Abstract: In the last two decades, metal matrix nanocomposites have witnessed tremendous growth. Particulate-reinforced nanocomposites have been extensively employed in the automotive industry for their capability to withstand high temperature and pressure conditions. Several manufacturing approaches have been used to fabricate them. Non-homogeneous particle dispersion and poor interface bonding are the main drawbacks of conventional manufacturing techniques. A critical review of nanocomposite manufacturing processes is presented; the distinction between ex-situ and in-situ processes is discussed in some detail. Moreover, in-situ gas/liquid processes are elaborated and their advantages are discussed. The thermodynamics and kinetics of the reaction between the precursor gas and the liquid metal have been analyzed and their role on particle formation studied. This critical review will provide the reader with an overview of nanocomposite manufacturing methods along with a clear understanding of advantages and disadvantages.
Abstract: The paper reports the results of an extensive characterization of the Ti6Al4V-SiCf composite produced by hot isostatic pressing (HIP) to assess its capability to withstand the in-service conditions of turbine blades operating at middle temperatures in aeronautical engines. The microstructure of composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 673-873 K, has been investigated by means of different techniques. Particular attention was paid to the micro-chemical evolution of fibre-matrix interface which is scarcely affected also by the most severe heat treatments examined here. This leads to stable mechanical properties as evidenced by hardness, tensile and FIMEC instrumented indentation tests. Therefore, the composite can operate at the maximum temperature (873 K) foreseen for its aeronautical applications without remarkable modifications of its microstructure and degradation of mechanical properties. The mechanical characterization has been completed by internal friction and dynamic modulus measurements carried out both at constant and increasing temperature, from 80 to 1173 K.
Abstract: A model based on the geometry of the phases is introduced in order to investigate the mechanical properties of interpenetrating microstructures. In order to characterize the elastic and elastic-plastic properties of the composite a self consistent unit cell model is applied on a wide range of volume fractions for an Al/TiO2 composite. Besides the volume fraction a microstructural based parameter is used, the matricity, to describe the mutual circumvention of both phases. Computations are carried out for different temperatures and void volume fractions. In addition a conservative fracture criterion based on critical normal stresses is applied to derive realistic stress strain curves.
Abstract: The present study focuses on the influence of the PEO (Plasma Electrolytic Oxidation) treatment on the tribological behaviour of the AA2618/20 % vol. Al2O3p composite, dry sliding against induction hardened UNI C55 steel. Particle-reinforced Al based composites offer a higher wear resistance by comparison with the corresponding unreinforced alloys, however, the presence of critical loads and/or velocities which lead to transition towards severe wear regime, was often observed. In such conditions, the composite can show higher wear rates than those of unreinforced alloys. For this reason, surface modifications, such as PEO, might contribute to improve wear resistance. In this paper, topography, microstructure, phase constitution and surface hardness of the PEO-treated composite were investigated and its tribological behaviour was studied by dry sliding tests using a block-on-ring tribometer. The results were compared with those from the uncoated composite, demonstrating a very positive effect of the PEO treatment, which moved transitions from mild to severe wear towards more severe test conditions, in terms of both load and velocity.
Abstract: Magnesium alloys containing rare earth elements are known to have high specific strength and corrosion resistance. The addition of SiC ceramic particles makes the metal matrix composite stronger with better wear and creep resistance and a still good machinability. The role of the reinforcement particles to the enhanced strength can be quantitatively evaluated using transmission electron microscopy (TEM). This paper presents a quantitative strengthening evaluation in a SiC Mg-RE composite alloy. The different contributions were determined by TEM inspections. The microstructure strengthening mechanism was studied after room temperature compression specimens. The way of combining the different contributions and the comparison to the measured yield stress, is also discussed and justified.
Abstract: The widespread use of metal matrix composites (MMC) is often limited due to the difficulties related to their joining by means of traditional fusion welding processes. The aim of this work was to evaluate the effect on microstructure and mechanical properties (hardness and tensile strength) of two different friction welding techniques used for joining two Al-based metal matrix composites. In particular, Friction Stir Welding was applied to a 6061 (Al-Mg-Si) alloy matrix, reinforced with 20vol.% of Al2O3 particles (W6A20A), while Linear Friction Welding was applied to a 2124 (Al-Cu-Mg) alloy matrix reinforced with 25vol.% of SiC particles (AMC225xe). Both the welding processes permitted to obtain substantially defect-free joints, whose microstructures was found to be dependent on both the initial microstructure of the composites and the welding processes. Hardness decrease was in the order of 40% for the FSW joint and of 10% for the LFW joint, mainly due to overaging of the matrix induced by the frictional heating, while the joint efficiency in respect to the ultimate tensile strength was 72% and 82%, respectively. Elongation to failure increased in the FSW joint due to coarsening of precipitates, whereas it decreased in the LFW joints due to the fibrosity in the thermomechanically altered zone. Fracture surface analysis showed good matrix/reinforcement interface for both composites.
Abstract: The aim of this paper is to investigate the behaviour in terms of drilling forces and roughness of Metal Matrix Composites (MMC) in hot drilling machining. In particular, Al2009/(SiC)w, Al6061/(SiC)w, and Al6061(Al2O3)p metal matrix composites were used, and the adopted temperature were in the range 20°C-160°C. A comparison with drilling at room temperature has been discussed. The results have shown the sensible influence of the working temperature on drilling forces and on surface material properties. In the case of Al2009/(SiC)w a minimum in the drilling forces has been found, making possible the dry machining and improving the cutting conditions. Instead, for Al6061/(SiC)W and Al6061(Al2O3)p in the used temperature range no minimum appears.
Abstract: In this work the microstructural evolution of an A360 alloy reinforced with 10vol.% SiC particulate is described. During the material solidification, mechanical vibration, in the range of 0-41 times the gravity acceleration, g, has been applied to a steel die. It has been observed that vibrations can promote a quite homogeneous SiC dispersion on macroscopic scale. On the other hand, by using too high vibrations’ intensity, segregation phenomena have been pointed out in the castings. Furthermore, it has been evidenced that the reinforcement distribution is influenced by mechanical entrapment of the particles at grain boundaries and in the interdendritic channel. The metallographic analysis has emphasized a finer microstructure with increasing vibrations’ intensity. By comparing simulated and experimental temperature curves of the mould in the different cases, different HTC made the best fit. By increasing the vibrations’ intensity, the HTC increases in the temperature range of solidification of the composite.
Abstract: Aluminum nitride (AlN) possesses superior thermal and electrical properties and is an ideal candidate for high-temperature, as well as for packaging and optoelectronic applications. Aluminum based composites reinforced with AlN have been manufactured via an in situ gas-assisted process, where a nitrogen-bearing gas is injected in the molten aluminum at 1273-1323 K. The process is carried out in an inert atmosphere in order to avoid oxygen contamination. Addition of Mg lowered the oxygen content in the melt by forming MgO and thus favoring the nitridation reaction. Particle size formed in the matrix varied from 1- 3 μm to sub-micron scale depending on the gas injection time. Longer bubbling times give rise to improved reinforcement dispersion. Addition of Si is detrimental for the synthesis of AlN; Mg2Si phase precipitates, replacing the formation of MgO and hindering the nitridation reaction. The challenges of controlling the kinetics are discussed.