Tips on Metal Matrix Composites (MMC)

 

What is Casting?

 

Casting is a manufacturing process by which a liquid metal (molten metal, melt) is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The metal product made by this process is also known as a casting. Casting is often used for making the products with complex shapes that would be otherwise difficult or uneconomical to make by other methods.

 

 

What is Metal Matrix Composite?

 

Composite is the materials fabricated by mixing the different starting materials. The composite has properties superior to the monolithic material. The composites with plastic matrix, with metal and with ceramics are called plastic matrix composite, metal matrix composite, and ceramics matrix composite, respectively. Many of the plastic matrix composite are the ones which contain the fibrous carbon or glass in the plastics (resin).  They are called fiber reinforced plastics (FRP). Metal matrix composites are generally applied for the use at elevated temperature.

 

 

Industrial application of Metal Matrix Composite

 

Ceramics particle- dispersed aluminum alloy composites are used for some of brake discs which requires wear resistance, exothermic and friction force. Replacing the cast iron disc with the composite contributes the weight reduction of the part, preserving its performance.  Ceramics fiber-reinforced aluminum alloy composites are used for some of engine parts, such as piston, cylinder block and connecting rod, which requires high temperature strength and wear resistance.

 

 

Composite casting technology

 

Practical application of metal matrix composite is outstripped by FRP due to the difficulty of the fabrication. Solid phase process such as powder metallurgy, diffusion bonding, HIP are expensive because it needs expensive starting materials such as powder or foil matrix, and it has many manufacturing process. Liquid phase process (casting process) is generally less expensive than solid phase process.  In the casting process, high temperature melt is used. High temperature often promotes the chemical reaction between the melt and the reinforcements. The reaction leads to the degradation or disappear of the reinforcements. The typical example of the reaction is shown in Fig.1. The expected properties of the composite wouldn乫t be obtained if this reaction occurs. Generally, the wettability between the melt and ceramics is bad, so special techniques would be required when you want to obtain the metal matrix composite in which small ceramic particles or whiskers are homogeneously dispersed into the matrix. Needless to say, examination of their microstructure is very important to conjecture their properties.

 

 

Fig. 1丂New phase (gray) formed by the reaction between ceramics fibers (black) and magnesium alloy(white) in the composite fabricated by squeeze casting.

 

 

This section gives an outline of each casting technology to fabricate the composites.

 

Figure 2 shows a schematic illustration of the squeeze casting. Ceramic fiber or particle preform is placed in the permanent mold, then the melt is poured into the mold, followed by applying the high pressure using a plunger while the melt solidifies. The preform consists of the reinforcement and cavity, and the cavity allows the melt to infiltrate into the preform. By forging the melt, the melt infiltration into the preform can be achieved even though the wettability between the melt and the reinforcement is poor. In addition, near net shape with high process speed can be achieved in sound, fully dense castings. Fiber-reinforced aluminum alloy composites produced by the squeeze casting are applied to the parts, such as automobile engines, conveyor rollers for carrying cokes and coals, etc. The melt infiltration with lower pressure (ex. gas pressure) can be accomplished by increasing the size of reinforcement and mixing the metal powder in the preform. Vacuum assist casting is a melt infiltration process using the suction by the reduced pressure. Centrifugal casting is a technique using the centrifugal force in the cylindrical mold to infiltrate the melt into the reinforcement preform or to disperse the reinforcement outside or inside of the castings by the difference in specific gravity.

 

Spontaneous infiltration is a melt infiltration process without pressure. Although this process is the simplest and most advantageous in cost, it is necessary to use the special agent (powder in the preform, atmosphere gas, etc) to accomplish the infiltration because the wettability between the melt and ceramics are generally not good. The thermit reaction between the agent and melt promotes the infiltration. Famous example of this technique is the Lanxide process (PRIMEX process TM, DIMOX process TM) .

 

Vortex method is a technique involving the melt stirring to disperse the reinforcement (particle or whisker) in the matrix. Propeller as illustrated in Fig.3 or electromagnetic force is used to stir the melt. Famous development example of the composite by this technique is DuralcanTM , which SiC or Al2O3 particles are dispersed in aluminum alloy. Compocasting is same technique as the vortex method, except for stirring at the temperature that the matrix consists of solid and liquid. Because the wettability between the melt and ceramics are not good, the wettability must be improved to disperse the reinforcement homogeneously by these techniques. Addition of alloying elements and metal coating on the reinforcement have been attempted to improve the wettability.   

 

In situ process is a technique to synthesize the reinforcement by the reaction between the melt and the additives or atmosphere. The synthesized reinforcement is thermodynamically stable in the melt; the strong bond between the matrix and the reinforcement can be obtained.

 

Cast-in insertion is a technique to join the different materials by pouring the melt into the mold in which the bulk material (reinforcement) is placed. Although the difference with the infiltration process is sometimes unclear, we think that the reinforcement size is relatively large and its macroscopic shape and dimension is functionally significant for the cast-in insertion.

 

Simultaneous pouring is a technique to obtain the structural gradient in the castings by pouring several kinds of the melt in a mold. Cast iron and aluminum alloy castings are fabricated by this technique.

 

Each technology listed in this section is independently used or combined. The materials and shape of the matrix and reinforcement are selected considering applied properties, cost, wettability, bond strength, etc.

 

 

Appearance of short alumina fiber preform乮冇55亊30mm乯

Scanning electron image of the short alumina fiber preform

Squeeze casting

Fig. 2  Schematic illustration of the squeeze casting.

 

 

 

Fig.3  Vortex method

 

 

Examination of microstructure of metal matrix composite

 

Measurement of reinforcement volume fraction in composite

For squeeze casting, the melt infiltration into the preform might cause the contraction and deformation the preform when the preform strength is not enough. For voltex method, some of the added reinforcement into the melt would be ejected due to the bad wettability between the melt and the reinforcement. These things lead to the difference between the expected and obtained volume fraction of the reinforcement in the composite. Therefore, the volume fraction in the composite must be exactly measured. Analyzing of the micrograph of the composite with image analyzer is one of method to measure the volume fraction. The area fraction of the reinforcement can be converted to the volume fraction.  Most popular method would be Archimedean method. Density of the composite can be measured from the buoyancy in a liquid using the Archimedes乫 principle. Accurate value can be easily obtained by this method if the density of matrix and the reinforcement is known.  Porosities or defects must be avoided to obtain the accurate value.

 

Observation of microstructure

Optical and scanning electron micrograph are used to observe the dispersion of the reinforcements in the composite. Figure 4 shows the optical micrographs of the short alumina fiber reinforced aluminum alloy composite fabricated by squeeze casting. Long fibers can be observed in the microstructure parallel to the pressed plane (Fig.4(a)), while sections of the fiber can be observed in the microstructure perpendicular to the pressed plane (Fig.4(b)) ; the nearly planar random nature of the fiber orientation can be clearly seen. In many cases, the fiber distribution in the composite is inherited from the preform.

 

 

(a) Parallel to pressed plane

(b) Perpendicular to pressed plane

 

Fig.4  Optical micrographs of short alumina fiber reinforced aluminum alloy composite fabricated by squeeze casting (fiber volume fraction; 15vol%).

 

 

Bundle, plain or satin fiber are used as the reinforcements for continuous fiber reinforced composites. Squeeze casting is often used to fabricate this composite. For the unidirectionally reinforced composite, the fiber contact is often shown caused by the contraction of the preform during melt infiltration.  The stress concentrates at the points of direct fiber contact, and stress transmission between the fiber and the matrix becomes difficult. To prevent the fiber contact, dispersion of fine particles or whiskers between the fibers in the composite using a preform of the fibers having particles or whiskers attached to their surfaces has been proposed. The composite is called a hybrid composite. Figure 5 shows a SEM image of a preform to fabricate the hybrid composite. The particles can be seen attached to the fiber surfaces thus preventing fiber-to-fiber contact. Figure 6 shows the optical micrographs of alumina continuous fiber reinforced aluminum alloy composite fabricated by squeeze casting (fiber volume fraction; 40vol%).

 

Fig. 5  SEM image of a preform to fabricate the hybrid composite. The alumina particles can be seen attached to the alumina fiber surfaces.

 

    

(a)  Composite without particles

(b)  Hybrid composite

Fig. 6  Optical micrographs of alumina continuous fiber reinforced aluminum alloy composite fabricated by squeeze casting (fiber volume fraction; 40vol%).

 

 

 

Flat surface must be obtained to observe the microstructure. If a specimen is polished for too long on the pad its surface may become rippled due to the difference of the hardness between the ceramic reinforcement and metal matrix. Plating the conductive materials such as carbon, copper or gold on the surface by PVD or ion plating would be effective to observe the microstructure of the composite because many of the ceramic reinforcements and inorganic binder retained on the reinforcement are insulators.  Reaction between the reinforcements and the matrix is sometimes too minor to be detected by the optical microscopy. Scanning and transmission electron microscopy(SEM, TEM) or electron probe X-ray microanalysis (EPMA)  would be effective in that case. Crystallographic information obtained by electron diffraction.  

 

A large number of dislocations can also be seen in the matrix near the fiber in the composite, as shown in Fig. 7. The dislocations are probably formed by the thermal expansion mismatch between the matrix and the fiber. It is reported that the incorporation of ceramic fibers into the heat-treated alloy reduces the incubation time for the nucleation of precipitates by the dislocation, leading to the achievement of the peak hardness in a shorter time.

 

 

Fig. 7  TEM photograph of a region near a fiber-matrix interface in the short alumina fiber reinforced aluminum alloy composite fabricated by squeeze casting. Dislocation can be observed in the aluminum matrix near the fiber (arrows).

 

 

Extracting the reinforcement from the composite by acid or alkali is another method to observe the reinforcement in the composite. Acid or alkali must be carefully selected so that they do not attack the reinforcement. Figure 8 shows SEM image of the extracts from the alumina fiber reinforced aluminum alloy composite by 30% nitric acid. No fiber damage or interfacial reaction can be seen. You can see not only fibers but also eutectic silicon which is originally contained in the aluminum alloy. Some eutectic silicon can be seen near or on the fiber surface. This indicates that the eutectic silicon tends to crystallize on the fiber surface and then grow near the fibers. If the pronounced interfacial reaction occurred as shown in Fig. 1, the reaction products would be seen in the extracts. In this case, it would be possible to identify the product by X-ray diffractometry.

 

 

 

Fig. 8  SEM image of the extracts from the alumina fiber reinforced aluminum alloy composite by 30% nitric acid.

 

 

* This article is based on 乬K. Asano et al.:  Trends of composite casting technology and joining technology for castings in Japan丆International Journal of Cast Metals Research Vol.21(2008), No.1-4(p.219-225)乭.