Showing papers on "Metal matrix composite published in 2009"

Fabrication and properties of dispersed carbon nanotube–aluminum composites

Abstract: Powder metallurgy techniques have emerged as promising routes for the fabrication of carbon nanotube (CNT) reinforced metal matrix composites. In this work, planetary ball milling was used to disperse 2 wt% MWCNT in aluminum (Al) powder. Despite the success of ball milling in dispersing CNTs in Al powder, it is often accompanied with considerable strain hardening of the Al powder, which may have implications on the final properties of the composite. Both un-annealed and annealed Al–2 wt% CNT composites were investigated. It was found that, ball-milled and extruded (un-annealed) samples of Al–2 wt% CNT demonstrated high notch-sensitivity and consistently fractured outside the gauge length during tensile testing. In contrast, extruded samples annealed at 400 and at 500 °C for 10 h prior to testing, exhibited more ductile behavior and no notch sensitivity. Under the present investigated processing conditions, ball milling for 3 h followed by hot extrusion and annealing at 500 °C resulted in enhancements of around 21% in tensile strength compared with pure aluminum with the same process history. The ball-milling conditions used were found to result in the creation of a nanostructure in all samples produced, as shown by XRD and TEM analysis. Such nanostructure was retained after prolonged exposures to temperatures up to 500 °C. The tensile testing fracture surfaces showed uniform dispersion and alignment of the CNTs in the aluminum matrix but also showed CNTs acting as nucleation sites for void formation during tensile testing. This has contributed to the observation of CNT pull-out due to the poor bond between the CNTs and the matrix.

Development of high-performance A356/nano-Al2O3 composites

Abstract: In this study, a new method was used in stir casting to fabricate nano-Al2O3 particulate reinforced aluminum composites and avoid agglomeration and segregation of particles. Different volume fractions of nano-alumina particles were incorporated into the A356 aluminum alloy by a mechanical stirrer and then cylindrical specimens were cast and tested. The microstructural characterization of the composite samples showed uniform distribution of reinforcement, grain refinement of aluminum matrix, and presence of the minimal porosity. The effects of nano-Al2O3 particle content on the mechanical properties of the composites were investigated. Based on experiments, it was revealed that the presence of nano-Al2O3 reinforcement led to significant improvement in hardness, 0.2% yield strength, UTS and ductility. This combination of enhancement in UTS and ductility exhibited by nano-Al2O3 reinforced aluminum is due to uniform distribution of reinforcement and grain refinement of aluminum matrix.

The use of Al–Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Abstract: Pure Al and 6061 aluminium alloy based Al2O3 particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al2O3 within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.

Synthesis of multi-walled CNT reinforced aluminium alloy composite via friction stir processing

Abstract: Friction stir processing is used to produce an aluminium alloy reinforced with multi-walled carbon nanotubes. Microscopy by SEM and TEM indicates that the nanotubes are embedded into Al-alloy matrix produced in the stir zone, and their multi-walled microstructure survived the thermo-mechanical conditions imposed during processing. Increasing the tool rotation speed from 1500 and 2500 rpm and increasing the tool shoulder penetration depth improved homogeneity of nanotubes in the Al-alloy matrix, however a fully uniform distribution could not be achieved when regularly tangled nanotubes were used.

Microstructural and mechanical characterization of Al–MWCNT composites produced by mechanical milling

Abstract: Novel Al-based nanocomposites reinforced with multi-walled carbon nanotubes were produced by mechanical milling. Next, pressure-less sintering at 823 K under vacuum and hot extrusion at 773 K were carried out. The interface between Al matrix and the multi-walled carbon nanotubes was examined using transmission electron microscopy. The values of yield strength (σy), maximum strength (σmax) and microhardness Vickers (HVN) of the composites were evaluated and reported as a function of carbon nanotubes content. The concentration of multi-walled carbon nanotubes has an important effect on the mechanical properties of the nanocomposite. Formation of aluminum carbide in the nanocomposites was observed. Possible strengthening mechanisms are presented and discussed.

Microstructure and mechanical properties of Ni–P coated Si3N4 reinforced Al6061 composites

Abstract: Silicon nitride is currently a popular choice as reinforcement to develop light alloy composites owing to its high hot hardness and excellent wear and corrosion resistance. However, meager information is available as regards the development of aluminum alloy–silicon nitride composites by stir cast technique. This route of processing metal matrix composites (MMCs) poses challenges in particular, poor wettability of the ceramic reinforcement in the matrix alloy. To overcome this problem, researchers are currently focusing on use of metallic coated ceramic reinforcement. In the light of the above, this paper discusses the development of Ni–P coated silicon nitride reinforced Al6061 composites by stir cast method. XRD, metallographic, EDAX studies, microhardness, and tensile strength tests of the developed composites have been carried out. A maximum of 10 wt% of Ni–P coated silicon nitride has been dispersed uniformly in the matrix alloy. A drastic improvement in both microhardness and tensile strength is observed.

Microstructure and properties of hard and wear resistant MMC coatings deposited by laser cladding

Abstract: Laser coating (laser cladding) is a useful method to produce metal matrix composite (MMC) coatings. The selection of the metallic matrix can be done on the base of the intended application and environment, whereas the additional wear resistance improvement is provided by mixing various carbides with the metallic matrix. In the present work, various types of MMC coatings were prepared by laser coating methods. The coatings consisted of vanadium, tungsten, titanium and chromium carbide hard phases mixed with metallic tool steel M2, Stellite 21, NiCrBSi-alloy and Inconel 625. Different levels of carbide contents were used. The abrasion resistance of the MMC coatings has been tested using a rubber-wheel abrasion apparatus. The wear surfaces were examined and the microstructures of MMC coatings were analysed in order to determine microstructures and carbide dissolution. The best abrasion results were achieved by the correct choice of carbide for each matrix material.

Structural characterization of a mechanically milled carbon nanotube/aluminum mixture

Abstract: The structural evolution of carbon nanotubes (CNTs) during mechanical milling was investigated using SEM, TEM, XRD, XPS and Raman spectroscopy. The study showed that milling of the CNTs alone introduces defects but preserves the tubular structure. When milling the CNTs with aluminum (Al) powder in order to produce a composite, Raman spectroscopy has shown that most of the nanotubes are destroyed. During sintering of the CNT/Al milled mixture, the carbon atoms available from the destruction of the nanotubes react with the Al to form aluminum carbide (Al4C3). The effect of milling on the Al matrix was also studied.

Microstructure and mechanical behavior of die casting AZ91D-Fly ash cenosphere composites

Abstract: The feasibility of incorporating fly ash cenospheres in die cast magnesium alloy has been demonstrated. The effects of fly ash cenosphere additions on the microstructure and some of the salient physical and mechanical properties of magnesium alloy (AZ91D) metal matrix composites were investigated. The control AZ91D alloy and associated composites, containing 5, 10, and 15 wt.% of fly ash cenospheres (added), were synthesized using a die casting technique. A microstructural comparison showed that microstructural refinement – occurred due to the fly ash additions and became more pronounced with an increase in the percentage of the fly ash added. The metal matrix areas nearer to the fly ash particles exhibited a greater degree of refinement than was observed in the areas further away from these particles. Both filled and unfilled fly ash cenospheres, and porosity were observed in the composite microstructures. The composite specimen densities decreased and the coefficient of thermal expansion did not change significantly as the volume percent of fly ash was increased within the range investigated. The hardness values of the composite specimens exhibited an increase in proportion to the increase in percentage of added fly ash. The tensile strength of the composites also increased as the concentration of fly ash cenospheres was increased. In contrast, the Young’s modulus of these composite samples, as measured by non-destructive pulse-echo method, decreased as the percentage of fly ash in the composite was increased. SEM micrographs of the tensile fracture surfaces showed broken cenospheres on the fracture surface and evidence of ‘pull outs’, where fly ash particles were previously embedded in the matrix. Compression testing results showed that the presence of 5 wt.% cenospheres decreased the compressive strength and compressive yield strength of the composite relative to that of the AZ91D matrix alloy. Surprisingly, a significant change in compression strength was not observed for the composites with 10 and 15 wt.% cenospheres in comparison to the AZ91D matrix alloy. In contrast to the tensile tests, no cenosphere remnants were observed on the compressive test fracture surface of the composites. This observation suggests that the fracture of the composite was initiated within the AZ91D matrix by normal void nucleation and growth, followed by crack propagation through the matrix, avoiding any of the cenospheres, leading to composite fracture of the matrix.

High-temperature mechanical properties of aluminium alloys reinforced with boron carbide particles

Abstract: The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys (6061 and 7015) at high temperatures were studied. Boron carbide particles were used as reinforcement. All composites were produced by hot extrusion. The tensile properties and fracture analysis of these materials were investigated at room temperature and at high temperature to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy.

Development and property evaluation of aluminum alloy reinforced with nano-ZrO2 metal matrix composites (NMMCs)

Abstract: In the present research, nano-ZrO2 particulates were dispersed in aluminum alloy (LM 13) by melt deposition technique followed by hot extrusion. The size of the particles dispersed varies from 50 to 80 nm and amount of addition varies from 3 to 15 wt.% in steps of 3%. Microstructural studies of the nano-composite developed indicate that there is uniform distribution of the reinforcement in the matrix alloy with significant grain refinement and retention of residual porosity. Mechanical properties reveal that the presence of nano-ZrO2 particulates has improved significantly the strength and hardness with slight reduction in ductility as compared against the matrix alloy. Fractography of the specimens showed that the fracture behavior of matrix alloy has changed from ductile intergranular mode to cleavage mode of fracture. Results of the thermal and electrical tests on the composite developed indicate that, thermal conductivity and electrical resistance both decreased with increase in reinforcement content.

Dual strengthening mechanisms induced by carbon nanotubes in roll bonded aluminum composites

Abstract: The dual role of carbon nanotubes (CNTs) in strengthening roll bonded aluminum composites has been elucidated in this study. An increase in the elastic modulus by 59% has been observed at 2 vol.% CNT addition in aluminum, whereas tensile strength increases by 250% with 9.5 vol.% CNT addition. CNTs play a dual role in the strengthening mechanism in Al–CNT composite foil, which can be correlated to the degree of dispersion of CNTs in the matrix. Better CNT dispersion leads to improvement of elastic properties. In contrast, CNT clusters in the aluminum matrix impede dislocation motion, causing strain hardening and thus improvement in the tensile strength. Dislocation density of the composites has been computed as a function of CNT content to show the effect on strain hardening of the metal matrix–CNT composite.

Processing and characterization of MWCNT reinforced aluminum matrix composites

Abstract: Metal matrix composites comprising aluminum matrix and multi-wall carbon nanotubes (MWCNTs) as reinforcements are fabricated using cold uniaxial compaction followed by sintering and cold extrusion as secondary processes. The MWCNTs are pretreated with sodium dodecyl sulfate for improved adhesion with aluminum powder. The effect of sintering temperature on the microstructure is explored using differential scanning calorimetric spectrum. The tensile yield and ultimate strength of Al-MWCNTs increased to 90% with 2 wt% addition of MWCNTs. Various theories for the strengthening and stiffening of Al-MWCNTs composites are explored.

Design, Fabrication, and Properties of High Damping Metal Matrix Composites—A Review

Abstract: Nowadays it is commonly considered that high damping materials which have both the good mechanical properties as structural materials and the high damping capacity for vibration damping are the most direct vibration damping solution. In metals and alloys however, exhibiting simultaneously high damping capacity and good mechanical properties has been noted to be normally incompatible because the microscopic mechanisms responsible for internal friction (namely damping capacity) are dependent upon the parameters that control mechanical strength. To achieve a compromise, one of the most important methods is to develop two-phase composites, in which each phase plays a specific role: damping or mechanical strength. In this review, we have summarized the development of the design concept of high damping composite materials and the investigation of their fabrication and properties, including mechanical and damping properties, and suggested a new design concept of high damping composite materials where the hard ceramic additives exhibit high damping capacity at room temperature owing to the stress-induced reorientation of high density point defects in the ceramic phases and the high damping capacity of the composite comes mainly from the ceramic phases.