Synthesis and characterization of novel ZnAl22 syntactic foam composites via casting

New syntactic foam composites comprising ZnAl22 (Zn–22 wt.%Al) eutectoid alloy and varying volume fraction (6–50 vol.%) of Ni-coated fly ash microballoons were successfully prepared through stir casting (vortex) method. Solution treatment at 400 °C for 0.5 h followed by quenching resulted in changing the dendritic structure of the foam composite matrix to fine grains. The effect of microballoon volume fraction on the microstructure, compressive properties and strain rate sensitivity of the foam composites was characterized. The experimental results showed that the density of the foam composite containing 50 vol.% microballoons was 3.3 g/cm3, the plateau stress averaged 100 MPa over a region of 10–60% strain, densification began at approximately 60% strain and the energy absorption at 60% strain was 65.5 MJ/m3. The strain rate sensitivity of foam composites was investigated under compression loading. Their responses to strain rate were tested over a range of strain rates from 4.16 × 10−4 s−1 to 1.25 × 10−3 s−1 at room temperature. Within this range, for the composites containing up to 25 vol.% microballoons, the yield strength increased with an increase of strain rate. However, the yield strength of the foam composites with 35 and 50 vol.% slightly decreased with increasing the strain rate. These novel foam composites developed in the present study displayed superior compressive properties and energy absorption capacity compared to those of the conventional foams, suggesting their potential use in the aerospace, automotive, and biomedical applications. The preparation of syntactic ZnAl22 syntactic foam composites by stir casting may open up the possibility of producing syntactic foam comprising other metal matrices with other microballoons in foundries.