Metallic syntactic foams synthesis, characterization and mechanical properties

In this study, we report two procedures for producing lab-scale syntactic steel by melt infiltration of millimeter-sized alumina microspheres: mechanical pressure infiltration and gravity-fed infiltration. Both methods yield foam with uniform distributions of microspheres and negligible unintended porosity. The most critical parameters in the manufacture of the syntactic steel foams are the melt temperature and the preheat temperature of the microspheres prior to infiltration. The preheatment temperature of the microspheres must be close to the melting temperature of steel.;Syntactic steel foams with relative density of about half of solid steel densities were produced using monosized microspheres randomly situated in a mold. Microspheres with a diameter of 1.27 mm were used for the mechanical pressure infiltration method and microspheres with a diameter of 4.45 mm for the gravity-fed infiltration method. Different steel chemical compositions were selected to produce steel foams of different inherent yield strength: including several ferritic-pearlitic steels and one TRIP steel (TRansformation-Induced Plasticity). The resultant foams were characterized by chemical and microstructural analysis. The microstructure of the samples consisted of blends of ferritic and pearlitic constituents in varying proportions for the ferritic-pearlitic steels, while the cast TRIP steel matrix presented an austenitic microstructure.;The basic mechanical properties of the steel syntactic foams were studied under compression loading. The pearlitic syntactic foams have greater compression strength and energy absorption capacity than the ferritic syntactic foams, but the TRIP steel syntactic foam exhibited the highest compression strength and highest energy absorption capacity. The properties of the steel syntactic foams were compared to those of other steel foams, aluminum foams and other cellular structures reported in the literature.;We present also the compression and impact behavior of aluminum syntactic foams (ASF) produced by gravity-fed infiltration of millimeter-sized ceramic microspheres. Aluminum syntactic foams with relative density of 0.46 were produced using monosized microspheres (4.45 mm and 3.05 mm) randomly situated in a mold and two types of aluminum alloy matrices: 1100 and 6061. The impact behavior was experimentally investigated using a drop-weight testing machine. The impact tests were carried out using a hemispherical indenter (16.1 mm diameter) on ASF plates (93 mm x 93 mm x 12.7 mm thick). We have studied the influence of the type of aluminum matrix, size of microspheres and the addition of a face sheet into the impact behavior of ASF. Results show that 1100 Al alloy outperforms 6061 Al alloy, it can absorb higher amount of energy at higher velocities (penetration); at lower velocities both absorb the same amount of energy (equal-energy interval). The use of smaller microspheres decreases the amount of energy absorbed compared to larger microspheres. The use of face sheet increases significantly the energy absorption capacity aluminum syntactic foams.;Metal syntactic foam (steel and aluminum) offers potential advantages over conventional aluminum foams and other metallic cellular structures, the inherent strength of metal combined with the reduced density and lack of defects in the regular structure of the syntactic foam presents an attractive material with excellent strength, modulus and energy absorption. However, thus far there have been few reports describing efforts to produce steel and aluminum syntactic foams, and these have relied on powder metallurgical approaches as opposed to molten state processing. Problems and challenges for achieving metal syntactic foam with lower relative densities, higher energy absorption capacity and the scaling up of the synthesis processes are discussed.