Effect of Porosity on Thermal Conductivity of Al?Si?Fe?X Alloy Powder Compacts

Abstract

The thermal conductivity and thermal diffusivity of hot- and cold-pressed Al–17Si–5Fe–3.5Cu–1.1Mg–0.6Zr (mass%) alloy powder compacts were investigated as a function of the porosity volume fraction. Samples with a very low degree of porosity were produced by hot-pressing air atomized alloy powder with a particle size of 45–100 μm. The same powder was used to produce highly porous compacts by cold compaction using a manual press. The thermal diffusivity of the powder compacts was measured using a sinusoidal modulation photopyroelectric technique in a configuration that is similar to the laser flash method. The thermal diffusivity of the material decreases by a factor of about 13 with an increasing porosity of 25 vol% and a factor of about 300 at 60 vol % porosity. Since the calculated specific heat (weighted average of mass specific heat values of major alloy compounds) is much less porosity dependent, the porosity dependence of the thermal conductivity is similar to the thermal diffusivity and decreases exponentially with increasing porosity. Microstructural characterization of high porosity samples prepared by cold compaction indicated that the distribution of pores is not uniform over the cross-section. An interconnecting network of open and closed pores in the form of channels created pockets of porosity,clearpage 2.3pc which are largely responsible for a drastic reduction of thermal conductivity 4pc with increasing porosity.