Flake Powder Metallurgy Research Articles

Enhanced mechanical properties and high electrical conductivity in multiwalled carbon nanotubes reinforced copper matrix nanolaminated composites

Multiwalled carbon nanotubes/copper (MWCNTs/Cu) composites with a nanolaminated structure have been successfully prepared via flake powder metallurgy. The key strategies are to achieve uniform dispersion of carbon nanotubes in copper matrix and laminated structure, leading to high strengthening efficiencies and architecture toughening. As a result, the composites show balanced failure strength and elongation and high electrical conductivity. The tensile strength of 1.0 vol% MWCNTs/Cu laminated composite is 395 MPa, 87% higher than that of coarse-grained Cu. At the same time, the enhancement on strength does not cause serious deterioration in failure elongation and electrical conductivity. A satisfied uniform elongation in excess of 20% and an electrical conductivity more than 90% International Annealed Copper Standard (IACS) are retained in the composite. Characterizations by in-situ digital image correlation and X-ray tomography indicate efficient stress transferring during loading as well as laminated structure guiding crack propagation, contributing to the good strength-elongation balance. Additionally, the electrical conductivity of the composites is anisotropic because of the laminated structure.

Tailoring the structure and mechanical properties of graphene nanosheet/aluminum composites by flake powder metallurgy via shift-speed ball milling

Graphene nanosheet (GNS)/aluminum composites were fabricated via shift-speed ball milling (SSBM), consisting of a long-term low-speed ball milling (LSBM) and a short-term high-speed ball milling (HSBM). During the early stage of LSBM, Al powders were flattened into flakes, while the agglomerated GNSs were gradually dispersed onto Al flakes. After an inflection point of LSBM time, the dispersed GNSs got re-agglomerated and seriously damaged due to the accumulated work-hardening of Al flakes. During HSBM, the GNS/Al flakes were cold-welded into lamellar-structured particles, preserving the GNS dispersion states. It was demonstrated that the 0.5 vol.% GNS/Al composites via SSBM with 6 h LSBM had proper combination of ultrafine-grained Al matrices with well-preserved, uniformly-dispersed GNSs. Exceptional properties were achieved with a good ductility of 13.5% at a tensile strength of 295 MPa. Therefore, such flake powder metallurgy via SSBM proved to be a smart and effective fabrication strategy for nano-reinforced metal matrix composites.

High-strength CNT/Al-Zn-Mg-Cu composites with improved ductility achieved by flake powder metallurgy via elemental alloying

The uniform dispersion of carbon nanotubes (CNTs) is usually accompanied by severe grain refinement of Al matrix, which leads to the low ductility of CNT/Al alloy composites. To accommodate this dilemma, a flake powder metallurgy route via elemental alloying was proposed to fabricate CNT/Al-Zn-Mg-Cu composites with improved ductility and high strength. CNT/Al flake powders were firstly obtained by ball milling at a low speed to achieve uniform dispersed of CNTs, and then milled with Zn, Mg and Cu elemental flake powders at a high speed to achieve lamellar CNT/Al-Zn-Mg-Cu particles, which were consolidated and homogenized to obtain bulk CNT/Al-Zn-Mg-Cu composites. Compared with the CNT/AA7075 counterparts fabricated by directly using CNTs and atomized AA7075 powders, the CNT/Al-Zn-Mg-Cu composites exhibited improved ductility with high modulus and strength, due to the well-protected and uniformly aligned CNTs, and good dislocation storage capability of the elongated ultrafine grains.

Young’s Modulus Enhancement and Measurement in CNT/Al Nanocomposites

Young’s modulus is a critical parameter for designing lightweight structure, but Al and its alloys only demonstrate a limited value of 70–72 GPa. The introduction of carbon nanotubes (CNTs) is an effective way to make Al and its alloys stiffer. However, little research attention has been paid to Young’s modulus of CNT/Al nanocomposites attributed to the uncertain measurement and unconvincing stiffening effect of CNTs. In this work, improved Young’s modulus of 82.4 ± 0.4 GPa has been achieved in 1.5 wt% CNT/Al nanocomposite fabricated by flake powder metallurgy, which was determined by resonance test and 13.5% higher than 72.6 ± 0.64 GPa of Al matrix. A comparative study and statistical analysis further revealed that Young’s modulus determined by tensile test was relatively imprecise (83.1 ± 4.0 GPa) due to the low-stress microplasticity or interface decohesion during tensile deformation of CNT/Al nanocomposite, while the value (98–100 GPa) was highly overestimated by nanoindentation due to the “pile-up” effect. This work shows an in-depth discussion on studying Young’s modulus of CNT/Al nanocomposites.

Uniform dispersion of multi-layer graphene reinforced pure titanium matrix composites via flake powder metallurgy

In this study, a micro-laminated structure of titanium matrix composite with uniformly dowel-like and aligned multi-layer graphene (MLG) was fabricated by flake powder metallurgy. Flake ball milling method was applied to efficiently obtain flaky Ti powder with embedded MLGs flakes. Spark plasma sintering (SPS) was applied to consolidate mixed powders, in order to obtain the laminated billet which can enable the MLGs to preserve its original structure. Subsequent hot-rolling (HR) was applied to form the tight interface and reduce the internal defection of composites. Results showed that the composites interface owns in-situ formed TiC layer between MLGs and matrix. Also, the composites with micro-laminated structure showed significant improvement of strength. Consequently, a uniform disperse of MLG enabled the as-designed composites to exhibit 280% increase in yield strength (~2 GPa), 96% increase in nano-hardness and 16% increase in elastic modulus as compared to monolithic flake pure Ti (HR normal direction (ND)). The micro-laminated structure and well-dispersed MLGs were believed to be beneficial to harden and strengthen Ti matrix, and the relevant strengthening mechanisms of the composites were carefully discussed.

Effect of flake powder metallurgy on thermal conductivity of graphite flakes reinforced aluminum matrix composites

The optimization of metal–matrix composite material is linked firstly with the intrinsic properties of the matrix and the reinforcement used and secondly with the reinforcement–matrix interfacial zone and the distribution/orientation of the reinforcement inside the metal–matrix. Flake powder metallurgy was used to fabricate graphite flake reinforced aluminum matrix (Al/GF) composites fabricated by vacuum hot pressing. Two types of aluminum powders morphology were used: spherical (AlS) and flake (AlF) powders. A higher thermal conductivity in the in-plane direction of the graphite flakes was obtained for Al/GF composite materials fabricated with aluminum flake powder. In addition to a better orientation of the GF in the flake aluminum matrix, a 3D puckered surface and plane surface are formed at the Al/GF interface in, respectively, AlS/GF and AlF/GF composite materials. Due to the morphology incompatibility between the graphite flakes and the spherical powder, the damaged inner structure of GF contributes to a limited enhancement of thermal conductivity in AlS/GF composite materials.

Design of an efficient flake powder metallurgy route to fabricate CNT/6061Al composites

A flake powder metallurgy route that combines high-shear pre-dispersion process with shift-speed ball milling was designed to fabricate carbon nanotube reinforced aluminum composites. In order to mitigate the dispersion responsibility of ball milling and improve the dispersion efficiency, high-shear pre-dispersion process utilizing the mechanism of dry particle coating was firstly applied to CNTs and 6061Al powder mixtures for a few minutes to de-agglomerate CNTs and uniformly coat them onto surface of 6061Al powder. It was demonstrated that, for the pre-dispersed powders, more rapid and uniform dispersion of CNTs was achieved during following shift-speed ball milling process, which in turn protected CNTs from additional damage and led to better interfacial bonding and higher reinforcing efficiency. As results, compared to counterpart without pre-dispersion, the as-extruded bulk 1.5 wt% CNT/6061Al composites exhibited simultaneous enhancement in Young's modulus, tensile strength and ductility.

Enhanced interfacial bonding and mechanical properties in CNT/Al composites fabricated by flake powder metallurgy

A flake powder metallurgy route consists of a slurry based dispersion process and a short time ball milling process was proposed to fabricate strong and ductile CNT/Al composites reinforced with high content CNTs. To improve the interfacial bonding, the CNT/Al nanoflake powders prepared through slurry based dispersion process would go through a short time high energy ball milling process to break native Al2O3 skin and embed CNTs into Al matrix. With the dispersion homogeneity and well-maintained structural integrity of CNTs achieved by the slurry based dispersion, and improvement of interfacial bonding from non-bonding to physical bonding/diffusion assisted bonding and partial reaction bonding through the short time ball milling, an enhancement of tensile strength from 298 to 406 MPa and ductility from 1.9 to 8.8% was obtained for the 2 h ball-milled 3 vol.% CNT/Al composites, compared to the composites fabricated by sole slurry based dispersion process. It also outperformed that fabricated by direct high energy ball milling, indicating the flake powder metallurgy method could provide a good coordination between the CNT dispersion homogeneity, structural integrity and interfacial bonding.

Prediction of effect of reinforcement content, flake size and flake time on the density and hardness of flake AA2024-SiC nanocomposites using neural networks

It is known that the size and morphology of matrix powders and reinforcement content affects the physical and mechanical properties of ceramic nanoparticle reinforced metal matrix composites. Therefore, an artificial neural network model has been developed for the prediction of density and hardness of AA2024-SiC nanocomposites fabricated by flake powder metallurgy. The weight percentage of SiC nanoparticles, AA2024 matrix size and ball milling time were selected as the inputs and the sintered density and hardness of the flake AA2024-SiC nanocomposites as the output of the model. Prediction model ofAA2024-SiC nanocompositeswas able to predict the density and hardness with a mean absolute percentage error (MAPE) of 0.18% and 0.99%, respectively. The root mean score error (RMSE) of ANN model developed for AA2024-SiC nanocomposites were 0.06% and 0.835 for the density and hardness, respectively. The results of present study shows that the density and hardness of SiC nanoparticle reinforced AA2024 matrix flake nanocomposites can be predicted with high accuracy using neural network model.

Microstructure, microhardness and thermal expansion of CNT/Al composites prepared by flake powder metallurgy

Carbon nanotube/aluminum (CNT/Al) composites are prepared by a combination of flake powder metallurgy and hot-isostatic-pressing. The specimens are investigated by several techniques including Raman spectroscopy, optical microscopy, scanning- and transmission electron microscopy. The composites show a layered-microstructure with a stacking of CNT/Al flakes with a CNT-rich layer between two flakes. The individual Al grains forming the flakes are about 500 nm in size. The CNTs are well dispersed within a flake and they bridge the micro-cracks. The results reveal that the coefficient of thermal expansion (CTE) decreases markedly upon the increase in carbon content, reaching 15.4 × 10−6 K−1 for the specimen with a carbon content of 2.0 wt% (2.9 vol%), i.e. a 30% decrease compared to the CTE of pure Al. This could arise from the layered-microstructure resulting from the utilization of Al flakes as opposed to rounded particles.

Grain refinement and superplastic behavior of carbon nanotube reinforced aluminum alloy composite processed by cold rolling

The carbon nanotube reinforced aluminum matrix composite (CNT/6061Al) fabricated by flake powder metallurgy was cold rolled to reduce grain size, and was examined by high temperature tensile experiments within temperature from 325 to 450°C and strain rate from 4.17E−2 to 2.09s-1 to explore the effect of grain refinement on the superplastic deformation behavior. The average grain size was reduced from 580nm to 300nm with better homogeneity after cold rolling, and elongation to failure at 400°C and 4.17E−1s-1 was improved by 40% compared to that without cold rolling. The temperature of 375°C and intermediate strain rate turned out to be appropriate deformation conditions for the refined microstructure, in which both large elongation to failure and uniform elongation was obtained. Stress exponent was changed to 2 and activation energy was determined to be 76.7kJ/mol, indicating grain boundary sliding was the main deformation mechanism.

Effect of carbon nanotubes and high temperature extrusion on the microstructure evolution of Al-Cu alloy

CNTs/Al composites were prepared using flake powder metallurgy. The effects of CNTs and high temperature extrusion on mechanical properties and density of the material were investigated through analyzing crystal size, distribution and the change (size, chemistry and crystal structure) of precipitates and CNTs. Results show that hardness and density of the composites can be increased via extrusion and CNTs addition. Moreover, extrusion leads to refined precipitates with uniform distribution and partial dissolution compared to the as-sintered. The grains become smaller after addition of CNTs. Four types of phases distributed homogeneously in CNTs/Al composites: CNTs, Al2Cu, Al2O3 and nano-sized Al4C3. The intermetallic compound Al4C3 as interlayer is beneficial to strengthen adhesion between CNTs reinforcement and the Al matrix. The uniformly distributed dispersions of CNTs, Al2Cu, Al2O3 and Al4C3 can refine the crystalline structure, thus improving mechanical performance. Combined strengthening mechanisms are identified in the composites, containing grain boundary strengthening, dispersion strengthening and better load transfer from the Al-Cu matrix to CNTs though Al4C3.

Superplastic behavior of carbon nanotube reinforced aluminum composites fabricated by flake powder metallurgy

Superplastic deformation of a carbon nanotube reinforced aluminum alloy matrix composite (CNT/6061 Al) fabricated through Flake powder metallurgy (Flake PM) was investigated using uniaxial tensile tests under different temperatures and different strain rates. The result showed that the maximum elongation at 853K was 106% at strain rate of 4.17E−1s−1, while the maximum elongation at 673K was 89% obtained at 4.17E−1s−1. Threshold stress was evident at lower temperature but insignificant at higher temperature. Strain rate sensitivity and apparent activation energy was determined to be 0.33 and 87.3kJ/mol, respectively, indicating the grain boundary diffusion controlled mechanism was dominant for this material. With the increase of temperature, partial liquid phase appeared and the reaction of CNT was intensified, which indicated the superplastic deformation of CNT/6061 Al should be conducted at lower temperature (673K).

Surface layer characteristics of CNT/Al–Mg–Si alloy composites treated by stress peening

To determine the effect of carbon nanotubes (CNTs) on the characteristics of CNT/Al composites treated by stress peening, a 1.5wt% CNT/Al–Mg–Si alloy composite and an Al–Mg–Si alloy were fabricated via flake powder metallurgy process. The evolution of residual stress fields, surface topography, and microhardness of peened layers were explored before and after the peening treatments. The differences in microstructure, including domain size, microstrain, and dislocation density, were determined via Rietveld refinement. After stress peening, the maximum residual stress values of CNT/Al–Mg–Si composite and Al–Mg–Si alloy were improved by 101MPa, 83MPa, respectively, compared with traditional shot peening. Meanwhile, the magnitude of the residual stress of CNT/Al–Mg–Si composite was generally higher than that of Al–Mg–Si alloy. Results also reveal that pre-stress and CNTs addition could improve the compressive residual stress fields treated by stress peening. Nevertheless, in the peened layers, the domain sizes were further refined (12%) and the microstrain was increased by applying prestress and CNTs. The difference in microhardness of peened layers was consistent with the change law of surface layer characteristics.

Balanced strength and ductility in CNT/Al composites achieved by flake powder metallurgy via shift-speed ball milling

Flake powder metallurgy via shift-speed ball milling (SSBM) was proposed to combine the mechanisms of low-speed and high-speed ball milling (LSBM and HSBM) for carbon nanotube (CNT)-reinforced, strong and ductile aluminum composites. LSBM featured mild ball-to-powder collision and slow flattening of spherical Al powders into flakes, leaving enough time for CNTs to be uniformly dispersed on the surface of Al flakes and causing little damage to CNTs; HSBM featured much stronger collision and cold welding of Al flakes, leading to clustered CNTs within the welded Al particles with serious damage but better inter-flake bonding. Therefore, the coordination of CNT dispersion, integrity and interfacial bonding could be achieved by a smart powder processing of SSBM, namely a longer starting LSBM and a shorter following HSBM. As demonstrated, such SSBM resulted in comparable strength in 1.5wt.% CNT/Al composites but doubled and tripled ductility as those fabricated via LSBM and HSBM.

An investigation on wear behavior of Cu-graphite nanocomposites prepared by flake powder metallurgy

PurposeThe purpose of this study is to investigate the effect of nanographite content and the sliding time on the wear behavior of Cu-graphite nanocomposites.Design/methodology/approachCu-graphite nanocomposites were produced by flake powder metallurgy from the copper and nanographite powders in the range of 0-5 wt.% of graphite. The effect of sliding time and content of B4C particles on the wear behavior of Cu-graphite nanocomposites was analyzed.FindingsResults show that the wear mechanisms of copper and Cu-graphite nanocomposites are adhesive wear and delaminating wear, respectively. It was observed that the wear loss increased with increasing load. Wear loss of the Cu-graphite nanocomposites decreased with increase in graphite content. This was related to change in wear mechanism from adhesive to delamination wear.Originality/valueAlthough there are a wide range of applications for Cu-based materials, there is a scarcity of literatures on the wear behavior of Cu-nanographite composites that can be used for electrical contact applications.

Determining the effect of flake matrix size and Al2O3 content on microstructure and mechanical properties of Al2O3 nanoparticle reinforced Al matrix composites

ABSTRACTIn this study, Al2024 matrix composites reinforced with Al2O3 nanoparticle contents ranging from 1 to 5 wt% were produced via a new method called as flake powder metallurgy (FPM). The effect of flake size and Al2O3 nanoparticle content on the reinforcement distribution, microstructure, physical, and mechanical properties of the composites were studied. SEM analysis was performed to investigate the microstructure of metal matrix and the distribution of nanoparticles. The hot-pressed density increased with decreasing the matrix size. The hardness of the Al2024–Al2O3 nanocomposites fabricated by using fine matrix powders increased as compared to the Al2024–Al2O3 nanocomposites produced by using coarse matrix powders. It has been found that the FPM method proposed in this study revealed to be an effective method for the production of nanoparticle reinforced metal matrix composites.

Abnormal Effects of Temperature and Strain Rate on the Ductility of a Carbon Nanotubes Reinforced Al Alloy Composite

Carbon nanotubes reinforced aluminum alloy (CNTs/Al alloy) composite was fabricated by the method of flake powder metallurgy. With Gleeble-3500 system, hot compression tests at different temperatures and strain rates were conducted to investigate the effect of temperature and strain rate on the deformation behaviors of the CNTs/Al alloy composite. Experimental results show that the composite’s ductility is worse at higher deformation temperature within range of 300 oC-450 oC. Additionally, the composite’s ductility is better at higher strain rate, which is against general knowledge. The microstructure before and after deformation were characterized by SEM and TEM. It demonstrates that the grain size of the composite is always in the nanoscale. The abnormal effects of temperature and strain rate on the ductility may be explained by the evolution of work hardening capability at different deformation conditions.

Synthesis of Carbon Nanotube-Reinforced Al2024 Matrix Nanocomposite Using Flake Powder Metallurgy Method

In current work, the flake powder metallurgy method was applied to achieve the uniform dispersion of carbon nanotubes (CNTs) within the Al2024 powder. For this purpose, the flake morphology of Al2024 powder with suitable diameter-to-thickness ratio (D/t = 85) was obtained after ball milling for 4 hours at 250 rpm and ball-to-powder ratio = 10. Then, the surface of matrix was modified by a hydrophilic polymer [polyvinyl alcohol (PVA)] to obtain the sufficient –OH group on its surface. Additionally, the refluxing of CNTs in nitric acid was performed at 393 K (120 °C) for 6 hours to functionalize the reinforcement by –COOH agent. After preparation of initial materials, the Al2024–1.5 wt pct CNTs suspension was stirred in a slurry at pH 3 until the color was changed in steady state from ink-like to transparent at pH 5. The hydrogen bonding was formed between the –OH groups of PVA coated Al2024 and –COOH groups of functionalized MWCNTs during the mixing step. Also, the temporary polarity could be considered between H+ and $${{\rm{C}}_{12}}{{\rm{H}}_{25}}{\rm{SO}}_4^ -$$ ions on the surface of constituents, which led to improvement in the CNT distribution due to the changing of suspension pH. Consequently, the homogenous dispersion of CNTs in Al2024 flaky powders resulted in a chemical reaction of constituents without any destructive effects of mechanical forces. The morphological changes of Al2024 powders were studied by scanning electron microscopy (SEM), and surface treatments were evaluated by Fourier transform infrared and Raman spectroscopies. The dispersion of nanocomposite powder was investigated through field emission SEM. Also, X-ray diffraction analysis was used to investigate the initial Al2024 powder and formed phases after the ball milling process.

Fabrication of NanoSiC-Reinforced Al2024 Matrix Composites by a Novel Production Method

In this paper, flake powder metallurgy technique is described for fabricating near-net-shape SiC nanoparticle-reinforced Al2024 matrix composites. The effect of the particle size on the flake Al2024 matrix powders and the amount of SiC nanoparticles in the particle distribution, microstructure, relative density, and hardness of SiC nanoparticle-reinforced Al2024 matrix composites were investigated. The flake Al2024 matrix powders were mixed with three different as-received Al2024 powders particle sizes by ball milling. Microstructural characterization revealed a flake-like microstructure and dispersed SiC nanoparticles between the Al2024 matrix powders with grain boundaries containing sufficient area for the reinforcement particles. The hot pressed density increased as the matrix size decreased due to the small frictional forces observed between the fine flake particles. The hardness of Al2024/SiC nanocomposites increased as the size of the Al2024 matrix powders decreased.