SiC-particulate aluminum composite foams produced from powder compacts: foaming and compression behavior

The foaming behavior of SiC-particulate (SiC_p) aluminum composite powder compacts containing titanium hydride blowing agent was investigated by heating to 750°C in a pre-heated furnace. Aluminum powder compacts were also prepared and foamed using similar compaction and foaming parameters in order to determine the effect of SiC_p-addition on the foaming and compression behavior. The SiC_p-addition (10 wt%) was found to increase the linear expansion of the Al powder compacts presumably by increasing the surface as well as the bulk viscosities. The compression tests conducted on Al and 10 and 20% SiC_p foams further showed a more brittle compression behavior of SiC_p/Al foams as compared with Al foams. The collapse stresses of Al and 10% SiC_p/Al foams were also predicted using the equations developed for the open and closed cell foams. Predictions have shown that Al foam samples behaved similar to open cell foams, while 10% SiC_p/Al foam collapse stress values were found between those of open and closed cell foams, biasing towards those of the open cell foams.     

Compressive properties of closed cell Al alloy–fly ash particle composite foams

Abstract

The closed cell aluminium alloy–fly ash particle composite (Al/FA) foams containing 1·5 wt-% fly ash were manufactured by molten body transitional foaming process. The quasi-static compressive properties of Al/FA have been investigated. Results show the compressive stress–strain curves of Al/FA foams exhibit three regions, i.e. the elastic region, the plastic plateau region and the densification region. A linear relationship between the densification strain and the relative density was obtained. The relation between the plastic collapse stress and the relative density can be described with Gibson and Ashby’s model. The energy absorption capacities of the Al/FA foams gradually increase with increasing strain and relative density.     

Compressive behaviour of open cell Al–Al2O3 composite foams fabricated by sintering and dissolution process

Abstract

The sintering and dissolution process (SDP) was used to produce the fine open cell Al–Al₂O₃ composite and pure Al foams with the relative density of 0·25–0·40 and the pore size of 112–400 μm. The composite foam exhibited much higher yield strength and Young's modulus than the pure Al foam, and thus had an elevated plateau stress. Moreover, the composite foam showed a unique dependence of the compression stress on the pore size, i.e. it increased with increasing pore size, which was quite different from that for the common metal foams.     

Closed-cell Al alloy composite foams: Production and characterization

Foamy Al alloy SiCp composites of different densities ranging from 0.4 to 0.7 g/cm³ were manufactured by melt-foaming process, which consisted of direct CaCO₃ addition into the molten A356 aluminum bath. Mechanical properties and morphological observations indicated that the three-stage deformation mechanism of typical cellular foams is dominant in the produced A356 aluminum foams. Middle-stage stress plateau shrinkage plus compressive strength and bending stress enhancements were observed in denser foams. With the same Al/SiCp ratio, energy absorption ability and plastic collapse strength of the closed-cell foams were increased with the foam density. Doubling cell-face bending effects resulted in larger compressive than bending strengths in the closed-cell foams; while stiffness lowering was due to the cell-face stretching conditions.     

ECAPT 工艺对 SiCp/ Al 基复合材料组织和性能的影响

探究了用等径角挤扭(ECAPT)法制备的SiC_p/Al基复合材料组织演化过程和力学性能。比较了2种体积分数的SiC_p(8.75%和35%)复合材料在ECAPT中组织演化规律,测试了其硬度值及致密度。结果表明,ECAPT形变量大,细化晶粒能力强,SiC_p分布均匀性好,其致密效果SiC_p体积分数大的(35%)比小的(8.75%)差;并且体积分数为35%的SiCp复合材料存在"拱桥效应",但颗粒细化效果优于体积分数为8.75%的SiC_p。     

Studies on the fracture and flexural strength of Al/Sip composite

Pure aluminum matrix composite reinforced with a high volume fraction of silicon particles (Al/Si_p) was fabricated by gas-pressured infiltration. The results of four point flexural strength tests show that Al/Si_p has low flexural strength. The analysis of the fractograph reveals the fracture mechanism of Al/Si_p. The fracture of Al/Si_p is primarily dominated by the fracture of brittle silicon particles and the subsequent link up of damage through the matrix. The pre-existent microcracks in silicon particles that were made during the process of compacting will also lower the flexural strength of Al/Si_p composite. The hybrid particle reinforced pure aluminum matrix composite (Al/Si_p+SiC_p) was fabricated in the same way. Results show the flexural strength can be improved by 11.3% compared with Al/Si_p when 6 vol.% silicon particles are replaced by silicon carbide particles with the same volume fraction and size. The reason is that SiC_p with higher fracture stress and higher elastic modulus can prevent the rapid expansion of cracks through the composite and lower the stress in silicon particles.     

Static and cyclic creep behaviour of SiC whisker reinforced aluminium composite

Abstract

Static and cyclic creep tests were carried out in tension at 573–673 K on a 20 vol.-%SiC whisker reinforced aluminium (Al/SiC_w ) composite. The Al/SiC_w composite exhibited an apparent stress exponent of 18·1–19·0 at 573–673 K and an apparent activation energy of 325 kJ mol^-1 for static creep, whereas an apparent stress exponent of 19·6 at 623 K and an apparent activation energy of 376 kJ mol^-1 were observed for cyclic creep. A cyclic creep retardation (CCR) behaviour was observed for the Al/SiC_w composite. The steady state creep rate for cyclic creep was three orders of magnitude lower than that for static creep. Furthermore, the steady state creep rates of the composite tended to decrease continuously with increasing percentage unloading amount. The static creep data of the Al/SiC_w composite were rationalised by the substructure invariant model with a true stress exponent of 8 together with a threshold stress. The CCR behaviour can be explained by the storage of anelastic strain delaying non-recoverable creep during the onload cycles.     

Closed cell ZA27–SiC foam made through stir-casting technique

Closed cell zinc aluminum alloy (ZA27)–SiC composite foam has been synthesized using conventional stir-casting technique and CaH₂ as foaming agent. The synthesized foams are characterized in terms of its micro-architectural characteristics and deformation responses under compressive loading. It is observed that ZA27–SiC foams could be easily foamed without any difficulty. The density of the developed foam ranges from 0.25 gm/cc to 0.45 gm/cc due to the variation of CaH₂ percentage. The plateau stress and energy absorption of these foams follow power law relationship with relative density. Wherein, the densification strain follows a linear relationship with the relative density.     

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.     

Fabrication and investigation of influence of CaCO3 as foaming agent on Al-SiCp foam

Closed cell aluminum foams have been used in various disciplines of engineering. Aluminum foams provide high strength with the advantage of low weight. In the current research, CaCO₃ is used as a foaming agent for producing closed-cell aluminum foams. For the fabrication of homogenous foam, optimization of process parameters was done. The effect of SiC as a thickening agent on structural property of foams viz. density and porosity have been inspected. Foams with density 0.40–0.86 g/cm³ were produced. The produced foams were studied under axial compression tests for evaluating mechanical properties. It can be inferred from the results that by adding 3 wt.% CaCO₃, the uniform viscosity of melt was achieved and a homogeneous foam structure is achieved with optimum porosity. Also, 5 wt.% addition of CaCO₃ in melt and stirring speed at 1400 rpm tend to increase porosity and decrease cell wall thickness. The optimum values for thickening agent SiC, foaming agent CaCO₃ at stirring speed 1400 rpm were found out to be 15 wt.%, 3 wt.%. The effect of relative density, the addition of thickening and foaming agent is studied.     

An experimental study on deformation behavior below 0.2% offset yield stress in some SiCp/Al composites and their unreinforced matrix alloys

The deformation behaviors below 0.2% offset yield stress in some silicon carbide particulate reinforced aluminum composites (SiCp/Al) and their unreinforced matrix alloys were investigated experimentally in this work. The results of the study showed that incorporation of SiC particulate into aluminum matrix can enhance the plastic flow stress (PFS) in macroplastic stage but slightly lower PFS in microplastic stage. With increase in the volume fraction of SiC particulate (V_p), the 0.2% offset yield stress (σ_0.2) increases while the resistance to microplastic deformation (σ_10⁻⁵) first decreases and then increases. The composite with smaller particle size presents higher PFS both in micro- and macro-plastic stages. It was also found that heat treatment remarkably influence both micro- and macro-plastic behaviors of the composites. Quenching followed by artificial aging can significantly enhance PFS both in micro- and macro-plastic stages for the age hardened alloy based composites (SiCp/2024Al) but has no obvious effect for the non-age hardened alloy based composites (SiCp/Al). For both the SiCp/2024Al composite and unreinforced 2024Al alloy, PFS exist a ‘peak value’ with variation of aging time, implying that like the conventional yield strength, PFS in microplastic stage of the composite is also strongly controlled by the precipitates formed in matrix during aging treatment. The effects of thermal cycling on PFS are dependent to the V_p. In large V_p case (35%), with increase in cyclic number PFS slightly decreases but in small V_p case (15%) PFS slightly increases as the cyclic number increases. The PFS in microplastic stage is very sensitive to the microstructure features. The lower residual thermal stresses, small density of moveable dislocations and harder matrix would be beneficial to the increase of PFS in microplastic stage in the composites.     

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.     

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.     

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.     

Multiple NDE Techniques for the Measurement of Constituent Volume Fractions in Metal Matrix Composites

Multiple nondestructive evaluation (NDE) techniques are developed for determining the constituent volume fractions of silicon carbide particulate (SiC_p) reinforced aluminum (Al) matrix composites. The composite constituents studied include SiC_p reinforcement and intermetallic compounds formed during the processing stage. The proposed techniques employ measurements of ultrasonic velocity and eddy current conductivity, together with theoretical models that relate the effective anisotropic properties of the composites to their microstructures. The SiC_p volume fraction is calculated first by coupling the measured velocity with the two-phase (SiC_p and Al matrix) model prediction. The intermetallics concentration then is calculated using the three-phase (SiC_p, intermetallics and Al matrix) model with the measured conductivity. The methods are shown to be reliable in determining the volume fractions of the reinforcement and the intermetallic phase. The techniques may be adopted in a production unit for the quality assessment of the metal matrix particulate composite extrusions.     

Fabrication of an electronic package box of SiCP/Al composites with high volume SiCP

In this paper, a SiC_P preform was prepared by Powder Injection Molding (PIM), and the melting aluminum was injected into the SiC_P preform by the pressure infiltration method to manufacture an electronic package box of SiC_P (65%)/Al composites. SiC_P (65%)/Al composite prepared by pressure infiltration has full density and a homogeneous microstructure. The relative density of the composite is higher than 99%, the thermal expansion coefficient and thermal conductivity of the composite are 8.0×10⁻⁶/K and nearly 130 W/(m · K) at room temperature, respectively, which meet the requirements of electronic packaging. Translated from Journal of Acta Materiae Compositae Sinica, 2006, 23(6): 109–113 (in Chinese)     

Metal matrix composite layers formed by laser processing of commercial purity Ti–SiCp in nitrogen environment

Abstract

Metal matrix composite layer formation by means of laser alloying using 6 μm particle size SiC powder (SiC_p) preplaced on titanium surfaces in a nitrogen environment produced golden coloured tracks and a complete solution of SiC_p in the melt zones under a range of processing conditions. The melt layers consisted of dendrites at the top (titanium nitride based) followed by threadlike particle structures (titanium silicides), and the sizes of dendrites and the threadlike particles werefound to increase with increasing laser power density. The surface layer of the dendrites developed a hardness 4·5–9 times that of the base metal (150 HV), and the deep underlying threadlike structures had a plateau of hardness of aboout 2·8–4 times the base hardness. The metal matrix composite layers were found to be 2–4 times thicker than those produced previously in a helium environment under similar processing conditions. The exothermic reactions due to the formation of titanium nitride, titanium carbide, or titanium carbonitride along with titanium silicide during laser melting of SiC_p coated titanium surfaces under a nitrogen environment are considered to be responsible for the greater melt depth and complete dissolution of ceramic particles, by increasing the temperature of the melt.

MST/3208     

Analyses on fracture characteristics of SiCp-6061Al/6061Al composites extruded by different ratios

Two 6061 Al alloy matrix composites reinforced with rods that are themselves composites of the same Al alloy reinforced with a high volume fraction of SiC particles were studied. After vacuum pressure infiltration, one was hot extruded at a ratio of 10 : 1 and the other at a ratio of 60 : 1. The fracture characteristics of the two SiC_p-6061Al/6061Al composites were examined in detail. It was found that increasing the hot extrusion ratio of this kind of composite can improve the bonding between the SiC_p-6061Al bars and the 6061Al matrix. The strengths of the SiC_p-6061Al bars and the 6061Al matrix were considered to increase with increasing extrusion ratio. Thus, the SiC_p-6061Al/6061Al composite extruded at a ratio of 60 : 1 shows fracture characteristics which are different from the composite extruded at a ratio of 10 : 1. The former has a higher fracture toughness, and its crack opening displacement versus load curve indicates a higher elastic modulus and maximum load. After application of the maximum external load, there is a slow decrease with increasing crack opening displacement in the case of the 60 : 1 extruded composite, but the load can be maintained for wide crack opening displacement in the case of the 10 : 1 extruded composite.     

Microstructural characterisation of novel 6351 Al–Al4SiC4 in-situ composite

Abstract

6351 Al–Al₄SiC₄ composite has been developed through stir casting route by incorporation of fine TiC powder in 6351 Al melt. Simultaneous effects of the generation of in-situ particles (Al₄SiC₄ and Al₃Ti) and grain refinement were observed. The in-situ generated Al₄SiC₄ particles were found to act as nucleation sites for primary α (causing grain refinement) along with engulfment effect promoting uniform particle distribution. As the volume fraction of Al₄SiC₄ particles increased, the dendritic solidification was suppressed (more equiaxed grains appeared) and overall grain size of the matrix decreased. Besides, the precipitation of Al₃Ti occurred at the dislocation enriched region. Accordingly, hardness of the composite was improved with increasing content of Al₄SiC₄ particles.     

Studies on machinability of Al/Si<Subscript>p</Subscript> + SiC<Subscript>p</Subscript> composite materials

This paper presents the experimental results on the machinability of silicon and silicon carbide particles (SiC_p) reinforced aluminium matrix composites (Al/Si_p + SiC_p) during milling process using a carbide tool. The total volume fraction of the reinforcements is 65 vol%. The milling forces, flank wear of the tool and the machined surface quality of composites with different volume fraction of SiC_p were measured during experiments. The machined surfaces of composites were examined through SEM. The results showed that the flexural strength and Vickers hardness are improved when certain volume fraction of silicon particles are replaced by silicon carbide particles with the same volume fraction and particle size and the effect of SiC_p on machinability is optimal when 9 vol% silicon particles in Al/Si_p was replaced by silicon carbide particles with the same volume fraction and the same particle size. Cracks and pits were found on the machined surfaces of composites due to the intrinsic brittleness of silicon particles.