  A three-dimensional model of a partially melted powder bed with particles
stochastically distributed in size and space coordinates has been developed.
Numerical simulation of temperature distributions in stainless steel AISI 316L
and Al-12Si powders in vacuum, air and argon has been performed to analyze
unsteady heat transfer in a porous medium. The numerical model demonstrates a
large effect of heat transfer through the gas phase in case of powders with low
thermal conductivities like stainless steels. At the porosity level of 65\% and
above, the mechanism of heat transfer drastically changes and a linear
dependence of thermal conductivity on porosity frequently used in literature
becomes incorrect. The effects of the consolidation coefficient and size
distribution on effective heat transfer in powders are discussed. The obtained
dependencies of the effective thermal conductivity on porosity and the
consolidation coefficient could be used in additive manufacturing applications.
