There is an on-going debate on the relationships between the alloying/inoculating elements and the graphite structure in cast irons. Since properties of cast irons are dependent on graphite morphology, establishing correlations between process variables to graphite morphology is vital to develop cast irons with optimized 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.

As part of a program funded by U.S. DOE’s Vehicle Technology Office, researchers from Caterpillar and Argonne National Laboratory used high-energy X-ray tomographyat the 1-IDX-ray beamline at the U.S. Department of Energy’s Advanced Photon Source at Argonne to take 2D and 3D images at 2 micrometer resolution. Large volumes (in mm3size) of the sample were analyzed in a relatively short time and provided results with high accuracy.

These measurements can be readily extended to in situ studies of microstructural evolution at elevated temperatures, to provide further understanding of cast iron solidification process.

3D structural analysis revealed that the compacted graphite (CG) can grow to a coral-tree-like morphology as large as a few millimeters in the iron matrix.The study results showed that high-energy X-ray tomography can reveal previously unknown behaviors of graphite in cast iron, such as the growth of nodules, as it undergoes various treatments.The analysis of the 3D structure reveals information, which can either not be seen or possibly misinterpreted with standard 2D analysis.

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

“3D Quantitative Analysis of Graphite Morphologyin High Strength Cast Iron By High Energy X-ray Tomography,”Scripta Materialia, 106:5-8 (2015). DOI: 10.1016/jscriptamat.2015.03.017

“Application of X-ray computed tomography for the characterization of graphite morphology in compact-graphite iron”, Materials Characterizations, August (2016). DOI.org/10.1016/j.matchar.2016.08.007

Powder-based printing methods increase porosity in titanium alloys, which reduces the finished parts resistance to fatigue or cyclic strain causing breakage. Scientists wanted to find the optimal printing parameters in electron beam melting (EMB) systems to control the size of the melt pool to reduce or eliminate porosity.

Researchers from Carnegie Mellon University used microtomography at the 2-BM X-ray beamline at the U.S. Department of Energy’s Advanced Photon Source at Argonne NationalLaboratory to study 5 samples and pre-printed powder of the most common titanium alloy, Ti-6Al-4V.The samples were all printed with varying deposition parameters of laser beam power, speed and spacing.

Printing parameters did significantly impact porosity, but not eliminate it. Printing larger melt pools at lower speeds produces fewer, smaller pores. Results suggest porosity initiates in the powder processing.

1-by 15 millimeters samples were scanned in 2-D in 2 minutes to produce 1,500 images at a resolution of thousands of pores at two microns. Industry lab equipment, such as electron microscopy, would have required a much smaller sample volume size, taken hours to scan and would not have provided the depth information.

“Evaluating the Effect of Processing Parameters on Porosity in Electron Beam Melted Ti-6Al-4V via Synchrotron X-ray Microtomography,” The Minerals, Metals & Materials Society, 68:765-771 (2016). DOI: 10.1007/s11837-015-1802-0

To understand the mechanisms responsible for the formation of various structure defects in additively manufactured parts, it is essential to develop and apply in situ characterization techniques to monitor the dynamic microstructural evolution in real time. However, due to the highly transient nature of the laser-metal interaction, experimentally characterizing the dynamics of the laser powder bed fusion (LPBF) process has been challenging.

Researchers from Carnegie Mellon University, Missouri University of Science and Technology, and Argonne National Laboratory used high-speed X-ray imaging and diffraction techniques at the 32-ID-B beamline at the U.S. Department of Energy’s Advanced Photon Source at Argonne.

The team demonstrated that quantitative structural information on melt pool size/shape, powder ejection, solidification, and phase transformation can be obtained from high resolution, time-resolved x-ray images and diffraction patterns.

The experiment platform and the data analysis algorithms they developed will help researchers not only understand the physics underpinning the formation of different defects, but also build high-fidelity models to guide the process optimization for manufacturing parts with different geometries and dimensions.

High-energy X-ray imaging is needed to see beneath the powder bed and analyze the interactions that affect component quality and performance, including melting and partial vaporization of metallic powders, flow of the molten metals, powder spatter ejection, rapid solidification, and non-equilibrium phase transition.

Laser beam interactions with metal occur at the millisecond timescale and produce complex physics reactions. The Advanced Photon Source is the only place in the US with the capability to do high-speed X-ray studies of additive manufacturing (AM) processes.

The X-ray data can be fed into physics-based computer models to predict the outcome of changes to AM processing parameters.

Cang Zhao, Kamel Fezzaa, Ross Cunningham, Haidan Wen, Francesco De Carlo, Lianyi Chen, Anthony Rollett, Tao Sun, “Real-time monitoring of laser powder bed fusion process using high-speed X-ray imaging and diffraction”, Scientific Reports, 7, (2017) 3602

High-speed imaging of LPBF process

Limited data exists to study fatigue crack growth in laser engineered net shaped (LENS) fabricated Ti-6AL-4V alloys to base damage tolerant designs on. This hinders widespread adoption of LENS for high-integrity structural applications.

Researchers from General Dynamics, the Air Force Research Laboratory, Worcester Polytechnic Institute, Nutonian Inc., Babcock Power and the Advanced Photon Source (APS) used high-energy absorption contrast microtomography at the 1-ID X-ray beamline at the U.S. Department of Energy’s APS at Argonne National Laboratory to study fatigue cracks in-situ. 3D images were taken with micron-level resolution under load and at different stages of crack propagation. Fractured samples were elevated using ex-situ fractography, and in-situ results were compared to conventional measurements based on compliance and direct current potential drop.

X-ray microtomography, particularly when combined with ex-situ fractographic analysis can accurately calculate local small fatigue crack growth rates in LENS printed alloys.

Local crack growth variations indicated the need for non-destructive and 3D observations to accurately understand fatigue crack growth in LENS fabricated Ti-Gal-4V. In-situ X-ray measurements can confirm and/or highlight deficiencies in conventional defect measurement techniques.

3D microtomography was found to provide higher-resolution data of crack formation over time and with more accuracy of growth rates at earlier stages of crack growth than traditional ex-situ striation measurements.

“Characterization of fatigue crack growth behavior in LENS fabricated Ti-6Al-4V using high-energy synchrotron X-ray microtomography,” Additive Manufacturing, 12: 132-141 (2016). DOI: 10.1016/j.addma.2016.09.002