Preparation of aluminum/silicon carbide metal matrix composites using centrifugal atomization

Abstract

This paper describes the development of a new technique to produce metal matrix composites (MMCs) by injecting silicon carbide particles into molten aluminum just prior to centrifugal atomization. A centrifugal atomization apparatus has been constructed for this study. Silicon carbide particles are injected during atomization of 6061 aluminum alloy to form metal matrix composite powder. The prepared aluminum/silicon carbide powder contains 18 vol.% of SiC particles and 1.2 vol.% of voids. The particle grain size is almost independent from the particle size.

Introduction

Aluminum metal matrix composites (MMC) reinforced with silicon carbide (SiC) particles have up to 20% improvement in yield strength, have a lower coefficient of thermal expansion and a higher modulus of elasticity and are more wear resistant than the corresponding non-reinforced matrix alloy systems [1], [2]. For these reasons they are currently being used in a number of specialty products. These include break discs made from castings and bicycle frames made from extrusions. However, aluminum silicon carbide composites (Al/SiC) are not accepted for a wide range of applications because of their low fracture toughness and poor fatigue properties [1], [2], [3]. Brittle interfacial reactions which are lower in strength than the reinforcing particles may be responsible for the lower fracture toughness of melt formed MMC [4]. Agglomerations of particles in MMC provide excellent nucleation points for fatigue cracks to start. This reduces the fatigue life of Al/SiC MMC. Both cast and powder metallurgy formed MMC suffer from agglomeration problems. These toughness and fatigue properties can be improved by reducing particle agglomeration and interfacial reactions between the matrix and the reinforcing particles.

Several methods are currently used to produce Al/SiC MMC. These include stir casting, preform infiltration, powder metallurgy and spray forming [5], [6], [7], [8]. Stir casting involves mixing SiC particles with aluminum while it is in a liquid or semi-liquid state. Stirring is required in this process due to the difference in density between the two materials (ρ_Al = 2400 kg/m³ and ρ_SiC = 3200 kg/m³). Without stirring, SiC particles tend to settle to the bottom of the melt causing an uneven distribution of the particles [5]. In addition, it has been observed that the solidification front can push particles during cooling [6], [7]. This leads to agglomerations of particles, which can be detrimental to the mechanical properties of the composite.

The preform infiltration method of producing Al/SiC MMC involves filling the cavities of a rigid porous SiC structure with molten aluminum via the capillary action. The aluminum can be pulled into the preform with a vacuum or pushed into the preform with a high-pressure flow [5]. The resulting composites typically have volume fractions of SiC greater than 55% and have low coefficients of thermal expansion. These composites have applications in electronic packaging [8].

The powder metallurgy method of producing Al/SiC MMC involves mixing aluminum powder with SiC, compacting and sintering. Mixing of the powders may involve dry blending, wet (slurry) blending or ball milling [9]. Agglomeration of the SiC particles is often a problem in blend processing. The small size of SiC particles cause them to become electrically charged, agglomerate and collect in between the larger aluminum particles. Additives are often added to reduce the electrical attraction of the SiC particles [10]. The compacted and sintered composite is often rolled or extruded into final shape. The powder metallurgy method of producing Al/SiC MMC is more costly than the casting method of production.

Spray forming is another method of producing Al/SiC MMCs. This method involves spray atomization of molten aluminum and injection of SiC particles into the stream of molten aluminum particles (Fig. 1a). The spray is collected on a plate where it is quickly solidified [11]. In this process segregation due to gravity is avoided. Interfacial reactions between SiC and aluminum are minimized due to the short time the SiC particles are in contact with the molten aluminum. However, the mechanical properties of composites produced by this method have not been impressive [12]. This is due to different cooling rates and improper collision angles causing that many of the particles to not be engulfed by the molten aluminum during their flight. The result is that, particles are not strongly bonded to the matrix [13].

Here we present a new method based on centrifugal atomization of the molten matrix while injecting SiC particles just prior to atomization (Fig. 1b). This method has two advantages. One is that it reduces particle agglomeration by creating a pre-combined powder metallurgy MMC with evenly dispersed particles, which does not require mixing.

The second is that it reduces the SiC particles contact time with the molten aluminum, therefore, the reaction times are minimized. With potential cooling rates of above 105 K/s, the amount of time over which harmful reactions can take place is greatly reduced [14]. In order to have a successful process, the interfacial reaction between SiC particles and the matrix should be minimized. This is achieved by controlling the thermodynamic activities of the elements. In the A1/SiC system, intermediate phases such as Al₄C₃ and A1₄SiC₄ may form during the thermal exposure either as a continuous layer or isolated precipitates. At temperatures above the melting point of A1, SiC reacts with molten A1 [15]. One reaction that can occur at the particle matrix interface is:

4Al(l)+3SiC(s) → Al₄C₃(s)+3Si(l)

It has also been suggested that interfacial reactions occur with free carbon present in the SiC particles according to the reaction:

4Al(l)+3C → Al₄C₃(s)

These reactions are minimized in SiC MMC's where the matrix is composed of an aluminum casting alloy. This is because the high content of silicon in casting alloys suppresses the disassociation of silicon from carbon in the SiC particles. Excess silicon in the melt also increases the formation temperature of Al₄C₃ to above 800 °C [16].