Aluminium foam sandwich had been prepared by modified powder compact melting process (powder pack rolling melting process). During the process, the effect of reduction on relative density of the precursor was studied by Archimedes' principle. The foaming behaviour and pore microstructure of precursor were determined. It is found that with the reduction increasing, the relative density of the precursor increases and the uniformity of the precursor can also get improved. The optimized reduction is 80% because the relative density and its distribution hardly change when the reduction is larger than 80%. The foaming process can be divided into three stages, pore forming, pore growing and pore cracking. During the stage II, the expansion factor of the precursor reaches a maximum 2.37. The microstructure of pore wall varies at different stages. At the second stage and third stage, the microstructure evolves into dendritic structure and eutectic phase, which is similar to the modified microstructure of cast AlSi12 alloy. (C) 2015 Published by Elsevier B.V.