Production of aluminum foams with hierarchical porosity by a combination of two different manufacturing methods

The increasing demand of metallic foams has generated important research trending toward higher porosity percentage, smaller pores, and the inclusion of pores of different sizes (fractal or hierarchical porosity). In this work, hierarchically porous foams were produced combining infiltration and heat-treatment processes. First, macro-pores of 2 mm were obtained infiltrating NaCl space holders with quaternary Al-6Si-xCu-7Mg alloys, followed by NaCl dissolution, reaching a porosity of ∼68%. Then, these foams were over-heated 3 h at 520 °C, which generated in the matrix ∼28% of micro-pores of 50–150 μm through the incipient melting of second phases. These ratios of relative quantities and sizes between small and large pores led to fractal dimensions for the porous network between 1.43 and 1.56. Micro-pores allowed to increase total porosity almost in 10% and decrease the relative density of the foams till 0.23. Mechanical behavior of such hierarchically porous foams were very low due to the flow of molten material through the macro-pores and the generation of interconnected pores. A variation in the manufacturing method applying the heat-treatment before NaCl dissolution avoided the flow of molten metal. Such hierarchical foams presented lower micro-porosities (from 3 to 10%), with fractal dimensions only reaching values from 0.74 to 1.28. Although in this case total porosities and relative densities were only slightly improved, the drastic decline of the mechanical properties was avoided. These hierarchical foams, with different porous fractal dimensions, could establish the basics for new applications of Al alloys.