The RheoMetal process (previously called the Rapid S- and RSF- process) is a novelmethod to produce cost effective, high quality, semisolid slurries for component casting. TheRheoMetal process uses an Enthalpy Exchange Material (EEM) as cooling agent to absorb heat andproduce a slurry. Critical process parameters to create a slurry by robust melting of the EEM arealloy content, stirring speed, EEM to melt ratio, EEM temperature, EEM microstructuralcharacteristics and melt superheat.In this paper, the melting sequence and melting rate of the EEM was studied experimentally. Theeffect of EEM composition, as well as superheat, on evolution of shape and dimension of the EEMduring stirring was investigated. Initial material freezing onto the EEM was observed, followed by astationary phase with subsequent gradual melting of the EEM. It was shown that the characteristicsof freeze-on layer were strongly correlated to melt superheat, EEM temperature, as well as materialcomposition, hence also has significant influence on the melting sequence.

The melting sequence of the Enthalpy Exchange Material (EEM) and formation of slurry in the RheoMetal^TM process was investigated. The EEM was extracted, together with a portion of the slurry at different times before complete melting, and quenched. The EEM initially increased in size due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a constant diameter of the EEM and thereafter subsequent melting. Microstructural characterization of the size and morphology of different phases in the EEM and the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum slightly tilted by about 30° toward the upstream direction, caused by the rotation of the EEM. Energy Dispersion Spectroscopy analysis showed that the freeze-on layer had a composition corresponding to a higher melting point than the EEM.

Microstructural investigation of the EEM showed that the temperature rapidly increased to 495 ºC, causing incipient melting of Al₂Cu and Al₅Mg₈Si₆Cu₂ phases in grain boundary regions. Following the incipient melting, the temperature in the EEM increased further and binary Al-Si eutectic started to melt to form a region of a fully developed coherent mushy state. Experimental results and a thermal model indicated that as the dendrites spheroidized and the interface at the EEM/freeze-on layer reached a mushy state with 25% solid fraction, coherency was lost and disintegration of the freeze-on layer took place. Subsequently, in the absence of the shielding effect from the freeze-on Layer, the EEM disintegrates at a higher solid fraction, estimated to be 50%. The fast and complex slurry generation in the RheoMetal^TM process is a hybrid process with both rheocasting and thixocasting elements in the process.

Four Al-Si-Mg-Fe alloys with Si contents varying from 1.6 to 4.5 wt pct were rheocast, using the RheoMetal™ process to prepare slurry and cast in a vertical high-pressure die casting machine. Particle size and Si concentration in the α-Al particles in the slurry and in the as-rheocast component were investigated. A uniform distribution of Si in the globular α ₁-Al particles was achieved in the slurry. In the rheocast samples, measurement of the α ₁-Al particles showed that these particles did not increase significantly in size during pouring and secondary solidification. The two additional α-Al particles types, α ₂-Al particles and α ₃-Al particles, were identified as being a result of two discrete nucleation events taking place after slurry production. The Si concentration in the α ₂-Al and α ₃-Al particles indicated that the larger α ₂-Al particles precipitated before the α ₃-Al particles. In addition, in the as-rheocast condition, the Si distribution inside the α ₁-Al particles showed three distinct zones; an unaffected zone, a transition zone, and in some cases the start of a dendritic/cellular zone. The phenomenon of dendritic growth of globular α ₁-Al particles during secondary solidification occurred concomitantly with the final eutectic reaction and increased with increasing amount of the Al-Si eutectic phase.

The relationship between microstructural characteristics and thermal diffusivity of a rheocast component was investigated for two Al-Si-Mg-Fe alloys with low Si contents. The microstructural investigation of the components clearly depicted the formation of coarse solute-lean globular α1-Al particles during the slurry fabrication process and fine solute-rich α2-Al and α3-Al particles during the secondary solidification in the die cavity. The microstructural characterization was quantified based on the amount of α1-Al particles in different locations of the component. The result clearly revealed a presence of both longitudinal and transverse macrosegregation of solute-lean α1-Al particles in the rheocast component. The study of thermal diffusivity and hardness revealed that the regions of the component with a high fraction of α1-Al particles had a higher thermal diffusivity but a lower hardness. Silicon in the solid solution was observed to be a critical factor in reducing the thermal diffusivity. The comparison between the effect of longitudinal and transverse segregation on thermal diffusivity showed that the transverse segregation had a stronger impact.

In the current paper, a low-Si aluminium alloy (1.4-2.4% Si) was used to fabricate acomplex shape telecom component using Semi-Solid High-Pressure Die Cast (SSM-HPDC),process. Microstructure and fracture characteristics were investigated. The cast material exhibitedmicrostructural inhomogeneity, in particular macrosegregation in the form of liquid surfacesegregation bands in addition to sub-surface pore bands and gross centre porosity. Tensilespecimens were taken from the cast components. Elongation and microstructural inhomogeneitywere investigated and correlated. Fracture surfaces of the tensile specimen were examined underscanning electron microscope (SEM). The study showed that both near surface liquid segregationbands and subsurface porosity strongly affected the fracture behaviour. Dominant for loss ofductility was gross centre porosity. The centre porosity was found to be a combination of trappedgas and insufficient, irregular feeding patterns.

The growing demand for increasingly more cost and energy effective electronics components is a challenge for the manufacturing industry. To achieve higher thermal conductivity in telecom components, an aluminum alloy with a composition of Al-2Si-0.8Cu-0.8Fe-0.3Mn was created for rheocasting. Yield strength and thermal conductivity of the material were investigated in the as cast, T5 and T6 heat-treated conditions. The results showed that in the as-cast condition thermal conductivity of 168 W/mK and yield strength of 67 MPa was achieved at room temperature. A T5 treatment at 200°C and 250°C increased thermal conductivity to 174 W/mK and 182 W/mK, respectively, while only a slight increase in yield strength was observed. Moreover, a T6 treatment resulted in similar thermal conductivity as the T5 treatment at 250°C with no significant improvement in yield strength. Therefore, the T5 treatment at 250°C was suggested as an optimum condition for the current alloy composition.

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.

The relation between microstructural inhomogeneity and thermal conductivity of a rheocast componentmanufactured from two different aluminum alloys was investigated. The formation of two different primarya-Al particles was observed and related to multistage solidification process during slurry preparationand die cavity filling process. The microstructural inhomogeneity of the component was quantified as thefraction of a1-Al particles in the primary Al phase. A high fraction of coarse solute-lean a1-Al particles inthe primary Al phase caused a higher thermal conductivity of the component in the near-to-gate region. Avariation in thermal conductivity through the rheocast component of 10% was discovered. The effect of aninhomogeneous temperature-dependent thermal conductivity on the thermal performance of a largerheocast heatsink for electronics cooling in an operation environment was studied by means of simulation.Design guidelines were developed to account for the thermal performance of heatsinks with inhomogeneousthermal conductivity, as caused by the rheocasting process. Under the modeling assumptions, the simulationresults showed over 2.5% improvement in heatsink thermal resistance when the higher conductivity nearto-gate region was located at the top of the heatsink. Assuming homogeneous thermo-physical properties ina rheocast heatsink may lead to greater than 3.5% error in the estimation of maximum thermal resistanceof the heatsink. The variation in thermal conductivity within a large rheocast heatsink was found to beimportant for obtaining of a robust component design.