The X-rays can also unambiguously classify the particle type involved in the behavior, which is critical to identifying the structure-process relationship. These insights hold the key to manipulating the atomic structure of the graphite through manufacturing treatments such as changing the chemistry of the melt and altering the inoculants added to the liquid cast iron.

Cast iron can be modified through the manufacturing process to optimize its mechanical and physical properties, such as strength and durability. This property makes it a material of choice for use in the transportation and machinery industries, which rely on cast iron’s resistance to wear, deformation, and rusting to design high-performance bridges, tools, and engine parts.

However, controversy still exists over the correlation between manufacturing casting parameters and desirable properties. Limited by typical industrial 2-D imaging techniques or time-consuming 3-D laboratory studies, researchers have been unable to pinpoint the exact processing parameters needed to elicit the ideal properties for each cast iron application.

The tomographic technique can provide a way to get those answers.

By understanding the structure, it will be possible to develop alloys with improved mechanical and thermal properties. This implies that for applications such as vehicle engine and engine components, one could use less material and reduce overall vehicle weight, which would translate into fuel savings.

—Dileep Singh, group leader of thermal-mechanical research at Argonne National Laboratory’s Center for Transportation Research and technical lead of the study

For the transportation industry, the ability to modify manufacturing processes to create high-performance materials could aid in the development of more fuel-efficient engines or engine parts that can withstand heat better to have longer lifespans.

The research team included Richard Huff from Caterpillar and Argonne researchers Singh, Chihpin Chuang, and John Hryn from the Energy Systems Division and Jon Almer and Peter Kenesei from the X-Ray Science Division. Caterpillar supplied engine alloy castings for use in the proof-of-principle study. The Advanced Photon Source (APS), a US Department of Energy (DOE) Office of Science User Facility based at DOE’s Argonne National Laboratory was used as part of this research.

Synchrotron X-ray analysis has several advantages over the current techniques used to evaluate graphite microstructure.

Three-dimensional imaging of the structure of graphite, its spatial arrangement in the alloy, and its phase connectivity are key factors that determine the properties of cast iron. These parameters cannot be attained reliably by the current industry standard 2-D test.

Less frequently used, but more effective, is the use of focused ion beams (FIB) and transmission electron microscopy (TEM), which can provide high-resolution 3-D images, but is labor-intensive and time consuming and destroys the sample. High-energy X-rays penetrate inhomogeneous samples up to a centimeter thick under real operating conditions. This avoids the challenges of FIB and TEM techniques while also providing a better statistical representation of parameters in bulk material.

The research team found that the synchrotron characterization methods enable new insight into why compacted graphite iron, used by Caterpillar in heavy-duty engine components, can conduct heat better than ductile iron while maintaining good ductile strength. The answer lay in the shape, size, and distribution of the graphite particles in the cast iron. The analysis revealed that the compacted graphite can grow with a coral-tree-like morphology and span several hundred microns in the iron matrix.

(a) 2-inch casting block and 2-mm-diam rod that is cut out of the sample via electrical discharge machining. (b) A typical slice of tomographic image obtained in this study. (c) An unetched metallography image of the same region in (b) under optical microscope. (d) Reconstructed 3-D model of the graphite particles in (b), showing that the 2-D features observed in (b) and (c) belong to a coral tree-like structure with flat, rounded branches that span ∼200 μm in the iron matrix. (e) At left is the slicing surface and the graphite structure beneath the surface; at right is the same graphite structure but sliced in a different orientation showing that 2-D analysis could identify the same feature (red arrow) as either nodular graphite or compact graphite depending on where it is sliced. Source: Argonne National Laboratory. Click to enlarge.

The 3-D characterization of the material enables greater insight into the structure formation and structure-property relationships.

—Richard Huff

The Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, US Department of Energy, supported the work, and the Office of Science supported the use of the APS.

Resources

• Chihpin Chuang, Dileep Singh, Peter Kenesei, Jonathan Almer, John Hryn, Richard Huff (2015) “3D quantitative analysis of graphite morphology in high strength cast iron by high-energy X-ray tomography,” Scripta Materialia, Volume 106, Pages 5-8 doi: 10.1016/j.scriptamat.2015.03.017