Charalabos C. Doumanidis

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Former Designated Dean of College of Engineering and Computer Science

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Now showing 1 - 3 of 3
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    Constrained crystal growth during solidification of particles and splats in uniform droplet sprays
    (2020) Yiannos Ioannou; Hiroki Fukuda; Claus Rebholz; Yiliang Liao; Teiichi Ando; Charalabos C. Doumanidis
    Uniform droplet spraying (UDS) is a novel process used to produce ideally narrow (mono-size) distributions of molten metal droplets for various applications. The crystallite size is a primary determinant of mechanical properties in solidified alloy deposits and thus in need of predictive modeling. This project reports on employing UDS as a paradigm for solidification modeling of mono-size solid droplets in an oil bath, as well as planar and globular splats on a cooling substrate, for magnesium alloys AZ91D and Mg97ZnY2. The model combines a nucleation and dendrite fragmentation description from solidification theory, with a framework for constrained growth of crystalline domains confined by adjacent developing ones. The latter is based on differential attributes of the dynamic temperature field during solidification, derived from semi-analytical expressions for the simple droplet and splat geometries above. The modeling results are validated against measured domain size distributions on section micrographs and found to be within a − 10% to + 14% estimation error range. Further improvement of the model via numerical thermal descriptions for off-line material design and optimization in additive manufacturing is discussed, along with its use as a real-time structural observer for closed-loop control based on temperature measurements in UDS-based processes.
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    Multivariable control of ball-milled reactive material composition and structure
    (2020) Matteo Aureli; Constantine C. Doumanidis; Aseel Gamal Suliman Hussien; Syed Murtaza Jaffar; Nikolaos Kostoglou; Yiliang Liao; Claus Rebholz; Charalabos C. Doumanidisf
    In reactive bimetallic compounds such as Ni–Al multilayers, desirable thermo-kinetic properties upon ignition require simultaneously controlled geometric microstructure and material composition. This article establishes fundamental dynamical models of plastic deformation and material diffusion in ball milling processing of particulates from Ni and Al powders, for the purpose of designing and implementing feedback control strategies for process control. The role of heat dissipation from plastic yield and friction slip in affecting compressibility and diffusivity of the material is elucidated. The different sensitivity of compressibility and diffusivity to thermal power is exploited by introducing multivariable control of both bilayer thickness and penetration depth simultaneously, using a real-time computational model as an observer with adaptation informed by infrared measurements of external vial temperature. The proposed control scheme is tested on a laboratory low-energy ball milling system and demonstrated to effectively modulate power intensity and process duration to obtain the desired microstructure and material composition.
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    Synthesis of bulk reactive Ni–Al composites using high pressure torsion
    (2020-10-15) Renk, Oliver; Tkadletz, Michael; Kostoglou, Nikolaos; Gunduz, Emre Ibirahim; Fezzaa, Kamel; Sun, Tao; Stark, Andreas; Doumanidis, C. Charalabos; Eckert, Jurgen; Pippan, Reinhard; Mitterer, Christian; Rebholz, Claus
    Self-propagating exothermic reactions, for instance, in the nickel aluminum (NieAl) system, have been widely studied to create high-performance intermetallic compounds or for in-situ welding. Their easy ignition once the phase spacing is reduced below the micron scale makes top-down methods like high-energy ball milling, ideal to fabricate such reactive nanostructures. A major drawback of ball milling is the need of a sintering step to form bulk pieces of the reactive material. However, this is not possible, as the targeted reactions would already proceed. Therefore, we investigate the ability of high-pressure torsion as an alternative process, capable to produce bulk nanocomposites from powder mixtures. Severe straining of powder mixtures with a composition of 50 wt% Ni and 50 wt% Al enables the fabrication of self-reactive bulk samples with microstructures similar to those obtained from ball milling or magnetron sputtering. Samples deformed at ambient temperature are highly reactive and can be ignited significantly below the Al melting point, finally predominantly consisting of Al3Ni2 and Al3Ni, independent of the applied strain. Although the reaction proceeds first at the edge of the disk, the strain gradient present in the disks does not prevent the reaction of the whole sample.
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