Teiichi Ando

Microstructure of Al–Si–Mg alloy processed by rotary-die equal channel angular pressing

Abstract New rotary-die ECAP (RD-ECAP) was applied to a commercial Al–Si–Mg (AC4C) alloy to improve the ductility, and the microstructural developments and mechanical properties of ECAPed specimens were investigated. The as-cast Al alloy was used after machining to a cylindrical shape. ECAP was performed at 543, 603 and 673 K. The microstructure changes of samples were examined. The Vickers microhardness was measured with respect to RD-ECAP passes and the tensile test was conducted to check the high-temperature ductility. The samples after ECAP showed very clean external surface without any edge cracking. The grain size was drastically decreased and saturated into 2–3 μm after six passes because the dynamic recovery occurred during the process. After 10 passes, a very homogenous microstructure was obtained. The longation of the alloy increased from 25% to 125% after 10 passes, and the m value was 0.21 after 20 passes.

Billet temperature rise during equal-channel angular pressing

AbstractTheactualbillettemperatureduringequal-channelangularpressingwasmeasuredanddiscussed.Thetemperaturerisesofaluminumalloysandamagnesiumalloyat573Kwerebetween4and6Kat673K,whenthedeformationratewas1mms 1 .Adiabaticcompressionisresponsibleforabout10–20%ofthetotalgeneratedheat. 2002ActaMaterialiaInc.PublishedbyElsevierScienceLtd.Allrightsreserved. Keywords:Equal-channelangularpressing;Temperaturerise;Mechanicalwork;Adiabaticcompression;Heattransfer 1. IntroductionEqual-channelangularpressing(ECAP)isanattractivedeformationprocessthatcanproduceultrafine-grained bulk materials without chang-ingthecross-sectionaldimensionsofabillet.InECAP,abilletisrepeatedlypassedthroughtwodiechannelsofequalcross-sectionconnectedatanangle[1–10].MostofthestudiesonECAPpub-lishedtodateareconcernedmainlywiththeabilityto produce ultrafine-grained microstructures [1]andtheuniquedeformationgeometry[6]oftheECAPprocess.Numericalsimulationsofthede-formationduringECAPhavealsobeenpresented[8]. The production of an ultrafine-grained mi-crostructure in ECAP results from the intenseplastic strains introduced in the material thatdriverecrystallizationinthestrainedmaterial.ThemechanismsofmicrostructuralevolutioninECAPwere discussed for four different deformationgeometriesbyLangdonetal.[11].OneofthefactorsthataffectthegrainsizeinECAPis thetemperatureofthebillet inwhichdeformationandrecrystallizationtakeplace.Prac-tically,thebillettemperatureiscontrolledthroughthe billet preheating temperature. However, theexactbillettemperatureduringanECAPopera-tion may be significantly above the preheatingtemperatureduetotheheatgeneratedbytheme-chanicalwork.Itisthereforenecessarytomeasurethe actual billet temperature during ECAP forprecisecontrolofrecrystallization,andhenceofthegrainsize.ThetemperatureriseduringECAPisconsideredtodependonboththebilletmate-rial and the deformation rate. This study was

Modeling of Droplet-Based Processing for the Production of High-Performance Particulate Materials using the Level Set Method

The dynamic and thermal processes of an Mg-Zn-Y alloy droplets spreading and solidification are investigated using the level set method in order to understand their effects on the phase change process in a uniform droplet spray process. The level set method, driven with the solidification velocity predicted by a free dendritic growth model, is capable of tracking the evolution of the solidification front within the deformed droplet. It is found that the solidification process heavily depends on the initial thermal state of the droplet, the latent heat released during solidification, and the heat loss to the substrate. A rapid solidification occurs in the initial microseconds before a slow solidification process takes place.

Grain refinement and stabilization in spray-formed AISI 1020 steel

Abstract An AISI 1020-grade mild steel, with a small amount of aluminum, was spray-formed by nitrogen gas-atomization and deposition. The spray-formed 1020 steel contained 0.05 mass% of nitrogen and 0.06 mass% of aluminum. Rolling the spray deposit at 1123 K for a thickness reduction of 70% and subsequent normalizing at low austenitic temperatures produced a fully dense steel having a refined ferrite grain size as small as 3 μm. This grain refinement resulted from the pinning of prior austenite grain boundaries by fine AlN particles which precipitated during the thermomechanical treatments. Room-temperature tensile properties (YS: 550 MPa, UTS: 630 MPa, elongation: 23%), exceeding those of conventional microalloyed high-strength low alloy steels, were achieved in the normalized state. The AlN-pinned fine-grained microstructure survived subsequent re-austenizing at temperatures as high as 1273 K. When tensile-tested in the austenite region at 1123 K, the microalloyed 1020 steel showed a large elongation exceeding 200%.

A Numerical Investigation Into Cold Spray Bonding Processes

Specific mechanisms underlying the critical velocity in cold gas particle spray applications are still being discussed, mainly due to limited access to in situ experimental observation and the complexity of modeling the particle impact process. In this work, particle bonding in the cold spray (CS) process was investigated by the finite element (FE) method. An effective interfacial cohesive strength parameter was defined in the particle–substrate contact regions. Impact of four different metals was simulated, using a range of impact velocities and varying the effective cohesive strength values. Deformation patterns of the particle and the substrate were characterized. It was shown that the use of interfacial cohesive strength leads to a critical particle impact velocity that demarcates a boundary between rebounding and bonding type responses of the system. Such critical bonding velocities were predicted for different interfacial cohesive strength values, suggesting that the bonding strength in particle–substrate interfaces could span a range that depends on the surface conditions of the particle and the substrate. It was also predicted that the quality of the particle bonding could be increased if the impact velocity exceeds the critical velocity. A method to predict a lower bound for the interfacial bonding energy was also presented. It was shown that the interfacial bonding energy for the different materials considered would have to be at least on the order of 10–60 J/m2 for cohesion to take place. The general methodology presented in this work can be extended to investigate various materials and impact conditions.

Fabrication of micro/nano structured aluminum–nickel energetic composites by means of ultrasonic powder consolidation

Abstract Al–Ni energetic composites, applicable to localized heating, were fabricated from atomized powders and hammer-milled flakes by means of ultrasonic powder consolidation, a novel low-temperature rapid powder consolidation technique. Full-density consolidation was achieved in 1 s of consolidation time at temperatures as low as 573 K without causing appreciable reactions between Al and Ni. The consolidated composite structure consists of Ni powder particles or flakes uniformly distributed in a metallurgically consolidated Al matrix. Consolidated materials were evaluated using spark and continuous-heating ignition tests. Ultrasonic powder consolidation is potentially applicable to the fabrication of a wide range of metastable materials that cannot be processed at elevated temperatures.

Miniature thermal matches: from nanoheaters to reactive fractals

Fine thermal actuation by miniature heat sources enables applications from electronics fabrication to tumor cauterization. This paper introduces the concept of nanoheaters, i.e., reactive bimetallic material dots (0D), ignited electrically to exothermically release precise heat amounts where and when needed. This concept is extended to nanoheater wires (1D) and foils (2D), as well as bulk nanoheaters (3D) manufactured via ball milling and ultrasonic consolidation of nickel and aluminum powders. The fractal structure of such powders and consolidates, with self-similar, multiscale Apollonian or lamellar packaging, is discovered to hold the key for their ignition sensitivity: nanoscale structures ignite first, to produce enough heat and raise the temperature of submicron formations, which then ignite microscale regions and so on; while inert areas quench and arrest the self-propagating exothermic reaction. Therefore, such engineered fractal reactive heaters lend themselves to affordable, high-throughput manufacture and controllable, safe, efficient, supplyless in situ thermal release. This can be transformative for innovations from self-healing composites and self-heating packages to underwater construction and mining.

A Method to Predict the Thickness of Poorly-Bonded Material Along Spray and Spray-Layer Boundaries in Cold Spray Deposition

Multi-traverse CS provides a unique means for the production of thick coatings and bulk materials from powders. However, the material along spray and spray-layer boundaries is often poorly bonded as it is laid by the leading and trailing peripheries of the spray that carry powder particles with insufficient kinetic energy. For the same reason, the splats in the very first layer deposited on the substrate may not be bonded well either. A mathematical spray model was developed based on an axisymmetric Gaussian mass flow rate distribution and a stepped deposition yield to predict the thickness of such poorly-bonded layers in multi-traverse CS deposition. The predicted thickness of poorly-bonded layers in a multi-traverse Cu coating falls in the range of experimental values. The model also predicts that the material that contains poorly bonded splats could exceed 20% of the total volume of the coating.

Continuous‐Heating Ignition Testing of Hybrid Al‐Ni‐CuO Reactive Composites Fabricated by Ultrasonic Powder Consolidation

Powder-based fabrication of hybrid reactive composites combining bimetallic and thermite endothermic reactions requires a consolidation process that produces dense composites with no reactions among the constituents. Reactive composites 2Al-3CuO-x(Al-Ni) (x = 1 - 4) were fabricated from nano-thick Al and Ni flakes and CuO nanoparticles by ultrasonic powder consolidation and tested for their ignition characteristics in continuous heating. The hybrid bimetallic thermite composites with x ≥ 2 ignited well below the melting point of aluminum, while maintaining large heat outputs. Combining the large heat output of the Al-metal oxide thermite reaction and the low ignition temperature of Al-Ni exothermic reactions in single reactive composites, the hybrid bimetallic-thermite composites are suited for controlled local heating, as in micro-joining, where small, easy-to-ignite, high-output heat sources are required.

On Simulation of Multi-Particle Impact Interactions in the Cold Spray Process

The cold spray process consists of coating build-up by sequential impact, deformation and bonding of many particles. Therefore, formation and properties of a deposited layer are not only affected by the impact behavior of a single particle, but also by subsequent impact events. To investigate the material behavior under such conditions, impact of multiple particles in cold spray was studied here by the finite element method. Effects of high strain rates and temperature on material yield and failure criteria were considered. Particle conditions prior to impact were derived from fluid dynamics calculations. To predict sticking behavior of the particle, an interfacial cohesive strength parameter was defined between the particle and the substrate. The effects of temperature and particle positioning were examined for three particle impacts. In addition, simulations involving 100 consecutive particle impacts were carried out, and findings were compared with experimental observations. Results showed that subsequent impacts have a large effect on the previously impacted particles for cohesion, degree of deformation, and residual stresses.Copyright