Plastic Deformation Of Particles Research Articles

Effect of current on the sintering of pre-oxidized copper powders by SPS

As mentioned in the literature, SPS (Spark Plasma Sintering) technology combines uniaxial charge and pulsed currents to achieve the rapid sintering of powders. Although the utilization of current is often reported, an understanding of its role in the sintering mechanisms is still a subject of controversy. In fact, the oxide layer around metal particles is sometimes considered to be a dielectric gap in which discharges may occur: these discharges can clean the surface of particles and enhance densification. In this paper, an oxide layer was grown on the particle surface of a copper powder, and the growth enabled the study of the role of current on such dielectric layers. The powders were sintered by SPS using graphite tools in a reference cycle with constant current and a relatively low pressure of 4 MPa to limit the plastic deformation of particles. Three sintering configurations were compared to this reference experiment: (i) one without current in a HP process, (ii) one with a strongly pulsed current using graphite and alumina tools and (iii) one with a high heating rate. The results indicate that current does not alter the sintering mechanism. No cleaning effect of the particle surface was observed. The oxide layers remain in the sintered samples and form collars around the necks. The oxide layer modifies the sintering mechanism; it leads to the delay of consolidation phenomena on the surface.

Crystallization Evolution of Cold-Sprayed Pure Ni Coatings

Cold spraying is a coating technology on the basis of aerodynamics and high-speed impact dynamics. Spray particles (usually 1-50 μm in diameter) are accelerated to high velocity (typically 300-1200 m/s) by a high-speed gas (preheated air, nitrogen, or helium) flow that is generated through a convergent-divergent de Laval type nozzle. The coating forms through the intensive plastic deformation of particles impacting on the substrate at temperatures well below the melting point of the spray material. In the present paper, the main processing parameters affecting the crystallization behavior of pure Ni cold spray deposits on IN718 alloy are described. Various experimental conditions have been analyzed: gas temperature and pressure, nozzle to substrate distance. In particular, the study deals with those conditions leading to a strong grain refinement, with an acceptable level of the deposits mechanical properties. In precise spray conditions, a shift toward amorphous phases has been observed and studied. A systematic analysis of microstructural evolution, performed through TEM observations, as a function of processing parameters is presented.

Characteristics and Mechanism of Cu Films Fabricated at Room Temperature by Aerosol Deposition.

We were successful in growing a dense Cu film on Al2O3 substrates at room temperature using an aerosol deposition (AD) method. The characteristics of Cu films were investigated through electrical resistivity and X-ray photoelectron spectroscopy (XPS). The resistivity of Cu films was low (9.2–12.5 μΩ cm), but it was five to seven times higher than that of bulk copper. The deterioration of the resistivity indicates that a Cu2O phase with CuO occurs due to a particle-to-particle collision. Moreover, the growth of Cu films was investigated by observing their microstructures. At the initial stage in the AD process, the impacted particles were flattened and deformed on a rough Al2O3 substrate. The continuous collision of impacted particles leads to the densification of deposited coating layers due to the plastic deformation of particles. The bonding between the Cu particles and the rough Al2O3 substrate was explained in terms of the adhesive properties on the surface roughness of Al2O3 substrates. It was revealed that the roughness of substrates was considerably associated with the mechanical interlocking between Cu particles and rough Al2O3 substrate.

Effect of Milling Time on Microstructure and Properties of AA6061/MWCNTS Composite Powders / Wpływ Czasu Mielenia Na Strukturę I Własności Proszk Ów Kompozytowyc H AA6061/MWCNTS

The main purpose of this work is to determine the effect of milling time on microstructure as well as technological properties of aluminium matrix nanocomposites reinforced with multi-walled carbon nanotubes (MWCNTs) using powder metallurgy techniques, including mechanical alloying. The main problem of the study is the agglomeration and uneven distribution of carbon nanotubes in the matrix material and interface reactivity also. In order to reach uniform dispersion of carbon nanotubes in aluminium alloy matrix, 5÷20 h of mechanical milling in the planetary mill was used. It was found that the mechanical milling process has a strong influence on the characteristics of powders, by changing the globular morphology of as-received powder during mechanical milling process to flattened one, due to particle plastic deformation followed by cold welding and fracturing of deformed and hardened enough particles, which allows to obtain equiaxial particles again. The obtained composites are characterised by the structure of evenly distributed, disperse reinforcing particles in fine grain matrix of AA6061, facilitate the obtainment of higher values of mechanical properties, compared to the initial alloy. On the basis of micro-hardness, analysis has found that a small addition of carbon nanotubes increases nanocomposite hardness.

Mechanical and Microstructural Behavior of Cold-Sprayed Titanium- and Nickel-Based Coatings

Cold spraying is a coating technology that can deposit materials with unique properties. The coating forms through intensive plastic deformation of particles impacting on a substrate at temperature well below the melting point of the sprayed material. Recently, various studies have been published regarding the microstructural and mechanical evolution of metal-matrix composite coatings produced by cold spraying. Herein, we describe the principal results of the available literature in the field of cold-sprayed composites. It is shown that more research is required to solve various questions in this field, for example, the different deformation modes of the material exhibited for various processing conditions, the reinforcing percentage of different material combinations, and the mechanical properties resulting from these complex systems. In the present study, this issue is approached and described for cold-sprayed Ni- and Ti-based composites. Materials were produced with varying ceramic phase (BN and TiAl3) fraction. The variation of the grain size, adhesion strength, porosity, and hardness of the deposits as a function of the ceramic phase fraction and processing parameters (impacting particle speed) is described. The interaction mechanisms between the cold-sprayed particles and the metal matrix during the coating process are presented and described. The results demonstrate a beneficial effect on grain size and porosity with increasing reinforcing phase percentage, as well as narrow processing parameter ranges to achieve the optimal properties with respect to the pure parent materials.

Processing conditions affecting residual stresses and fatigue properties of cold spray deposits

Cold spraying is an innovative coating technology mainly based on the high-speed impact of metallic particles on different substrates. Through the employment of low-temperature gases (air, He, N2), spray particles (usually 1–50 μm in diameter) are accelerated to a high velocity (typically 300–1200 m/s) that is generated through a convergent-divergent de-Laval-type nozzle. Severe plastic deformation of particles impacting on the substrate occurs at temperature well below the melting point leading to the unique mechanical properties experienced by such kinds of coatings. In the present paper, the main processing parameters affecting the fatigue behavior of metal-metal cold spray deposits are described. In the present paper, the effect of processing parameters on fatigue properties were analyzed for different materials such as AA7075 sprayed on AA7075 substrate, AA2024 sprayed on AA7075 substrate, and AA2024 sprayed on AZ91 magnesium alloy substrate. Many experimental conditions have been analyzed in terms of particle dimensions and composition, substrate temperature and composition, gas temperature and pressure, and nozzle properties. In particular, they were analyzed those conditions leading to an increase in fatigue life.

Hot-pressure sintering, microstructure and chemical durability of Ce0.5Eu0.5PO4 monazite ceramics

In an attempt to provide a low temperature and rapid densification technique for immobilizing high-level radioactive waste (HLW), the hot-pressure sintering process was applied in this paper. Ce0.5Eu0.5PO4 monazite ceramics with relative densities in excess of 98% were fabricated by hot-pressing at temperature as low as 1150°C (30MPa) for 2h. The agglomerates of about 5µm in length on surface of the ceramic sintered at 1000°C could indicate the first steps of the grain growth. The morphology of the ceramic became regular and the grain sizes obviously coarsened as the temperature increased to 1150°C. The grain boundary became unclear from 1150 to 1250°C, which might be caused by the plastic deformation of particle. In addition, the normalized elementary release rates of Ce and Eu decrease with increasing time and remain almost unchanged in a low value below 4×10−5gm−2d−1 and 2×10−4gm−2d−1 after 21d. Preferential leaching of P has been observed.

Canola meal moisture-resistant fuel pellets: Study on the effects of process variables and additives on the pellet quality and compression characteristics

This study focuses on pelletization of waste canola meal biomass to increase the bulk density, thereby reducing the transportation and storage costs, thus provide better material feeding in gasification reactors with less dust formation. The effects of feed constituents of canola meal such as protein, fiber, fat, lignin and feed moisture content as well as added binder, lubricant and densification process parameters on the strength and durability of the densified product are investigated. The increased durability (99%) of canola meal pellets was a result of added binder (5wt%) and the inherent protein (40wt%) and lignin (12wt%) content in the feed. From the compression data at different temperature and pressure, Kawakita and Lüdde model (1971) was developed to classify the feed material into groups. The R2 value ≥0.999 showed good model fit. It was found that at temperature >70°C, the particle undergoes rearrangement followed by fragmentation and particle plastic deformation during the compression process. The effects of coating agent on pellet durability, hardness and moisture uptake were studied to produce moisture-resistant pellets. Finally, the pellets were gasified in a fixed bed reactor using different gasifying agents such as steam, oxygen (O2) and carbon dioxide (CO2) and their effects were assessed. Carbon dioxide was found to give maximum carbon efficiency (CE) up to 82.7% and 50.7MJ/m3 LHV of gas at a temperature of 750°C and equivalence ratio (ER) of 0.4., whereas O2 gave 66.5% of CE with 44.7MJ/m3 LHV of gas at 650°C and 0.4 ER and steam produced gas with LHV 40.8MJ/m3 with CE 27.4% at 650°C and 0.2 ER. Thus, by producing moisture-resistant canola meal pellets with reasonable fuel characteristics, pelletization of canola meal provides a promising alternative for the utilization of canola meal waste as an alternative source of renewable energy.

Sintering Kinetics of the Powder during Fields-Activated Micro-Forming and Sintering (Micro-FAST) of Copper Micro-Gears

Forming of micro-components from powder with fields-activated sintering technology (FAST) renders different forming and sintering mechanisms, comparing to that occurring during the forming of macro-sized components with a similar technology. Establishing a good understanding of these mechanisms would help process design and control aiming at achieving desired quality of the components to be formed. This paper presents a study and the results on the sintering kinetics of the powder during Micro-FAST for the fabrication of micro-gears (the module is 0.2 and the pitch diameter 1.6 mm) from copper powder. The results showed that the densification of copper powder is related largely to the bulk plastic-deformations of the particles and the melting of the particles at contact interfaces. Particularly, it is revealed that plastic deformations of the copper particles mainly occurred at approximately 340 °C and melting of the particle-interfaces at approximately 640 °C. Differently, in a densification process with a traditional powder sintering method, grain growth and neck growth would, normally, be two dominant mechanisms that achieve the densification of powder.

Charge Transfer to Metal Nanoparticles Bouncing from Conductive Surfaces

The charging behavior of metal nanoparticles bouncing from conductive surfaces was investigated in a single-stage-low-pressure-impactor. Ag and Pt particles of a fixed particle size between 20 nm and 100 nm were impacted on targets of bulk Au and Pt, respectively, and the resulting contact charge was measured as a function of impact velocity. The influence of target hardness was revealed by the comparison to measurements with soft nanostructured layers obtained by direct current sputtering of Au and Pt on mica discs. From the dataset, regions of elastic, elastoplastic, and fully plastic particle deformation were identified, and the size-dependent effective yield stress for the respective particle materials was calculated. The influence of electron back-tunneling in the separation phase of the collision on the effective contact charge is discussed.Copyright 2014 American Association for Aerosol Research

Mechanical and microstructural behavior of nanocomposites produced via cold spray

Cold spraying is an innovative coating technology mainly based on the high speed impact of metals and ceramic particles on different substrates. Through the employment of low temperature gases (Air, He, N2) spray particles (usually 1–50μm in diameter) are accelerated to a high velocity (typically 300–1200m/s) that is generated through a convergent–divergent de Laval type nozzle. Severe plastic deformation of particles impacting on the substrate occurs at temperature well below the melting point leading to the unique mechanical properties experienced by such kinds of coatings. In the present paper the main processing parameters affecting the microstructural and mechanical behavior of metal–metal cold spray deposits are described. The effect of processing parameters on grain refinement and mechanical properties were analyzed for different particles (Ti–TiAl3, Al–Al2O3, Ni–Cr3C2, Ni–BN, Cu–Al2O3, Co–SiC). The results belonging to the properties of the formed nanocomposites were compared with those of the pure parent materials sprayed in the same conditions. Many experimental conditions have been analyzed in terms of particle dimensions and composition, substrate temperature and composition, gas temperature and pressure, nozzle properties. In particular, those conditions leading to a strong grain refinement with an acceptable level of the mechanical deposit properties such as porosity, adhesion strength and hardness were underlined.

Processing Conditions Affecting Grain Size and Mechanical Properties in Nanocomposites Produced via Cold Spray

Cold spray is a coating technology based on aerodynamics and high-speed impact dynamics. In this process, spray particles (usually 1-50 μm in diameter) are accelerated to a high velocity (typically 300-1200 m/s) by a high-speed gas (pre-heated air, nitrogen, or helium) flow that is generated through a convergent-divergent de Laval-type nozzle. A coating is formed through the intensive plastic deformation of particles impacting on a substrate at a temperature below the melting point of the spray material. In the present paper the main processing parameters affecting the microstructural and mechanical behavior of metal-metal cold spray deposits are described. The effect of process parameters on grain refinement and mechanical properties were analyzed for composite particles of Al-Al2O3, Ni-BN, Cu-Al2O3, and Co-SiC. The properties of the formed nanocomposites were compared with those of the parent materials sprayed under the same conditions. The process conditions, leading to a strong grain refinement with an acceptable level of the deposit mechanical properties such as porosity and adhesion strength, are discussed.

Mechanical properties of Cold Spray deposited NARloy-Z copper alloy

This work focuses on the properties of a Cu-3wt.% Ag-0.5wt.% Zr alloy deposited by a Cold Spray process. Helium was used as the carrier gas and main gas to obtain a high deposition efficiency, low porosity and high mechanical properties. The effect of deposition parameters and heat treating on the microstructure and mechanical properties was explored. An ultimate tensile strength higher than 650MPa, a yield strength of about 600MPa and a Vickers micro hardness HV0.2 higher than 220 were obtained, values which compare quite favorably with those corresponding to a classically forged material. The deposits microstructure and more particularly the plastic deformation of particles upon impact were observed by optical and electronical microscopy. The residual stress level was also investigated as a function of heat treating.

Effects of tungsten and aluminium on the oxidation and phase formation in mechanically alloyed Ti(C,N)–W–Al systems

Abstract The effects of milling parameters and composition of the powder mixtures on the transformations of Ti(C,N)–W–Al powders processed by high energy ball milling were investigated by XRD, SEM and TEM. The strain energy and the fine particle size contributed to the high chemical reactivity with oxygen of the powders milled for 12–24 h. Powders milled for 48 h were chemically stable. The affinity with oxygen decreased after W dissolution in Ti(C,N), and the subsequent decrease in lattice strains. Aluminium lowered the lattice strains, and subsequently the strain energy stored in the deformed crystals of Ti(C0.5N0.05) and W milled above 25 °C. Fracturing of hard particles dominated in the early stage of milling in the absence of Al, whereas with Al, plastic deformation of particles and cold welding of Ti(C,N) and W particles by the softer Al prevailed at the same time.

Ash deposition modeling using a visco-elastic approach

In pulverized fuel (PF) combustors and gasifiers, ash particles may deposit on heat exchangers reducing efficiency, performance and, in some cases, damaging boilers and gasifiers. In general, ash particle composition, size (fragmentation) and mechanical properties (hardness and stickiness) are affected by the history of each particle along their trajectory. In the present work, visco-elastic models are used to predict the Young Modulus according to time, temperature and ash composition. In commercial CFD programs, particle adhesion is usually modeled either as fully trapped or bouncing off according to a specific restitution coefficient usually given as input to the wall boundary surface. The authors apply the mechanical adhesion approach proposed by Johnson (JKR theory) which takes into account the surface energy, particle size and the hardness of both the particle and the impacted surface. The energy restitution coefficients are calculated as the remaining kinetic energy after elastic–plastic particle deformation and work of adhesion due to inertia impaction. To implement this approach, a specific Lagrangian particle tracking code was developed to track particle and predict their fate post-processing CFD flow data fields. Numerical results are validated against hollow glass particle and biomass ash experiments carried out in the entrained flow reactor in use at The Energy Research Center of The Netherlands (ECN). Hollow glass particle were used to mimic the high silica content ash obtained from co-firing biomass facilities. The final goal of this work is to present a comprehensive approach to mathematically describe deposition and deposit build-up by means of mechanical and rheological models for visco-elastic solids. Numerical results compare fairly well to available experimental data of the average particle dispersion.

An experimental evaluation of an effective medium based compaction equation

Tablet production involves compression of free flowing powder in an enclosed cavity of defined geometry. The complexity of the powder bed system necessitates that a way be found to better understand what occurs during compression. One such approach is by means of compaction equations, of which, the Heckel and Kawakita equations are the best known. This work attempts to experimentally evaluate the applicability of the effective medium (EM) equation introduced by Frenning et al. (2009) to powder systems. Two powder types (sodium chloride and lactose monohydrate), each consisting of three size fractions (<40, 125–212 and 212–300μm) were characterised and compressed to a pressure of 500MPa. These powders were chosen because of their differing mechanical properties. An invariance which is inherent in the EM equation is exposed by varying the starting points of compression, and can yield insights into compression mechanisms. Such invariant regions were observed once plastic particle deformation started to dominate the compression behaviour, and enabled the determination of the point where particle rearrangement stops.

Study of the Influence of Particle Velocity on Adhesive Strength of Cold Spray Deposits

The adhesion mechanism of deposit/substrate interface prepared by the cold spray method is not fully understood at present. It seems that the adhesion strength is mainly determined by the mechanical (including the plastic deformation of particle and substrate) and thermal interaction between particle and substrate when the particles impact onto the substrate with a high velocity. In order to understand the adhesion mechanism, a novel adhesive strength test was developed to measure the higher bonding strength of cold sprayed coatings in this study. The method breaks through the limits imposed by glue strength in the conventional adhesive strength test, and it can be used to measure the coatings with a higher adhesive strength. The particle velocity was obtained with DPV-2000 measurement and CFD simulation. The relationships between the adhesion strength of deposits/substrate interface and particle velocity were discussed. The results show that stronger adhesion strength can be obtained with the increase of particle velocity. There are two available ways to improve the adhesion strength. One is to increase the temperature of working gas, and another is to employ helium gas as the working gas instead of nitrogen gas.

A protocol for the classification of powder compression characteristics

In this paper, a structured protocol for powder compression analysis as a test to assess the mechanical properties of particles in a formulation development programme is presented. First, the sequence of classification steps of the protocol is described, and secondly, the protocol is illustrated using compression data of six powders of two model substances, sodium chloride and mannitol. From powder compression data, a set of compression variables are derived, and by using critical values of these variables, the stages expressed during the compression of the powders are identified and the powders are classified into groups with respect to the expression of particle rearrangement, particle fragmentation and particle plastic deformation during compression. It is concluded that the proposed protocol could, in a satisfactorily way, describe and distinguish between the powders regarding their compression behaviour. Hence, the protocol could be a valuable tool for the formulation scientist to comprehensively assess important functionality-related characteristics of drugs and excipients.

Elastic and plastic properties of powder materials: a continuum model

A continuum model based on the physical hypotheses of the discrete contact model is developed to describe the elastic and plastic properties of isotropic powder materials, taking into account the inhomogeneous deformation of the solid phase. The localization of elastic and plastic deformation is described by hypothesizing that deformable and nondeformable volumes form in the solid phase and that the resistance to deformation is associated only with the deformable volume. An analytical dependence of the deformable volume on the density of powder material is provided. The proposed model ensures high-accuracy fit to the experimental compaction curves at the stages of interparticle slip and plastic deformation of particles. At the beginning of plastic deformation, a particle is regarded as a cast one already plastically deformed and hardened to the level of the effective yield strength of the solid phase. The particle is further hardened as a cold-worked cast material is. Good agreement is reached between calculated and experimental data on the elastic moduli and plastic compaction of powders during isostatic pressing and deposition in a high-pressure chamber.

Cooling as a possible method for increasing the flowability of certain pharmaceutical and other powders.

Measurements have been made of the rates of flow under gravity of powdered sugars and of fatty acids through circular orifices at temperatures between -25 and 110 degrees C. The flow rates decrease considerably as the powders are heated; conversely they increase as the powders are cooled from a high ambient temperature of 40 to -25 degrees C. The results are ascribed to changes in the cohesiveness of the powders caused by softening and plastic deformation of particles at elevated temperatures. Lactose, paracetamol and griseofulvin granule formations have been reported to show similar effects and the possibility is discussed of using cooling to increase the flowability of particular powders in pharmaceutical production. The rates of flow of the powders are given by the expression(formula: see text)where W is the flow rate in g min-1, Do is the orifice diameter in cm, rho is the particle density in g cm-3, g is the gravitational constant, and A and n are numerical terms which depend on temperature and particle size.