Low-Pressure Investment Casting of Ductile Iron: Effect of Controlled Filling on Graphite Morphology and Fracture Mechanisms

Abstract This study investigates the effect of low-pressure investment casting (LPIC), a novel process, on the integrity and mechanical properties of ductile iron castings in comparison with conventional gravity investment casting (GIC). Although investment casting is widely used for high-precision components, its application to ductile iron remains limited by turbulence-induced defects and insufficient control of solidification conditions. To eliminate compositional variability, EN-GJS-500 ductile iron castings were produced by both processes from a single melt and evaluated by tensile, Charpy impact and hardness testing, light microscopy and scanning electron microscopy (SEM) of the fracture surfaces. Compared with GIC, LPIC increased the ultimate tensile strength by approximately 22% (from 564 to 687 MPa) and the elongation by approximately 52%, whereas the yield strength and hardness remained comparable. The impact strength decreased by approximately 29%, although with considerably lower scatter. The LPIC castings exhibited a higher nodule count, a smaller mean nodule diameter (28 µm vs. 51 µm) and a more homogeneous graphite distribution. Fractography revealed a transition in the dominant fracture mechanism from graphite nodule fracture in GIC to matrix-controlled failure in LPIC. These findings suggest that the benefits of LPIC extend beyond a reduced defect population and include a modification of the graphite–matrix interaction and load transfer. The combination of laminar filling and pressure-assisted solidification is considered to reduce bifilm formation and enhance feeding. LPIC therefore represents a promising route for improving the performance of ductile iron investment castings in demanding engineering applications.