Mechanical properties and microstructure of semi-solid processed cast iron

Attempts to develop appropriate semi-solid processing techniques to manufacture iron castings have not been successful despite extensive laboratory studies done in the past. This research aimed at producing equiaxed primary phase structure in gray cast iron slurry using a cooling plate under a mechanism explained by the crystal separation theory, and casting the slurry into moulds made from different materials to form cylindrical components. A boron nitride coating was applied onto the surface of the cooling plate to improve melt-flow and prevent the melt from sticking along the flow path. Primary particle size, number, volume fraction and hardness for cast components were investigated with respect to radial and longitudinal position along sectioned surfaces. Results on size, shape and grain count of graphite precipitated in the matrix are also discussed, comparing resulting component microstructure with that of castings made through conventional sand casting. Pouring of melt onto the cooling plate was done at a temperature a little higher than liquidus to initiate nucleation of primary phase along the flow path. The casting technique enhanced nucleation rate, leading to finer and higher volume fraction of the primary particles before the slurry flowed into the mould. The nature of flow along the cooling plate ensured that agglomeration of primary particles was minimized, resulting in highly globular grains hence better mechanical properties.