Synthesis of bulk reactive Ni-Al composites using high pressure torsion

17 USC 105 interim-entered record; under review.The article of record as published may be found at http://dx.doi.org/10.1016/j.jallcom.2020.157503Self-propagating exothermic reactions, for instance in the nickel-aluminum (Ni-Al) 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 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 signifi cantly 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 reaction of the whole sample.COMETAustrian Federal MinistriesDepartment of Energy National Nuclear Security AdministrationERC Advanced Grant INTELHYBCOMET programERC-2013-ADG-340025DENA0002377Project No 859480DE-AC02-06CH1135

Sampled weighted attraction control of distributed thermal scan welding

summary:This article addresses the problem of distributed-parameter control for a class of infinite-dimensional manufacturing processes with scanned thermal actuation, such as scan welding. This new process is implemented on a robotic GTAW laboratory setup with infrared pyrometry, and simulated by a flexible numerical computation program. An analytical linearized model, based on convolution of Green’s fields, is expressed in multivariable state-space form, with its time-variant parameters identified in-process. A robust controller design compensates for model uncertainty, and a sampled weighted attraction method is introduced for heat source guidance based on real-time thermal optimization of the heat input distribution. The distributed thermal regulation strategy with infrared feedback is validated both computationally and experimentally in scan welding tests

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A Prelude to Fractal Engineering and Biomedicine Introduction ". . .How is it in perception that the partible appears as whole and the whole is partitioned . . .because, is said, there is no measure to mind; the mind being impartible divides and perceives all. . . Nothing is in cognition not previously in the sense [s]. . ." Aristoteles De Anima (p. [407][408][409][410][411][412][413][414][415][416][417] Since classical antiquity, perception of self-similarity in multidimensional space-time forms in nature and humanity has been an appealing subject in psychophysics, especially through its association with esthetics (Birkhoff's theory [1]). Scale-invariant visual and aural patterns have been perpetually tempting and provoking our cognition, with eventual perceptions of beauty arising from conquering of similarity relationships through a comprehensible model of their generator law by our mind. Ostensibly stemming from the self-similar cortical and lobe anatomy of the human brain itself and the concomitant electrochemical function of neural physiology with iterative feedback patterns, science and art always shared such an original common root: Tantalizing of human mind by intelligible regular, mathematically elegant, recursively repetitive self-similar forms, reflecting both esthetic appeal as, e.g., in visual and performing arts, and simultaneously functional performance as in, e.g., natural phenomena and human activity. Harmonious beauty and optimal functionality of selfsimilar structures, as illustrated below, appear to be dual sides of a kernel cognitive element in the human mind's affinity to creation. Description and Generation Mathematical abstraction, since the early days of Euclidean geometry of simple, continuous, smooth, finite-featured forms, has tackled complex, discrete, nondifferentiable, multiscale selfsimilar forms with golden ratio recursion, helical spirals, harmonic waves, Pythagorean trees, Hippocrates menisci, Apollonian packs Equally importantly, this definitive work on spatial fractal forms and distributions illustrated their generative connection to the temporal dynamics of chaotic iterative systems. Strict or approximate (asymptotic) self-similar fractals are produced via infinite or finite repetitions of discrete recursion or continuous feedback-based, deterministic or stochastic chaos generator laws with certain features: (i) lack of an absolute characteristic spatial/ temporal dimension (e.g., a length or time), yielding scaleinvariant applicability; (ii) nonlinearity (either smooth via a frequency power law or a probability density function, or hard via a nondifferentiable function) with nonintegrability and nonunique invertibility, causing branching bifurcations of the response; (iii) unstable amplification of random variability in its initial, boundary, or functional conditions, dominating the eventual response; and (iv) domains of dynamic repellers and basins of attractors (e.g., strange attractors with nondifferentiable contours) of lower dimensionality, shaping the equilibrium orbits of the chaotic process, thus leaving a fractal structure as its fingerprint. Paradigms of such fractal-producing recursive chaos algorithms evolved from logistical parabolas, modulus functions, limit cycle frequency-doubling (Feigenbaum) fig trees to circle inversion, three-body (clover leaf) and multi-attractor games, turtle algorithms, origami folding, etc. Such laws are central in discrete/ atomistic modeling techniques in nanotechnology, including Monte Carlo, molecular dynamics, density functional theory, etc., essentially mimicking natural phenomena and processes. Natural Paradigms Throughout natural evolution from astronomical macrospace to the nanoscale and subatomic world, numerous such chaotic phenomena have produced a variety of formations exhibiting an intriguing fractal architecture across dimensional scales: In inanimate nature galaxies and galactic foams

Real-Time Computational Model of Ball-Milled Fractal Structures Introduction and Literature

Ball milling (BM) offers a flexible process for nanomanufacturing of reactive bimetallic multiscale particulates (nanoheaters) for self-heated microjoining engineering materials and biomedical tooling. This paper introduces a mechanics-based process model relating the chaotic dynamics of BM with the random fractal structures of the produced particulates, emphasizing its fundamental concepts, underlying assumptions, and computation methods. To represent Apollonian globular and lamellar structures, the simulation employs warped ellipsoidal (WE) primitives of elasto-plastic strain-hardening materials, with Maxwell-Boltzmann distributions of ball kinetics and thermal transformation of hysteretic plastic, frictional, and residual stored energetics. Interparticle collisions are modeled via modified Hertzian contact impact mechanics, with local plastic deformation yielding welded microjoints and resulting in cluster assembly into particulates. The model tracks the size and diversity of such particulate populations as the process evolves via sequential collision and integration events. The simulation was shown to run in realtime computation speeds on modest hardware, and match successfully the fractal dimension and contour shape of experimental ball-milled Al-Ni particulate micrographs. Thus, the model serves as a base for the design of a feedback control system for continuous BM

A Review on New 3-D Printed Materials’ Geometries for Catalysis and Adsorption: Paradigms from Reforming Reactions and CO2 Capture

“Bottom-up” additive manufacturing (AM) is the technology whereby a digitally designed structure is built layer-by-layer, i.e., differently than by traditional manufacturing techniques based on subtractive manufacturing. AM, as exemplified by 3D printing, has gained significant importance for scientists, among others, in the fields of catalysis and separation. Undoubtedly, it constitutes an enabling pathway by which new complex, promising and innovative structures can be built. According to recent studies, 3D printing technologies have been utilized in enhancing the heat, mass transfer, adsorption capacity and surface area in CO2 adsorption and separation applications and catalytic reactions. However, intense work is needed in the field to address further challenges in dealing with the materials and metrological features of the structures involved. Although few studies have been performed, the promise is there for future research to decrease carbon emissions and footprint. This review provides an overview on how AM is linked to the chemistry of catalysis and separation with particular emphasis on reforming reactions and carbon adsorption and how efficient it could be in enhancing their performance

Thermal and mass balance in reactive thermal processing of nickel aluminide coatings on steel substrates

Nickel aluminide coatings were produced on steel substrates by reactive thermal processing of pre-plated precursor layers of nickel and aluminum using plasma arc as the heat source. Controlled rapid heating melted the outer aluminum layer, which then dissolved nickel to facilitate the nucleation and growth of a nickel aluminide. The resultant coating microstructures varied from a duplex or triplex structure, consisting of either NiAl3 and a eutectic; Ni2Al3, NiAl3 and a eutectic; to a fully monolithic Ni2Al3 structure, with the latter resulting at high heat input rates and/or low heat-source traverse rates. The temperature of the reaction layer was simulated for the experimental conditions by a numerical model based on Green\u27s function analysis, The nickel concentration at the liquid-solid interface just before any nickel aluminide nucleation was calculated by assuming local equilibrium interface conditions between the liquid layer and the fcc nickel-rich solution. The depth of nickel dissolution, which consequently determines the extent of nickel aluminide growth, was also predicted by the model. Numerical results of the nickel dissolution compared well with experimental observations

Enhanced diffusion and phase transformations during ultrasonic welding of zinc and aluminum

Elevated-temperature (513 K) ultrasonic welding of aluminum with zinc was investigated. The interface exhibits structures indicative of enhanced interdiffusion and local melting of Al-Zn solid solution. EDS analysis yielded an interdiffusivity value of 1.9 μm2/s. The enhanced diffusion and melting can be attributed to high strain rate (∼103 s -1) plastic deformation in aluminum, which may increase the instantaneous vacancy concentration as high as ∼10-1. © 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Effects of high-speed deformation on the phase stability and interdiffusion in ultrasonically joined aluminum and zinc foils

The effects of high strain-rate deformation on the phase stability and interdiffusion were investigated for Al-Zn welds produced by ultrasonic welding at 513 K. The welds exhibited three distinct regions: a featureless region indicative of local melting on the zinc side, a solidified mushy layer and a layer of fee grains enriched with zinc. Al-Zn phase diagrams calculated from vacancy-modified Gibbs free energy curves indicate that local melting at the weld interface may result even at 513 K if the vacancy concentrations in the fee and hcp solutions approach 0.07 as a result of high strain-rate deformation. EDS analysis of the weld interface yielded an interdiffusivity of 1.9 μm 2/s, which is five orders of magnitude larger than the normal diffusivity of zinc in aluminum at 513 K. Application of the mono-vacancy diffusion mechanism to the diffusion data also yields a vacancy concentration of 0.07, indicating that such a high vacancy concentration may indeed resulted during the ultrasonic welding at 513 K. © 2005 Materials Research Society

Thermal and mass modeling of the laser-point sealing process in MEMS packaging

This article addresses the reactive thermal processing of locally heated intermetallic seals for micro-electro-mechanical systems (MEMS) packaging. Traditional post-packaging of MEMS involves high temperature brazing or soldering which results in degrading the quality of the thermally sensitive device. Focused laser heating of multilayer metals, therefore, serves as an attractive alternative due to a decrease in heat affected/degraded area. In this work a model is presented to decipher the dynamics of the thermal reaction and to serve as a tool for optimization and control of the multilayer sealing process. The analytical model is based on three-dimensional space- and time-dependent temperature and concentration Green\u27s fields and is solved numerically. The laser heat distribution is represented as a spatially varying Gaussian heat source to simulate the temperature and concentration evolution that occurs during the thermal process. The rapid heating induced by the laser source results in melting the sealing layer of lowest melting temperature, and then dissolves the layer in contact with the molten region to facilitate the nucleation and growth of an intermetallic compound. The simulation results show the model can successfully predict the temperature of the layers as well as the amount of barrier layer dissolution, which consequently determines the extent of the intermetallic compound growth in the bonding/sealing process. Copyright © 2004 by ASME