Corrosion properties of two Al-Si alloys processed by Rheo-high pressure die cast (HPDC) method were examined using polarization and electrochemical impedance spectroscopy (EIS) techniques on as-cast and ground surfaces. The effects of the silicon content, transverse and longitudinal macrosegregation on the corrosion resistance of the alloys were determined. Microstructural studies revealed that samples from different positions contain different fractions of solid and liquid parts of the initial slurry. Electrochemical behavior of as-cast, ground surface, and bulk material was shown to be different due to the presence of a segregated skin layer and surface quality.
This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. Citation for the original published paper (version of record):Payandeh, M., Sjölander, E., Jarfors, A., Wessén, M. (2016) Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy. International Journal of Cast Metals AbstractThermal conductivity of a rheocast telecom component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. Conventionally liquid die cast samples were used as reference material. In the as-rheocast condition, a thermal conductivity of 153 W/mK at room temperature was measured. A T5 treatment at 250 or 300°C increased thermal conductivity to 174 W/mK. A T6 treatment resulted in further increase in thermal conductivity to 182 W/mK. The liquid die cast material exhibited lower thermal conductivity and higher hardness for all conditions compared to the as-rheocast material.The microstructural investigation revealed that the rheocast material consisted of coarse α1-Al particles formed during slurry preparation and fine α2-Al particles formed during solidification in the die cavity. Macrosegregation was observed as different the ratio of α1-Al particles to α2-Al particles in different locations in the rheocast component. The relation between microstructural characteristics and thermal diffusivity was investigated by determination of local thermal conductivity in the rheocast component and ratio of α1-Al particles to α2-Al particles. The results revealed that regions of rheocast component with a high amount of α1-Al particles showed higher thermal conductivity. WDS measurement showed that α1-Al particles contains lower concentrations of both Si and Cu inside compare to α2-Al particles. The reduced amount of solutes in the α1-Al particles was therefore determined as the root cause to higher thermal conductivity.Silicon precipitation was confirmed using calorimetry and dilatometry to take place between 200 and 250°C. A linear relation between the fraction of Si precipitates formed and the increase in thermal diffusivity was obtained. Silicon in solid solution is shown to have a strong influence (negative) on thermal conductivity. 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 for Si precipitation.
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.
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