Effect of dual size particle distribution on processing and properties of AZ91D/SiCp magnesium matrix composites prepared by rotation cylinder method

Abstract

Spiral fluidity and hardness and wear experiments were carried out to investigate the effect of dual size (5 and 50 μ m) SiC particle distributions on the fluidity, hardness, and wear resistance of Mg - 9.1Al - 0.7Zn (wt-%) alloy containing 10 vol.-% SiC particles, with the aim of tailoring properties to specific applications. Although a decrease in the fluidity of the composites is observed, as expected, in the presence of SiC particles, the fluidity of the composites with dual size particle distributions was in some instances better than that of composites containing the same volume fraction of single size particles. The hardness and wear resistance of the composites with dual size distributions were weakly dependent on the mixing ratio. In terms of complete molten processing and tailored mechanical properties, the optimum mixing ratio of 5 and 50 μm particles appears to be 1:2.     

Fluidity of aluminium alloy AlSi7Mg–SiC particulate composite melts

Abstract

A spiral test was used to evaluate the flowability of an AlSi7 Mg0·6 alloy reinforced via 10–30 vol.-%SiC particles having average diameters of 9, 13, and 23 μm. The results show that the composite melt has the same ability to flow into long thin sections as unreinforced aluminium, when the particle content is <20 vol.-%. The flowability of the particulate metal matrix composite is significantly reduced for particle volume fractions >20 vol.-% and approaching 30 vol.-% as a result of increased viscosity. At these higher levels of SiC, an increase in pouring temperature cannot compensate for the reduced flowability. The particle distribution in the solidified material depends on the length which the metal flows before solidification. After 150–200 mm of flow, dendritic grains growing in the composite melt push the particles so that the solidified microstructure consists of open areas without particles. The particle free areas increase in diameter with the flow distance.

MST/3005     

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.     

Rotating cylinder manufacturing method and investment casting of SiCp/AZ91HP magnesium composites

Abstract

This paper describes the rotating cylinder method for manufacturing, and investment casting for forming, composite slurry. Microstructural features, such as SiC particle distribution and grain refinement of the as cast composites, were investigated. Also the effect of SiC particle fraction and size, and process parameters on the microstructure and the mechanical properties are discussed. Attempts were made to evaluate the thermal stability of oxides against molten AZ91HP magnesium alloy. The oxides examined included CaO, CaZrO₃, and silica bonded Al₂O₃ and zircon flour. Finally, the tensile properties, hardness, and wear resistance of the as cast composites were evaluated and the results are compared with those of the as cast alloy.     

Fractal theory study on morphological dependence of viscosity of semisolid slurries

Abstract

A theoretical model is developed which enables the effect of particle morphology on the viscosity of semisolid slurries to be assessed. Hausdroff dimensionality is introduced as a means of characterising the morphology of a solid particle. The effective solid volume fraction, defined as the volume fraction of actual solid plus the entrapped liquid, is derived analytically. The viscosity of semisolid slurries is calculated by the adaptation of Thomas's empirical approximation. The viscosity of a semisolid slurry is found to be a non-linear function of the Hausdroff dimensionality. Slurries with particle morphologies of lower Hausdroff dimensionality have a higher viscosity for a given solid fraction. The external variables, such as shear rate and cooling rate, affect the flow behaviour of semisolid slurries primarily by changing the Hausdroff dimensionality and the linear size of solid particles. The effect of particle size on the viscosity is also discussed.     

Processing of Al/SiC composites in continuous solid–liquid co-existent state by SPS and their thermal properties

Silicon carbide (SiC)-particle-dispersed-aluminum (Al) matrix composites were fabricated in a unique fabrication method, where the powder mixture of SiC, pure Al and Al–5mass% Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction was detected at the interface between the SiC particle and the Al matrix. The relative packing density of the Al–matrix composite containing SiC was higher than 99% in a volume fraction range of SiC between 40% and 55%. Thermal conductivity of the composite increased with increasing the SiC content in the composite at a SiC fraction range between 40 vol.% and 50 vol.%. The highest thermal conductivity was obtained for Al–50 vol.% SiC composite and reached 252 W/mK. The coefficient of thermal expansion of the composites falls in the upper line of Kerner’s model, indicating strong bonding between the SiC particle and the Al matrix in the composite.     

The properties of AlN-filled epoxy molding compounds by the effects of filler size distribution

AlN filler was compared with crystalline silica as a filler for advanced epoxy molding compounds. Properties such as the thermal conductivity, dielectric constant, CTE, flexural strength, elastic modullus and water absorption ratio of water-resistant grade AlN-filled molding compounds according to the contents or size of AlN and the filler size distribution were evaluated. A spiral flow test was also carried out to measure the change in viscosity according to the AlN size distribution for improved fluidity. The properties of EMC that is filled with a 70 vol.% of 12 micron AlN was compared with a crystalline silica-filled EMC. Thermal conductivity was improved by 2.2 times, the dielectric constant was reduced to less than one-half, the flexural strength was improved, and the CTE was also reduced. A binary mixture of an AlN-filled (65 vol.%) EMC showed improved fluidity, thermal conductivity, dielectric constant, flexural strength and water resistance compared to a single-size AlN-filled EMC. The maximum improvement was obtained when the fraction of small particles in the binary mixture of the AlN is 0.2–0.3. The CTE of EMC was decreased by increasing the volume fraction of small particles in the binary mixture of the AlN.     

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.     

Fluidity and microstructure of an Al–10% B<Subscript>4</Subscript>C composite

The fluidity evolution of an Al–10 vol.% B₄C experimental composite during long holding periods has been investigated by using a vacuum fluidity test. It was found that the fluidity of the composite melt decreased with the increase of the holding time. The microstructure of the fluidity samples was examined by optical metallography, quantitative image analysis, and electron microscopy. Two secondary reaction-induced phases were identified and the volume fraction changes of the solid phases during the holding periods were quantified. The relationship between the fluidity, volume fraction, and surface area of solid phase particles was established. In addition, the particle distribution along the entire length was examined in the fluidity samples. The mechanism of the particle redistribution during flow and solidification is presently discussed.     

Microstructure and abrasive wear behaviour of B<Subscript>4</Subscript>C particle reinforced 2014 Al matrix composites

In this study, the microstructure and abrasive wear properties of varying volume fraction of particles up to 12% B₄C particle reinforced 2014 aluminium alloy metal matrix composites produced by stircasting method was investigated. The density, porosity and hardness of composites were also examined. Wear behaviour of B₄C particle reinforced aluminium alloy composites was investigated by a block-on-disc abrasion test apparatus where the samples slid against the abrasive suspension mixture (contained 10 vol.% SiC particles and 90 vol.% oil) at room conditions. Wear tests performed under 92 N against the abrasive suspension mixture with a novel three body abrasive. For wear behaviour, the volume loss and specific rate of the samples have been measured and the effects of sliding time and the content of B₄C particles on the abrasive wear properties of the composites have been evaluated. The dominant wear mechanisms were identified using SEM. Microscopic observation of the microstructures revealed that dispersion of B₄C particles was generally uniform while increasing volume fraction led to agglomeration of the particles and porosity. The density of the composite decreased with increasing reinforcement volume fraction but the porosity and hardness increased with increasing particle content. Moreover, the specific wear rate of composite decreased with increasing particle volume fraction. The wear resistance of the composite was found to be considerably higher than that of the matrix alloy and increased with increasing particle content.     

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.     

Design aspects of processing of aluminium 6061 based metal matrix composites via powder metallurgy

Abstract

Acceptance of metal matrix composites for industrial applications depends upon improving properties using an economic production route, which includes the processing design. Two powder metallurgical routes have been used in the manufacture of Al 6061 metal matrix composites. The first involves blending, vacuum canning, and hot pressing from prealloyed powders and the second involves blending of elemental powders, liquid phase sintering, and subsequent hot rolling. These composites comprise 7·5 or 15 vol.-% of 7, 23, or 45 μm SiC particles. In this paper, the composite microstructure at each stage of the different processing routes has been examined and the aging behaviour investigated. Effects on the tensile properties of fabrication techniques, SiC particle size, and volume fraction are presented and discussed.

MST/3020     

Microstructure and tensile properties of graded Al–Mg2 Si in situ composites fabricated by centrifugal casting

Abstract

Al–Mg₂ Si in situ composite tubes were fabricated by a centrifugal casting process. The microstructure and tensile strength of the composite tubes were examined revealing an inhomogeneous distribution of Mg₂ Si particles along the radial direction. Adjacent to the rapidly cooled area, near the outer periphery, in situ particles were forced inwards leaving behind a particle free region in the middle part of the tubes. Increasing Mg₂ Si weight percentage in the aluminium matrix induced dendrite formation in the rapidly cooled area and the width of the particle free region in the middle part of the cylinder wall decreased. The particle distribution gradient in the inner region also decreased with increasing Mg₂ Si content. The width of the particle free region and the particle distribution gradient were both influenced by the gradually varying viscosity of the liquid alloy as it solidified. The tensile strength of the centrifugally cast tubes increased from inner to outer periphery. High volume fractions of Mg₂ Si particles of between 50 and 70 vol.-% reduced the strength of the composites.     

Manufacturing of Al/SiCp composite foams using calcium carbonate as foaming agent

Abstract

The aluminium composite foams reinforced by different volume fractions of SiC particles were manufactured with the direct foaming route of melt using different contents of CaCO₃ foaming agent. The density of produced foams changed from 0·43 to 0·76 g cm^?3. The microstructural features and compressive properties of the Al/SiC_p composite foams were investigated. Compressive stress–strain curve of Al/SiC_p composite foams is not smooth and exhibits some serrations. At the same relative density of composite foams, the plateau stress of the composite foams increases with increasing volume fraction of SiCp and decreasing weight percentage of CaCO₃. The relation between plateau stress, relative density, weight percentage of CaCO₃ and SiCp volume fraction of Al/SiCp composite foams with a given particle size was investigated.     

Thermal properties of diamond/SiC/Al composites with high volume fractions

The diamond/SiC/Al composites with high volume fractions and a large ratio of diamond to SiC particle size (7.8:1) were fabricated by gas pressure infiltration. The results show that the fine SiC particles occupy efficiently the interstitial positions around coarse diamond particles; the main fracture mechanism of the composite is matrix ductile fracture, and diamond brittle fracture was observed which confirms a high interfacial bonding strength; the diamond/SiC/Al composites with 80% and 66.7% volume fraction of diamond in the reinforcement have the higher volume fraction in the reinforcement and lower coefficient of thermal expansion compared to the diamond/Al composite. Turner and Kerner models are not in good agreement with the experimental data for the composites based on reinforcement with two phases different in shape and component. When the effect of the coating layer considered, differential effective medium (DEM) model is confirmed a reliable model in designing a composite with a given thermal conductivity based on reinforcement with two phases different in size.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Production of SiC particulate reinforced aluminium composites by melt spinning

Melt spinning is successfully used for the preparation of a rapidly solidified SiC particle reinforced AlSi7Mg0.3 alloy. The composites are prepared by introducing SiC particles in a semi-solid matrix slurry (SiC volume fractions up to 0.15, particle size 10 or 20 m). Duralcan material (SiC volume fraction 0.20, particle size 12 m) was also used. After stirring in the semi-solid state the composites are heated above the liquidus temperature and subsequently melt-spun. Featureless, columnar and dendritic zones can be identified in the ribbons. A finer dendritic structure is found around the SiC particles. The SiC particles tend to segregate to the air side of the ribbons and the segregation effect is influenced by particle size and volume fraction. As interface velocities are higher than the critical velocities predicted by models on interface pushing, it is concluded that fluid flow in the melt puddle is responsible for the segregation effect.     

Thermal expansion coefficient and wear performance of aluminium/SiC composites with bimodal particle distributions

Abstract

High volume fraction metal matrix composites were produced by infiltration of liquid aluminium into preforms made by mixing and packing SiC particulates having two different average diameters (170 and 16 μm). The maximum particle volume fraction (0.74) was attained for a mixture containing 67% of coarse particles. The variation of particle volume fraction with percentage of coarse particles can be reasonably well understood in terms of a simple model. Experimental results for the threshold pressure indicate that it is mainly determined by the local volume fraction of fine particles, a result which is shown to be compatible with the model used to estimate the particle volume fraction. On the other hand, the coefficient of thermal expansion (CTE) is mainly determined by the total particle volume fraction of the composite. Wear performance was evaluated through sliding wear of the composite against a static alumina ball. The results indicate that, in this case, the key parameter is the coarse particle content of the composite.     

Fluidity of mica particle dispersed aluminium alloy

Cast aluminium alloy-mica particle composites were made by dispersing mica particles in a vortex produced by stirring the liquid Al-4 wt% Cu-1.5 wt% Mg alloy and then casting the melt containing the suspended particles into permanent moulds. Spiral fluidity and casting fluidity of the alloy containing mica particles in suspension were determined. Both the spiral fluidity and the casting fluidity of the base alloy were found to decrease with an increase in volume or weight percent of mica particles (of a given size), and with a decrease in particle size (for a given amount of particles). The fluidities of Al-4 wt% Cu-1.5 wt% Mg alloys containing suspended mica particles were found to correlate very well with the surface area of suspended mica particles. The regression equation for spiral fluidity Y (cm) as a function of surface area of mica particles per gram of spiral X (cm² g^–1) at 700° C was found to be Y=42.62–0.42X with a correlation coefficient of 0.9634. The regression equations for casting fluidity Y (cm) as a functiono of surface area of mica particles per gram of fluidity test piece X (cm² g^–1) at 710 and 670° C were found to be Y=19.71–0.17X and Y=13.52–0.105X with correlation coefficients of 0.9194 and 0.9612 respectively. The percentage decrease in casting fluidity of composite melts containing up to 2.5 wt% mica with a drop in temperature is quite similar to the corresponding decrease in the casting fluidity of base alloy melts (without mica). The change in fluidity due to mica dispersions has been discussed in terms of changes in viscosity of the composite melts. However, the fluidities of these composite alloys containing up to 2.5 wt% mica are adequate for making a variety of simple castings including bearings for which these alloys have been developed.     

Kinetic modelling of binder burnout for optimisation of slurry–powder manufacturing process of Ti/SiC composites

Abstract

A slurry–powder metallurgy method is currently under development as a low cost production route for Ti/SiC composites. One of the critical steps in the process is the complete removal of a fugitive binder used to form the powder slurry. A diffusion-controlled, shrinking-core model has been developed for the prediction of the binder burnout kinetics in a metal powder compact with relatively large particles. This model is suitable for a polymeric binder that decomposes mainly to a monomer. The model considers the degradation of the polymer and the diffusion of the monomer in the core that contains a powder structure filled with a liquid polymer–monomer solution. It is found that the dominant mechanism is the liquid diffusion of monomer and the burnout time increases significantly with the compact size. The model also predicts a longer burnout time for a larger particle volume fraction.