Effect of Pressure on Graphite Morphology and Mechanical Properties of Squeeze Cast Hyper-Eutectic Grey Cast Iron

摘要

Squeeze casting is a process in which a high external pressure is applied and maintained on the molten metal during the whole solidification process resulting in the lower porosity, higher solidification rate and improved mechanical properties. In this research, the effects of applied pressure on the morphology of graphite flakes in a squeeze cast hyper-eutectic grey cast iron were investigated. The molten grey cast irons were solidified under atmospheric pressure as well as 25, 50 and 75 MPa externally applied pressures. The results indicated that the pressure increase during solidification decreased the amount of free graphite flakes, reduced the size of pro-eutectic Kish graphite flakes and increased the density of the castings. D-type graphite enclosing austenite dendrites, resembling that expected in hypo-eutectic grey cast iron, was observed near the surfaces of the castings solidified under externally applied pressures. Moreover, some compacted and spheroidal graphite particles were observed in the castings solidified under 50 and 75 MPa pressures. Furthermore, by increasing the squeeze casting pressure from atmospheric pressure to 75 MPa, the hardness, ultimate tensile strength and elongation increased by about 13, 68 and 128%, respectively. The results have been discussed in terms of increased cooling rate, expected change in the phase diagram of the alloy, different atomic structure of the interfaces of graphite and austenite as well as some proposed theories for formation of eutectic cells and spheroidal graphite.