Squeeze-casting conditions of Al/Al2O3 metal matrix composites with variations of the preform drying process

The characteristics of the preform play a role in determining the final properties of MMCs. Effects of organic binder and microwave drying on preform microstructure have been examined by SEM. In the preform with organic binder, flocking processes are observed during drying. The preform has a uniform distribution of binder and dries quickly with microwave drying owing to its internal and volumetric heating patterns. The fundamental manufacturing process and controlling parameters of squeeze casting, including preform temperature, mould temperature, applied pressure and molten metal temperature, have been studied in Al/Al₂O₃ composites. MMCs have poor mechanical properties with too high temperatures of preform and molten metal due to thermal shocking of the preform, oxidation of the matrix and thermal damage to the fibers. Mould temperature barely affects the tensile strength of MMCs. High applied pressure reduces voids and solidifies the matrix faster. Conditions for squeeze casting to achieve optimal processing, are suggested. The tensile strength of MMCs can be improved by up to about 20% compared with the unreinforced matrix alloy.     

Pressure infiltration processes to synthesize metal matrix composites – A review of metal matrix composites,the technology and process simulation

Metal matrix composites (MMCs) acquire their improved physical and mechanical properties through the careful reinforcement of their matrices by a variety of light but strong and stable reinforcement materials. The pressure infiltration process (PIP) is one of the most important techniques used for making MMCs with a high reinforcement content in which a molten metal or alloy is injected and solidified in a mold packed with continuous or discontinuous reinforcement materials. Several factors affect the quality of MMCs made by this process. These include, but are not limited to, the reinforcement type, preform geometry, applied pressure and pressure control, as well as the transport phenomena of the molten metal. This paper presents a review of the various aspects of MMCs, the process in terms of the technological details, the latest developments in the reinforcement materials used and the simulation models developed for pressure infiltration manufacturing of MMCs.     

金属基复合材料热弹塑性变分渐近细观力学模型

基于变分渐近法建立具有周期性微结构的金属基复合材料（MMCs）细观力学模型及相应的增量方程,以准确预测其典型的热弹塑性行为和初始屈服面。利用细、宏观尺度比很小的特点,对单胞变分能量泛函变化进行渐近扩展,计算得到有效瞬时弹塑性刚度矩阵和热应力矩阵;利用迭代均质化及局域化技术模拟MMCs的非线性热弹塑性性能,并通过有限元技术实现相应的数值模型。算例分析表明:该模型能较好地预测MMCs的初始屈服面,并模拟热弹塑性耦合行为,研究成果为MMCs的进一步研究和实际应用提供了技术支撑。      

Asymmetric behaviour of fibrous metal matrix composites

Abstract

Experimental investigations have illustrated that unidirectional metal matrix composites (MMCs) show asymmetric behaviour under uniaxial tensile and compressive loading. This asymmetry occurs when the material is loaded along the fibre direction and also when loaded in the transverse direction. In this paper, results from finite element micromechanical models are presented. The models were used to study the asymmetric behaviour of unidirectional fibre reinforced MMCs subjected to longitudinal and transverse loading. The effects of the thermal residual stresses arising from the manufacturing process were included in the study. Also, the influence of the degree of bonding of fibre to matrix was examined, from perfectly bonded to completely debonded. Results reveal that thermal residual stresses are responsible for the asymmetric behaviour of the MMCs in the longitudinal direction. In transverse loading, both the degree of interface bonding and residual stresses account for the asymmetric behaviour. The predicted stress–strain response of the MMC shows good agreement with the available experimental data for both tènsile and compressive loading. Results also suggest that in order to predict accurately the yielding behaviour of MMCs, the current symmetric yield criteria require modification.     

Size dependent strengthening mechanisms in carbon nanotube reinforced metal matrix composites

The matrix grain size plays a dual role in metal matrix composites (MMCs). Contrary to enhance the strength of matrix, grain refinement can weaken the thermal expansion mismatch strengthening induced by the reinforcement. In this article, a dislocation density based model is developed to describe the factors affecting the strengthening mechanisms in Carbon nanotube (CNT)-reinforced MMCs with different matrix grain sizes. Two kinds of thermal expansion mismatch strengthening mechanisms are considered, i.e., geometrically necessary dislocations (GNDs) are distributed in entire matrix and GNDs are limited in dislocation punched zones (DPZs). In addition, comparisons between the predictions and some available experimental results are also performed.     

Improvement of the bonding interface in hybrid fiber/particle preform reinforced Al matrix composite

The bonding interface between the reinforcement and the matrix alloy in hybrid AZS fiber/SiC particle preform based aluminum metal matrix composites (Al MMCs) has been investigated as a function of reinforced particle size and the binder content. It is observed that high binder and large particle will result in a poor bonding interface. This has deleterious effects on the mechanical properties of the cast MMCs. Estimation of the binder thickness indicates that there exists a critical particle size above which the particles are not appropriate to be used in fabricating the hybrid fiber/particle preform based MMCs.     

Grey-fuzzy algorithm to optimise machining parameters in drilling of hybrid metal matrix composites

Metal matrix composites (MMCs) are difficult to machine due to their abrasive properties. With the projected widespread application of MMCs, it is necessary to develop an appropriate technology for their effective machining. The present investigation focuses on finding the optimal machining parameters setting in drilling of hybrid aluminium metal matrix composites using the grey-fuzzy algorithm. This proposed algorithm, coupling the grey relational analysis with the fuzzy logic, obtains a grey-fuzzy reasoning grade to evaluate the multiple performance characteristics according to the grey relational coefficient of each performance characteristics. The Taguchi method of experimental design is a widely accepted technique used for producing high quality products at low cost, therefore a L₂₇ 3-level orthogonal array is used for the experiments. The optimisation of multiple responses in complex processes is common; therefore, to reduce the degree of uncertainty during the decision making, fuzzy rule-based reasoning is integrated with the Taguchi’s method. The response table, response graph and analysis of variance (ANOVA) are used to find the optimal setting and the influence of machining parameters on the multiple performance characteristics. Experimental results have shown that the required performance characteristics in the drilling process are improved by using this approach.     

Void formation in metal matrix composites by solidification and shrinkage of an AlSi7 matrix between densely packed particles

Particle reinforced metals are developed as heat sink materials for advanced thermal management applications. Metal matrix composites combine the high thermal conductivity of a metal with a low coefficient of thermal expansion of ceramic reinforcements. SiC and carbon diamond particle reinforced aluminum offer suitable thermal properties for heat sink applications. These composites are produced by liquid metal infiltration of a densely packed particle preform. Wettability, interface bonding strength and thermal mismatch are critical for void formation which leads to thermal fatigue damage under operation. The evolution of voids in AlSiC and AlCD has been studied by in-situ high resolution synchrotron tomography during matrix solidification. Large irregularly shaped matrix voids form during eutectic solidification. These voids help alleviate thermal expansion mismatch stresses by visco-plastic matrix deformation during cooling to RT after solidification, if sufficient interface bonding strength is assumed.     

Thermal residual stresses in metal matrix composites: A review

Recently, metal matrix composites (MMCs) have generated a considerable interest in the materials field because of their attractive physical and mechanical properties. However, during the fabrication of MMCs, thermal residual stresses are reportedly developed in the matrix as a result of the mismatch of the thermal expansion coefficients between the reinforcement and the matrix. It is well established that these residual stresses have a significant effect on the composite properties. For example, due to the presence of thermal residual stresses, it is almost never possible to achieve the maximum elastic response of the composites. In addition, yield stress and fracture toughness of the composites are significantly affected by thermal residual stresses. In this paper, a critical review of the published literature on thermal residual stresses in MMCs and their effect on composite properties are presented. Also, experimental and numerical techniques that are currently available to measure and estimate thermal residual stresses are reviewed and discussed.     

Simulation of void formation in liquid composite molding processes

Air entrapment within and between fiber tows during preform permeation in liquid composite molding (LCM) processes leads to undesirable quality in the resulting composite material with defects such as discontinuous material properties, failure zones, and visual flaws. Essential to designing processing conditions for void-free filling is the development of an accurate prediction of local air entrapment locations as the resin permeates the preform. To this end, the study presents a numerical simulation of the infiltrating dual-scale resin flow through the actual architecture of plain weave fibrous preforms accounting for the capillary effects within the fiber bundles. The numerical simulations consider two-dimensional cross sections and full three-dimensional representations of the preform to investigate the relative size and location of entrapped voids for a wide range of flow, preform geometry, and resin material properties. Based on the studies, a generalized paradigm is presented for predicting the void content as a function of the Capillary and Reynolds numbers governing the materials and processing. Optimum conditions for minimizing air entrapment during processing are also presented and discussed.     

Modeling of oxidation in metal matrix composites

Oxidation in metal matrix composites (MMCs) is modeled by Fickian diffusion of oxygen in both the oxide layer and metal matrix. The oxidation process and the resulting immobilized oxygen at the interface is accounted for by the introduction of a jump discontinuity in the oxygen concentration across the interface. The problems of a uniformly growing oxide layer from the surfaces of a semi-infinite solid, outward oxygen diffusion and oxidation from a cylindrical cavity, as well as inward oxygen diffusion and oxidation from the surface of a cylinder, are considered as benchmark problems in oxidation of MMCs. In addition to the modeling results, a series of experimental results of oxidation of a unidirectional SiC/Ti matrix MMC are presented. The measured oxide thicknesses at different times and temperatures are used to calibrate and validate the model under development. While the present paper deals only with the oxidation problem, the coupled oxidation—thermomechanical problem, and especially the influence of oxidation on fatigue life of MMCs, will be presented in a future communication.     

Thermal stress relaxation in magnesium matrix composites controlled by dislocation breakaway

In metal matrix composites (MMCs) thermal stress relaxation can be achieved either by interface debonding, crack propagation or by dislocation motion. The present paper shows that in the case of magnesium matrix, interface thermal stresses are relaxed by dislocation motion. Moreover the results obtained by mechanical spectroscopy prove that this dislocation motion is controlled by a solid friction mechanism, which is not thermally activated. This point is very interesting for the development of MMCs, which exhibit a high damping capacity over a wide frequency range. Dislocation hysteretic motion in the magnesium matrix is evidenced by the dependence of the mechanical loss on the stress amplitude. The obtained relationship obeys perfectly to the Granato–Lücke model for dislocation breakaway.     

On the impact toughness of Al-15 vol.% B4C metal matrix composites

The present work was performed on ten metal matrix composites (MMCs) produced using the new powder injection technique. These MMCs were divided into two series in which pure aluminum was the matrix for one series, while an experimental 6063 alloy was the matrix for the second series. Small amounts of Ti, Zr and Sc were added to those composites, either individually or combined. In all cases the volume fraction of the reinforced B₄C particles was in the range 12–15 vol. %. The molten metal was cast in an L-shaped metallic mold preheated at 350°C. Unnotched rectangular impact samples (1 cm × 1 cm × 5 cm) were prepared from these castings and heat treated. Samples were tested using instrumental impact testing machine. Microstructure and fracture surface were examined using Hitachi SU-8000 FESEM. The results show that the presence of Ti improves the wettability of the B₄C particles and their adherence to the matrix. Repeated remelting at 730 °C applying vigorous mechanical stirring could lead to fragmentation of some of the B₄C particles. Aluminum based composites exhibited better toughness compared to those obtained from 6063 based composites in all the studied conditions. The composite impact toughness was controlled by the precipitation and coarsening of hardening phase particles namely Mg₂Si, Al₃Zr and/or Al₃Sc. Cracks in the fracture surface were observed to be initiated at the particle/matrix interfaces and propagate either through the B₄C particles or through the protective layers. No complete debonding was reported due the presence of Zr/Ti/Sc rich layers which improved the particle/matrix adhesion.     

Design with metal matrix composites

Abstract

Applications for metal matrix composites (MMCs) have not emerged at the rate needed to justify the development costs. A reason for this may be that material developments have not been adequately linked to identified commercial needs. It is certainly true that some of the expectations raised about the potential offered by MMCs have been misguided. As the MMC business contracts, there is an ever greater need for a systematic method of linking material properties to the needs of engineering designers. This paper presents a methodology for evaluating materials in design, with the aim of linking MMCs to applications. The methodology has two main components: first, the use of performance indices and materials selection charts for specific design goals, to compare existing MMCs with competing materials; and secondly, the conceptual design of new MMC systems guided by those design goals. A selection of case studies illustrates that in mechanical applications the gains in using MMCs are frequently marginal, whereas in design for thermal management and vibration control, the materials can show very substantial improvements in performance. The methodology is general, and could be applied to other material systems.

MST/3094     

Particulate reinforced metal matrix composites — a review

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.     

Dislocation-induced damping in metal matrix composites

The damping response of crystalline metals and alloys is generally associated with the presence of defects in the crystal lattice. The disturbance of these defects, usually in response to an applied cyclic load, dissipates energy, a mechanism known as internal friction. The various defects commonly found in crystalline materials include point defects (e.g. vacancies), line defects (e.g. dislocations), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions). Among these, dislocations are noteworthy because they play a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behaviour of the materials. Among the various structural materials actively being developed, metal matrix composites (MMCs) have received considerable attention as a result of their potential to combine reinforcement properties of strength and environmental resistance, with matrix properties of ductility and toughness. Of interest is the generally observed phenomenon that MMCs exhibit unusually high concentrations of dislocations, an observation typically attributed to the difference in coefficient of thermal expansion between matrix and reinforcement. The objectives of the present paper are to provide an overview of the sources of dislocation generation in MMCs, and to provide insight into the effects that dislocations have on the damping response of MMCs. The presence of dislocations in MMCs is highlighted on the basis of transmission electron microscopy studies, and the dislocation damping mechanisms are discussed in light of the Granato-Lücke theory.     

Prediction of the failure properties of short fiber reinforced composites with metal and polymer matrix

The statistical strength of short fiber reinforced composites such as metal matrix composites (MMCs) and polymer matrix composites (PMCs) with different fiber volume fractions is investigated in this work using combined cell models (CCM) and Weibull statistical approach. Statistical combined cell models (SCCM) have been developed, which were originally developed for fiber fracture in composites. This allows to calculate separately the two types of unit cells, i.e. unit cells with unbroken fibers, and with broken ones. The global mechanical behavior of metal or PMCs reinforced with randomly oriented short fibers (e.g., an Al/15 vol.% Al₂O₃ aluminium matrix composite and PMC with 20 or 30 vol.% glass-fibers) is calculated based on the rule of mixture. In all cases, predictions of the behavior by the SCCM are compared with experiments and they show good agreement.     

Stiffness of particulate reinforced metal matrix composites with damaged reinforcements

Abstract

During tensile plastic deformation particulate reinforced metal matrix composites (MMCs) undergo reinforcement damage and a parallel reduction in stiffness. An analytical model is developed to calculate this stiffness reduction using the equivalent inclusion technique proposed by Eshelby. The model considers both damaged and undamaged reinforcement particles as ellipsoidal inclusions but with different stiffness tensors. The effect of the aspect ratio of the reinforcing particles has been accounted for in the model. The model is very flexible and can meet different specific damage situations by designing a suitable stiffness tensor for the damaged reinforcements. Finite element analysis is used to modify a numerical stiffness tensor for cracked reinforcement particles. The model is compared with an earlier model of modulus reduction in MMC materials and with a few experimental measurements made on a 15 vol.-%SiC particulate reinforced aluminium alloy 2618 MMC.     

Thermal expansion characteristics of cast Cu based metal matrix composites

Like any other metal/alloy, copper and its alloys also soften at elevated temperatures. Reinforcing with ceramic or carbon fibres is one of the suggested solutions to overcome this. Very limited literature is available on Cu based metal matrix composites (MMCs); none of these pertain to liquid phase fabrication. Hence, a systematic investigation was carried out on MMCs based on copper, with alumino-silicate fibres and carbon fibres as reinforcements. The MMCs thus produced exhibit a uniform distribution of reinforcement in the matrix. Coefficient of thermal expansion (CTE) values are lower than that of pure copper.     

Metal matrix composites for aeroengines

Abstract

Metal matrix composites (MMCs) will play a significant role in the future of gas turbine aeroengine development. This paper outlines the benefits and some of the potential applications for Al and Ti MMCs and discusses issues involved in the introduction of this relatively new class of composite materials into engine components. The potential for cost savings and performance improvements which may be achieved by the introduction of silicon carbide particulate reinforced Al alloys (Al–SiC_p MMC)are discussed, where the properties of the composite can be tailored to meet the requirements of a specific application. Some of the processing implications for Ti matrix composites (Ti MMCs) are explained for a range of component applications. Illustrations are given of how the manufacturing process can be controlled for a complex component. Finally, the influence of raw material, manufacturing, and component costs on the successful introduction of MMCs into aeroengines is discussed.