Effect of cooling rate on the microstructure and graphite morphology in the ductile iron-steel hollow sphere syntactic foams

In this study, ductile iron-steel hollow sphere syntactic foams were manufactured using sand mold casting technique. The steel hollow spheres were synthesized through fluidized bed and in this process, commercial iron powder was coated on the expanded polystyrene spheres. After coating, De-bonding and sintering processes were carried out and the steel hollow sphere produced with 4, 6, and 8 mm in diameter. Then, the steel hollow spheres were randomly filed in the cavities of prepared mold and ductile iron infiltrated the interstitial space between the spheres. Thus, ductile iron- steel hollow sphere syntactic foams with 4, 6, and 8 mm spheres sizes. Microstructural properties of manufactured foams were investigated by optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). In addition, cooling rates in the syntactic foams with different sphere sizes were calculated and effects of cooling rate on the graphite morphology and microstructure were studied. The results revealed that decreasing the spheres size caused to increasing the cooling rate. Furthermore, by increasing the cooling rate, surface fraction of graphite, graphite circularity, and surface fraction of iron carbide decrease and graphite nodule count increases.