Solidification and Re-melting Phenomena During Slurry Preparation Using the RheoMetal™ Process

Abstract

The melting sequence of the enthalpy exchange material (EEM) and formation of a slurry in the RheoMetalä process was investigated. The EEM was extracted and quenched, together with a portion of the slurry at different processing times before complete melting. The EEM initially increased in size/diameter due to melt freezing onto its surface, forming a freeze-on layer. The initial growth of this layer was followed by a period of a constant diameter of the EEM with subsequent melting and decrease of diameter. Microstructural characterization of the size and morphology of different phases in the EEM and in the freeze-on layer was made. Dendritic equiaxed grains and eutectic regions containing Si particles and Cu-bearing particles and Fe-rich particles were observed in the as-cast EEM. The freeze-on layer consisted of dendritic aluminum tilted by about 30 deg in 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 an alloy with higher melting point than the EEM and thus shielding the EEM from the surrounding melt. Microstructural changes in the EEM showed that temperature rapidly increased to 768 K (495°C), indicated by incipient melting of the lowest temperature melting eutectic in triple junction grain boundary regions with Al 2 Cu and Al 5 Mg 8 Si 6 Cu 2 phases present. As the EEM temperature increased further the 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 near to the interface at the EEM/freeze-on layer reached a mushy state with 25 pct 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 continued to disintegrate with a coherency limit of a solid fraction estimated to be 50 pct.

Influence of Grain Refinement on Slurry Formation and Surface Segregation in Semi-solid Al-7Si-0.3Mg Castings

“…In the New Rheocasting process the grain refiner effect decreased as the pouring temperature was reduced because of the large thermal undercooling obtained when the liquid contacts the cold wall of the cup. [7] The disintegration of the freeze-on layer and EEM result in additional crystals introduced into the liquid as observed by Payandeh et al, [23] which are unlikely affected by the grain refiner particles in the liquid. The addition of Al-5Ti-1B grain refiner introduces soluble TiAl 3 and insoluble TiB 2 particles into the liquid.…”

“…[22] In this work, the previous shearing of the slurry may result in the formation of very few and near-globular crystals in the shot sleeve. Payandeh et al [23] observed that the freeze-on layer microstructure is composed of long and bent columnar dendrites. Disintegration and growth of the freeze-on layer dendrites occur during the Rheometalä process and cavity filling, and the initial size and shape of the crystals are changed.…”

“…A short time after immersion of the EEM, columnar a-Al dendrites and equiaxed a 1 -Al grains were observed in microstructures of quenched EEM and slurry, respectively. [23] Therefore, during the initial stages of Rheometalä slurry preparation, a columnar to equiaxed growth transition occurs. A large thermal gradient is established between the EEM (~200°C) and the liquid (650°C) after immersion of the EEM, resulting in a thermally undercooled region in the immediate liquid surrounding the EEM.…”

A Comparison Between Semisolid Casting Methods for Aluminium Alloys

“…Critical understanding, for the RheoMetal process, are the consequences of the deviation from equilibrium. The first issue is that as the EEM is immersed into the melt, a freeze-on layer forms [26][27][28]. This layer has a composition which is given by the composition of the solidus line in the phase diagram at the slurry forming temperature.…”

“…Due to that, the majority of the solid phase is formed at very high temperature; the primary slurry particles are lean in solutes. For an A356-type of alloy, the time to homogenise the Si and Mg content in the slurry particle s of a 70 µm diameter is approximately 30 s [28]. This is of the same order of time as it takes to make the slurry.…”