Magnesium Alloy (AZ91D) Matrix Syntactic Foams Reinforced by Dispersion Particles of Silicon Carbide

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foams made of magnesium alloy and silicon carbide have been studied, but still do not have enough available literature [5][6][11]. This new type of material consists of hollow particles that will be dispersed into the matrix of the composite. The ratio of inner to outer radius of hollow particles is over 0.9 and the volume fractions can be around 60%. The density of this materials is in the range 1–1.5 g/cc, which directly compete with polymer matrix composites [5][6][11].

The properties of MMSFs depend on a number of parameters: particle shell material, shell wall thickness to diameter ratio, matrix alloy, processing parameters, entrapped voids, and heat treatment conditions. The properties of MMSFs can be normalized with respect to the properties of the matrix alloy processed under similar conditions. However, the effect of porosity is the most significant aspect to understand. In most cases, the densities of aluminum and magnesium matrix MMSFs occur within a narrow range or 1–2.2 g/cc and they are expected to compete with each other for applications based on their lower density at the same level of desired mechanical properties.[5][6][7][11]

Cenospheres are the most widely used particles in syntactic foams due to their low cost; they are recovered from ash generated in thermal power plants. The first part of the process they go through is the beneficiation process to remove impurities and select only the low density intact cenospheres particles. However, even with this quality process some particles may come with defects that cannot be noticed before the manufacturing process of the matrix which can cause damage to the structure of the matrix due to unwanted particles proprieties. To solve this problem has been studied the use of hollow particles of alumina, silica, and silicon carbide that can provide advantage in improving interfacial bonding with the matrix [5][7].

Hollow particles are now available in a range of large size from nanometers to several millimeters. The main interest in these hollow particles is due to their more stable property in the process, which reduce the chance of damage in the particles’ structures, and its efficient processing in order to have better compatibility with the matrix metal. Those characteristics allow better properties to the matrix in contrast to the cenospheres [5][7].

The microstructure of AZ91D alloy used in the fabrication of syntactic foams is shown in the figure 1 that follow in annex. There are precipitates of Mg17Al12 intermetallic alloy, that are in the γ-phase, which are seen in α-solid solution in the microstructure. The precipitates are surrounded by a eutectic α + γ mixture. The precipitates are aluminum rich compared to the surrounding matrix. In this alloy system, Mn is also present in the matrix and the formation of precipitates of Al8Mn5 is also observed under appropriate conditions[5][11].

In a recent study about syntactic foams made of magnesium alloys and silicon carbide hollow spherical particles presented quasi-static and dynamic compression properties. The AZ91D/SiCHP syntactic foams presented better performance than aluminum matrix syntactic foams (AMSFs) on the unit weight basis and 44% specific compressive strength higher[5][6][11].

3. The Manufacturing Process

The syntactic foam in general are produced by a process of solidification that has been used for many years. The solidification synthesis of metal matrix foam have many methods that can be summarized by the figure 2 that is in the appendix [5][6][8].

There is other types of manufacturing process that has been applied to other types of materials as Aluminum matrix syntactic foams, that gave been manufactured by melt infiltration, and Iron and Titanium matrix syntactic foams , that have been manufactured by powder metallurgy. Those methods have its own characteristics and requirements and should be just apply to materials that follows those requirements .

The melt infiltration is a rapid process suitable for metals with low melting points. However, this produces creates a dense matrix with the reinforcements distributed uniformly in it. The resulting is a metal matrix syntactic foams with a better structural and compressive properties but the volume of this matrix ratio between the metal matrix and the ceramic spheres cannot be varied. Another method that can be used is the powder metallurgy, it is a longer method in contrast to the melt infiltration , that can be applied to metals with higher melting points. In addition, the volume ratio between the metal matrix and the ceramic spheres can be varied in a wide range and the mechanical properties can be designed to meet the specific needs. However, this method can result in considerable amounts of defects, such as voids and oxides [13].

Therefore, can be seen that high compaction pressures can lead to significant damages to the ceramic spheres. In this research the focus is on the pressureless infiltration technique that was used to produce the syntactic foam composed of magnesium alloy AZ91D reinforced with silicon carbide (SiCHP) that is able to overcome the issues of damages in the particles [6]

The technique of infiltration to produce MMSFs is common due to capacity of it to produce a material with high volume fraction of reinforcement. The infiltration consist of a liquid metal that is forced into the narrow crevices between the surfaces of hallow spheres, which previously has been formed into shape with fixed volume percentages of hallow spheres [5][6][8][11].

The pressure required to infiltrate the preform is determined in function of the liquid vapor surface tension, contact angle of the liquid melt reinforcement couple, size and volume percentage of spherical reinforcement[5][8][11].

In the case of magnesium alloy AZ91D, with melting temperatures around 990ºF, reinforced with SiCHP the infiltration need to be spontaneous due to the fragile reinforcement of the material, because high pressure can cause damage to the particles. The average diameter and wall thickness of the spheres are 2 mm and 130 mm, respectively. In this process the melt and preform operating temperature and atmosphere composition are controlled in such a way to reduce the contact angle between the liquid and the reinforcement [5][6][8][11].

The steps of the pressureless infiltration technique start with the mold cavity with sufficient venting that is filled to maximum with the hollow spheres. The filled mold is closed except for vents where the metal enters. High temperature mesh can be positioned over the entry to prevent the hollow spheres from exiting the chamber during casting. The mold and spheres are preheated to ensure that the molten metal will not freeze in the mold during the spilled process in the mold. The