Bimetallic diffusion modeling and temperature regulation during ball milling

Abstract

This work establishes a computationally efficient model of temperature and diffusion dynamics in ball milled bimetallic Ni-Al particulates. Mechanical contact conditions between adjacent bimetallic domains and temperature fields generated upon impact define the diffusive fluxes through the domains' interfaces. The concentration distributions are studied via Green's function methods, by defining mirror images which reflect the boundary diffusive resistance conditions. Predictions of the model are validated against experimental open-loop scanning electron micrographs and X-ray diffraction spectra at steady-state. Dynamic models of diffusion saturation and internal temperature are developed for the design of closed-loop controller via simulation. This feedback controller is implemented on a laboratory device, equipped with infrared thermometry for external vial temperature measurement, as a self-tuning regulator. The controller adapts an efficiency parameter of the model that thus serves as a real-time observer for internal temperature and diffusion saturation, which are physically inaccessible during processing. Experimental open-loop results are employed to set thresholds of diffusion penetration to avoid early exothermic reaction during fabrication. By controlling the process duration, the regulation system successfully reproduces the material composition of the open-loop reference tests, to ensure desired thermodynamic properties of the ball milled products and safety of the operation.