Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy

Abstract

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.

Mechanisms of Improving the Thermal and Electrical Conductivity of Aluminum Alloy Cylinder Heads

“…Furthermore, similar to artificial aging, the time-dependent mechanical response of direct aging has been found to be contingent on the heat treatment temperature. For as-cast 319-type alloys, appreciable strengthening was achieved at relatively low aging temperatures, whereas softening was reported when heat treatments were applied on the order of 250 °C [28,235,244,251]. Nonetheless, direct aging has been found to produce considerable improvements in thermal and electrical conductivity, with greater enhancements achieved as the treatment temperature and time increased [28,[250][251][252].…”

Effect of fillets on heat transfer in a rheocast aluminium heatsink

“…The percentage difference defined by Eq. (1), (1) served as the indicator of the thermal resistance improvement/degradation when comparing heatsink HS1 with fillets and HS2 without fillets, at the same hotspot distribution.…”

“…Cast aluminium (Al) heatsinks (HS) for electronics cooling applications have a number of advantages compared to extruded and machined alternatives, including lower cost and higher geometry complexity [1]. Rheocasting is a semi-solid casting technology.…”

A Comparison Between Semisolid Casting Methods for Aluminium Alloys

“…Firstly, thermal conductivity is increased with up to 17% compared to the same material cast using HPDC or gravity die-casting. [20,29,30] The downside is that yield strength is commonly slightly lower than for HPDC casting, but ductility is improved [25,31].…”

“…The intricate differences in the slurry making process between GISS, RheoMetal and SEED processes make them better for some applications and more difficult in others. Important to remember is that this is valid for both physical properties such as thermal conductivity [20] and mechanical properties [14], making the matching between component and process important.…”