The goal of the present work is the refinement of the pore morphology of aluminum integral foam castings. Integral foam molding, a modified high pressure die casting process, is used where a mixture of melt and blowing agent particles (magnesium hydride, MgH2) is injected at high velocity into a permanent steel mold. At the mold surface, decomposition of the blowing agent and pore formation is suppressed due to the high solidification rate whereas solidification of the core is much slower allowing blowing agent decomposition, pore nucleation, and growth. Blowing agent particles not only act as gas suppliers but also represent pore nuclei. Thus, microcellular foam cores can be attained by increasing the number of MgH2 particles. But increasing the number of powder particles by powder milling strongly decreases the flowability and strong particle agglomeration as a result of the increasing cohesive forces leads to inhomogeneous foams. Flowability of the powder can be restored by coating it with SiO2-nano-particles resulting in a homogeneous microcellular foam morphology.