Transmission x-ray diffraction for real-time sensing of the solidification of a casting

A transmission x-ray diffraction (XRD) technique has been developed which may be used, in real time, to locate the molten metal-solidifying metal interface in single-crystal castings. We proved feasibility of the sensor on samples of aluminum and copper in a gradient furnace, and found that the liquid-solid boundary could easily be identified even though the samples were surrounded by a mold wall and encased in a furnace. Subsequently, we have successfully attached the XRD sensor to a small industrial turbine blade casting furnace. The liquid-solid interface can be identified and followed, in real-time during the single-crystal casting process. The high energy x-rays (up to 320 keV) permit in-furnace transmission XRD to be performed on a 6 mm thick nickel alloy specimen. The diffraction pattern was clearly seen, even though the entrance x-ray and diffraction paths through the furnace included 20 mm of Pyrex (furnace ports), 3.2 mm of molybdenum ( furnace windings), 9 mm of aluminum oxide (hot zone heater coil support), and 12.8 mm of mold material (refractory oxides). The x-ray source to imager distance was 1180 mm. An analytical model for transmission XRD has been developed. It is useful for assessing the feasibility of particular sensing applications.