Effects of reinforcement distribution on low and high temperature tensile properties of Al356/SiCp cast composites produced by a novel reinforcement dispersion technique

A major challenge for full utilization of the potentials of SiC_p reinforced metal matrix composites is the uniform dispersion of very fine SiC particles in the matrix alloys. In this study, a novel method for gradual in situ release of properly wetted SiC particles with average size of less than 3 μm in the liquid metal was employed which greatly overcame this challenge. SiC particles were injected into the melt in three different forms, i.e., untreated SiC_p, milled particulate Al-SiC_p composite powder, and milled particulate Al-SiC_p-Mg composite powder. The resultant composite slurries were then cast in either a fully liquid state (stir casting) or semisolid state (compocasting). Subsequently, the effects of the type of the injected powder and the casting method on the microstructural and mechanical characteristics of the cast composites at room temperature and 300 °C were investigated. The results demonstrated that distribution of SiC particles in the matrix were greatly improved by injecting milled composite powders instead of untreated SiC particles into the melt. Also casting the composite slurries in a semisolid state instead of fully liquid state slightly improved the distribution. The ultimate tensile strength, yield strength and elongation at room temperature of Al356/5 vol.% SiC_p composite manufactured by compocasting of the (Al-SiC_p-Mg)_cp-injected melt were increased by 113%, 90% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiC_p injected melt. The improvements in these properties at 300 °C were about 100%, 103% and 129%, respectively. Almost all the composite samples retained more than 90% of their strengths at 300 °C, whereas the monolithic samples lost more than 25% of their strength at this temperature. The composites manufactured by compocasting of (Al-SiC_p-Mg)_cp-injected melts exhibited a typical ductile fracture surface with equiaxed dimples at both room temperature and 300 °C.     

Pressure infiltration casting process and thermophysical properties of high volume fraction SiCp/Al metal matrix composites

Abstract

SiC_p/Al composites containing high volume fraction SiC particles were fabricated using a pressure infiltration casting process, and their thermophysical properties, such as thermal conductivity and coefficient of thermal expansion (CTE), were characterised. High volume fraction SiC particulate preforms containing 50–70 vol.-%SiC particles were fabricated by ball milling and a pressing process, controlling the size of SiC particles and contents of an inorganic binder. 50–70 vol.-%SiC_p/Al composites were fabricated by high pressure infiltration casting an Al melt into the SiC particulate preforms. Complete infiltration of the Al melt into SiC preform was successfully achieved through the optimisation of process parameters, such as temperature of Al melt, preheat temperature of preform, and infiltration pressure and infiltration time after pouring. Microstructures of 50–70 vol.-%SiC_p/Al composites showed that pores resided preferentially at interfaces between the SiC particles and Al matrix with increasing volume fraction of SiC particles. The measured coefficients of thermal expansion of SiC_p/Al composites were in good agreement with the estimated values based on Turner's model. The measured thermal conductivity of SiC_p/Al composites agreed well with estimated values based on the 'rule of mixture' up to 70 vol.-% of SiC particles, while they were lower than the estimated values above 70 vol.-% of SiC particles, mainly due to the residual pores at SiC/Al interfaces. The high volume fraction SiC_p/Al composite is a good candidate material to substitute for conventional thermal management materials in advanced electronic packages due to their tailorable thermophysical properties.     

Development of Al356/SiCp cast composites by injection of SiCp containing composite powders

One of the great challenges of producing cast metal matrix composites is the agglomeration tendency of the reinforcements. This would normally result in poor distribution of the particles, high porosity content, and low mechanical properties. In the present work, a new method for uniform distribution of very fine SiC particles with average size of less than 3 μm was employed. The key idea was to allow for gradual in situ release of properly wetted SiC particles in the liquid metal. For this purpose, SiC particles were injected into the melt in three different forms, i.e., untreated SiC_p, milled particulate Al–SiC_p composite powder, and milled particulate Al–SiC_p–Mg composite powder. The resultant composite slurries were then cast from either fully liquid (stir casting) or semisolid (compocasting) state. Consequently, the effects of the casting method and the type of the injected powder on the microstructural characteristics as well as the mechanical properties of the cast composites were investigated. The results showed that the distribution of SiC particles in the matrix and the porosity content of the composites were greatly improved by injecting milled composite powders instead of untreated-SiC particles into the melt. Casting from semisolid state instead of fully liquid state had similar effects. The average size of SiC particles incorporated into the matrix was also significantly reduced from about 8 to 3 μm by injecting milled composite powders. The ultimate tensile strength, yield strength and elongation of Al356/5 vol.%SiC_p composite manufactured by compocasting of the (Al–SiC_p–Mg)_cp injected melt were increased by 90%, 103% and 135%, respectively, compared to those of the composite manufactured by stir casting of the untreated-SiC_p injected melt.     

Incorporation of new technique for processing of Al/SiCp composites based on evaporative pattern casting (EPC) method

Abstract

A336 Al matrix composites containing different volume fraction and mean mass particle size of SiC particles as the reinforcing phase were synthesised by evaporative pattern casting (EPC) route. The process consisted of fabricating of EPS/SiC_p composite pattern followed by EPC of A336 Al alloy. The EPS/SiC_p pattern was made by blending SiC particles with expandable polystyrene (EPS) beads and placing them in expanding mould heating with steam until EPS beads expand completely. Uniform distributed SiC particles around the EPS beads and locally movement of them during pouring and degradation leads to homogenous distribution of particles in final Al/SiC_p composite. Higher modulus, strength and hardness were observed in the composites than the unreinforced Al alloy part. The fracture surfaces of the composite samples exhibited dimple surfaces and fracture in SiC particles.     

Microstructure and mechanical properties of SiC<Subscript>P</Subscript>/SiC and SiC<Subscript>W</Subscript>/SiC composites by CVI

37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were prepared by chemical vapor infiltration (CVI) process through depositing SiC matrix in the porous particulate and whisker preforms, respectively. The particulate (or whisker) preforms has two types of pores; one is small pores of several micrometers at inter-particulates (or whiskers) and the other one is large pores of hundreds micrometers at inter-agglomerates. The microstructure and mechanical properties of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were studied. 37.2 vol.% SiC_P/SiC (or 25.0 vol.% SiC_W/SiC) consisted of the particulate (or whisker) reinforced SiC agglomerates, SiC matrix phase located inter-agglomerates and two types of pores located inter-particulates (or whiskers) and inter-agglomerates. The density, fracture toughness evaluated by SENB method, and flexural strength of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were 2.94 and 2.88 g/cm³, 6.18 and 8.34 MPa m^1/2, and 373 and 425 MPa, respectively. The main toughening mechanism was crack deflection and bridging.     

Vacuum brazing of aluminium metal matrix composite (55 vol.% SiCp/A356) using aluminium-based filler alloy

Aluminium matrix composites with high volume fractions of SiC particles, as the reinforcements, are potentially suitable materials for electronic packaging. These composites, due to their poor weldability, however, have very limited applications. The microstructure and shear strengths of the bonds made in 55 vol.% SiC_p/A356 composite, using an aluminium based filler alloy containing Cu, Si, Mg and Ni, were investigated in this paper. The brazing temperature had a clear effect on the bond integrity, and the samples brazed at 560 °C demonstrated good bonding between the filler alloy and the SiC particles. The maximum shear strength achieved in this work was 102 MPa.     

Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications

The fabrication process and thermal properties of 50–71 vol% SiC_p/Al metal matrix composites (MMCs) for electronic packaging applications have been investigated. The preforms consisted with 50–71 vol% SiC particles were fabricated by the ball milling and pressing method. The SiC particles were mixed with SiO₂ as an inorganic binder, and cationic starch as a organic binder in distilled water. The mixtures were consolidated in a mold by pressing and dried in two step process, followed by calcination at 1100 °C. The SiC_p/Al composites were fabricated by the infiltration of Al melt into SiC preforms using squeeze casting process. The thermal conductivity ranged 120–177 W/mK and coefficient of thermal expansion ranged 6–10 × 10^–6/K were obtained in 50–71 vol% SiC_p/Al MMCs. The thermal conductivity of SiC_p/Al composite decreased with increasing volume fraction of SiC_p and with increasing the amount of inorganic binder. The coefficient of thermal expansion of SiC_p/Al composite decreased with increasing volume fraction of SiC_p, while thermal conductivity was insensitive to the amount of inorganic binder. The experimental values of the coefficient of thermal expansion and thermal conductivity were in good agreement with the calculated coefficient of thermal expansion based on Turner's model and the calculated thermal conductivity based on Maxwell's model.     

Effects of particle size and properties on the microstructures,mechanical properties,and fracture mechanisms of 7075Al hybrid composites prepared by squeeze casting

To investigate the effects of particle size and properties on the mechanical properties of 7075Al matrix composites, hybrid composites reinforced using three different reinforcement combinations, 40 vol. % 7 μm SiC_p with 5 vol. % 7 μm SiC_p, 35 μm SiC_p, and 35 μm Ti, were prepared using squeeze casting. The failure mechanisms and the microstructure–property relationships of hybrid composites were studied using SEM, TEM, and tensile tests. The composite containing Ti particles achieved the highest tensile strength of 626 MPa and an elongation of 1.2 %. Fracture mechanism analyses imply that the reduced strength for the 35 μm SiC_p-containing composite are caused by the inefficient load transfer capability resulting from the preferential breakage of larger-sized SiC_p particles during the deformation process. In contrast, micro-zones formed by Ti particles at the center and matrix alloy with few dislocations around release stress and deform synergistically during deformation, which decreases the breakage of SiC_p and improves the plastic deformation ability of the matrix alloy, resulting in a good combination of strength and ductility.     

Microstructure and mechanical properties of Mg – Al9Zn/SiCp composite produced by vacuum stir casting process

Abstract

15 vol.-% SiC particle reinforced cast Mg – 9AlZn (AZ91C) composite was produced by a vacuum stir casting process, and the microstructure and mechanical properties of the composite investigated. The stirring process was carried out at a speed of 750 – 1500 rev min^-1 with a stainless steel impeller for 25 min in a vacuum of 20 – 40 mbar. SiC particles in the composite exhibited a reasonably homogeneous distribution and were well wetted by magnesium. The Mg – Al9Zn/15SiC_p composite showed significant improvement in yield strength and elastic modulus following T4 heat treatment. The ultimate tensile strength of the composite was low, but close to that of unreinforced magnesium alloy. Mg/SiC interfacial reactions and reaction mechanisms are discussed. No evident interfacial products were found at a low process temperature of 700°C. However, significant chemical reactions at the Mg/SiC interface occurred when the composite melt was maintained at 750°C, and complex reaction products were formed. The fluidity of the composite melt deteriorated seriously after the interfacial reactions occurred.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Investigation of thermal properties of Al–Si matrix reinforced fine SiCp composites

Abstract

The characterisation of thermal expansion coefficient and thermal conductivity of Al–Si matrix alloy and Al–Si alloy reinforced with fine SiC_p (5 and 20 wt-%) composites fabricated by stir casting process are investigated. The results show that with increasing temperature up to 350°C, thermal expansion of composites increases and slowly reduces when the temperature reaches to 500°C. The values of both thermal expansion and conductivity of composites are less than those for Al–Si matrix. Microstructure and particles/matrix interface properties play an important role in the thermal properties of composites. Thermal properties of composites are strongly dependent on the weight percentage of SiC_p.     

Preliminary investigation on joining performance of SiCp-reinforced aluminium metal matrix composite (Al/SiCp–MMC) by vacuum brazing

The present study has investigated the joining performance of SiC particulate reinforced aluminium metal matrix composite (Al/SiC_p–MMC) by vacuum brazing process. After the joints brazed with Al–Si–Mg foil brazing filler metal at different brazing batches, both the mechanical properties and the microstructures of brazed joints were estimated. Moreover, the influence of SiC_p size, SiC_p volume percentage and the brazing parameters on bonding quality of the joints have also been discussed in detail. The results have revealed that the bonding quality either in SiC_p/Al interface or in SiC_p/SiC_p interface belongs to weak bonding, and the results also show that under the same brazing parameters condition, the strength of brazed joints decreases along with increasing the SiC_p volume percentage. In addition, the results also indicate that for a constant SiC_p volume percentage the strength of brazed joints decreases when SiC_p size increases. These results are very useful for the joining design of discontinuously reinforced metal matrix composites and further for the optimum design of composition of composites.     

Thermal expansion of a novel hybrid SiC foam–SiC particles–Al composites

A new type of hybrid SiC foam–SiC particles–Al composites (V_SiC = 53, 56.2 and 59.9%) to be used as an electronic packaging substrate material were fabricated by squeeze casting technique, and their thermal expansion behavior was evaluated. The coefficients of thermal expansion (CTEs) of the hybrid composites in the range of 20–100 °C were found to be between 6.6 and 7.7 ppm/°C. The measured CTEs are much lower than those of SiC particle-reinforced aluminum (SiC_p–Al) composites with the same content of SiC because of the characteristic interpenetrating structure of the hybrid composites. A material of such a low CTE is ideal for electronic packaging because of the low thermal mismatch (and therefore, low thermal stresses) between the electronic component and the substrate. To achieve similar CTEs in SiC_p–Al composites, the volume fraction of SiC would be much higher than that in the hybrid composites.     

Mechanical properties of Cu/SiCp composites fabricated by composite electroforming

The silicon carbide particles (SiC_p) dispersion-strengthened copper matrix composites (Cu/SiC_p) were fabricated by composite electroforming technology. The microstructure, tensile property and wear behavior of Cu/SiC_p composites were investigated. The results showed that composites with different SiC_p contents were obtained. The microstructure of the composite presented a uniform distribution of SiC_p in the matrix and good interfacial integrity. The hardness, yield strength and rigidity of the composites increased with increasing SiC_p content, but at the cost of ductility. Increasing SiC_p content in the composite was not always beneficial to the ultimate tensile strength and wear resistance. The ultimate tensile strength increased with SiC_p content increasing up to 16 vol.% and decreased as exceeding 16 vol.%, and the wear mass loss decreased with SiC_p content increasing up to 21 vol.% and increased again as exceeding 21 vol.%.     

Refining SiCp in reinforced Al–SiC composites using equal-channel angular pressing

Aluminum–silicon carbide composite (Al–SiC_p) is one of the most promising metal matrix composites for their enhanced mechanical properties and wear resistance. In the present study, Al–SiC (average size 55 μm) composites with 5% and 10% by volume were fabricated by stir casting technique. The equal-channel angular pressing (ECAP) was then applied on the cast composites at room temperature in order to study the effect of ECAP passes on the SiC_p size and distribution. The ECAP process was successfully carried out up to 12(8) passes for Al–5%(10%)SiC samples. Microstructure study revealed that the highest refinement by breakage of SiC_p was achieved after the first ECAP pass and that further refinement took place in the next passes. More breakage of the SiC_p was found in the composite richer in reinforcing particles so that the SiC_p reached approximately 1 μm in the Al–10%SiC after 8 passes and 4 μm in Al–5%SiC after 12 ECAP passes. The distribution of SiC reinforcement particles also improved after applying ECAP. The factors including decrease in reinforcing particle size, improvement in their distribution, decrease in porosity in addition to strain hardening and grain refining of the matrix resulted in enhancement of tensile and compressive strengths as well as hardness by more than threefold for the Al–5%SiC after 12 passes and for Al–10%SiC after 8 passes compared to the cast composites. Additionally, the composite remained ductile after the ECAP process. The fracture surface indicated good bond between the matrix and the reinforcement.     

Preparation and properties of nano-SiCp/A356 composites synthesised with a new process

Nano-SiC_p reinforced A356 aluminium alloy composites were prepared by solid–liquid mixed casting. Nano-SiC particles 40?nm in diameter were pre-oxidised at 850°C. Millimetre-sized composite granules were then fabricated by milling a mixture of nano-SiC_p and Al powders, and then remelting to form an alloy melt, which was treated by mechanical stirring and ultrasonic vibration to prepare the composites. Results showed that nano-SiC particles were dispersed pretty well within the matrix and no serious agglomeration was observed. As the weight fraction of SiC_p increased, the tensile strength, yield strength and elongation of the composites increased as well, by 22%, 62% and 24%, respectively, when compared to A356 alloy with 2wt-% nano-SiC_p.     

Effect of heat treatment on strength and abrasive wear behaviour of Al6061-SiC<Subscript>p</Subscript> composites

In recent years, aluminum alloy based metal matrix composites (MMC) are gaining importance in several aerospace and automobile applications. Aluminum 6061 has been used as matrix material owing to its excellent mechanical properties coupled with good formability and its wide applications in industrial sector. Addition of SiC_p as reinforcement in Al6061 alloy system improves its hardness, tensile strength and wear resistance. In the present investigation Al6061-SiC_p composites was fabricated by liquid metallurgy route with percentages of SiC_p varying from 4 wt% to 10 wt% in steps of 2 wt%. The cast matrix alloy and its composites have been subjected to solutionizing treatment at a temperature of 530°C for 1 h followed by quenching in different media such as air, water and ice. The quenched samples are then subjected to both natural and artificial ageing. Microstructural studies have been carried out to understand the nature of structure. Mechanical properties such as microhardness, tensile strength, and abrasive wear tests have been conducted both on matrix Al6061 and Al6061-SiC_p composites before and after heat treatment. However, under identical heat treatment conditions, adopted Al6061-SiC_p composites exhibited better microhardness and tensile strength reduced wear loss when compared with Al matrix alloy.     

Corrosion behavior of Al–60 vol.% SiCp composites in NaCl solution

In this study, corrosion behavior of pure Al and Al–4 wt.% Mg alloy matrix composites, comprising 60 vol.% SiC particles, has been investigated. Composites were produced by pressure infiltration technique at 750 °C. The corrosion tests were carried out in 3.5 wt.% NaCl environment up to 28 days. The weight loss of the composites increased with increasing duration time up to 3–5 days then remained constant. Scanning electron microscopy (SEM) analysis showed that Al–4 wt.% Mg alloyed matrix composite exhibited higher corrosion resistance than pure Al matrix composite although potentiodynamic polarisation measurements showed higher i_corr values of Al–4 wt.% Mg alloyed matrix composites than pure Al matrix composites. Experimental results revealed that precipitation of Mg₂Si as a result of reaction between Al–Mg alloy and SiC particle has a beneficial effect on corrosion resistance of Al–4Mg alloy matrix composites due to interruption of the continuity of the matrix channels within the pressure infiltrated composites.     

Brazing of SiC and SiCf/SiC composites performed with 84Si-16Ti eutectic alloy: microstructure and strength

The microstructure and strength of brazed joints for monolithic SiC and SiC_f/SiC composites are presented and discussed; the brazing technique is based on the use of the 84Si-16Ti (at%) eutectic alloy. The rather low melting point of the used alloy allows to avoid a degradation of the fibre/matrix-interfaces in the composite materials. All the joints did not show any discontinuities and defects at the interface and revealed a fine eutectic structure. Moreover, in the case of composites, the joint layer appeared well adherent both to the matrix and the fibre interphase, and the brazing alloy infiltration looked sufficiently controlled. High resolving electron microscopic investigations of the microstructure and of the nanochemistry (HREM, EELS, esp. ELNES) revealed atomically sharp interfaces without interdiffusion or phase formation at the interlayer leading to the conclusion that direct chemical bonds are responsible for the adhesion. The joints of SiC_f/SiC composites showed 71 ± 10 MPa shear strength at RT and nearly the same values at 600°C.     

Extrusion textures in Al, 6061 alloy and 6061/SiCp nanocomposites

The 6061 alloy matrix composites reinforced with 10 wt.% and 15 wt.% of SiC nanoparticles with an average diameter of ~ 500 nm were hot extruded in strip shape from ball milled powders. The microstructures and textures of the hot extruded nanocomposites have been investigated by means of three dimensional orientation distribution functions and electron backscatter diffraction (EBSD) techniques. Pure Al and 6061 alloy extruded strips from atomised powders have been produced for comparison purposes. The results show that the non-deformable SiC particulates have a strong influence on the formation of extrusion textures in the matrix. Pure Al and 6061 alloy develop a typical β fibre texture after extrusion in strip shape. For 6061/SiC_p nanocomposites, the intensities of major texture components decrease with increasing amount of SiC particles. The total intensities of Brass, Dillamore and S components have decreased by 19% for 6061/10 wt.% SiC_p and 40% for 6061/15 wt.% SiC_p composites when compared with the 6061 alloy. EBSD analysis on local grain orientations shows limited Al grain rotations in SiC rich zones and decreased texture intensities.