Graphite Phase Formation in Al Alloyed Ductile Irons

The influence of aluminum, within the range of 0.08 - 6.16 wt.%, on the microstructure and hardness of as-cast ductile cast iron has been studied. A number of irons of different composition have been made by green sand casting and gravity die casting of appropriate design to provide the experimental materials. It has been found that Al addition and higher cooling rates in permanent mould casting both promote pearlite formation in the as-cast irons. Detailed microscopic observation revealed that foreign particles (with a high concentration of Al, Si and Mg) within the graphite spherulites probably behaved as nucleation sites, and that Al addition can promote spheroidal graphite nucleation and growth. Furthermore, it was illustrated that spherulites with a fan-like structure consist of aggregates of graphite platelets and interplatelet regions.