Fast X-ray Microscopy to Quantify the Nucleation of Hot Cracking

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

NON-TECHNICAL SUMMARY: This project will use state-of-the-art facilities for ultra-high speed imaging with high energy synchrotron x-rays for the purpose of understanding the problem of hot cracking during the solidification of metals and alloys. Hot cracking means that, instead of obtaining fully solid metal during casting or 3D printing, cracks are left behind that weaken the material. It may also mean that the cracked component must be discarded and/or trimmed, which is wasteful. The main focus will be on studying cracking during 3D printing with laser light where mega-Hertz imaging provides micro-second resolution of the sort best known for stop-action movies of bullets penetrating through armor. Such ultra-high speed imaging has made rapid progress in recent years thanks to the advent of new cameras and improvements in x-ray detection and has already made substantial contributions to our understanding of the melting process. It is thus extremely well suited to imaging the sudden onset and growth of cracks. The sensitivity of cracking to variations in solidification speed and chemical composition will be investigated. Computer simulation will be used to test hypotheses about how the cracking happens with respect to the materials microstructure. The new understanding gained in this work has broad impacts in the casting industry in general. It is also likely to stimulate new theoretical analysis of the problem, which often happens when a new experimental technique is applied. In addition to supporting a doctoral student, undergraduates will be recruited to assist with the work, which involves a good deal of detailed analysis of sequences of images. The work will also be disseminated to the twenty-plus companies that are members of CMU's NextManufacturing Center and have a strong direct interest in additive manufacturing. As the analysis proceeds, the main results will be incorporated into the PI's teaching, which will help ensure that CMU's MS and engineering minor programs in additive manufacturing stay up to date. TECHNICAL SUMMARY: This proposal will use ultra-fast x-ray microscopy, with the high energy, high intensity synchrotron x-rays, to test the hypothesis that the nucleation of solidification cracking is variable and depends on the morphology of the solid near the end of the freezing process. Given the lack of direct measurement of the nucleation point and the arbitrary aspect of nucleation in cracking theories, even a measurement of the solid fraction at which cracking starts will be novel. Measuring the degree to which the nucleation of cracking depends on the extent of columnar versus equiaxed growth will further extend our fundamental knowledge of the problem. We will also probe for heterogeneous nucleation of the cracks from, e.g., the small vapor bubbles that are often observed in laser melting of Al-based alloys. The expected results include direct visualization of solidification cracking in a variety of materials as a function of temperature gradient and cooling rate, which are controlled by laser power and scan speed. We will mainly focus on aluminum alloys, partly because many of the structural Al alloys are prone to cracking and partly for ease of imaging, with stainless steel or nickel alloys for comparison purposes. We will also use lattice-Boltzmann simulations (from a previous DMREF project) to model the solidification and quantify the mushy zone, specifically the shape of the liquid zones at high solid fractions. The most direct impact will be on laser powder bed printing but the potential for broader impact on casting technologies also exists. Through collaboration, we will seek access to different types of modified powders that are intended to avoid cracking through, e.g., promoting equiaxed microstructures. We will use computed tomography and sectioning for 3D characterization. The ultra-fast x-ray microscopy will be mostly carried out at the Advanced Photon Source because this facility has the best combination of high energy and intensity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.