Thermal diffusivity of the aluminium alloy Al-10Si-Mn-Mg (Silafont 36) in the solid and liquid states
Peter Hofer and Erhard Kaschnitz
The thermal diffusivity of the aluminium alloy Al-10Si-Mn-Mg (Silafont 36) was measured in a temperature range from room temperature to 750°C using the laser-flash technique. These data are required as input parameters for the numerical simulation of the solidification of castings. A commercial laserflash system (NETZSCH LFA 427) was used for the measurements. A short laser pulse of 300 μs was applied to heat the bottom surface of a disk shaped specimen, resulting in a time-dependent temperature increase at the top surface. Corrections for the laser pulse length as well as heat loss due to surface radiation and heat conduction were applied in order to evaluate the half time value by fitting a transient heat conduction model to the measured values of the temperature increase. Thermal diffusivity was calculated from specimen thickness, corrected for thermal expansion of the specimen, and the half time value. A sapphire crucible was used to contain the specimen in the mushy region and in the liquid state. As the laser is firing from below at the bottom surface of the specimen, the thickness of the melt has to be slight to avoid significant buoyancy. The thermal diffusivity of the alloy above the eutectic temperature and in the liquid state is drastically lower than in the solid state.
The samples for the laser-flash measurements were retained from high pressure die casting parts. As in these parts wall thickness varies, samples were taken from both thick and thin walled areas. Samples from the ingot material were also taken as reference. In the solid state, noticeable differences in the thermal diffusivity were obtained between specimens cut from different regions of the castings. This is probably due to porosity and an individual local solidification time causing a spatially different microstructure
Keywords: Aluminium alloy; Al-10Si-Mn-Mg; eutectic temperature; high temperature; laser-flash method; liquid metal; liquidus temperature; solidus temperature; thermal diffusivity; thermal expansion.