feedstocks in the as received and reheated conditions has revealed that the transition from dendritic to non-dendritic occurs through the formation of ripened dendrites, rosettes and ripened rosettes. Formation of divorced eutectic is observed during the initial stages of remelting in all the alloys. Electrical conductivity measurement can be used as an instantaneous non-destructive evaluation (DE) tool to assess the presence of divorced eutectic. The eutectic entrapped inside the primary alpha globules is finer than that present outside. Feedstock, which is neither grain refined nor modified, requires longer holding time to obtain the non-dendritic microstructure on remelting. [ABSTRACT FROM AUTHOR]

There have been many challenges in aluminum die casting to establish casting processes to produce high-integrity components from aluminum alloys. Advances in new casting technology mainly have been in pressure die casting; in particular, to obtain mold filling at low speed. This can be achieved by using innovative filling processes with aluminum alloys in the liquid or semisolid state. Different techniques such as high-pressure die casting (HPDC), Cosworth process, low-pressure systems, squeeze casting, indirect squeeze casting, metal compression forming (MCF), and semisolid metal (SSM) processing have been developed. Semisolid forming includes thixoforming and rheoforming. During the semisolid casting process, preheating by induction is needed to obtain the same temperature and the same liquid fraction through the billet in a short time. Thixocasting in the semisolid state helps to avoid turbulence during mold filling. [ABSTRACT FROM AUTHOR]